Investigation of Cracks in 

Acela Coach Car Brake Discs: Test and Analysis 

Volume I - Final Report 

Offices of Safety and 

Research and Development 

Washington, DC 20590

DOT/FRA/ORD-06/07.I November 30, 2005 This document is available to the U.S. 

 Final Report public through the National Technical 

 Information Service, Springfield, VA 
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1. REPORT DATE (DD-MM-YYYY)

November 30, 2005 

2. REPORT TYPE

Final 

3. DATES COVERED (From - To)

April 2005 - November 2005 

4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER 

DTFR-53-02-D-00077 and DTFR53-05-D-00205 

Investigation of Cracks in Acela Coach Car Brake Disc: Test and Analysis 

Volume I – Final Report 

5b. GRANT NUMBER 

5c. PROGRAM ELEMENT NUMBER 

6. AUTHOR(S) 

Sherrock, Eric T.; Owings, Raymond; Clemenzi, Jacinda L.; and Kesler, Kevin 

5d. PROJECT NUMBER 

5e. TASK NUMBER 

Task 8 and Task 6, respectively 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT 

 NUMBER

ENSCO, Inc. 

Applied Technology and Engineering Division 

5400 Port Royal Road 

Springfield, VA 22151 

ENSCO-DOT-FR-05-07 

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)

U.S. Department of Transportation 

Federal Railroad Administration 

Office of Research and Development 11. SPONSOR/MONITOR’S REPORT 

1120 Vermont Avenue, NW - Mail Stop 20 NUMBER(S)

Washington, DC 20590 DOT/FRA/ORD-06/07.I

12. DISTRIBUTION / AVAILABILITY STATEMENT 

This document is available to the public through the National Technical Information Service, Springfield, VA 22161. 

This document is also available on the FRA web site at www.fra.dot.gov. 

13. SUPPLEMENTARY NOTES

14. ABSTRACT

In April 2005, visual and laboratory tests identified cracks in the spokes of several brake discs on coach cars within Amtrak’s Acela trainsets, the high-speed 

trainsets operating on the Northeast Corridor. Amtrak halted operations of the Acela fleet until an assessment of the cracked spokes could be made. With the 

support of the Federal Railroad Administration, Amtrak launched an extensive test program that relied on a cooperative effort between several organizations, 

including the Northeast Corridor Maintenance Services Company, Bombardier, Alstom Transportation, the manufacturers of the brake system, and ENSCO, 

Inc. The test program involved a three-phase over-the-road test effort, finite element analyses, and a series of laboratory tests. The first and second phases 

focused on characterizing the mechanical and thermal load environment associated with WABTEC/SAB-WABCO supplied brake discs employed on the Acela 

equipment. In the third phase, the Knorr Brake Corporation provided a replacement disc, and an axle equipped with brake discs of this alternative design was 

also evaluated. This report documents the background of this issue, as well as the development and implementation of the study. The results of the test 

program, also detailed in this report, allowed for the identification of the Knorr brake disc as an acceptable alternative to the WABTEC/SAB-WABCO supplied 

disc, enabling Amtrak to return the Acela fleet to service. 

15. SUBJECT TERMS

Brake disc, cracked spoke, Acela brake disc, center and outer brake discs, WABTEC/SAB-WABCO supplied brake disc, Knorr brake disc, out-of-plane 

bending, in-plane bending, analysis and testing of brake discs 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION 

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a. REPORT 

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Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39.18
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iii

ACKNOWLEDGEMENTS 

This report documents the efforts of a task force involving many people at several organizations who 

were brought together under the leadership of Mr. David Schramm, Mr. Gregory Gagarin, and Mr. 

Michael Trosino of the National Railroad Passenger Corporation (Amtrak), Dr. Magdy El-Sibaie and Mr. 

Ronald Newman of the Federal Railroad Administration, Mr. Norbert Behety at the NorthEast Corridor 

Maintenance Services Company, Mr. Frank Duschinsky of Bombardier Transportation, and Mr. Richard 

Bowie at Knorr Brake Corporation. 

The authors would like to express their gratitude to Mr. Terry Welsh and the fabrication team at Knorr 

Brake Corporation for their support during this test effort. Although many individuals were committed to 

this intensive effort, the authors also wish to recognize and extend a special thanks to Mr. Michael Tomas 

of Amtrak for his invaluable assistance with instrumentation, testing, and data analysis. 

The investigative team would like to express gratitude to Knorr Brake Corporation and Faiveley 

Transport for their efforts to investigate the causes of the cracked brake discs and present Amtrak with a 

solution to the problem in an open and expeditious fashion. In particular, the leadership provided by Mr. 

Richard Bowie during this investigation proved invaluable. Without his efforts to facilitate 

communications and coordinate the activities of the brake suppliers, the introduction of a practical 

solution to the problem faced by Amtrak would not have occurred in such a timely fashion. 

v

TABLE OF CONTENTS 

List of Figures..................................................................................................................................... vi 

List of Tables ...................................................................................................................................... xiii 

Executive Summary ............................................................................................................................ 1 

1.0 Introduction............................................................................................................................... 5 

1.1 Background ................................................................................................................... 5 

1.2 Test Objectives and Approach ........................................................................................ 8 

1.3 Test Participants .............................................................................................................. 10 

1.4 Report Organization ........................................................................................................ 11 

2.0 Test Method .............................................................................................................................. 13 

 2.1 Over-The-Road Testing................................................................................................... 13 

 2.2 Over-The-Road Testing Sequence .................................................................................. 14 

 2.3 Test Instrumentation........................................................................................................ 15 

 2.4 Component Level Testing ............................................................................................... 38

 2.5 Procedures Used During Over-The-Road Testing .......................................................... 41 

 

3.0 Test Results ............................................................................................................................... 47 

 3.1 Phase 1 Test Results........................................................................................................ 47 

3.2 Phase 2 Test Results........................................................................................................ 72 

 3.3 Phase 3 Test Results........................................................................................................ 95 

 3.4 Further Consideration of Test Results............................................................................. 127 

 

4.0 Observations.............................................................................................................................. 147 

 4.1 Accelerations................................................................................................................... 147 

 4.2 Spoke Strains................................................................................................................... 148 

 4.3 Axle Strains ..................................................................................................................... 149 

 4.4 Temperature of Friction Ring.......................................................................................... 150 

 4.5 Finite Element Analysis Model....................................................................................... 150 

 4.6 Brake Caliper Links......................................................................................................... 150 

5.0 Conclusions............................................................................................................................... 151 

Technical Definitions.......................................................................................................................... 153 

Acronyms and Abbreviations ............................................................................................................. 154 
vi

LIST OF FIGURES 

Figure 1.1 Typical Acela Coach, Bogie, and Brake Disc.............................................................. 6 

Figure 1.2 WABTEC/SAB-WABCO Supplied Brake Disc.......................................................... 6 

Figure 1.3 Example of a Crack in Spoke of WABTEC/SAB-WABCO Supplied Brake Disc 

 Used on Acela Coach Car ............................................................................................ 7 

Figure 1.4 Knorr Brake Disc ......................................................................................................... 9 

Figure 1.5 Spokes of Brake Disc Provided by Knorr .................................................................... 9 

Figure 2.1 Test Consist–Acela Trainset 10 ................................................................................... 13 

Figure 2.2 Spoke Designations for the WABTEC/SAB-WABCO Supplied Brake Disc ............. 15 

Figure 2.3 Location of Strain Gages on WABTEC/SAB-WABCO Supplied Discs During 

 Phase 1 of Study........................................................................................................... 16 

Figure 2.4 Strain Gage Being Applied to a Spoke of the WABTEC/SAB-WABCO Supplied 

 Brake Disc.................................................................................................................... 16 

Figure 2.5 Strain Gage on Surface (Longitudinal Direction) of the Axle with WABTEC/SAB- 

 WABCO Supplied Discs.............................................................................................. 17 

Figure 2.6 Location of Strain Gages on WABTEC/SAB-WABCO Test Axle During Phase 2 

 of Study........................................................................................................................ 18 

Figure 2.7 Illustration of Strain Gage Locations on Knorr Disc Used During Phase 3 of Acela 

 Brake Disc Study ......................................................................................................... 19 

Figure 2.8 Strain Gage Being Applied to Spoke 3 of the Knorr Brake Disc................................. 19 

Figure 2.9a Location of Strain Gages on WABTEC/SAB-WABCO Test Axle During Phase 3 of 

 Acela Brake Disc Study ............................................................................................... 20 

Figure 2.9b Location of Strain Gages on Knorr Test Axle During Phase 3 of Acela Brake Disc 

 Study ............................................................................................................................ 21 

Figure 2.10 Brake Caliper Support Links and Mounting Tube (Shown on Removed Truck 

 for Clarity)................................................................................................................... 22 

Figure 2.11 Tri-Axial Accelerometer Originally Used to Measure Lateral and Vertical 

 Accelerations on Ends of Test Axle............................................................................. 23 

Figure 2.12 Lateral Accelerometer Mounted Near Center Disc on WABTEC/SAB-WABCO 

 Test Axle During Phase 1 of Acela Brake Disc Study................................................. 24 

Figure 2.13 Acceleration Measurement Locations on the Caliper Assemblies............................... 25 

Figure 2.14 Tri-Axial Accelerometer Mounted on Center Brake Caliper Assembly Near Actuator 

 For WABTEC/SAB-WABCO Test Axle, Phase 1 of Acela Brake Disc Study........... 25 
vii

LIST OF FIGURES 

Figure 2.15 Tri-Axial Accelerometer Mounted on Brake Caliper Assemblies Near Brake Pads 

 For WABTEC/SAB-WABCO Test Axle, Phase 1 of Acela Brake Disc Study........... 26 

Figure 2.16 Tri-Axial Accelerometer Mounted on Brake Mounting Tube Over WABTEC/SAB- 

 WABCO/Knorr Test Axles, Phase 3 of Acela Brake Disc Study................................ 27 

Figure 2.17 Tri-Axial Accelerometer on Truck Frame Over Primary Suspension of Knorr 

 Test Axle During Phase 3 of Acela Brake Disc Study................................................. 28 

Figure 2.18 Infrared Sensors Mounted to Measure Surface Temperatures of 

 WABTEC/SAB-WABCO Supplied Discs in Center and Outer Positions During 

 Phase 1 of Acela Brake Disc Study............................................................................. 29 

Figure 2.19 Thermocouple Applied to Spoke on Instrumented WABTEC/SAB-WABCO 

 Supplied Disc During Phase 1 of Acela Brake Disc Study.......................................... 30 

Figure 2.20 Thermocouple Applied on Back of Friction Ring of Instrumented WABTEC/SAB- 

 WABCO Supplied Disc During Phase 1 of Acela Brake Disc Study .......................... 30 

Figure 2.21 Pressure Transducer Used to Measure Brake Cylinder Pressure on Coach Car 3413 . 32 

Figure 2.22 Slip Ring/Resolver Assembly with Wheel Position Output......................................... 33 

Figure 2.23 Instrumented Spoke Phase Based on Wheel Angular Position, WABTEC/SAB- 

 WABCO Supplied Discs ............................................................................................. 34 

Figure 2.24 Instrumented Spoke Phase Based on Wheel Angular Position, Knorr Disc ................ 34 

Figure 2.25 Resolver Output During One Revolution of Instrumented Axle.................................. 35 

Figure 2.26 Example of Video Collected Over WABTEC/SAB-WABCO Test Axle ................... 36 

Figure 2.27 Illustration of Concept of Compressive Pre-Strain ...................................................... 39 

Figure 2.28 Illustration of Typical Setup for Modal Assessment of Brake Discs........................... 40 

Figure 3.1 Out-of-Plane Bending Mode Determined from FEA................................................... 48 

Figure 3.2 Location of Highest Strain for Out-of-Plane Bending Mode Determined from 

 Finite Element Analysis ............................................................................................... 48 

Figure 3.3 Vertical Accelerations Measured on Instrumented Axle 1 Near MP AP 8.5 

 (File 051605_08.ABT)–May 16, 2005......................................................................... 49 

Figure 3.4 Lateral Accelerations Measured on Instrumented Axle 1 Near MP AP 8.5 

 (File 051605_08.ABT)–May 16, 2005......................................................................... 50 

Figure 3.5 Lateral Accelerations Measured on Instrumented Axle 1 Between Boston and 

 Rte. 128 Station (File 051705_02.ABT)–May 17, 2005.............................................. 50 

Figure 3.6a Distribution of Vertical Accelerations Above 50 g Measured on Instrumented 

 Axle 1 with WABTEC/SAB-WABCO Supplied Brake Discs in Trail Position– 

 Washington, DC, to Boston, MA, on May 16, 2005.................................................... 51 

Figure 3.6b Distribution of Vertical Accelerations Above 50 g Measured on Instrumented 

 Axle 1 with WABTEC/SAB-WABCO Supplied Brake Discs in Lead Position– 

 Boston, MA, to Washington, DC, on May 17, 2005.................................................... 52 
viii

LIST OF FIGURES 

Figure 3.7 Designation of Strain Gages Used on Spoke 6 of Center and Outer Instrumented 

 WABTEC/SAB-WABCO Supplied Discs During Phase 1 Testing ............................ 53 

Figure 3.8 Strains on Spoke 6 of Center and Outer WABTEC/SAB-WABCO Supplied 

 Brake Discs During Brake Applications (File 051605_09.ABT)–May 16, 2005 ........ 54 

Figure 3.9 Vertical Accelerations and BOP Strains on Spoke 6 of Center and Outer 

 WABTEC/SAB-WABCO Supplied Brake Discs During Vertical Shock 

 (File 051705_19.ABT)–May 17, 2005......................................................................... 56 

Figure 3.10a Comparison of BOP Amplitudes of WABTEC/SAB-WABCO Supplied Center 

 and Outer Discs Measured During Vertical Impacts, Left End Shock Greater than 

 Right End Shock–May 17, 2005 .................................................................................. 57 

Figure 3.10b Comparison of BOP Amplitudes of WABTEC/SAB-WABCO Supplied Center 

 and Outer Discs Measured During Vertical Impacts, Right End Shock Greater than 

 Left End Shock–May 17, 2005 .................................................................................... 58 

Figure 3.11 Strain Recorded on Gage R1 of Center WABTEC/SAB-WABCO Supplied Brake 

 Disc During Braking Near MP E17 (File 051705_17.ABT)–May 17, 2005 ............... 59 

Figure 3.12 BOP Strain Recorded on Center and Outer WABTEC/SAB-WABCO Supplied 

 Brake Discs During Braking Near MP E17 (File 051705_17.ABT)– 

 May 17, 2005 ............................................................................................................... 59 

Figure 3.13 Terminology Used to Describe Sustained Oscillations During Braking...................... 60 

Figure 3.14 Accelerations Measured on Center Caliper During Normal Brake Application 

 (File 051705_19.ABT)–May 17, 2005......................................................................... 61 

Figure 3.15 Accelerations Measured on Center and Outer Calipers Near Pads During 

 Normal Brake Application (File 051705_19.ABT)–May 17, 2005 ............................. 62 

Figure 3.16 Accelerations Measured on Center Caliper During Brake Application with High 

 BOP Strains (File 051705_17.ABT)–May 17, 2005.................................................... 63 

Figure 3.17 Accelerations Measured on Center and Outer Calipers Near Pads During Brake 

 Application with High BOP Strains (File 051705_19.ABT)–May 17, 2005............... 63 

Figure 3.18 Friction Ring and Spoke Temperatures Measured on Center 

 WABTEC/SAB-WABCO Supplied Brake Disc During Brake Applications 

 (File 051605_09.ABT)–May 16, 2005......................................................................... 64 

Figure 3.19 Temperature Measurements Made During Testing Between Newark, NJ, and 

 Philadelphia, PA–May 17, 2005 .................................................................................. 65

Figure 3.20 Extreme Temperature Measurements Made During Testing Between 

 Wilmington, DE, and Baltimore, MD – May 17, 2005................................................ 66

Figure 3.21 Brake Disc Temperature versus Initial Spoke Tensile Strain Before Brake 

 Applications, Center WABTEC/SAB-WABCO Supplied Disc, Spoke 6– 

 May 17, 2005 ............................................................................................................... 67 

Figure 3.22 Brake Cylinder Pressure and BOP Strains Recorded on WABTEC/SAB-WABCO 

 Supplied Brake Discs During Braking Near MP E17 (File 051705_17.ABT)– 

 May 17, 2005 ............................................................................................................... 68 
ix

LIST OF FIGURES 

Figure 3.23a Strains Measured During Spreader Bar Test, Load Aligned with Spokes 

 of Center Disc .............................................................................................................. 69

Figure 3.23b Strains Measured During Spreader Bar Test, Load Aligned with Spokes 

 of Outer Disc................................................................................................................ 69

Figure 3.24 Comparison of Vertical Shock and BOP Strains Recorded with 3,000 Hz and 

 10,000 Hz Sampling Rates (Files 052605_18.AB2 [3,000 Hz Data] 

 and File 052605_18.001 [10,000 Hz Data])–May 26, 2005 ........................................ 73

Figure 3.25a Distribution of Vertical Accelerations Above 50 g Measured on Instrumented 

 Axle 1 with WABTEC/SAB-WABCO Supplied Brake Discs in Lead Position– 

 Washington, DC, to Boston, MA, on May 26, 2005.................................................... 74

Figure 3.25b Distribution of Vertical Accelerations Above 50 g Measured on Instrumented 

 Axle 1 with WABTEC/SAB-WABCO Supplied Brake Discs in Trail Position– 

 Boston, MA, to Washington, DC, on May 27, 2005.................................................... 74 

Figure 3.26a Distribution of Lateral Accelerations Above 30 g Measured on Instrumented 

 Axle 1 with WABTEC/SAB-WABCO Supplied Brake Discs in Lead Position– 

 Washington, DC, to Boston, MA, on May 26, 2005.................................................... 75 

Figure 3.26b Distribution of Lateral Accelerations Above 30 g Measured on Instrumented 

 Axle 1 with WABTEC/SAB-WABCO Supplied Brake Discs in Trail Position– 

 Boston, MA, to Washington, DC, on May 27, 2005.................................................... 76

Figure 3.27 Peak Lateral and Vertical Accelerations Observed in Near-Constant Speed 

 Segments as a Function of Speed–May 26, 2005......................................................... 77 

Figure 3.28 Vertical to Lateral Acceleration Ratios Determined for Data Collected in 

 Near-Constant Speed Segments at 122 mph–May 26, 2005........................................ 78 

Figure 3.29 Designation of Strain Gages Used on Spoke 6 and Spoke 3 of Instrumented 

 Center WABTEC/SAB-WABCO Supplied Disc During Phase 2 Testing .................. 79 

Figure 3.30 Vertical Shock and BOP Strains Recorded on Center WABTEC/SAB-WABCO 

 Supplied Brake Disc During Phase 2 Testing (File 052605_18.AB2)– 

 May 26, 2005 ............................................................................................................... 80 

Figure 3.31 BOP Strains Recorded During Braking on Center WABTEC/SAB-WABCO 

 Supplied Brake Disc Near MP AB 158 During Phase 2 Testing 

 (File 052605_15.AB2)–May 26, 2005 ......................................................................... 81 

Figure 3.32 BOP Strains on WABTEC/SAB-WABCO Supplied Center Disc Resulting from 

 Vertical Shocks on Both Ends of Test Axle (File 052705_08.AB2)– 

 May 27, 2005 ............................................................................................................... 83 

Figure 3.33 BOP Strains as a Function of Acceleration Differences Measured on 

 May 26, 2005 ............................................................................................................... 84 

Figure 3.34 Correlation of BOP Strain Amplitudes and Acceleration Differences Based on 

 Measurements Collected on May 26, 2005.................................................................. 85 

Figure 3.35 Definition of Wheelset Acceleration Terminology...................................................... 86 

Figure 3.36 Bending Moments in Generic Wheelset ...................................................................... 87 
x

LIST OF FIGURES 

Figure 3.37 Axle Bending Strain as a Function of Bearing Force and Moment Arm ..................... 88 

Figure 3.38 Typical Axle Bending Strains Recorded Over Tangent Track 

 (File 052605_18.AB2)–May 26, 2005 ......................................................................... 88 

Figure 3.39 Axle Bending Strains Recorded During Case of Small Oscillating BOP Strains 

 During Brake Application (File 052605_18.AB2)–May 26, 2005 .............................. 89 

Figure 3.40 Axle Bending Strains Recorded During Response of WABTEC/SAB-WABCO 

 Supplied Brake Disc to Vertical Impact (File 052605_18.AB2)–May 26, 2005 ......... 90 

Figure 3.41 Brake Disc Temperature versus Initial Spoke Tensile Strain Before Brake 

 Applications, Center WABTEC/SAB-WABCO Supplied Disc, Spoke 6– 

 May 26, 2005 ............................................................................................................... 91 

Figure 3.42 Brake Disc Temperature versus Initial Spoke Tensile Strain Before Brake 

Applications, Center WABTEC/SAB-WABCO Supplied Disc, Spoke 6– 

 May 27, 2005 ............................................................................................................... 91 

Figure 3.43 Analysis of Sound Recorded During Sustained Oscillations While Braking Near 

 MP AB 179–May 26, 2005 .......................................................................................... 92 

Figure 3.44 Strain Gage Location and Cross Section Associated with Spoke of Knorr 

 Brake Disc.................................................................................................................... 95 

Figure 3.45 Strain Gage Location and Cross Section Associated with Spoke of 

 WABTEC/SAB-WABCO Supplied Brake Disc.......................................................... 95 

Figure 3.46a Distribution of Vertical Accelerations Above 50 g Measured on Instrumented 

 Axles 1 and 2 in Lead Position–Washington, DC, to Boston, MA, on 

 June 17, 2005 ............................................................................................................... 99 

Figure 3.46b Distribution of Vertical Accelerations Above 50 g Measured on Instrumented 

 Axles 1 and 2 in Lead Position–Boston, MA, to Washington, DC, on 

 June 18, 2005 ............................................................................................................... 100 

Figure 3.47a Distribution of Lateral Accelerations Above 30 g Measured on Instrumented 

 Axles 1 and 2 in Lead Position–Washington, DC, to Boston, MA, on 

 June 17, 2005 ............................................................................................................... 101 

Figure 3.47b Distribution of Lateral Accelerations Above 30 g Measured on Instrumented 

 Axles 1 and 2 in Lead Position–Boston, MA, to Washington, DC, on 

 June 18, 2005 ............................................................................................................... 102 

Figure 3.48 Vertical Shock and BOP Spoke Strain Response of WABTEC/SAB-WABCO 

 Supplied Brake Disc Measured Near Lane Interlocking (File 061805_30.AB3)– 

 June 18, 2005 ............................................................................................................... 103 

Figure 3.49 Vertical Shock and BOP Spoke Strain Response of Knorr Brake Disc Measured 

Near Lane Interlocking (File 061805_30.AB3)–June 18, 2005................................... 104 

Figure 3.50 Comparison of WABTEC/SAB-WABCO Supplied Disc and Knorr Disc 

 During a Brake Application From 116 mph with Sustained Oscillations 

 (File 061805_03.AB3)–June 18, 2005 ......................................................................... 108 
xi

LIST OF FIGURES 

Figure 3.51 Comparison of Cross Sections of Spokes from WABTEC/SAB-WABCO 

 Supplied and Knorr Brake Discs.................................................................................. 108 

Figure 3.52 Comparison of Truck Frame Lateral Accelerations and Axle Strains 

 (File 061805_03.AB3)–June 18, 2005 ......................................................................... 110 

Figure 3.53 PSD of Axle Strains Collected Over Tangent Section of Track 

 (File 061805_03.AB3)–June 18, 2005 ......................................................................... 111 

Figure 3.54 PSD of Axle Strains of WABTEC/SAB-WABCO Supplied Disc and Knorr Disc 

 During a Brake Application from 116 mph with Sustained Oscillations 

 (File 061805_03.AB3)–June 18, 2005 ......................................................................... 112 

Figure 3.55 Temperature Profiles from Back of WABTEC/SAB-WABCO Supplied and 

 Knorr Brake Discs During Shakedown Run (File 061605_18.AB3)– 

 June 16, 2005 ............................................................................................................... 113

Figure 3.56 Comparison of Disc Temperature Changes versus Dissipated Kinetic Energy for 

 Knorr and WABTEC/SAB-WABCO Supplied Brake Discs–June 18, 2005............... 114 

Figure 3.57 Time History and PSD of Brake Mounting Tube Vertical Accelerations During 

 Sustained Oscillations While Braking (File 061805_24.AB3)–June 18, 2005............ 116 

Figure 3.58 Time History and PSD of Brake Mounting Tube Lateral Accelerations During 

 Sustained Oscillations While Braking (File 061805_24.AB3)–June 18, 2005............ 117 

Figure 3.59 Time History and PSD of Brake Caliper Pad Vertical Accelerations During 

 Sustained Oscillations While Braking (File 061805_24.AB3)–June 18, 2005............ 118 

Figure 3.60 Time History and PSD of Brake Caliper Pad Lateral Accelerations During 

 Sustained Oscillations While Braking (File 061805_24.AB3)–June 18, 2005............ 118 

Figure 3.61 Illustration of Instrumented Brake Links and Locations within Truck ........................ 119 

Figure 3.62 Brake Link Strain Level–Force Relationship............................................................... 120 

Figure 3.63 Frequency Spectrum of Strains Recorded on Caliper Link During 

 Sustained Oscillations While Braking (File 061805_24.AB3)–June 18, 2005............ 122 

Figure 3.64 Strains Recorded on Caliper Links During Sustained Oscillations of 

 WABTEC/SAB-WABCO Supplied Brake Disc While Braking 

 (File 061805_24.AB3)–June 18, 2005 ......................................................................... 123 

Figure 3.65 Brake Cylinder Pressure and Strains Recorded on Side of 

 WABTEC/SAB-WABCO Supplied and Knorr Brake Disc Spokes During Short 

 Case of Sustained Oscillations (File 061705_36.AB3)–June 17, 2005 ....................... 124 

Figure 3.66 Relationship of Peak-to-Peak Sustained Oscillations to Brake Cylinder 

 Pressure, Data Collected Throughout Test Effort ........................................................ 124 

Figure 3.67 Analysis of Sound Recorded Over Both Test Axles During Mild Case 

 of Sustained Oscillations on WABTEC/SAB-WABCO Supplied Disc During 

 Braking Near MP AB 179–June 17, 2005.................................................................... 125 

Figure 3.68 Mean Strain versus Alternating Strain, Center and Outer WABTEC/SAB-WABCO 

 Supplied Brake Discs, Spoke 6–May 16, 2005............................................................ 131 
xii

LIST OF FIGURES 

Figure 3.69 Mean Strain versus Alternating Strain, Center and Outer WABTEC/SAB-WABCO 

 Supplied Brake Discs, Spoke 6–May 17, 2005............................................................ 132 

Figure 3.70 Mean Strain versus Alternating Strain, Center WABTEC/SAB-WABCO Supplied 

 Brake Disc, Spokes 6 and 3–May 26, 2005 ................................................................. 133 

Figure 3.71 Mean Strain versus Alternating Strain, Center WABTEC/SAB-WABCO Supplied 

 Brake Disc, Spokes 6 and 3–May 27, 2005 ................................................................. 134 

Figure 3.72 Mean Strain versus Alternating Strain, Center WABTEC/SAB-WABCO 

 Supplied Brake Disc, Spokes 6 and 3–June 17, 2005 .................................................. 135 

Figure 3.73 Mean Strain versus Alternating Strain, Center WABTEC/SAB-WABCO 

 Supplied Brake Disc, Spokes 6 and 3–June 18, 2005 .................................................. 136 

Figure 3.74 Comparison of Goodman Lines for WABTEC/SAB-WABCO Supplied 

 and Knorr Brake Discs................................................................................................. 137 

Figure 3.75 Inception of Sustained Oscillations During Braking, Near MP AP 3.5 

 (File 061705_10.AB3)–June 17, 2005 ......................................................................... 140 

Figure 3.76 Inception of Sustained Oscillations During Braking, Near MP AB 143 

 (File 061705_36.AB3)–June 17, 2005 ......................................................................... 141 

Figure 3.77 Inception of Sustained Oscillations During Braking, Near MP AB 159 

 (File 061805_07.AB3)–June 18, 2005 ......................................................................... 142 

Figure 3.78 Inception of Sustained Oscillations During Braking, Near MP AN 55 

 (File 061805_24.AB3)–June 18, 2005 ......................................................................... 143 
xiii

LIST OF TABLES 

Table 2.1 Summary of Test Runs Conducted on NEC During Acela Brake Disc Test Effort..... 14 

Table 2.2 Data Acquisition Sampling Rates and Filter Settings .................................................. 38 

Table 2.3 Channel Designation During Phase 1 Test Runs, May 16-17, 2005............................ 43 

Table 2.4 Channel Designation During Phase 2 Test Runs, May 26-27, 2005............................ 44 

Table 2.5 Channel Designation During Phase 3 Test Runs, June 17-18, 2005............................ 45 

Table 3.1 Summary of Measurements Collected from Center WABTEC/SAB-WABCO 

 Supplied Brake Disc During Brake Applications Exhibiting Signs of High BOP 

 Strain–May 17, 2005.................................................................................................... 60 

Table 3.2 Summary of Measurements Collected from Center WABTEC/SAB-WABCO 

 Supplied Brake Disc During Brake Applications Exhibiting Signs of High BOP 

 Strain–May 26, 2005.................................................................................................... 82 

Table 3.3 Vibration Frequencies Identified During Impact Tests Conducted on 

 WABTEC/SAB-WABCO Supplied Brake Disc.......................................................... 97 

Table 3.4 Vibration Frequencies Identified During Impact Tests Conducted on Knorr 

 Brake Disc.................................................................................................................... 98 

Table 3.5 Natural Frequencies Identified by Hammer/Strain Measurement Test for the 

 Knorr Brake Disc ......................................................................................................... 98 

Table 3.6 Summary of Observations of High BOP Strain Sustained Oscillations 

 During Braking ............................................................................................................ 105 

Table 3.7 Summary of Measurements Collected from Center WABTEC/SAB-WABCO 

 Supplied Brake Disc During Brake Applications Exhibiting Signs of High BOP 

 Strain–June 17, 2005.................................................................................................... 106 

Table 3.8 Summary of Measurements Collected from Center WABTEC/SAB-WABCO 

 Supplied Brake Disc During Brake Applications Exhibiting Signs of High BOP 

 Strain–June 18, 2005.................................................................................................... 107 

Table 3.9 Properties of Cross Sections of Spokes of WABTEC/SAB-WABCO Supplied 

 and Knorr Brake Discs................................................................................................. 109 

Table 3.10 Estimate of Change in Tension Per Unit Change in Temperature in Spoke 6 of 

 WABTEC/SAB-WABCO Supplied and Knorr Brake Discs–June 17–18, 2005......... 115 

Table 3.11 Details of Braking Events Selected for Further Analysis of Instrumented 

 Caliper Links................................................................................................................ 121 

Table 3.12 Strain Changes in Right and Left Links During Selected Brake Applications ............ 121 

Table 3.13 Key Stress and Strain Values for WABTEC/SAB-WABCO Supplied Brake Disc..... 129 

Table 3.14 Key Stress and Strain Values for Knorr Brake Disc .................................................... 129 

Table 3.15 Number of Cycles Identified in Goodman Analysis of Mean/Oscillating 

 Spoke Strains................................................................................................................ 130 

Table 3.16 List of Cases of Sustained Oscillations During Braking Considered in Detail............ 138 

Table 3.17 Summary of Frequencies Determined During Acela Brake Disc Test Program.......... 144 

1

EXECUTIVE SUMMARY 

In April 2005, visual and laboratory tests identified cracks in the spokes of several brake discs on various 

coach cars within Acela trainsets, the high-speed trainsets operated by the National Passenger Railroad 

Administration (Amtrak) on the Northeast Corridor (NEC). The cracks were initially discovered during 

an inspection of Acela Trainset 17 following its use in the annual recertification test. Crack propagation 

through each of the six spokes joining the friction ring to the hub of a brake disc would lead to a situation 

in which no rigid connection existed between the braking surface of the disc and the hub, and therefore 

the axle itself. Not only would this result in an elimination of contribution to braking effort from a disc, it 

could also result in significant damage caused to the truck, therefore presenting a potential derailment 

scenario should a portion of the disc separate from the axle. This situation would be exacerbated by the 

high speeds associated with the operation of the Acela fleet. Although inspections revealed individual 

spokes in which cracks had propagated completely through the material and exhibited signs of corrosion, 

indicating the existence of the cracks for a significant period of time, no disc within the fleet had the 

braking surface separate from the hub. 

Amtrak decided to halt operations of the Acela fleet until an accurate assessment of the cracked spokes 

could be made. With the support of the Federal Railroad Administration (FRA), Amtrak, working in 

conjunction with Northeast Corridor Maintenance Services Company (NECMSC), Bombardier, Alstom 

Transportation, and the manufacturers of the brake system, Knorr Brake Corporation (Knorr), Faiveley 

Transport (Faiveley) and Wabtec Corporation (Wabtec), launched an extensive test program. ENSCO, 

Inc. was given the responsibility of instrumentation, testing, and analysis. 

The test program was comprised of a three-phase over-the-road test effort, finite element analyses and a 

series of laboratory tests. The initial phase of the over-the-road test effort focused on characterizing the 

mechanical and thermal load environment associated with the WABTEC/SAB-WABCO supplied disc 

employed on the Acela equipment to date. The second phase of the effort employed additional strain 

measurements to focus attention on the interaction of the center disc with the axle and on frequencies 

observed on the WABTEC/SAB-WABCO supplied disc during Phase 1. In the final phase of the effort, 

Knorr provided a replacement disc, and an axle equipped with brake discs of this alternate design was 

included in the test program. Direct comparisons were made between the Knorr disc and the 

WABTEC/SAB-WABCO supplied disc during over-the-road service. 

The following key observations and conclusions were drawn from testing and analysis: 

1. A detailed metallurgical analysis of a section of a cracked brake disc conducted at the onset of the 

investigation revealed that cracks developed due to a progression of bending fatigue that 

originated along a decarburized surface layer. The report states: “Although the decarburized 

surface provided a site for fatigue initiation, the root cause of the cracking is the level of bending 

stresses at the crack sites.” 

The inspection revealed that, with the exception of the thin decarburized surface layer, the brake 

disc casting met mechanical and chemical specifications. 

2. Sustained out-of-plane bending oscillations were observed on the WABTEC/SAB-WABCO 

supplied disc during some brake applications. The sustained out-of-plane bending did not occur 

during all brake applications for the WABTEC/SAB-WABCO supplied disc, and, once it was 

observed, it was not always repeatable during a given test run. The magnitude of the oscillating 

strain was observed to grow proportionally with higher brake pressure. During braking events, 
2

these oscillating strains were added to the mean (tension inducing) strains associated with the 

heating of the disc friction surface. The magnitude of the sustained oscillations on the center disc 

was approximately three times that observed on the outboard disc. Braking-induced oscillations 

were sustained through the duration of brake application. This activity was only observed on the 

WABTEC/SAB-WABCO supplied disc when the test axle was in the leading position of the 

truck. 

This activity was not observed at any time for the Knorr disc. These responses are relevant to the 

fatigue life evaluation. The absence of this activity in the Knorr disc is expected to provide a 

beneficial effect on the fatigue life and a reduction in the likelihood of crack formation. 

3. Accelerations at the journal box were observed at several track locations with most peaks in the 

range of 50 to 100 g vertical and 30 to 50 g lateral. Up to 21 peaks were measured in the vertical 

direction in excess of 100 g per one-way trip between Washington and Boston, with the 

maximum vertical acceleration reaching 189 g. These were generally observed at switches, 

bridge approaches, and other special track work. Because these measurements were made in May 

and June, further stiffening of the ballast as a consequence of cold weather may result in higher 

loads. 

For center and outer WABTEC/SAB-WABCO supplied discs, when an asymmetric vertical 

shock occurred independent of braking, it frequently induced large out-of-plane bending 

oscillations of the spokes. These oscillations were observed to be of similar amplitudes on the 

center and outer discs. These oscillations were observed at a frequency close to the natural outof-plane bending frequency of the brake disc determined during laboratory testing. Because 

shocks of this nature are independent of braking, these oscillations did occur about a wide range 

of mean strain levels. 

A similar response to vertical shocks was observed for the Knorr disc, but the amplitudes of these 

strains were typically 40 percent of the strains observed on the WABTEC/SAB-WABCO 

supplied disc, and these strains damped quicker. These responses are relevant to the fatigue life 

evaluation of the two disc designs. The reduced level of strains observed in the Knorr disc is 

expected to provide a beneficial effect on the fatigue life and a reduction in the likelihood of 

crack formation. 

Lateral shocks did not appear to induce similar oscillations in either of the disc designs tested. 

4. Temperatures of the brake disc friction ring and strains associated with this thermal heating under 

test braking conditions were within normal ranges and on the order of what was expected by the 

brake suppliers. 

5. The installed WABTEC/SAB-WABCO supplied disc retains a compressive preload in the spokes, 

which laboratory tests confirmed. 

6. Finite element analysis indicated the highest strains associated with out-of-plane bending at the 

location of the observed cracking. Discontinuities found on the surface of some spokes arising 

from a mold parting line also appeared in this location. 

Based on the testing performed and subsequent analysis, the conclusion of the test program is that the 

mechanical and thermal responses of the spokes of the Knorr-designed brake disc measured during testing 

with the Acela Express vehicles were within predicted and acceptable values. Although experiencing 

slightly lower thermal strains, the WABTEC/SAB-WABCO supplied brake disc experienced significantly 
3

higher mechanical strains at times. The lower mechanical strains observed in the Knorr designed brake 

disc, in comparison with those observed in the WABTEC/SAB-WABCO supplied brake disc and 

associated with the absence of sustained oscillations during braking and the response to vertical shocks, 

are expected to provide a beneficial effect on the fatigue life associated with the Knorr brake disc design, 

as well as a reduction in the likelihood of crack formation. 

The findings discussed in this report permitted Amtrak to arrive at an acceptable alternative that allowed 

the Acela equipment to return to service. 

5 

1.0 INTRODUCTION 

1.1 Background 

The Acela high-speed trainsets have been an important part of America’s passenger rail service since 

March 1996 when the Amtrak announced the acquisition of the equipment from Bombardier 

Transportation and Alstom Transportation (the Consortium). A fleet of 20 trainsets, each consisting of 6 

passenger-carrying coach cars and 1 power car on each end, currently moves passengers through the NEC 

between Washington, DC, and Boston, MA, at speeds up to 150 mph (240 kph). The trainsets employ 

tilting systems that lean the coach cars into curves at high speeds to provide a comfortable ride for 

passengers. 

Following nearly 1 year of testing on the NEC, FRA granted approval for conditional revenue service 

operations of the Acela trainsets on September 18, 2000, in accordance with the Federal Track Safety 

Standards (49 CFR Part 213). On December 11, 2000, Amtrak introduced the high-speed trainsets into 

service under the name Acela Express. 

In compliance with §213.333 of the Track Safety Standards, tests are conducted annually with a typical 

Acela trainset over the length of the NEC at revenue speeds to confirm the safe operation of the 

equipment. Following a series of technical discussions between Amtrak and FRA, it was decided that the 

annual recertification test conducted in 2005 would be run with a speed profile desired by Amtrak for 

future revenue service; this speed profile employed slightly higher speeds in selected curves over the 

route referred to as cant deficiency testing. The 2005 annual recertification test runs were made between 

Washington and Boston with Acela Trainset 17 over a period from April 11 to 14, 2005. 

On the evening of April 14, 2005, upon successful completion of the last scheduled high cant deficiency 

and qualification run round trip between Washington and Boston, Amtrak’s Acela Express Trainset 17 

was undergoing a post-test general inspection at the Ivy City Maintenance Facility by FRA Inspector 

Steve Clay and FRA Motive Power and Equipment (MP&E) Safety Specialist Richard Thomas, both 

from FRA’s Region 2, when they identified a crack in the spoke of a brake rotor disc utilized on the coach 

cars. Subsequent to finding the first cracked disc spoke, other cracked spokes were identified on Trainset 

17. After a discussion between personnel from FRA’s Headquarters MP&E Division, Region 2, and 

Amtrak, the remaining trainsets were inspected, and pervasive cracking was identified on brake disc 

spokes. Details associated with the initial discovery of the cracks have been well documented.1

 Cracks 

found on the spokes have been classified for the purpose of inspection.2

Figure 1.1 provides a drawing of the WABTEC/SAB-WABCO supplied disc, including an indication of 

its location on an Acela coach car; Figure 1.2 shows photographs of this disc. During braking of the 

Acela trainset, three sets of calipers each force a set of brake pads to make contact with the large friction 

ring on both sides of the three brake discs installed on each axle. 

 

1

Rich Bowie, Definition of Acceptable Cracks for Acela Monoblock Discs, rev. 6, Knorr Brake Corporation, May 

12, 2005, p. 4. 

2

Norbert Behety, TS10 Brake Discs Inspection Procedure, rev. 1, Northeast Corridor Maintenance Services 

Company, May 9, 2005, p. 8. 
6 

Figure 1.1. Typical Acela Coach, Bogie and Brake Disc 

Figure 1.2. WABTEC/SAB-WABCO Supplied Brake Disc 
7 

Figure 1.3. Example of a Crack in Spoke of WABTEC/SAB-WABCO Supplied Brake Disc 

Used on Acela Coach Car 

The brake discs are considered a monoblock design, one in which the disc is made from a one-piece 

casting as opposed to designs that have the friction ring bolted to the hub. Figure 1.3 illustrates an 

example of a crack identified during the inspections. Many cracks observed on the brake discs were in a 

location corresponding to discontinuities found on the surface of some spokes arising from a mold parting 

line. As can be seen by considering the crack shown in Figure 1.3, crack propagation through each of the 

spokes joining the friction ring to the hub of the disc would lead to a situation in which no rigid 

connection exists between the braking surface of the disc and the hub, and therefore the axle itself. Not 

only would this result in an elimination of contribution to braking effort from this disc, it could also result 

in significant damage caused to the truck, therefore presenting a potential derailment scenario, should a 

portion of the disc separate from the axle. This situation would be exacerbated by the high speeds 

associated with the operation of the Acela fleet. Although inspections revealed individual spokes in 

which cracks had propagated completely through the material, exhibiting signs of corrosion, which 

indicated that cracks have propagated for a significant period of time, no disc within the fleet had the 

braking surface separate from the hub. 

Upon discovery of the cracks, Amtrak voluntarily removed all 20 Acela trainsets from service until the 

cracking issue could be further understood and resolved; Acela trainset operations ceased on the morning 

of April 15, 2005. Amtrak, working in conjunction with the NECMSC, the Consortium and the 

manufacturers of the brake system, Knorr, Sab-Wabco (later acquired by Faiveley) and Wabtec, 

determined that additional information related to the cause of the cracks was necessary to determine the 

best course of action and, if necessary, undertake a re-design of the brake disc. 

Wabtec contracted Metallurgical Technologies, Inc., P.A. (MTI) to conduct a detailed analysis of one of 

the cracked discs. 3 A section of a cracked brake disc (S/N 339958) was provided to MTI, and a thorough 

investigation of the nature of the cracks was conducted. The investigation employed several techniques, 

including magnetic particle examinations, cross-sectional metallographic analysis, energy dispersive xray spectrographic analysis, material hardness/tensile-strength testing, and chemical analysis. The 

 

3

 T.C. Tschanz, P.E., and S. Pendergrass, P.E., Analysis of Cast WC700 Brake Disc, Metallurgical Technologies 

Inc., P.A. Report No. 2005381, May 25, 2005, p. 7. 
8 

investigation revealed that cracks developed due to a progression of bending fatigue that originated along 

a decarburized surface layer. The report states: “Although the decarburized surface layer provided a site 

for fatigue initiation, the root cause of the cracking is the level of bending stresses at the crack sites.” 

The inspection revealed that, with the exception of the thin decarburized surface layer, the brake disc 

casting met mechanical and chemical specifications. 

As a result of discussions between Amtrak and FRA, it was agreed that an over-the-road test was 

warranted. ENSCO, Inc. was given the responsibility to instrument Acela Trainset 10 and conduct overthe-road testing to fully quantify the service environment. Appendix A provides the sequence of test 

plans developed during the program. Amtrak and FRA also tasked ENSCO to analyze test data. 

After an intensive 3-month safety review and test program orchestrated by FRA, Amtrak, and the 

equipment suppliers, a rotor of alternative design was identified and accepted that permitted the trainsets 

to be returned safely to service. The first trainset was returned to service on the NEC on July 11, 2005, 

and all trainsets were subsequently retrofitted and returned to service by the end of September 2005. 

This report documents the over-the-road test program, subsequent analysis of the data, and selection of an 

alternative rotor design. 

1.2 Test Objectives and Approach 

The original goals of this test and evaluation were to gather data from which to determine the causes and 

contributing factors associated with the observed cracking of the WABTEC/SAB-WABCO supplied disc 

design and to characterize the load environment associated and the mechanical response of the discs 

during conditions resulting from 7-inch and 9-inch cant deficiency speed profiles on the NEC. As test 

data was gathered, the program evolved primarily into a three-phased effort. 

Phase 1 was focused on characterizing the mechanical and thermal load environment associated with the 

WABTEC/SAB-WABCO supplied disc employed on the Acela equipment to date, identifying the strains 

in the WABTEC/SAB-WABCO supplied disc spokes themselves, and measuring the accelerations 

generated from the vehicle-track interaction associated with train operation. One objective was to 

determine if a relationship existed between accelerations and measured strains, and what strains and 

sources of strain might contribute to spoke failure. A second objective was to characterize the 

environment in terms of the accelerations so that a replacement disc could be developed and applied that 

could be successful in that environment. Phase 1 involved the complete instrumentation of the center and 

an outer disc of a single axle, their calipers, and the accelerations observed on the axle and its bearings. 

As a result of testing, it was observed that some fairly significant strains appeared to be associated with an 

out-of-plane bending of the disc. This bending activity appeared to be associated with both braking and 

impact events and was observed to occur at a frequency of approximately 227 Hz. 

In Phase 2, to better characterize the strains associated with out-of-plane bending, additional strain 

measurements were made on the axle and on opposing spokes of the center disc; the highest strains 

observed during Phase 1 occurred on the center disc. Attention focused on the interaction of the center 

disc with the axle and on the investigation of the resonant frequency associated with the WABTEC/SABWABCO supplied disc and those frequencies observed during Phase 1. Sampling rates higher than those 

in Phase 1 were employed to ensure that all of the relevant information was captured. In addition, some 

laboratory tests were performed to investigate the natural frequencies of the WABTEC/SAB-WABCO 

supplied disc. 
9 

In Phase 3, Knorr identified and provided a replacement disc, and an axle equipped with brake discs of 

this alternate design was included in the test program. Figure 1.4 provides an illustration of the Knorr 

disc; Figure 1.5 shows a photograph of the spokes of the Knorr disc. The goal of this portion of the 

evaluation was to determine if the Knorr disc design demonstrated substantial improvements over the 

original disc design and was likely to be successful in service. The axle with the set of alternative discs 

was placed on an adjacent car within the test train, and the instrumentation suite was expanded to 

accommodate measurements on the axle equipped with the Knorr disc and on the axle equipped with the 

WABTEC/SAB-WABCO supplied disc. A final test series was run, during which direct comparisons 

were made between the Knorr disc and the WABTEC/SAB-WABCO supplied disc in over-the-road tests. 

Data was collected in a manner that would allow decisions to be made about the potential for success of 

the Knorr design in revenue service. 

Figure 1.4. Knorr Brake Disc 

Figure 1.5. Spokes of Brake Disc Provided by Knorr 

Analyses were done in a short turn-around time, some in real-time in the field and others very quickly 

after the test as guidance toward future testing and analysis. In addition to the analysis of the test data, 

finite element analyses were performed for the original WABTEC/SAB-WABCO supplied disc to enable 

a better understanding of the test data. 
10 

1.3 Test Participants 

The test program described in this document was a large effort that required the efforts of many 

individuals from several organizations. The following provides the names of those most directly involved 

in the effort. The lead participant from each organization is italicized. 

Amtrak Bombardier FRA Knorr

Carl Andreason Sylvain Boily Dr. Thomas Blankenship (Safety) Dr. Frank Guenther (GE)

Clare Auvil Suzanne El Zawi Daniel Buckley (Safety) Frank Hellmer (GE) 

Earl Braxton Frank Duschinsky Stephen Carullo (Safety) Dr. Xaver Wirth (GE) 

Bob Costello Virgilio Hilario Steve Clay (Safety) Rich Bowie (US) 

Kathy Costello Denis Oakes Richard Cogswell (R&D Wade McLain (US) 

Ed Finn Mario Raymond Dr. Magdy El-Sibaie(R&D) Markus Seidl (US) 

Greg Gagarin, P.E. Larry Ewing (Safety) Terry Welsh (US) 

John Hines ENSCO Gary Fairbanks (Safety) Dave Welly (US) 

Bob Kennedy Jacinda Clemenzi Mahmood Fateh (R&D) Randall Wingate (US) 

Ed Lombardi (TD) Bill Jordan Leslie Fiorenzo (Safety) 

Al Mahler Kevin Kesler Paul Furman (Safety) NECMSC

Pat Malin Tim Martin Thomas Herrmann (Chf. Counsel) Norbert Behety 

Mark Murphy David McNew Peter Lapre (Safety) Sarabpreet Bumra 

Mike Minnich Boris Nejikovsky Daniel Lewis (Safety) Jeff Cook 

Donald Savidge Dr. Ray Owings John Mardente (Safety) Sylvain Pages 

Dave Schramm Eric Sherrock Shahram Mehrvarzi (R&D) 

Paul Steets Amit Singh Ronald Newman (Safety) Volpe

Mike Tomas Narayana Sundaram Edward Pritchard (Safety) Jeff Gordon 

Michael Trosino Brian Whitten Dr. Satya Singh (Safety) Brian Marquis 

 Richard Thomas (Safety) 

 Faiveley Charles Whalen (Safety) Wabtec

 Mike Cook Bill Slater 

Bjöern Neller 

 

 Notes:

 TD–Test Director 

 GE–Knorr-Bremse Germany 

 US–Knorr-Bremse America 

 
11 

1.4 Report Organization 

Chapter 2 discusses the test methodology, along with a review of the physical measurements recorded 

during testing. Measurements included the following: 

• Strains on brake discs, axles, and links supporting a caliper assembly 

• Temperature of the brake discs 

• Accelerations measured on the test axles, caliper assemblies, and caliper mounting components 

• Pressures throughout the brake system 

• Consist location and speed 

• Video images of the WABTEC/SAB-WABCO supplied disc 

• Sound measurements collected above the test axles 

Chapter 3 presents the results of the analysis of the collected test data; much of the information presented 

in Chapter 3 is supported by detailed analyses provided in the appendices. Chapter 4 presents the major 

observations of the test program. Chapter 5 presents conclusions based on the observations discussed in 

Chapter 4. 

Supplemental material is provided in the appendices. A list of the appendices follows: 

Appendix A. Record of Test Plans–The documentation of the test plans used in the test program. 

Electronic copy of test plans provided on CD-ROM. 

Appendix B. Instrumentation Suite–Information on the sensors used in the test program, including 

calibration sheets and documentation on instrumentation verification. 

Appendix C. Data Descriptions–Documentation providing formats of the data files generated in the test 

effort. 

Appendix D. Test Documents and Logs–Documentation of data files collected during each day of 

testing. Includes daily reports provided by Knorr. 

Appendix E. Finite Element Analysis Results–Supplemental information pertaining to modeling efforts. 

Appendix F. Accelerations–Supplemental information on analysis of accelerations collected during test 

program. 

Appendix G. Spoke Strains–Supplemental information on analysis of spoke strains collected during test 

program. 

Appendix H. Axle Strains–Supplemental information on analysis of axle strains collected during test 

program. 

Appendix I. Temperature–Additional analysis material compiled for temperature measurements 

collected during test program. 

Appendix J. Laboratory Testing–Material compiled from spreader bar testing on WABTEC/SABWABCO test axle at end of Phase 1, of pre-load stress determination efforts conducted by 

Amtrak, and of modal vibration tests conducted during test program. 
12 

Appendix K. Brake Support Links–Detailed treatments of information collected from instrumented links 

used in Phase 3 testing. 

Appendix L. Daily Handouts–Electronic copy of strip charts provided on CD-ROM. 

Appendix M. Background of the WABTEC/SAB-WABCO Supplied Brake Disc–Supplemental 

information provided by Faiveley on the design of its brake disc, including discussions of 

the source of pre-load stresses and the disc’s design. 

 
13 

2.0 TEST METHOD

The test program used three methods of investigating the behavior of the WABTEC/SAB-WABCO 

supplied and Knorr discs. The first and most important was testing of a full trainset under simulated 

revenue service conditions on the NEC. The second was a series of component level tests to investigate 

and explain the behavior observed during the NEC testing. Finally, finite element analyses (FEA) were 

conducted with models of brake discs to gain insight into the response of the discs to different load 

conditions. 

This chapter will focus on the over-the-road and component level tests. 

2.1 Over-the-Road Testing 

Acela Trainset 10 was used during all tests. Figure 2.1 depicts the test consist and the vehicle number of 

all coaches and power cars. The test axle employing the WABTEC/SAB-WABCO supplied discs, 

designated as Axle 1, was installed on the A-end truck of Coach 3413 in the axle 1 position. The test axle 

with the Knorr discs, designated as Axle 2, was located on the B-end truck of Coach 3534 in the axle 4 

position. The following illustrates the position of both test axles. 

All data collection instrumentation was housed in Coach Car 3413 throughout the test effort. 

Figure 2.1. Test Consist–Acela Trainset 10 
14 

2.2 Over-the-Road Testing Sequence 

A total of eight test runs were made during the over-the-road portion of the test program. Table 2.1 

summarizes the test runs made. 

Table 2.1. Summary of Test Runs Conducted on NEC During Acela Brake Disc Test Effort 

Date Test 

Day 

Cant 

Deficiency 

Profile 

(Inches) 

Instrumented Disc 

Axle 

Position 

(w/Respect 

to Test 

Vehicle) 

Description 

Trail Washington to Wilmington 5/14/2005(1) - 7 W/S-W 

(center & outer) Lead Wilmington to Washington 

5/16/2005 1 7 W/S-W 

 (center & outer) Trail Washington to Boston 

5/17/2005 2 7 W/S-W 

 (center & outer) Lead Boston to Washington 

5/26/2005 3 7 W/S-W 

 (center) Lead Washington to Boston 

5/27/2005 4 7 W/S-W 

(center) Trail Boston to Washington 

Lead Washington to New York 6/16/2005 5 7 W/S-W & Knorr 

(center) (center) Trail New York to Washington 

6/17/2005 6 9 W/S-W & Knorr 

(center) (center) Lead Washington to Boston 

6/18/2005 7 9 W/S-W & Knorr 

(center) (center) Lead Boston to Washington 

(1) Shakedown Run 
15 

2.3 Test Instrumentation 

The following sections provide details on measurements recorded at various times throughout the test 

program. Appendix B provides documentation for the sensors discussed in these sections. 

Instrumentation was installed at NECMSC’s Ivy City Maintenance Facility. 

2.3.1 Strains

Strains were measured on various components using single-axis, weldable strain gages designed and 

manufactured for high-temperature applications. The weldable gages, designated as HBWN-35-125-6-

1TR, employed Vishay Measurements’ WK-series self-temperature-compensated foil strain gages and 

were provided by Hitec Products, Inc. Appendix B provides specifications of the foil strain gages. 

Strain measurements were made at the following locations: 

• Phase 1 

Axle 1 from Car 3413, outfitted with WABTEC/SAB-WABCO supplied brake discs, was employed 

during testing. The center disc (serial number 070511) and the disc located closest to the ground 

brush, designated as the G disc (serial number 728610), were used during Phase 1. These brake discs, 

as well as all brake pads on the train, were in a new condition at the start of the test program. 

NECMSC provided train condition and serial number information. 

The strains on each side of a single spoke on the center brake disc and on the outer, or G, brake disc 

were recorded. During Phase 1, spokes 1 and 6 of each of the discs of interest were instrumented; 

spoke 6 was designated as the spoke of interest during the test, while spoke 1 was instrumented to 

serve as a backup in the event of damage to the gages placed on spoke 6. There was no need to 

collect data with the backup strain gages during Phase 1 of the study. All gages were located 1.2” 

below the lower edge of the friction ring, corresponding to the location of several observed cracks. 

Figure 2.2 illustrates the designation of the spokes on the WABTEC/SAB-WABCO supplied brake 

disc. Figure 2.3 provides an illustration of the location of all strain gages employed during Phase 1 of 

the study. Figure 2.4 shows the installation of a weldable strain gage on one of the spokes of a 

WABTEC/SAB-WABCO supplied disc. 

Figure 2.2. Spoke Designations for the WABTEC/SAB-WABCO Supplied Brake Disc 
16 

Figure 2.3. Location of Strain Gages on WABTEC/SAB-WABCO Supplied Discs 

During Phase 1 of Study 

 a) Spoke Grinding Process; b) Gage Being Applied to Web 

 Spoke to Left of Nut Unmodified on Spoke to Left of Nut 

Figure 2.4. Strain Gage Being Applied to a Spoke of the WABTEC/SAB-WABCO 

Supplied Brake Disc 

(Courtesy M. Perkins, Amtrak) 
17 

• Phase 2 

Following the completion of Phase 1, interest existed for focusing on the out-of-plane bending 

observed during the initial stage of the test. In order to accomplish this, the sides of spokes 3 and 4 of 

the WABTEC/SAB-WABCO supplied disc located at the center of the test axle were instrumented 

with strain gages in a manner similar to that employed in preparation for Phase 1. This was done to 

capture the bending of the spoke directly opposite spoke 6 and to determine the phase relationship 

between the strains recorded on the two spokes. As was done in the first part of the study, spoke 6 

and spoke 3 were designated as the spokes of interest during the test. Spoke 1 and spoke 4 served as 

a set of backup spokes, but it was not necessary to use these gages during this phase of the study.

In addition to the new strain gages on spokes of the center disc, four strain gages were mounted to the 

axle, two gages 180o apart as close to the center disc as possible and two gages 180o apart at a 

location approximately 1⁄4 of the distance between the two wheels. According to Knorr, the purpose 

of these gages was to observe the behavior of the axle, including the frequency associated with the 

second mode of axle bending. The strain gages were installed in a manner such that their location 

corresponded to spokes 6 and 3 of the center disc; due to the geometry of the axle, the geometric 

centers of the gages were located 9 inches from each other and centered within the space between the 

two discs. Figure 2.5 shows a picture of an axle-mounted strain gage. Figure 2.6 provides an 

illustration of the location of all strain gages employed during Phase 2 of the study. 

Figure 2.5. Strain Gage on Surface (Longitudinal Direction) of Axle with 

WABTEC/SAB-WABCO Supplied Discs 
18 

Figure 2.6. Location of Strain Gages on WABTEC/SAB-WABCO Test Axle During 

Phase 2 of Study 

In order to accommodate the additional channels of interest, signals from strain gages mounted to the 

outer brake disc were not recorded for the remainder of the study. 

• Phase 3 

Following the completion of Phase 2, a replacement disc was identified and provided by Knorr, and 

an axle equipped with brake discs of this alternate design was included in the series of tests. Brake 

pads used with the Knorr discs were replaced with new pads at the time of installation. The centermounted brake disc (serial number 097740 019) was the focus of the test effort. Knorr provided 

serial number information for this document. 

Strain gages were installed on the center-mounted disc at particular locations on spokes designated by 

Knorr; Figure 2.7 illustrates these locations. The strain gages were located to coincide with the sites 

of highest strain in the spoke under typical operating conditions determined by FEA treatments 

conducted by Knorr. 
19 

 

Figure 2.7. Illustration of Strain Gage Locations on Knorr Disc Used During Phase 3 

of Acela Brake Disc Study 

Gages located near the bottom of the spokes were located 17 millimeters above the surface of the hub; 

gages located at the top of the spoke were located as close to the transition into the friction rings as 

possible. As was done in the earlier parts of the study, spokes 6 and 3 on the Knorr instrumented 

discs were designated as the spokes of interest during the test. Spokes 1 and 4 served as a set of 

backup spokes, but no reason existed to use the gages on spokes 1 and 4 during Phase 3 of the test. 

Figure 2.8 illustrates the installation of a weldable strain gage on the side of spoke 3 of the Knorr 

disc. 

 a) Spoke 3 Following Grinding on Side b) Gage Applied to Side of Spoke 3 

Figure 2.8. Strain Gage Being Applied to Spoke 3 of the Knorr Brake Disc 

(Courtesy J. Gordon, VOLPE)
20 

In addition to gages on the spokes of the Knorr disc, gages were installed on the test axle containing 

the Knorr discs. The location of the gages along the axle corresponded to the location of those 

installed on the WABTEC/SAB-WABCO test axle; on the Knorr test axle, four strain gages were 

installed around the axle near the center-mounted disc separated by 90o. 

Figures 2.9a and 2.9b provide an illustration of the location of all strain gages employed on the 

WABTEC/SAB-WABCO and Knorr test axles during Phase 3 of the study. 

Figure 2.9a. Location of Strain Gages on WABTEC/SAB-WABCO Test Axle During 

Phase 3 of Acela Brake Disc Study 
21 

Figure 2.9b. Location of Strain Gages on Knorr Test Axle During Phase 3 of Acela Brake Disc Study

In addition to strain gages mounted to the instrumented discs and the test axles, strain gages were 

installed on two caliper hanger links (levers) at the request of Bombardier. Figure 2.10 shows the 

caliper hanger links installed on a truck removed from a coach car. One gage was installed on each of 

the hangers employed by the center caliper assembly associated with the WABTEC/SAB-WABCO 

test axle. The gages were installed on the side of the hanger facing the adjacent power car, and the 

center of each gage was located as close to the geometric center of the link as possible. 
22 

Figure 2.10. Brake Caliper Support Links and Mounting Tube 

(Shown on Removed Truck for Clarity)

2.3.2 Accelerations 

Many acceleration measurements were made throughout the designated vehicles during the test program. 

A variety of accelerometers from a number of sources were employed during the program. This section 

will review these measurements. Appendix B provides accelerometer calibration sheets. 

• Phase 1 

The lateral and vertical accelerations experienced by both ends of the WABTEC/SAB-WABCO test 

axle on the non-rotating bearing housing were originally measured using Silicon Designs Model No. 

2430-200 +/- 200 g tri-axial accelerometers (see Figure 2.11). 
23 

Figure 2.11. Tri-Axial Accelerometer Originally Used to Measure 

Lateral and Vertical Accelerations on Ends of Test Axle

During the shakedown run on May 14, the sensor located on the left end of the axle experienced 

trouble. During a stop in Baltimore, MD, the left end of the axle was re-instrumented with two +/- 

250 g single-axis piezoelectric accelerometers manufactured by Kistler and provided by Knorr. 

Amtrak personnel installed ICP® accelerometers, manufactured by PCB Piezotronics, Inc., on the 

ends of the WABTEC/SAB-WABCO test axle before the commencement of the test run on May 16 

for their own monitoring purposes. Amtrak’s instrumentation involved two single-axis 

accelerometers installed on each end of the test axle, one mounted in the lateral direction and one 

mounted in the vertical direction. 

During the trip from Washington to New York on May 16, the replacement accelerometers provided 

by Knorr showed signs of unacceptable noise levels. Upon arrival in Newark, NJ, ENSCO’s data 

acquisition hardware was modified to allow for the recording of the Amtrak accelerometers located 

on the left end of the test axle; the ENSCO-provided accelerometer installed on the right end of the 

test axle continued to be recorded. This configuration of sensors was used throughout the remainder 

of Phase 1 of the effort. 

The lateral accelerations of the WABTEC/SAB-WABCO test axle in the vicinity of the center brake 

disc were recorded using a Silicon Designs Model No. 2410-200 +/- 200 g single-axis accelerometer 

(see Figure 2.12). The sensor was affixed to a mounting block that was held to the axle with a kevlar 

strap. 
24 

Figure 2.12. Lateral Accelerometer Mounted Near Center Disc on WABTEC/SAB-WABCO 

Test Axle During Phase 1 of Acela Brake Disc Study 

At times throughout the study, the Silicon Designs accelerometer mounted to the axle displayed signs 

of saturation while traveling through selected curves. In an effort to improve the reliability of the 

axle lateral acceleration measurement, other types of sensors were evaluated throughout the 

remainder of the test effort. 

The lateral, vertical, and longitudinal accelerations experienced by the brake caliper assemblies 

associated with the center and outer instrumented discs were measured using Silicon Designs Model 

No. 2440-100 +/- 100 g tri-axial accelerometers. On the center caliper assembly, accelerations were 

monitored near one of the brake pads and on the upper portion of the assembly near the actuator. On 

the outer caliper assembly, only accelerations near one of the brake pads were measured. Figure 2.13 

illustrates the locations of these measurements. Figures 2.14 and 2.15 show mounting brackets and 

sensors affixed to the calipers. 
25 

Figure 2.13. Acceleration Measurement Locations on the Caliper Assemblies 

Figure 2.14. Tri-Axial Accelerometer Mounted on Center Brake Caliper Assembly Near Actuator 

for WABTEC/SAB-WABCO Test Axle, Phase 1 of Acela Brake Disc Study 
26 

Figure 2.15. Tri-Axial Accelerometer Mounted on Brake Caliper Assemblies Near Brake Pads 

for WABTEC/SAB-WABCO Test Axle, Phase 1 of Acela Brake Disc Study

• Phase 2 

During the second phase of the study, several modifications were made to the arrangements of the 

accelerometers employed on or near the WABTEC/SAB-WABCO test axle. These changes included 

the following: 

- It was agreed upon by all test participants that the sampling rate of ENSCO’s data collection 

system would be increased to 3,000 Hz and a second data collection system provided by Amtrak 

would be used to record selected channels at 10,000 Hz. Due to the higher sampling rates, it was 

felt that vertical accelerations in excess of 100 g would be observed on the ends of the test axle. 

To accommodate the possibility of accelerations in excess of the range associated with ENSCO’s 

tri-axial accelerometer, the Amtrak-provided accelerometers mounted to the right end of the test 

axle were recorded throughout Phase 2 of the study. 

- ENSCO installed two additional types of accelerometers to the mounting block pictured in Figure 

2.12 in an attempt to improve the reliability of the axle lateral acceleration measurement. The 

details associated with these additional sensors are as follows: 

• In an attempt to avoid mechanical frequencies associated with curving, an accelerometer with 

a high frequency response was obtained. ENSCO obtained a +/- 500 g low impedance 

voltage mode style accelerometer (Model No. 3030B4) from Dytran Instruments, Inc. 

Measurements from this sensor were referred to as AXLELAT2. 

• ENSCO also employed a piezoresistive, or strain-based, accelerometer to improve the 

reliability of lateral acceleration measurements on the axle. The sensor was a +/- 500 g 

accelerometer (Model No. 7264B-500) from Endevco. Measurements from this sensor were 

referred to as AXLELAT3. 

Brake 

Pad

Tri-Axial 

Accelerometer

Brake 

Cylinder

Mounting 

Bracket 

Brake 

Pad

Tri-Axial 

Accelerometer

Mounting 

Bracket 

Brake 

Cylinder
27 

- Due to the shift in focus to the WABTEC/SAB-WABCO supplied disc mounted in the center 

position of the test axle, acceleration measurements near the pad of the caliper mounted above the 

outer disc were no longer collected during Phase 2. 

• Phase 3 

During the third phase of the study, additional accelerometers were employed to allow for 

measurements at new locations, as well as over the additional Knorr test axle. These changes are 

summarized as follows: 

- Amtrak provided five additional ICP accelerometers, manufactured by PCB Piezotronics, Inc., for 

this portion of the effort. Two single-axis accelerometers were installed on each end of the Knorr 

test axle, one mounted in the lateral direction and one mounted in the vertical direction. In 

addition, one single-axis accelerometer was installed on the left end of the WABTEC/SABWABCO test axle in the longitudinal direction. 

- A Silicon Designs Model No. 2440-025 +/- 25 g tri-axial accelerometer was mounted on the 

brake mounting tube over each of the two test axles approximately 5 inches from the center of the 

tube (i.e., near the center caliper assembly). Figure 2.16 shows the location of the sensor over 

one of the test axles. 

Figure 2.16. Tri-Axial Accelerometer Mounted on Brake Mounting Tube Over 

WABTEC/SAB-WABCO/Knorr Test Axles, Phase 3 of Acela Brake Disc Study 

- A Silicon Designs Model No. 2440-025 +/- 25 g tri-axial accelerometer was mounted on the truck 

side frame, over each of the test axles to capture the movement of the truck frame. For the 

WABTEC/SAB-WABCO axle, the sensor was mounted over the L1 wheel; for the Knorr test 

axle, the accelerometer was mounted over the R4 wheel. The sensors were located directly over 

the primary suspension of each of the axles, approximately 8 inches from the end of the frame 

and approximately 4 inches from the side of the frame. Figure 2.17 shows the accelerometer 

mounted over the Knorr test axle. 
28 

 

Figure 2.17. Tri-Axial Accelerometer on Truck Frame Over Primary Suspension of Knorr Test 

Axle During Phase 3 of Acela Brake Disc Study 

- ENSCO installed two Vibra·Metrics Model No. 7002HG2K +/- 500 g accelerometers to monitor 

the lateral accelerations of the test axles near the center-mounted instrumented discs. On the 

Knorr test axle, this sensor was mounted in a fashion similar to the one pictured in Figure 2.11. 

On the WABTEC/SAB-WABCO test axle, the Dytran Instruments accelerometer was replaced 

with a Vibra·Metrics accelerometer to improve the reliability of the axle lateral acceleration 

measurement. During Phase 3 of the study, the Vibra·Metrics accelerometer was referred to as 

AXLELAT2. 

2.3.3 Temperatures

Temperature measurements were made at several locations on the test axle using two basic methods – the 

use of thermocouples and the use of infrared (IR) sensors. This section will discuss the measurement of 

temperatures in this program. 

• Phase 1 

During the initial phase of the study, the temperature of the instrumented WABTEC/SAB-WABCO 

supplied disc in the center position of the test axle was monitored at three locations at various times in 

the test–on the friction rings in contact with the pads, on one of the spokes of the disc, and on the 

back of the friction ring. The temperature of the instrumented WABTEC/SAB-WABCO supplied 

disc in the outer position of the test axle was monitored on the friction rings in contact with the pads. 

Each of the methods used to make these measurements will be discussed here. 

- Temperature measurements were made on the friction surface using two methods: 
29 

• IR Thermometers 

ENSCO employed four IR thermometers (Omega Engineering, Inc. Model No. OS553-V1-4) 

capable of measuring temperatures over a range of 0 °F to 1600 °F (-18 °C to 870 °C). The 

sensors were arranged such that a sensor was placed on each side of the disc of interest. 

Figure 2.18 illustrates the arrangement of the sensors. 

Figure 2.18. Infrared Sensors Mounted to Measure Surface Temperatures of WABTEC/SABWABCO Supplied Discs in Center and Outer Positions During Phase 1 of Acela Brake Disc Study 

The signal from the sensor was routed to a readout device mounted on the edge of the truck 

frame, which in turn supplied a 0-5 VDC signal to the data acquisition hardware for scaling 

and recording. The readout device also allows the user to specify the emissivity of the 

device; emissivity settings of 0.78 were established for all the sensors throughout the test 

program. 

A series of mounting brackets and support frames was fabricated and installed on the truck to 

mount the sensors and the readout devices. 

• Thermocouples 

Following a series of brake tests during the shakedown runs of May 14 and test run 

conducted on May 16, friction surface temperature measurements were made by touching the 

surface in question with a thermocouple affixed to a mounting pole. The thermocouple 

employed was the Omega Engineering, Inc. Model No. 5SRTC-KK-K-24-36 K-type 

thermocouple, capable of collecting temperatures up to 600° F (316° C). The thermocouple 

was attached to the Omega Engineering, Inc. Model No. 450-AKT readout device for all hand 

measurements. 

- Temperature measurements on the instrumented WABTEC/SAB-WABCO supplied disc mounted 

at the center of the test axle were initially made on the spokes instrumented with strain gages with 

the use of an Omega Engineering, Inc. Model No. 5SRTC-KK-K-24-36 K-type thermocouple. 

As observed in Figure 2.19, the thermocouple was welded in the vicinity of the strain gages 

installed on the same spoke. 
30 

Figure 2.19. Thermocouple Applied to Spoke on Instrumented WABTEC/SAB-WABCO 

Supplied Disc During Phase 1 of Acela Brake Disc Study 

Spoke temperatures from the center-mounted WABTEC/SAB-WABCO supplied disc were 

recorded during the shakedown runs of May 14 and the test run between Washington and Boston 

conducted on May 16. 

- Temperature measurements on the back of the friction ring on the instrumented WABTEC/SABWABCO supplied disc in the center position of the test axle commenced at FRA’s request 

following the test runs conducted on May 16. A new thermocouple was installed on the back of 

the friction ring (see Figure 2.20) approximately 3⁄4 of an inch below the edge of the friction ring, 

and this temperature was considered the disc temperature for the remainder of the test program. 

Figure 2.20. Thermocouple Applied on Back of Friction Ring of Instrumented 

WABTEC/SAB-WABCO Supplied Disc During Phase 1 of Acela Brake Disc Study
31 

The thermocouple used for this measurement was the Omega Engineering, Inc. Model No. WTK6-36 thermocouple, capable of measuring temperatures up to 900° F (480° C). 

• Phase 2 

During the second phase of the study, the focus of the test was on the WABTEC/SAB-WABCO 

supplied disc mounted at the center of the test axle. As a result of this change, the IR temperature 

sensors mounted on either side of the instrumented disc at the outer position of the test axle were 

neglected for the second phase of the study. Output from the IR sensors mounted near the center disc 

was recorded throughout the second phase of the test. 

The center disc temperature during the second phase of the test continued to be based on the 

thermocouple mounted near the edge of the friction ring pictured in Figure 2.20. 

• Phase 3 

During the third phase of the study, the focus of the test expanded to include both the 

WABTEC/SAB-WABCO and the Knorr test axles. None of the IR temperature sensors mounted near 

the WABTEC/SAB-WABCO test axle were monitored during Phase 3 because information collected 

during the earlier phases of the test effort indicated that friction surface temperatures were not 

excessive. 

The temperature of the instrumented Knorr disc was monitored in a similar fashion as that employed 

on the WABTEC/SAB-WABCO supplied instrumented disc throughout Phase 2 of the study. The 

type of thermocouple used for this measurement, as well as its location, was the same that was used to 

measure the temperature on the back of the friction ring on the WABTEC/SAB-WABCO supplied 

instrumented disc. 

2.3.4 Pressures

All brake system pressures monitored throughout the test program were measured and recorded using 

Omega Engineering, Inc. Model No. PX41C1-200G10T pressure transducers. The transducers are 

capable of producing an analog signal between 0 and 2.5 volts proportional to a pressure between 0 and 

200 psi. All pressure measurements were achieved by attaching the pressure transducer to one of several 

test points within the brake system. A Knorr test fitting was required for all connections; the T2 position 

on the test fitting was used at all connections. 

The measurements made during each of the phases of the test program are provided below: 

• Phase 1 

During the initial phase of the study, the following parameters were measured for the 

WABTEC/SAB-WABCO test axle: 

- Brake Cylinder Pressure–Measured by connecting a transducer to the test point labeled T1 on the 

manifold in the Dump Valve/Truck Cut-Out compartment at the A-end of Coach Car 3413. 

- Brake Pipe Pressure–Measured by connecting a transducer to the test point labeled BPTP on the 

main brake manifold of Coach Car 3413, located at the B-end of the vehicle. 

- Parking Brake Pressure–Measured by connecting a transducer to the test point labeled PBTP on 

the main brake manifold of Coach Car 3413, located at the B-end of the vehicle. 

Figure 2.21 shows the pressure transducer used to monitor the brake cylinder pressure on Coach Car 

3413. 
32 

Figure 2.21. Pressure Transducer Used to Measure Brake Cylinder Pressure on Coach Car 3413 

• Phase 2 

The same pressures monitored in Phase 1 of the study were monitored during the second phase of the 

study. 

• Phase 3 

During the third phase of the study, only the brake cylinder pressures associated with each of the test 

axles were recorded. The pressure sensor originally used to measure the parking brake pressure on 

Car 3413 was moved to the B-end of Car 3534 and connected to the test point labeled BCTP4. 

2.3.5 Axle Angular Position and Speed 

The slip ring assembly used to transfer strain, accelerometer, and temperature signals from the rotating 

axle to the non-rotating instrumentation station was equipped with an angle resolver, which produces a 

single cycle of an analog sine wave signal for each revolution of the axle. Figure 2.22 shows the slip 

ring/resolver assembly. 
33 

Figure 2.22. Slip Ring/Resolver Assembly with Wheel Position Output 

The output of the resolver was used to determine the angular position of the instrumented brake discs at 

all times. In order to determine the relationship between the output of the resolver and the position of the 

brake discs following the installation of the instrumented axle with the WABTEC/SAB-WABCO 

supplied disc, the axle was positioned over a drop table and the rails were lowered until the wheels no 

longer touched the rail. The signal from the slip ring resolver was then recorded as the wheel was rotated 

by hand, pausing as each spoke was positioned in a straight up position. The axle was rotated towards 

Power Car 2038, the same manner in which the axle would be rotating if Power Car 2038 were the lead 

unit in the consist. This established the important phase relationship between the angular position of the 

strain-gaged spoke 6 of each disc and the sine wave output from the resolver that is shown in Figure 2.23. 

From this relationship, the angular location of spoke 6 for the center and outer discs could be determined 

at any time during the NEC testing. Based on observations, spoke 6 of the instrumented outer disc 

appeared to be approximately 5° from the instrumented center disc in a counter-clockwise direction when 

viewing the test axle from the R1 wheel. 

A similar procedure was used in the determination of the relationship between the output of the resolver 

and the position of the brake discs on the instrumented axle with the Knorr brake discs. Figure 2.24 

shows the results of this exercise. 
34 

Figure 2.23. Instrumented Spoke Phase Based on Wheel Angular Position, 

WABTEC/SAB-WABCO Supplied Discs 

Figure 2.24. Instrumented Spoke Phase Based on Wheel Angular Position, Knorr Disc 

The phase shift between a pure sine wave and the output of the resolver when spoke 6 on the center 

WABTEC/SAB-WABCO supplied disc was straight up was 100°; a small misalignment of the center and 

outer WABTEC/SAB-WABCO supplied discs occurred between approximately 5-10.° The phase shift 
35 

between a pure sine wave and the output of the resolver when spoke 6 on the center Knorr disc was 

straight up was 96.°

As a method of recording speed, the sine wave generated by the resolver located within the slip ring on 

the right side of the WABTEC/SAB-WABCO test axle was recorded throughout the test effort, and a 

simple series of calculations was performed to determine the train’s speed. 

A signal reflective of speed was acquired by determining the time required for the sine wave produced by 

the slip ring/resolver assembly to produce two zero crossings; this represents the time required for the 

wheel to make a complete revolution, denoted as Trev in Figure 2.25. 

Figure 2.25. Resolver Output During One Revolution of Instrumented Axle 

Knowing the time associated with one revolution of the axle and the circumference of the wheel allows 

for the calculation of speed as follows: 

Speed = Circumference of Wheel/Trev

The value of the circumference of the wheel used during this investigation was 9.42 feet, corresponding to 

the tape measurement made before the study. 

2.3.6 Vehicle Location and Speed

Global Positioning System (GPS) information was used to determine location and speed of the test train 

at all times. This information was digitally recorded along with strain, accelerations, temperatures, and 

pressures. The antenna of the GPS receiver was mounted to the roof of Car 3413 near the end of the car 

above the test axle. 
36 

2.3.7 Video System

ENSCO installed a video monitoring system underneath Coach 3413 to record the behavior of the test 

axle with the WABTEC/SAB-WABCO supplied brake discs. The goal of this effort was to capture 

images of any behavior of the WABTEC/SAB-WABCO supplied discs that would lead to an indication of 

the cause of the condition of the spokes. Two PULNiX Model TMC-7DSP color charge-coupled device 

cameras were employed for the effort. Each of the cameras was outfitted with 3.5 mm fixed focal length 

lens and mounted to a platform that was bolted to a support beam under the floor of Coach 3414 above 

the test axle. The cameras were positioned so that each camera had one-half of the test axle in its field of 

view. 

The video signal of interest was used as input to GeoStamp, a device used to overlay location, heading, 

speed, and time information provided by a GPS receiver; the GeoStamp unit is manufactured by Intuitive 

Circuits, LLC. The Flexpak-SSII GPS receiver, manufactured by NovAtel, Inc., was employed for the 

video monitoring system. The video signal with integrated GPS location, heading, speed, and time 

information was displayed on a monitor and recorded with a videocassette recorder throughout the test 

effort. 

Figure 2.26 provides a sample frame from the video recorded over the WABTEC/SAB-WABCO test axle 

near MP 79 on the Philadelphia Division on May 16. Information provided on the video image and the 

views of the brake discs are labeled within the figure. 

Figure 2.26. Example of Video Collected Over WABTEC/SAB-WABCO Test Axle 
37 

Upon review of the video images, it was felt that little value existed in the video images recorded 

throughout the test effort. 

2.3.8 Sound Level Measurements 

During the second phase of testing, sound levels under the test vehicle were recorded using the OROS 

Model OR24 four channel sound and vibration analyzer. Sound level measurements were made to 

attempt to capture and analyze two types of noise: 

• The high frequency noise heard during the movement of the test train through certain curves. A high 

frequency noise, or a squeal, was heard during times in which signs of saturation were observed on 

some or all of the accelerometers mounted directly to the axle. 

• A moaning noise heard above many of the axles of the test train during braking. 

Two microphones were affixed to the camera mounting platform above the WABTEC/SAB-WABCO test 

axle during Phase 2, and the system was configured to record 15 seconds worth of sound upon each 

manual triggering of the system. 

During Phase 3 testing, two additional microphones were installed above the Knorr test axle, for a total of 

four microphones. During this round of testing, 25 seconds worth of sound were recorded upon each 

manual triggering of the system. 

2.3.9 Data Acquisition Hardware Settings

The temperature, strain, acceleration, pressure, and other pertinent measurements were filtered for antialiasing purposes and sampled at various rates during the course of the test program. The analog data was 

digitized in a 12 bit format based on a ± 5 volt input and stored in binary integer format in data files as 

described in Appendix C. Table 2.2 provides a summary of the data sampling rates and anti-alias filter 

settings employed for the test runs. 
38 

Table 2.2. Data Acquisition Sampling Rates and Filter Settings 

Date Segment 

Sampling 

Rate 

(Hz) 

Anti-Alias 

Filter Setting 

(Hz) 

5/14/2005 Washington–Wilmington–Washington (Shakedown 

Run) 1200 > 600 Hz 

5/16/2005 Washington–Boston 2000 500 Hz 

Boston–Baltimore 2000 500 Hz 

5/17/2005 Baltimore (MP96.5)–MP96 4000 1000 Hz 

MP96–Washington 3000 1000 Hz 

Washington–Boston Primary 32-Channel File 3000 750 Hz 5/26/2005 

 Secondary 16-Channel File 10000 No Filter 

Boston–Washington Primary 32-Channel File 3000 750 Hz 5/27/2005 

 Secondary 16-Channel File 10000 No Filter 

6/16/2005 Washington–New York–Washington 

(Shakedown Run) 3000 750 Hz 

6/17/2005 Washington–Boston 3000 750 Hz 

6/18/2005 Boston–Washington 3000 750 Hz 

2.4 Component Level Testing

A series of component level static tests was conducted to assist in the interpretation of the data and results 

from over-the-road testing. The procedures used during these tests will be described here. The following 

chapter will discuss the results from each of these tests. 

2.4.1 Spreader Bar Testing, WABTEC/SAB-WABCO Supplied Discs 

In order to confirm proper operation and assess the response of gages placed on the spokes of the center 

and outer WABTEC/SAB-WABCO supplied discs following the May 17 test run, a spreader bar test was 

conducted. This test was an exercise in which a hydraulic ram and load cell were placed between the 

center brake disc and the outer brake disc, and a series of normal spreading forces were applied to the two 

discs. Spreading forces were applied at the outer circumference of the discs at circumferential positions 

aligned with each spoke of the center disc and in line with each spoke of the outer disc. Strain readings 

were recorded from each disc and are included in Part A of Appendix J. 

A test of a similar nature was conducted on the instrumented Knorr disc on Instrumented Axle 2 before 

Phase 3 testing. This exercise used a hydraulic jack instead of a ram and did not employ a load cell, so no 

measure of applied load was recorded. This test was conducted only to confirm the operation of the strain 

gages installed on the Knorr disc. Results of this exercise confirmed proper operation of the strain gages 

but are not provided in this report. 
39 

2.4.2 Strain Measurements Made During Disc Removal and Installation

During the initial phase of the investigation, it was recognized that a measurement of mean strains in a 

spoke during service was not sufficient to yield a complete picture of the absolute strain, and 

consequently stress, in a spoke. Residual compressive stresses within the disc arising from several 

sources will subtract from those strains resulting from service loads; the sources of these compressive 

stresses are discussed in the document in Appendix M provided by Faiveley. A complete assessment of 

the stresses/strains associated with the discs required knowledge of the residual compressive stress or 

equivalent strain within the spoke. Figure 2.27 illustrates this concept. 

Figure 2.27. Illustration of Concept of Compressive Pre-Strain

During the preparation of Phase 3 tests focusing on both the WABTEC/SAB-WABCO supplied disc and 

the Knorr disc, Amtrak test personnel conducted a series of investigations to determine residual stresses 

within the discs. For the Knorr discs to be employed on the test train, a single axle was removed from 

Coach Car 3524 following the completion of Phase 2 testing with the intent to remove the three 

WABTEC/SAB-WABCO supplied brake disc rotors and replace them with discs of the Knorr design. 

The removed axle was shipped to the ORX facility in Tipton, PA, for the WABTEC/SAB-WABCO 

supplied discs to be pressed off the test axle and for the Knorr discs to be pressed on the test axle. Before 

the removal of the original WABTEC/SAB-WABCO supplied discs, Amtrak personnel installed strain 

gages on both sides of each spoke of the WABTEC/SAB-WABCO supplied center disc. The outputs of 

the strain gages prior to and following the removal of the disc were recorded, resulting in a measure of the 

strains found on the WABTEC/SAB-WABCO supplied disc caused by the pressing operation. 

Compressive Strain Tensile Strain

Compressive

Pre-Strain

Reference (Zero) Strain

For Test Measurements

Material Tensile Strain

Material (Zero) Strain

εp

εt

εm

m t p ε = ε −ε

Compressive

Pre-Strain

εp

Positive Strain Measured During Testing

Negative Strain Positive Strain

εt

Compressive Strain Tensile Strain

Compressive

Pre-Strain

Reference (Zero) Strain

For Test Measurements

Material Tensile Strain

Material (Zero) Strain

εp

εt

εm

m t p ε = ε −ε

Compressive

Pre-Strain

εp

Positive Strain Measured During Testing

Negative Strain Positive Strain

εt
40 

Using a procedure similar to the one described above, Amtrak personnel installed strain gages on both 

sides of each of the spokes of the Knorr brake disc designated for the center position before having it 

pressed on to the test axle. The outputs of the strain gages prior to and following the installation of the 

Knorr designed disc were recorded, resulting in a measure of the strains associated with the Knorrdesigned disc ensuing from the pressing of the disc on to the axle. 

Finally, Amtrak personnel cut all the strain-gaged spokes of the WABTEC/SAB-WABCO supplied center 

disc that was removed from the test axle to relieve any remaining stress in the spokes. The outputs of the 

strain gages prior to and following the severing of the spokes were recorded, resulting in a measure of the 

strains found on a WABTEC/SAB-WABCO supplied disc attributable to the manufacturing process. 

Chapter 3 will present results from these tests, with details of the testing provided in Appendix J. 

2.4.3 Modal Analysis

Simple laboratory vibration tests were performed using the OROS Model OR24 four-channel sound and 

vibration analyzer and associated software to identify some fundamental natural frequencies of both the 

WABTEC/SAB-WABCO supplied disc and the Knorr disc mounted on their respective axles. For these 

tests, each wheelset was free from its respective truck and was resting on the shop floor.

Figure 2.28 provides an illustration of a typical test set-up. A force input was provided to the center disc 

being studied by striking it with a 45-ounce dead blow hammer. Response to the force input was 

measured using accelerometers, magnetically mounted to the disc. In the example shown in Figure 2.27, 

the accelerometers were placed at the top and bottom of the brake disc near the outer edge, 180o across 

from each other. 

Test Conditions - 1 

Hammer Blow Accelerometer

Axis

A2

A1

Figure 2.28. Illustration of Typical Setup for Modal Assessment of Brake Discs

The frequency amplitude spectrum was computed for each acceleration signal over a 1-second window 

immediately following the force input. A comparison of the relationship between the responses of the 

two accelerometers was also computed. Appendix J, Part C, provides details of all of these tests. 

After completion of the Phase 3 over-the-road testing, an additional vibration-based test was conducted 

on the Knorr disc to investigate frequencies observed in the over-the-road test data. The center Knorr disc 
41 

on Instrumented Axle 2, still installed under Coach 3534, was used for this test. In this series of tests, the 

strains on spokes 3 and 6 were recorded and their time histories analyzed. Several successive hammer 

blows applied at approximately 5-second intervals to the friction ring provided excitation. The hammer 

blows were applied near spoke 3 with spoke 6 positioned within the brake pads. During the investigation, 

the brakes were not applied, and no incidental contact occurred between the Knorr disc and the brake 

pads. The results of the analysis of these strains will be presented in the following chapter and are 

detailed in Appendix J, Part C. 

2.5 Procedures Used During Over-the-Road Testing 

Before the commencement of each test, the following procedures were followed to establish/confirm 

instrumentation settings: 

• All strain gage and accelerometer signals were zeroed by collecting a sample of the signals, 

calculating an average reading, and subtracting this reading from all measurements within the data 

collection software. Data was recorded during this step.

• A shunt calibration was performed on all strain gage signals to establish proper scale factors and to 

confirm proper working order of the strain gages (NOTE: Scale factors for all other sensors were set 

based on information contained on calibration sheets.) Data was recorded during this step.

• A known signal (i.e., sine wave) of a fixed frequency was fed into one channel on each data 

acquisition board–anti-alias filter board set in order to confirm proper sampling rate settings. Data 

was recorded during this step.

• A known signal (i.e., sine wave) of variable frequency was fed into one channel on each data 

acquisition board–anti-alias filter board set in order to confirm proper anti-alias filter settings. Data 

was recorded during this step.

• The various temperatures being monitored throughout the test train were reviewed, and any 

temperature that had drifted significantly from any of the others would be checked and adjusted to an 

acceptable level. In the tests in which IR temperature sensors were employed, the emissivity setting 

of the individual sensors was confirmed.

Zeroes were checked every morning; strain gage signals typically varied by approximately 10 microstrain from the end of one test day to the beginning of the next test day. Zero settings of the sensors were 

not typically adjusted over the course of a test unless noted in the test logs. 

2.5.1 Data Processing Methods

ENSCO used two data processing programs for analyzing the data from the Acela brake disc tests, 

DaDisp and Matlab. DaDisp (www.dadisp.com) allows manipulation of large data sets and was used to 

create daily stripcharts and most of the graphs in this document. Matlab (www.mathworks.com) is a 

scripting language for mathematical analysis. Matlab was used to combine all the files for a test day and 

search through the data for interesting events. 
42 

When it was necessary to consider the frequency domain of certain data elements, a power spectral 

density (PSD) of the data was prepared. PSDs illustrated in this document were prepared by selecting an 

area of interest from the data set and extracting 16,384 points; it was important for the number of points to 

be a power of two. The PSD was then calculated on the extracted data, and the mean, or average, was 

removed. The y-axis of the plots are expressed in decibels (dB): 

y = ⋅ (PSD(x − x)) 10 10 log

where this expression is based on the definition of dB. Data was smoothed on selected plots by 

employing a five-point moving average. 

Stripcharts were produced each day for each data file. They consisted of each of the main sensors and 

were saved into PowerPoint files, which have been included on CD-ROM with this report in Appendix L. 

Stripcharts were created using DaDisp through May 26 and in Matlab for the remainder of the days of 

testing. 

2.5.2 Channel Assignments During Over-the-Road Testing

Tables 2.3, 2.4 and 2.5 provide summaries of the channel designations/assignments for tests conducted 

during Phases 1, 2 and 3, respectively. Appendix C provides formats of the recorded data files. 
43 

Table 2.3. Channel Designation During Phase 1 Test Runs, May 16-17, 2005 

Channel Short 

Name 

Chan 

No. Parameter Units 

CTRSPKF1 1 Strain gage, W/S-W center disc, spoke 6, F1 side uε

CTRSPKF2 2 Strain gage, W/S-W center disc, spoke 6, F2 side uε

CTRSPKR1 3 Strain gage, W/S-W center disc, spoke 6, R1 side uε

CTRSPKR2 4 Strain gage, W/S-W center disc, spoke 6, R2 side uε

OUTSPKF1 5 Strain gage, W/S-W outer disc, spoke 6, F1 side uε

OUTSPKF2 6 Strain gage, W/S-W outer disc, spoke 6, F2 side uε

OUTSPKR1 7 Strain gage, W/S-W outer disc, spoke 6, R1 side uε

OUTSPKR2 8 Strain gage, W/S-W outer disc, spoke 6, R2 side uε

CTRSPKTEMP 9 Thermocouple, W/S-W center disc, spoke 6 °F 

CTRRTRTEMPL 10 Temperature, IR, W/S-W center disc braking surface °F 

CTRRTRTEMPR 11 Temperature, IR, W/S-W center disc braking surface °F 

OUTRTRTEMPL 12 Temperature, IR, W/S-W outer disc braking surface °F 

OUTRTRTEMPR 13 Temperature, IR, W/S-W outer disc braking surface °F 

RAWSPEED 14 Speed sine wave from resolver Volts 

PROCSPEED 15 Computed Speed from resolver sine wave Mph 

AXLELAT 16 Lateral Acceleration, axle-mounted near W/S-W center disc g 

LBOXLAT 17 Lateral Acceleration, Axle Box left, Axle 1 g 

LBOXVERT 18 Vertical Acceleration, Axle Box left, Axle 1 g 

RBOXLAT 19 Lateral Acceleration, Axle Box right, Axle 1 g 

RBOXVERT 20 Vertical Acceleration, Axle Box right, Axle 1 g 

CTRCALPLAT 21 Lateral Acceleration, center caliper, near pad g 

CTRCALPVERT 22 Vertical Acceleration, center caliper, near pad g 

CTRCALPLONG 23 Longitudinal Acceleration, center caliper, near pad g 

CTRCALALAT 24 Lateral Acceleration, center caliper, near actuator g 

CTRCALAVERT 25 Vertical Acceleration, center caliper, near actuator g 

CTRCALALONG 26 Longitudinal Acceleration, center caliper, near actuator g 

OUTCALPLAT 27 Lateral Acceleration, outer caliper, near pad g 

OUTCALPVERT 28 Vertical Acceleration, outer caliper, near pad g 

OUTCALPLONG 29 Longitudinal Acceleration, outer caliper, near pad g 

PIPEPRESS 30 Brake Pipe Pressure psi 

CYLPRESS 31 Brake Cylinder Pressure, center caliper psi 

PARKPRESS 32 Brake Park Pressure, center caliper psi 
44 

Table 2.4. Channel Designation During Phase 2 Test Runs, May 26-27, 2005 

Channel Short 

Name 

Chan 

No. Parameter Units

CTRSPK6F1 1 Strain gage, W/S-W center disc, spoke 6, F1 side uε

CTRSPK6F2 2 Strain gage, W/S-W center disc, spoke 6, F2 side uε

CTRSPK6R1 3 Strain gage, W/S-W center disc, spoke 6, R1 side uε

CTRSPK6R2 4 Strain gage, W/S-W center disc, spoke 6, R2 side uε

CTRSPK3R1 5 Strain gage, W/S-W center disc, spoke 3, R1 side uε

CTRSPK3R2 6 Strain gage, W/S-W center disc, spoke 3, R2 side uε

AXLECSPK6 7 Strain gage, axle near center, disc adjacent spoke 6 uε

AXLECSPK3 8 Strain gage, axle near center, disc adjacent spoke 3 uε

AXLEOSPK6 9 Strain gage, axle near 1⁄4 location, adjacent spoke 6 uε

AXLEOSPK3 10 Strain gage, axle near 1⁄4 location, adjacent spoke 3 uε

CTRDSKTEMP 11 Thermocouple, back of friction ring °F 

CTRRTRTEMPL 12 Temperature, braking surface, IR, center rotor °F 

CTRRTRTEMPR 13 Temperature, braking surface, IR, center rotor °F 

SINE 14 Speed sine wave from resolver volts 

PROCSPEED 15 Computed Speed from resolver sine wave mph 

AXLELAT 16 Lateral Acceleration, axle mounted near rotor g 

LBOXLAT 17 Lateral Acceleration, Axle Box left g 

LBOXVERT 18 Vertical Acceleration, Axle Box left g 

RBOXLAT 19 Lateral Acceleration, Axle Box right g 

RBOXVERT 20 Vertical Acceleration, Axle Box right g 

CTRCALPLAT 21 Lateral Acceleration, center caliper, near pad g 

CTRCALPVERT 22 Vertical Acceleration, center caliper, near pad g 

CTRCALPLONG 23 Longitudinal Acceleration, center caliper, near pad g 

CTRCALALAT 24 Lateral Acceleration, center caliper, near actuator g 

CTRCALAVERT 25 Vertical Acceleration, center caliper, near actuator g 

CTRCALALONG 26 Longitudinal Accel, center caliper, near actuator g 

AXLELAT2 27 Lateral Acceleration, axle-mounted (piezo-electric) g 

SYNC 28 File Synchronization signal volts 

AXLELAT3 29 Lateral Acceleration, axle (strain-based) g 

PIPEPRESS 30 Brake Pipe Pressure psi 

CYLPRESS 31 Brake Cylinder Pressure, center caliper psi 

PARKPRESS 32 Brake Park Pressure, center caliper psi 

NOTE: Channels Recorded on with 10,000 Hz data collection system 
45 

Table 2.5. Channel Designation During Phase 3 Test Runs, June 17-18, 2005 

(NOTE: Axle 1–WABTEC/SAB-WABCO Supplied Brake Discs, Axle 2–Knorr Brake Discs)

Channel Short 

Name 

Chan 

No. Parameter Axle Units 

AXLELAT1-1 1 Lateral Acceleration, axle mounted 1 g 

TRFLLAT1 2 Lateral Acceleration, Truck Frame Left 1 g 

TRFLVERT1 3 Vertical Acceleration, Truck Frame Left 1 g 

TRFLLONG1 4 Longitudinal Acceleration, Truck Frame Left 1 g 

BRMTLAT1 5 Lateral Acceleration, Brake Mounting Tube 1 g 

BRMTVERT1 6 Vertical Acceleration, Brake Mounting Tube 1 g 

SINE1 7 Sine wave from resolver 1 volts 

SINE2 8 Sine wave from resolver 2 volts 

BRMTLONG1 9 Longitudinal Acceleration, Brake Mounting Tube 1 g 

CTRCALPLAT1 10 Lateral Acceleration, center caliper, near actuator 1 g 

CTRCALPVERT1 11 Vertical Acceleration, center caliper, near actuator 1 g 

CTRCALPLONG1 12 Longitudinal Accel, center caliper, near actuator 1 g 

CYLPRESS1 13 Brake Cylinder Pressure, center caliper 1 psi 

CTRDSKTEMP1 14 Thermocouple, center rotor 1 °F 

SYNC 15 File Synchronization signal volts 

AXLELAT2 16 Lateral Acceleration, axle mounted 2 g 

TRFLLAT2 17 Lateral Acceleration, Truck Frame Left 2 g 

TRFLVERT2 18 Vertical Acceleration, Truck Frame Left 2 g 

TRFLLONG2 19 Longitudinal Acceleration, Truck Frame Left 2 g 

BRMTLAT2 20 Lateral Acceleration, Brake Mounting Tube 2 g 

BRMTVERT2 21 Vertical Acceleration, Brake Mounting Tube 2 g 

BRMTLONG2 22 Longitudinal Acceleration, Brake Mounting Tube 2 g 

CTRCALPLAT2 23 Lateral Acceleration, center caliper, near actuator 2 g 

CTRCALPVERT2 24 Vertical Acceleration, center caliper, near actuator 2 g 

CTRCALPLONG2 25 Longitudinal Accel, center caliper, near actuator 2 g 

CTRDSKTEMP2 26 Thermocouple, center rotor 2 °F 

CYLPRESS2 27 Brake Cylinder Pressure, center caliper 2 psi 

CTRSPK6F1 28 Strain gage, center disc spoke 6, F1 1 uε

CTRSPK6F2 29 Strain gage, center disc spoke 6, F2 1 uε

CTRSPK6R1 30 Strain gage, center disc spoke 6, R1 1 uε

CTRSPK6R2 31 Strain gage, center disc spoke 6, R2 1 uε

BAD 32 Bad channel, unused 
46 

Table 2.5. Channel Designation During Phase 3 Test Runs, June 17-18, 2005 

(NOTE: Axle 1–WABTEC/SAB-WABCO Supplied Brake Discs, Axle 2–Knorr Brake Discs) 

(Cont’d) 

Channel Short 

Name 

Chan 

No. Parameter Axle Units 

CTRSPK3R1 33 Strain gage, center disc spoke 3, R1 1 uε

CTRSPK3R2 34 Strain gage, center disc spoke 3, R2 1 uε

AXLECSPK6 35 Strain gage, axle near center disc, adjacent spoke 6 1 uε

AXLECSPK3 36 Strain gage, axle near center disc, adjacent spoke 3 1 uε

AXLEOSPK6 37 Strain gage, axle near 1⁄4 location, adjacent spoke 6 1 uε

AXLEOSPK3 38 Strain gage, axle near 1⁄4 location, adjacent spoke 3 1 uε

AXLE1LLINK 39 Strain gage, center caliper left 1 uε

AXLE1RLINK 40 Strain gage, center caliper right 1 uε

CTR2SPK6R1 41 Strain gage, center disc spoke 6, R1 (SG1) 2 uε

CTR2SPK6R2 42 Strain gage, center disc spoke 6, R2 (SG2) 2 uε

CTR2SPK3R1 43 Strain gage, center disc spoke 3, R1 (SG3) 2 uε

CTR2SPK3R2 44 Strain gage, center disc spoke 3, R2 (SG3a) 2 uε

CTR2SPK6_4 45 Strain gage, center disc spoke 6 face, upper gage (SG4) 2 uε

CTR2SPK6_5 46 Strain gage, center disc spoke 6 face, lower gage (SG5) 2 uε

CTR2SPK6_6 47 Strain gage, center disc spoke 4, R2 position (SG6) 2 uε

AXLE2CSPK6 48 Strain gage, axle near center disc, adjacent spoke 6 2 uε

AXLE2CSPK3 49 Strain gage, axle near center disc, adjacent spoke 3 2 uε

AXLE2CSPK6+90 50 Strain gage, axle near 1⁄4 location, adjacent spoke 6 2 uε

AXLE2CSPK6-90 51 Strain gage, axle near 1⁄4 location, adjacent spoke 3 2 uε

AXLE2OSPK6 52 Strain gage, axle near center disc adjacent spoke 6 +90° 2 uε

AXLE2OSPK3 53 Strain gage, axle near center disc adjacent spoke 6 -90° 2 uε

LBOXLAT1 54 Lateral Acceleration, Axle Box left 1 g 

LBOXVERT1 55 Vertical Acceleration, Axle Box left 1 g 

RBOXLAT1 56 Lateral Acceleration, Axle Box right 1 g 

RBOXVERT1 57 Vertical Acceleration, Axle Box right 1 g 

LBOXLAT2 58 Lateral Acceleration, Axle Box left 2 g 

LBOXVERT2 59 Vertical Acceleration, Axle Box left 2 g 

RBOXLAT2 60 Lateral Acceleration, Axle Box right 2 g 

RBOXVERT2 61 Vertical Acceleration, Axle Box right 2 g 

AXLELAT1-2 62 Lateral Acceleration 2, axle mounted 1 g 

AXLELAT1-3 63 Lateral Acceleration 3, axle mounted 1 g 

AXLE1LLONG 64 Longitudinal Acceleration, axle mounted 1 g 

Speed 65 Calculated Speed for SINE 1 mph 
47 

3.0 TEST RESULTS

This section will provide an overview of the results determined from analysis of the data collected and 

events observed during this test program. More detailed results, as well as the original data sets, are 

contained in the various appendices to this report. Appendix D provides logs generated during each of 

these test runs. 

Section 3.1 presents results determined during Phase 1 testing; Section 3.2 provides results from Phase 2 

testing; and Section 3.3 includes results from Phase 3 testing. Each section provides information of the 

various quantities studied during the over-the-road testing, as well as results from any component level 

testing conducted during a particular phase of the investigation. Section 3.4 provides results compiled 

from data collected from multiple phases of testing.

3.1 Phase 1 Test Results 

Phase 1 involved the instrumentation of two WABTEC/SAB-WABCO supplied discs, the center and an 

outer disc of a single axle, their calipers, and the accelerations observed on the axle and its bearings. The 

objective of Phase 1 was the characterization of the mechanical and thermal load environment associated 

with the WABTEC/SAB-WABCO supplied discs. Following a shakedown run, a round-trip test was 

conducted between Washington and Boston on May 16 and 17. An FEA focusing on the 

WABTEC/SAB-WABCO supplied disc and a spreader bar test were also part of this phase of the effort. 

3.1.1 Finite Element Analysis 

During the preparation for the first set of over-the-road tests, ENSCO conducted an FEA of the 

WABTEC/SAB-WABCO supplied disc to gain insight into the response of the disc to various loadings. 

ENSCO developed a solid model of the WABTEC/SAB-WABCO supplied disc based on measurements 

made on removed brake discs using AutoDesk Inventor v. 9.0. This model was then turned into a finite 

element model, and ENSCO engineers used COSMOS DesignSTAR v. 4.5 to analyze its response to 

mechanical loading. The different modes of vibration and the frequencies associated with that vibration 

were then identified. ENSCO then provided the model to MSC Software Ltd. to confirm the results of 

ENSCO’s mechanical response analysis and to conduct an analysis of the response of the disc to heat 

input that would result from braking, as well as stresses that would result from high rotational speeds. 

ENSCO and MSC identified two different fundamental modes of vibration associated with the model of a 

single WABTEC/SAB-WABCO supplied disc. One fundamental frequency was calculated to be 206 Hz 

and was associated with the situation in which the hub of the brake disc was fixed (i.e., prevented from 

moving). This frequency was associated with a motion in which the disc vibrates outside of the plane of 

rotation; Figure 3.1 illustrates this vibration. The other fundamental frequency that was identified was 

585 Hz and was associated with the situation in which the hub of the brake disc was free to move. The 

motion associated with this frequency resembles the opening and closing of an umbrella. 

The second fundamental frequency associated with the case in which the hub of the disc was fixed was 

calculated to be 269 Hz. The motion associated with this frequency also resembled the opening and 

closing of an umbrella. Because this was determined using a model with a fixed hub, it is likely that the 

motion associated with this frequency is associated with excitation in the lateral direction. 
48 

Figure 3.1. Out-of-Plane Bending Mode Determined from FEA

An important result of the analysis was the identification of the location on the spoke at which the highest 

strains could be expected. This location is highlighted in red in Figure 3.2. This finding was important 

for two reasons–this location was close to the location of many of the cracks observed on the brake discs 

identified to date, and it corresponded to the location in which discontinuities arising from a mold parting 

line found on the surface of some spokes was observed. 

Figure 3.2. Location of Highest Strain for Out-of-Plane Bending Mode 

Determined from FEA 

The FEA predicted tensile strains in the spokes as a result of the increase in temperature due to heat 

conduction from the friction ring during braking. In this analysis, the disc was stationary and not 
49 

subjected to mechanical loads. The analysis also predicted a relatively low level of strain due to the 

rotation of the disc. Appendix E provides additional details associated with the FEA. 

Although the numerical studies highlighted above considered only a single disc and did not account for 

the entire brake disc/axle assembly or any effects of the mounting process or long-term usage, 

information provided by the FEA was very useful during the analysis of data collected during over-theroad testing. 

3.1.2 Axle Accelerations 

The test plan for Phase 1 testing called for the characterization of the dynamic environment of the 

wheelset and consequently the brake disc, in typical revenue service. Accelerometers were mounted to 

each end of the test axle, on the axle itself near the center brake disc, and on the calipers over the center 

and one outboard brake disc. Appendix F provides detailed analyses of accelerations measured 

throughout the test effort. In general, accelerations recorded during the tests were analyzed using 

percentile levels, peak levels, and PSDs.

As discussed in Chapter 2, the vertical load environment associated with the test axle was measured with 

accelerometers mounted on the bearing end caps located at both ends of the axle. During the early part of 

the initial test run between Washington and Boston on May 16, the accelerometers oriented in the lateral 

and vertical directions on the left end of the test axle displayed an unsatisfactory signal-to-noise ratio. 

Figure 3.3 provides an illustration of vertical accelerations measured between MP 8 and MP 9 south of 

Philadelphia. 

Figure 3.3. Vertical Accelerations Measured on Instrumented Axle 1 Near MP AP 8.5 

(File 051605_08.ABT)–May 16, 2005
50 

Figure 3.4 illustrates lateral accelerations measured on the axle at the same geographic location. 

 

Figure 3.4. Lateral Accelerations Measured on Instrumented Axle 1 Near 

MP AP 8.5 (File 051605_08.ABT)–May 16, 2005 

To address the issue observed on the accelerations measured on the left end of the axle, ENSCO’s data 

acquisition hardware was modified upon arrival in Newark, NJ, to allow for the recording of the Amtrak 

accelerometers previously mounted on the left end of the test axle for Amtrak’s monitoring purposes. 

The test was then able to proceed with satisfactory results. 

Another issue associated with measured axle accelerations was identified during the initial phases of the 

test–an apparent saturation of the signal produced by the axle mounted accelerometer intended to measure 

lateral accelerations, resulting in extremely high accelerations not observed at other locations on the axle. 

This effect is noticeable in Figure 3.4 between t=800 and t=900; Figure 3.5 shows a more severe example 

of this effect. 

Figure 3.5. Lateral Accelerations Measured on Instrumented Axle 1 Between 

Boston and Rte 128 Station (File 051705_02.ABT)–May 17, 2005

Consideration of the issue with the axle-mounted accelerometer led to the observations that this high 

amplitude signal only appeared while the test vehicle was moving through a curve and was usually 
51 

accompanied by an audible, high-pitched squealing. It was decided that these particular instances of high 

amplitude lateral accelerations should be disregarded and that lateral accelerations on the axle would be 

measured using sensors of alternative design during subsequent testing. 

With these issues in mind, a significant amount of information still existed regarding the load 

environment associated with the revenue service operations that was learned from the axle-mounted 

accelerometers. High magnitude shocks were identified by counting all the cases in which one or more 

consecutive data values exceed a given acceleration threshold. Each acceleration channel was analyzed 

independently, and values appearing to result from electrical noise or other instrumentation issues were 

neglected. Figure 3.6a illustrates the distribution of vertical accelerations above 50 g recorded on the test 

axle from Washington to Boston on May 16 with the test axle in a trailing position; Figure 3.6b illustrates 

the corresponding information for the data collected between Boston and Washington on May 17 with the 

test axle in a leading position. The data illustrated in Figure 3.6 is tabulated in Appendix F. 

Accelerations above 50 g were reviewed to ensure that any acceleration affected by the noise-related issue 

highlighted in Figure 3.2 was disregarded. 

Figure 3.6a. Distribution of Vertical Accelerations Above 50 g Measured on Instrumented Axle 1 

with WABTEC/SAB-WABCO Supplied Brake Discs in Trail Position– 

Washington, DC, to Boston, MA, on May 16, 2005
52 

Figure 3.6b. Distribution of Vertical Accelerations Above 50 g Measured on Instrumented Axle 1 

with WABTEC/SAB-WABCO Supplied Brake Discs in Lead Position– 

Boston, MA, to Washington, DC, on May 17, 2005 

As indicated in the plots provided in Figure 3.6, 172 vertical shocks above 50 g were measured on the 

WABTEC/SAB-WABCO test axle during the northbound test conducted on May 16, 92 on the left end 

axle box and 80 on the right end axle box, and 161 vertical shocks above 50 g were measured during the 

southbound test conducted on May 17, 79 on the left end axle box and 82 on the right end axle box. 

The highest vertical acceleration observed in Phase 1 testing was 126 g, measured on the left end of the 

axle on May 16 near Ham Interlocking at MP AN 55.5 while the test train was traveling at almost 111 

mph. Appendix F provides the details associated with all the accelerations above 100 g, including 

location, speed of the test train, and information regarding the measured strains at the particular location, 

in tabular form. 

Regarding lateral accelerations, six lateral shocks above 30 g were measured on the right end axle box of 

the WABTEC/SAB-WABCO test axle during Phase 1 testing; two were observed during the northbound 

test conducted on May 16, and four were observed during the southbound test conducted on May 17. 

Although many lateral accelerations above 30 g were measured on the left end axle box and on the middle 

of the test axle itself, these were affected by the signal quality issues highlighted in Figures 3.4 and 3.5 

and were deemed unreliable. 

The highest lateral acceleration observed on the right end axle box during Phase 1 testing was 39.1 g, 

measured on May 17 between Boston’s South Street Station and the Route 128 Station while the test train 

was traveling at a speed near 120 mph. 
53 

The following key observations were made regarding the accelerations measured on the axle during Phase 

1 testing: 

• The load environment associated with vertical impacts was much more severe than that associated 

with lateral impacts. The maximum vertical acceleration was almost three times that of the maximum 

lateral acceleration. In addition, the number of high amplitude vertical impacts was significantly 

higher than the number of high amplitude lateral impacts. 

• Based on analysis of the lateral acceleration measurements made on the WABTEC/SAB-WABCO 

test axle during Phase 1, the accelerometer mounted on the middle of the axle was subject to a signal 

quality issue from time to time that suggested saturation of the sensor. When the sensor was working 

properly, the accelerations measured on the middle of the axle were lower than the accelerations 

measured on the axle boxes on either end of the axle. 

3.1.3 Spoke Strains 

The analysis of strains collected during the test program was based the analysis of combinations of 

individual measurements to identify the effects of spoke tension, in-plane bending (BIP) of the spokes, 

and out-of-plane bending (BOP) of the spokes. These three types of spoke strain are used to characterize 

the response of the spokes throughout the test effort. Figure 3.7 illustrates the strain gage arrangement 

employed with spoke 6 on the instrumented brake discs in the center and outer positions. 

Figure 3.7. Designation of Strain Gages Used on Spoke 6 of Center and Outer 

Instrumented WABTEC/SAB-WABCO Supplied Discs During Phase 1 Testing 

The tensile strain in the spoke is produced by averaging the output of the four strain gages around the 

circumference of the spokes. In equation form, the tensile strain is given by: 

Average Tensile Strain εT = (εF1+ εF2+ εR1+ εR1) / 4

where: 

εF1 is the strain measured by gage F1 εF2 is the strain measured by gage F2 

εR1 is the strain measured by gage R1 εR2 is the strain measured by gage R2 
54 

Combining the readings of the strain gages in this manner effectively cancels the effects of BIP and BOP. 

Strain due to BIP of the spoke is calculated using the output from the two strain gages located within the 

plane of the disc, gages F1 and F2. The formulation of the BIP strain is given by: 

In-Plane Bending Strain εBIP = (εF1- εF2) / 2 

Similarly, strain associated with BOP of the spoke is calculated from the two strain gages located on the 

sides of the spoke, gages R1 and R2. The formulation of the BOP strain is given by: 

Out-of-Plane Bending Strain εBOP = (εR1- εR2) / 2 

Zero-to-peak strain levels are preserved in each of these formulations. 

Throughout this report, the strains presented are as measured and have not been corrected for existing 

strains, as defined in Figure 2.27, unless otherwise stipulated. 

Two types of strain clearly dominated the data collected during Phase 1 testing, tensile strains in the 

spoke resulting from the buildup of heat in the disc during braking and a BOP of the spoke produced both 

in braking and non-braking scenarios. BIP was generally small compared to the other types of strain. 

Figure 3.8 illustrates this observation, where the average tensile strain, BIP strain, and BOP strain 

measured on spoke 6 of both the center and outer WABTEC/SAB-WABCO supplied discs is illustrated 

for a series of brake applications on May 16 between Philadelphia, PA, and MP AN 60. 

Figure 3.8. Strains on Spoke 6 of Center and Outer WABTEC/SAB-WABCO Supplied Brake 

Discs During Brake Applications (File 051605_09.ABT)–May 16, 2005 
55 

Brake applications illustrated in Figure 3.8 are not presented as typical brake applications but were chosen 

to illustrate the general behavior of the measured strains. 

One can observe several items of interest in Figure 3.8, including the following: 

• As the brakes are applied and the speed of the train decreases, the average tensile strain increases, 

reflective of the fact that the disc is being heated up and the spoke is affected by the tensile strain 

resulting from the thermal expansion of the material. 

• The BOP strain for this braking scenario is relatively low, save for the spike observed in the middle of 

the time history of the strains. This resulted from a vertical impact that was measured on the test axle 

at that instance. The BOP strain does not exhibit the gradual rise observed on the average tensile 

strains during the brake applications–the same rise in strain is observed on both sides of the spoke and 

is removed in the subtraction of the two strains. 

• The BIP strain is small compared to average tensile and BOP strains. 

Upon review of the data, it was found that, although the average tensile strain experienced large changes 

only during braking, BOP strain could be relatively large in two different scenarios–when the test axle 

experienced a vertical impact and under selected brake applications. 

BOP Strains Due to Vertical Impacts 

Figure 3.9 illustrates the BOP strain responses of the center disc and outer disc to a 117.5 g vertical shock 

on the left end axle box (with a 39.5 g vertical shock on the right end axle box) between Newark, NJ, and 

Philadelphia, PA, on May 17 while traveling at a speed of 119 mph. 
56 

Figure 3.9. Vertical Accelerations and BOP Strains on Spoke 6 of Center and Outer 

WABTEC/SAB-WABCO Supplied Brake Discs During Vertical Shock (File 051705_19.ABT)– 

May 17, 2005

One can see the relatively large levels of strains that are measured on both discs as a result of the impact. 

Further analysis of this response of the discs to vertical shocks revealed that the frequency associated with 

this oscillating BOP strain was approximately 227 Hz. The frequency associated with BOP strain 

resulting from vertical impacts was close to the frequency associated with the BOP motion identified in 

the FEA for the single disc with a fixed hub, calculated to be 206 Hz. A natural frequency for the 

disc/axle assembly was not calculated using FEA, but the proximity of the measured frequency to that 

calculated for the single brake disc suggests a correlation. 

In this particular case, the strains measured on the center disc are comparable to those measured on the 

outer disc. In order to assess the relative response of the two discs to shocks, shocks greater that 50 g 

measured on either end of the test axle on May 17 were analyzed to determine the relative amplitude of 

BOP on the center disc to that found on the outer disc. Figure 3.10 illustrates this analysis. The 

maximum peak-to-peak strain of the center disc is plotted against the maximum peak-to-peak strain of the 

outer disc for selected impacts. 
57 

Figure 3.10a. Comparison of BOP Amplitudes of WABTEC/SAB-WABCO Supplied Center and 

Outer Discs Measured During Vertical Impacts, Left End Shock Greater Than Right End Shock–

May 17, 2005 

It can be seen that the responses of the center and outer discs are similar but that the amplitudes of the 

BOP strain oscillations on the center disc were generally a bit higher than those on the outer disc when 

the higher amplitude shock is at the opposite end of the axle from the outer instrumented disc. 
58 

Figure 3.10b. Comparison of BOP Amplitudes of WABTEC/SAB-WABCO Supplied Center and 

Outer Discs Measured During Vertical Impacts, Right End Shock Greater Than Left End Shock–

May 17, 2005

When the higher amplitude shocks were seen on the right end of the axle (i.e., closer to the outer disc), the 

response of the outer disc was generally equal to the response of the center disc, as indicated by the slope 

of the linear fit to the data points contained in Figure 3.10b. 

The highest strains on both discs resulting from vertical impacts were observed on May 17 approximately 

1700’ southwest of MP AN 79, near the Gunpow Interlocking, while traveling at 119 mph. The 

amplitude of the BOP strains on the center disc reached 1,365 microstrain, zero-to-peak. The amplitude 

of the BOP strains on the outer disc was 1,186 microstrain, zero-to-peak, at the same location. As stated 

earlier, the strains presented are as measured and have not been corrected for pre-load strains in the spoke. 

BOP Strains During Particular Braking Events 

During the test run from Boston to Washington on May 17, high BOP strains were observed on the 

WABTEC/SAB-WABCO supplied brake discs, especially the one in the center position, under several 

brake applications. Figure 3.11 illustrates the output of the R1 strain gage on the center disc recorded 

near MP E17 as the test train was slowing down from 100 mph. The figure also provides a detailed plot 

of the strain gage activity. The oscillatory nature of the strain is similar to the one illustrated in Figure 

3.9; however, it is superimposed over an increasing average tensile strain resulting from the buildup of 

heat during braking, and the oscillations persist for a much longer period of time than that seen in the 

response to the vertical shock shown in Figure 3.9. 
59 

Figure 3.11. Strain Recorded on Gage R1 of Center WABTEC/SAB-WABCO Supplied Brake Disc 

During Braking Near MP E17 (File 051705_17.ABT)–May 17, 2005 

The strain illustrated in Figure 3.11 is that measured on the side of the spoke of the center brake disc. 

Figure 3.12 provides the BOP strain measured on both the center and outer brake disc during this 

particular brake application. The upper portion of the illustration is the BOP strain measured on the two 

discs; the plot in the lower portion of the illustration is indicative of the amplitude of the oscillatory 

strains measured on each of the discs. It can be seen that the BOP strains measured on the center disc are 

between three and four times that of the strains recorded on the outer disc at the same time. It should be 

kept in mind that the overall rise in tensile strain experienced by the spoke is not observed in the BOP 

strains. The BOP strains represent the differences between the strains measured on either side of the 

spoke of interest. 

Figure 3.12. BOP Strain Recorded on Center and Outer WABTEC/SAB-WABCO Supplied 

Brake Discs During Braking Near MP E17 (File 051705_17.ABT)–May 17, 2005

Eight brake applications occurred during the test run conducted on May 17 that exhibited this type of 

substantial strain. Table 3.1 provides details associated with each of the events in which this behavior 

was observed. Appendix G provides additional details associated with these events. Figure 3.13 

illustrates the terminology employed in Table 3.1. 
60 

Figure 3.13. Terminology Used to Describe Sustained Oscillations During Braking 

Table 3.1. Summary of Measurements Collected from Center WABTEC/SAB-WABCO Supplied 

Brake Disc During Brake Applications Exhibiting Signs of High BOP Strain–May 17, 2005 

Measurements from Center Disc 

Geographic Location Duration 

(s) 

Max 

Avg-to-Pk 

Amplitude 

(με) 

Avg 

Strain 

@ Start 

(με) 

Avg 

Strain 

@ End 

(με) 

Disc 

Temp 

@ Start

 (oF) 

Disc 

Temp 

@ End

 (oF) 

Peak 

Disc 

Temp 

(oF) 

Brake 

Cyl 

Press 

 (psi) 

Speed at 

Start of 

Braking 

(mph) 

1685 SW of MP E17 28 850 445 1329 136.7 184.6 229.0 45 101 

190 SW of MP AN13 22 711 350 887 116.0 163.5 189.9 35 103 

Near N. Philadelphia 7 502 11 32 183.3 187.7 193.0 45 14 

516 SW of MP AP25 6 623 476 659 109.9 122.6 172.8 36 94 

2303 NE of MP AP65 20 700 785 1368 127.5 186.4 242.2 40 110 

947 NE of MP AP71 32 591 950 1702 172.3 227.3 283.1 37 119 

471 SW of MP AP79 3 576 875 958 160.9 161.8 171.9 31 123 

1573 W of MP AP91 12 502 617 886 131.4 136.7 181.5 32 70 

The highest amplitude of oscillations on the center disc was seen during the brake application near MP E 

17 during braking from 101 mph. In this case, BOP strain oscillations reached an average-to-peak 

amplitude of ± 850 microstrain or a peak-to-peak amplitude of 1,700 microstrain. These strains are as 

measured and have not been corrected for pre-load strains in the spoke. 

Several key observations were made during these particular brake applications: 

• No brake applications occurred on the May 16 test run in which this behavior was observed. The 

main difference between the two test runs was the direction of travel of the test vehicle and therefore 

the direction of rotation of the test axle. The test run conducted on May 16 was with the test axle in a 

trailing position, and the test run conducted on May 17 was with the test axle in a leading position. 

This behavior was investigated during a subsequent stage of the test program. 

• During these brake applications, the frequency associated with the oscillatory BOP strains was 

approximately 186 Hz. This was lower than the frequency of 227 Hz associated with the oscillatory 

strains resulting from vertical impacts.

• In each of these brake applications, the amplitude of the BOP strain measured on the center brake disc 

was higher than the BOP strain measured on the outer brake disc, just as Figure 3.12 shows.

• Because these oscillatory BOP strains occur during braking, they are superimposed on average tensile 

strains built up during the heating of the brake discs. The example provided in Figure 3.11 illustrates 

this action.
61 

3.1.4 Caliper Accelerations

As described in Section 2.3.2, accelerations were recorded near one of the brake pads and near the 

actuator on the test axle’s center caliper assembly and near one of the brake pads of the caliper used with 

the outer brake disc used for the test effort. Analysis of these signals indicated that little was of interest in 

the various caliper accelerations at times other than brake applications. Therefore, this section will 

present examples of the caliper accelerations collected during braking. 

Figure 3.14 illustrates the three accelerations measured at each of the locations on the center caliper 

during a brake application from 135 mph to a near stop made while the test train was moving southbound 

near MP AN 41 on May 17. It can be seen that no significant level of activity occurs at either location; 

lateral accelerations on the actuator reached a peak level of 15 g, but accelerations at other locations on 

the caliper did not exceed approximately 8 g. Considering that the caliper is mounted to the truck, this is 

not an excessive level of excitation. 

Figure 3.14. Accelerations Measured on Center Caliper During Normal Brake Application 

(File 051705_19.ABT)–May 17, 2005
62 

Figure 3.15. Accelerations Measured on Center and Outer Calipers Near Pads During Normal 

Brake Application (File 051705_19.ABT)–May 17, 2005

Figure 3.15 illustrates the three accelerations measured near the pads on both the center and outer caliper 

assembly during the same brake application pertaining to Figure 3.14. Again, it can be seen that no 

significant level of activity occurs on either caliper assembly. 

Figure 3.16 illustrates the three accelerations measured at each of the locations on the center caliper 

during the brake application illustrated in Figures 3.11 and 3.12 where high BOP strains were observed 

on the center WABTEC/SAB-WABCO supplied disc while the test train was slowing down from 100 

mph near MP E17 on May 17. Clearly the accelerations measured near the pads are much higher than 

those measured near the actuator. In addition, it can be seen that the lateral accelerations measured on the 

pad while the brake disc in question is experiencing high BOP strains can reach high levels, in this case 

up to approximately 30 g. The lateral accelerations measured near the pad are higher than vertical 

accelerations and much higher than longitudinal accelerations measured at the same location; 

accelerations measured in the longitudinal direction are relatively low during this type of phenomenon. 

The frequency associated with these accelerations is 186 Hz, the same as that observed in the BOP strains 

of the brake disc. 

Figure 3.17 illustrates the three accelerations measured near the pads on the center and outer caliper 

assemblies during a brake application near MP AN 13 from 100 mph. Accelerations measured on the 

center caliper assembly are much higher than those measured on the outer caliper assembly in this 

situation, and the relative levels of acceleration are very similar to the relative levels of BOP strain 

illustrated in Figure 3.12. 
63 

Figure 3.16. Accelerations Measured on Center Caliper During Brake Application with High BOP 

Strains (File 051705_19.ABT)–May 17, 2005 

Figure 3.17. Accelerations Measured on Center and Outer Calipers Near Pads During 

Brake Application with High BOP Strains (File 051705_19.ABT)–May 17, 2005
64 

From these illustrations and review of the caliper accelerations recorded during Phase 1 testing, the 

following observations were made: 

• Accelerations on the calipers can get quite high during incidents in which the brake disc is 

experiencing high levels of BOP strains if the brakes are applied. The highest accelerations were 

observed to be lateral accelerations recorded near the pad. 

• During typical brake applications in which high levels of strain are not observed on the brake discs, 

the accelerations measured on the caliper assemblies are relatively low. 

• Based on a review of the test data (print outs of which are provided in Appendix L) the calipers 

experience relatively low levels of excitation when the brakes are not applied. 

3.1.5 Temperatures 

As described in Section 2.3.3, temperature measurements were made on the center disc using the IR 

sensors mounted in either side of the disc and with a thermocouple located on the disc itself. On May 16, 

the thermocouple was attached to spoke 6 in the vicinity of the strain gages. During testing on May 16, 

the initial location of the thermocouple was deemed unsatisfactory because the temperature of the spoke 

was not yielding useful information. Figure 3.18 illustrates the spoke temperature measured on the center 

WABTEC/SAB-WABCO supplied disc by the thermocouple and the temperature of the left side of the 

disc on the friction ring measured by one of the IR sensors during a series of brake applications on May 

16 between Philadelphia, PA, and MP AN 60. The speed profile during this series of brake applications 

is provided for reference. 

Figure 3.18. Friction Ring and Spoke Temperatures Measured on Center WABTEC/SAB-WABCO 

Supplied Brake Disc During Brake Applications (File 051605_09.ABT)–May 16, 2005 

Brake applications illustrated in Figure 3.17 are not presented as typical brake applications but were 

chosen to illustrate the general behavior of the measured strains. 

One can observe that the spoke temperature is well below the temperature measured on the left friction 

ring. The highest spoke temperature recorded on May 16 was 185.5 °F following a brake application near 

MP AB215. This was not considered to be an excessive temperature by the brake vendors. 
65 

During the test run on May 16, thermocouple measurements were collected from the center disc on the 

left side friction ring following several brake applications. These measurements have been compared; 

Appendix I provides these results. In general, hand thermocouple measurements were lower than the 

temperatures measured with the IR sensor with the highest difference being close to 100 °F but the 

average difference being approximately 25 °F. 

Upon review of the data from the May 16 test run, it was decided that the thermocouple should be moved 

to the back of the friction ring for the remainder of testing to monitor higher disc temperatures. On the 

evening on May 16, the thermocouple was removed from spoke 6 of the center disc and relocated to the 

back of the friction ring as illustrated in Figure 2.19. 

Figure 3.19 illustrates the temperatures measured with the IR sensors on both sides of the center and outer 

brake discs and the temperature measured on the back of the center disc’s friction ring during the test run 

between Newark, NJ, and Philadelphia, PA, on May 17. As evidenced by the large temperature increase 

in the middle of this data sample, a full-service brake application was made during this portion of the test. 

Figure 3.19. Temperature Measurements Made During Testing Between Newark, NJ, 

and Philadelphia, PA–May 17, 2005

In general, the trends of the various temperature measurements agree with each other. The temperature 

measured on the back of the friction ring tends to be higher than the temperatures measured by the IR 

sensors. Many factors influenced this difference, including the IR sensors’ sensitivity to orientation or 

angle, with respect to the surface upon which measurements are made; the IR sensors’ sensitivity to 

surface finish; and the proper selection of emissivity settings. Two separate corrections that can be 

applied to temperatures measured by the IR sensors on the friction rings to bring them into agreement 
66 

with temperatures measured by the thermocouple on the back of the center disc’s friction ring were 

developed based on the data illustrated in Figure 3.19. Appendix I provides these corrections. One of 

these corrections involves a simple bias, or offset, shift while the other involves the application of both a 

scale factor and an offset shift. 

With these statements in mind, the IR sensor measurements, in combination with the temperatures 

measured on the back of the friction ring using the thermocouple, provided sufficient information to be 

able to make an assessment of the brake disc temperatures. The highest temperatures on the brake discs 

were observed between Wilmington, DE, and Baltimore, MD, on May 17 following the second of two 

brake applications from 125 mph to 30 mph between MP AP 65 and MP AP 72; Figure 3.20 shows this 

occurrence. The maximum temperature measured on the back of the friction ring of the center disc was 

close to 280 °F and was observed following the last of the brake applications shown in the illustration. 

The temperature measured on the left friction ring of the center brake disc reached a value of 

approximately 255 °F. The brake vendors did not identify these temperatures as excessive. 

Figure 3.20. Extreme Temperature Measurements Made During Testing Between 

Wilmington, DE, and Baltimore, MD–May 17, 2005

The temperatures measured at the single location on the back of the center disc’s friction ring were used 

to estimate the state of thermal stress in the spokes of the disc. It is recognized that the distribution of the 

temperature in the friction plates of the brake discs is expected to be complex and dependent on many 

aspects, including the geometry of the friction plate. It must be recognized that analyses conducted on 

data collected with the thermocouple data were conducted to extract meaningful observations from 

temperature data, and the results of these analyses are only considered to be approximations. 

Temperatures measured on May 17 on the back of the friction ring before brake application were 
67 

compared with the corresponding tensile strain in spoke 6. No direct measure of spoke temperature 

occurred following tests conducted on May 16. Figure 3.21 illustrates the comparison of temperature and 

spoke strain. Choosing to consider the temperatures before the brake applications allows for equalization 

of temperatures throughout the disc. By establishing a relationship between these two quantities, an 

estimate of the thermal strains in the spoke can be arrived at without a direct measurement of spoke 

temperature. These estimated thermal strains would not account for any existing pre-load strains in the 

spoke. 

Figure 3.21. Brake Disc Temperature versus Initial Spoke Tensile Strain Before Brake 

Applications, Center WABTEC/SAB-WABCO Supplied Disc, Spoke 6–May 17, 2005 

3.1.6 Brake Pressures 

As described in Section 2.3.4, three pressures within the brake system were monitored during Phase 1 

testing. In general, the pressures varied as follows: 

• Brake cylinder pressure was typically 0 psi until the brakes were applied. Brake cylinder pressure 

reached a maximum of 56 psi during full service brake applications. 

• Brake pipe pressure was typically 110 psi, falling to a minimum of 90 psi during brake applications. 

• Parking brake pressure measurements were reflective of the main reservoir pressure, 140 to 145 psi, 

when the brakes were released. When the brakes were applied, the parking brake pressure 

measurement was the same as the brake cylinder pressure.

Upon review of the data, nothing of particular interest was observed in brake pipe and parking brake 

pressure. According to the brake vendors, the brake system appeared to function to an acceptable level. 
68 

Review of brake cylinder pressure measurements revealed that the amplitude of the high BOP strains 

measured during seven brake applications on May 17 correlated with the brake application pressure 

reflected in the brake cylinder pressure. Figure 3.22 illustrates the BOP strains recorded on the center and 

outer WABTEC/SAB-WABCO supplied discs provided in Figure 3.12 along with the brake cylinder 

pressure recorded during the same time frame. It can clearly be seen that the amplitude of the strain 

oscillations increase with increasing brake pressure; the maximum amplitude oscillation is reached when 

the brakes are at full service pressure of 56 psi. Brake pressure, however, does not appear to be the 

trigger for this dynamic behavior. Based on the brake pressures at the start of this activity in the seven 

examples highlighted in Table 3.1, this activity can be initiated over a range of brake cylinder pressures, 

(31–45 psi). 

Figure 3.22. Brake Cylinder Pressure and BOP Strains Recorded on WABTEC/SAB-WABCO 

Supplied Brake Discs During Braking Near MP E17 (File 051705_17.ABT)–May 17, 2005 

3.1.7 Spreader Bar Test

Following the completion of the test run conducted on May 17, a series of tests was conducted on 

Instrumented Axle 1 in which a spreading force was placed at various locations between the center and 

outer WABTEC/SAB-WABCO supplied instrumented discs. Section 2.4.1 provides a description of the 

procedure followed for this investigation. 

Figure 3.23 provides strain measurements recorded from the instrumented spokes on each of the discs 

during both loadings. The graph in Figure 3.23a pertains to loadings aligned with spokes on the center 

disc, while Figure 3.23b pertains to loadings aligned with spokes on the outer disc. Part A of Appendix J 

includes strain readings illustrated in Figure 3.23. 
69 

Figure 3.23a. Strains Measured During Spreader Bar Test, Load Aligned with 

Spokes of Center Disc 

Figure 3.23b. Strains Measured During Spreader Bar Test, Load Aligned with 

Spokes of Outer Disc

Spreader Bar Test Results, 500 lb. Load Applied at Spokes on Center Disc

-30.00

-20.00

-10.00

0.00

10.00

20.00

30.00

40.00

Spoke 6 Spoke 1 Spoke 2 Spoke 3 Spoke 4 Spoke 5 Spoke 6

Load Point on Center Disc

Strain per 100 lbs (microstrain/100 lb)

Center Disc, Spoke 6, Strain F1

Center Disc, Spoke 6, Strain F2

Center Disc, Spoke 6, Strain R1

Center Disc, Spoke 6, Strain R2

Outer Disc, Spoke 6, Strain F1

Outer Disc, Spoke 6, Strain F2

Outer Disc, Spoke 6, Strain R1

Outer Disc, Spoke 6, Strain R2

Spreader Bar Test Results, 400 lb. Load Applied at Spokes on Outer Disc

-30.00

-20.00

-10.00

0.00

10.00

20.00

30.00

40.00

Spoke 6 Spoke 1 Spoke 2 Spoke 3 Spoke 4 Spoke 5 Spoke 6

Load Point on Outer Disc

Strain per 100 lbs (microstrain/100 lb)

Center Disc, Spoke 6, Strain F1

Center Disc, Spoke 6, Strain F2

Center Disc, Spoke 6, Strain R1

Center Disc, Spoke 6, Strain R2

Outer Disc, Spoke 6, Strain F1

Outer Disc, Spoke 6, Strain F2

Outer Disc, Spoke 6, Strain R1

Outer Disc, Spoke 6, Strain R2
70 

Based on the results presented in Figure 3.22, the following observations were made: 

• The strain gages on each of the instrumented spokes were working as expected following the first 

portion of the over-the-road testing. The lateral spreading force resulted in little strain on the faces of 

the spokes and strains of opposite nature (i.e., tension versus compression) on the web/rib of the 

spokes. 

• The strains measured by the R2 gage on spoke 6 of the outer disc tended to be approximately 20 

percent lower than strain measured by the R1 gage on spoke 6 of the center disc. This difference is 

largely reflective of the variability of strain measurements in individual spokes and is not significant 

enough to affect the results provided for the outer disc in this section. 

3.1.8 Concluding Comments

Important information was learned from the results of Phase 1 testing, including the following: 

• Based on a preliminary FEA, a frequency associated with a BOP was calculated to be 206 Hz. 

• High magnitude shocks occurring in accelerations measured on the axle were identified by counting 

cases in which one or more consecutive data values exceed a given acceleration threshold. 

Accelerations appearing to result from electrical noise or other instrumentation issues were neglected. 

It was found that the load environment experienced by the typical axle was more influenced by high 

vertical accelerations than by high lateral accelerations. 

During Phase 1 testing, 172 vertical shocks above 50 g were recorded on May 16, 5 of which were 

above 100 g. There were 161 vertical shocks above 50 g recorded on May 17, 8 of which were above 

100 g. The highest vertical acceleration observed in Phase 1 testing was 126 g. 

Six lateral shocks above 30 g were measured on the WABTEC/SAB-WABCO test axle during Phase 

1 testing; two were observed during the test conducted on May 16, and four were observed during the 

test conducted on May 17. The highest lateral acceleration measured during Phase 1 testing was 39.1 

g, measured on May 17. 

• Two types of strain dominated the data collected during Phase 1 testing, tensile strains in the spoke 

resulting from the buildup of heat in the disc during braking and BOP of the spoke produced in 

response to vertical shocks and in certain braking scenarios. 

The highest BOP strains on both the center and outer WABTEC/SAB-WABCO supplied discs 

resulting from vertical shocks reached 1,365 microstrain, zero-to-peak, and 1,186 microstrain, zeroto-peak, respectively at the same location. The frequency associated with oscillating BOP strains 

resulting from vertical shocks was approximately 227 Hz. The responses of the center and outer discs 

are similar, but the amplitudes of the BOP strain oscillations on the center disc were generally a bit 

higher than those on the outer disc when the higher amplitude shock is at the opposite end of the axle 

from the outer instrumented disc. When the higher amplitude shocks were seen on the right end of 

the axle (i.e., closer to the outer disc), the response of the outer disc was generally equal to the 

response of the center disc. 

High BOP strains were observed on the WABTEC/SAB-WABCO supplied brake discs, especially 

the one in the center position, under eight brake applications during the test run from Boston to 

Washington on May 17. This phenomenon was not observed on the northbound test on May 16. The 

nature of the strain is characterized by an oscillating strain superimposed over an increasing average 

tensile strain resulting from the buildup of heat during braking. These oscillations persist for a much 

longer period of time than that seen in the response to vertical shocks and have a frequency of 186 Hz 

associated with them. The amplitude of the oscillations on the center brake disc was between three 
71 

and four times that of the outer brake disc; the maximum amplitude of oscillating strain on the center 

disc observed during Phase 1 tests was 1,700 microstrain, peak-to-peak. The amplitude of these 

oscillations was proportional to the brake pressure measured throughout the application. 

Specified strains are as measured and have not been corrected for pre-load strains in the spoke. 

• Accelerations measured on the caliper assemblies near the brake pad were higher than those measured 

near the actuators. The highest accelerations near the caliper pads were measured during those 

braking events in which the high BOP strains were observed and appeared to correlate with the 

amplitude of the oscillations of the particular brake disc. Consequently, accelerations measured on 

the center caliper assembly during these incidents were higher than those measured on the outer 

caliper assembly. 

• Temperatures measured at several locations on the brake discs did not exceed temperatures expected 

by the brake vendors. 

In addition, several issues associated with instrumentation were identified, including the following: 

• The low signal-to-noise ratio exhibited in the output of the accelerometers on the left end axle box in 

the beginning of the test run conducted on May 16. 

• The intermittent saturation of the lateral accelerometer mounted directly to the test axle. 

• The discrepancy in temperatures measured by the IR sensors with those measured by the 

thermocouples. 

Although many things were learned during Phase 1 testing, several questions remained unanswered, 

including the following: 

• What was the mode of vibration experienced by the brake discs? Although the frequencies associated 

with the various motions suggested an out-of-phase bending motion identified in the initial FEA, 

insufficient data existed to confirm this. 

• What was the exact value of the natural frequency of the WABTEC/SAB-WABCO supplied brake 

disc? 

• What were the contributing factors to the high BOP strains observed during several brake 

applications, and why were they not observed during the northbound test conducted on May 16? 

These questions, in addition to questions raised by the results provided during Phase 1 testing, served as 

motivation for testing conducted during Phase 2 of the effort. 
72 

3.2 Phase 2 Test Results 

To better characterize the strains associated with BOP, focus shifted to the center WABTEC/SABWABCO supplied disc in Phase 2 testing where additional strain measurements were made on the spoke 

opposite that monitored during the initial test efforts. Strain gages were mounted to the axle at the request 

of the brake system suppliers to determine the contribution of axle bending to the results observed in 

Phase 1 testing. The sampling rate employed with the ENSCO-based data collection hardware was 

increased from 2,000 Hz to 3,000 Hz for Phase 2 testing so that all relevant information, especially those 

quantities that change quickly such as accelerations, was captured in an accurate manner. Amtrak 

provided a second data collection system that employed a sampling rate of 10,000 Hz to record selected 

parameters. 

A round trip test was conducted between Washington and Boston on May 26 and 27. The test run 

conducted on May 26 was a northbound run with the test axle in a leading position within the test vehicle; 

the test run conducted on May 27 was a southbound run with the test axle in the trailing position within 

the test vehicle. 

During the second phase of testing, sound levels under the test vehicle were recorded to attempt to capture 

and analyze the high frequency noise heard during times in which signs of saturation were observed on 

some or all of the accelerometers mounted directly to the axle. The sound recording equipment was also 

used to attempt to capture and analyze a moaning noise heard above many axles of the test train during 

braking in Phase 1 testing. 

3.2.1 Axle Accelerations 

As described in Chapter 2, the sampling rate employed with the ENSCO data collection hardware was 

increased from 2,000 Hz to 3,000 Hz for Phase 2 testing; an anti-alias filter with a cut-off frequency of 

750 Hz was used during Phase 2 testing. Amtrak provided a second data collection system that employed 

a sampling rate of 10,000 Hz to record axle accelerations and other selected parameters; no anti-alias 

filter was used with the Amtrak data collection system. Figure 3.24 illustrates a vertical shock measured 

on the left end axle box and corresponding BOP strain recorded on spoke 6 of the center disc, while the 

test train traveled through the Forest Interlocking near MP AB 223 at 120.4 mph during the May 26 test 

run. The ENSCO-based collection system recorded the amplitude of this shock as 116.6 g. As can be 

seen in the figure, the Amtrak-based collection system was able to record a higher acceleration during this 

event. This highlights an important consequence of the proper selection of a sampling rate–increasing the 

rate at which data is recorded, or increasing the bandwidth of the data, can result in higher recorded 

amplitudes of acceleration. 

Further analysis of acceleration data collected at the two sampling rates by test personnel and the brake 

vendors indicated that no additional information critical to the study was provided by the data collected 

with the 10,000 Hz sampling rate. This is evidenced by the negligible difference in strains recorded 

spoke 6 at the two different sampling rates shown in Figure 3.24. Subsequent testing in Phase 3 did not 

employ the data collected with 10,000 Hz in the official test record. In light of the decisions made 

regarding the data collected with the higher sampling rate, discussion of results in this section will be 

limited those based on data recorded at 3,000 Hz. 
73 

Figure 3.24. Comparison of Vertical Shock and BOP Strains Recorded with 3,000 Hz and 

10,000 Hz Sampling Rates (Files 052605_18.AB2 [3,000 Hz Data] and 

File 052605_18.001 [10,000 Hz Data])–May 26, 2005 

Figure 3.25a illustrates the distribution of vertical accelerations above 50 g recorded on the test axle from 

Washington to Boston on May 26 with the test axle in a leading position. Figure 3.25b illustrates the 

corresponding information for the data collected between Boston and Washington on May 27 with the test 

axle in a trailing position. Appendix F tabulates the data illustrated in Figure 3.25. 
74 

Figure 3.25a. Distribution of Vertical Accelerations Above 50 g Measured on Instrumented Axle 1 

with WABTEC/SAB-WABCO Supplied Brake Discs in Lead Position–Washington, DC, to 

Boston, MA, on May 26, 2005 

Figure 3.25b. Distribution of Vertical Accelerations Above 50 g Measured on Instrumented Axle 1 

with WABTEC/SAB-WABCO Supplied Brake Discs in Trail Position–Boston, MA, to 

Washington, DC, on May 27, 2005
75 

As shown in Figure 3.25, 276 vertical shocks above 50 g were measured on the WABTEC/SAB-WABCO 

test axle during the northbound test conducted on May 26, 115 on the left end axle box and 161 on the 

right end axle box, and 238 vertical shocks above 50 g were measured during the southbound test 

conducted on May 27, 101 on the left end axle box and 137 on the right end axle box. Regarding results 

from May 26, 16 shocks above 100 g were measured. Fifteen shocks over 100 g were measured on May 

27. 

The highest vertical acceleration observed in Phase 2 testing was 156 g, measured on the left end of the 

axle on May 26 near Union Interlocking near MP AN 20 while the test train was traveling at 113 mph. 

Appendix F provides the details associated with accelerations above 100 g in tabular form. 

As discussed in Section 2.3.2, two additional accelerometers were mounted to the axle in an attempt to 

improve the reliability of the measurement of the lateral accelerations on the axle. Upon review of the 

data, it was determined that neither of the two new accelerometers provided improved performance. 

Results pertaining to lateral accelerations during this phase of testing will be based upon the axle boxmounted sensors and the Silicon Designs +/- 200 g single-axis accelerometer employed during the first 

phase of testing. 

Figure 3.26a illustrates the distribution of lateral accelerations above 30 g recorded on the test axle from 

Washington to Boston on May 26 with the test axle in a leading position. Figure 3.26b illustrates the 

corresponding information for the data collected between Boston and Washington on May 27 with the test 

axle in a trailing position. Appendix F tabulates the data illustrated in Figure 3.26. 

Figure 3.26a. Distribution of Lateral Accelerations Above 30 g Measured on Instrumented Axle 1 

with WABTEC/SAB-WABCO Supplied Brake Discs in Lead Position–Washington, DC, to 

Boston, MA, on May 26, 2005 
76 

Figure 3.26b. Distribution of Lateral Accelerations Above 30 g Measured on Instrumented Axle 1 

with WABTEC/SAB-WABCO Supplied Brake Discs in Trail Position–Boston, MA, to 

Washington, DC, on May 27, 2005

Forty-four lateral shocks above 30 g were measured on the WABTEC/SAB-WABCO test axle during the 

northbound test conducted on May 26, 31 on the left end axle box and 13 on the right end axle box. No 

accelerations were measured above 30 g on the axle that were reliable measurements. Thirty-three lateral 

shocks above 30 g were measured on the WABTEC/SAB-WABCO test axle during the southbound test 

conducted on May 27, 19 on the left end axle box and 14 on the right end axle box. No accelerations 

were measured above 30 g directly on the axle that were considered reasonable. 

The highest lateral acceleration observed during Phase 2 testing was on the left end axle box. A 75.3 g 

shock was measured on May 26 at the Union Interlocking near MP AN20 while the test train was 

traveling at a speed near 112 mph. 

During Phase 1 testing, the observation was made that the load environment associated with vertical 

impacts was much more severe than that associated with lateral impacts, with peak vertical accelerations 

measuring almost three times the level of peak lateral accelerations. Accelerations measured during 

Phase 2 testing were analyzed in detail to see if this relationship between lateral and vertical accelerations 

extended beyond accelerations associated with large impacts. 

In order to characterize the typical load environment affecting the test axle, a number of segments of the 

data collected on May 26 and May 27 were identified in which near-constant speeds were maintained. 

Within each of these zones, the highest lateral and vertical accelerations were identified. Figure 3.27 

provides the results of this analysis conducted with data collected on May 26. Lateral accelerations 

identified in this process are shown in the upper portion of the figure, and the corresponding information 

for vertical accelerations is shown in the lower portion of the figure. The results contained in these plots 

do not represent the entire run but only those constant speed segments selected for consideration. 
77 

Figure 3.27. Peak Lateral and Vertical Accelerations Observed in Near-Constant Speed Segments 

as a Function of Speed–May 26, 2005 

A similar consideration was made for data collected on the southbound portion of the trip conducted on 

May 27. The results based on data collected on May 27 were similar to those shown in Figure 3.27 and 

are included in Appendix F. 

In addition to the peak lateral and vertical accelerations calculated in each near-constant speed data 

segment, several percentile values of acceleration were calculated. As an example, the 99.9 percentile 

value is the acceleration value below which 99.9 percent of all accelerations within the data segment 

reside. Calculating several percentile values of acceleration for each data segment considered in this 

analysis allowed for a more general characterization of the accelerations. Figure 3.28 illustrates the ratios 

of vertical to lateral accelerations for several percentile levels of acceleration determined within nearconstant speed segments recorded at 122 mph. In this illustration, vertical accelerations measured on the 

axle boxes were typically 2 to 3.7 times as large as the lateral accelerations measured at the same location. 

Similar results were found for other data segments considered. 
78 

Figure 3.28. Vertical to Lateral Acceleration Ratios Determined for Data Collected in NearConstant Speed Segments at 122 MPH–May 26, 2005 

In summary, results from Phase 2 testing confirmed that the load environment associated with vertical 

impacts was much more severe than that associated with lateral impacts. It was established that vertical 

accelerations are typically between 2 and 3.7 times lateral accelerations, regardless of the level of 

accelerations considered.
79 

3.2.2 Spoke Strains 

One of the goals of Phase 2 testing was to collect strain measurements on the WABTEC/SAB-WABCO 

supplied disc mounted in the center position of the axle from the spoke opposite the one that was subject 

of measurements in Phase 1. This was done so that sufficient information was available to be able to 

specify the motion of the brake disc. Figure 3.29 illustrates the arrangement of the strain gages on the 

spokes of the WABTEC/SAB-WABCO supplied disc mounted in the center position of the axle. 

Figure 3.29. Designation of Strain Gages Used on Spoke 6 and Spoke 3 of 

Instrumented Center WABTEC/SAB-WABCO Supplied Disc During Phase 2 Testing

During Phase 2 testing, noise spikes were observed on the signals from the F1 and F2 strain gages on 

spoke 6. Caution had to be exercised when considering results that made use of these gages to ensure that 

the noise spikes did not adversely affect findings. 

Figure 3.30 illustrates the BOP strains determined for spoke 6 and spoke 3 as a result of a vertical impact. 

In this particular example, a vertical shock on the left end axle box of 116.6 g was measured while the test 

train traveled through the Forest Interlocking near MP AB 223 at 120.4 mph during the May 26 test run 

(Figure 3.24 illustrates this shock). The plot in the upper portion of the figure illustrates the vertical 

accelerations on the test axle. Based on the behavior of the BOP strains on the opposing spokes, it is clear 

that an out-of-plane vibration of the brake disc is taking place. 

Figure 3.31 illustrates BOP strains observed during a brake application. In this example, the disc 

exhibited high BOP strains during a brake application from 150 mph near MP AB 158 during the 

northbound test run on May 26. Based on the relationship between the BOP strains recorded on the 

opposite spokes during this brake application, it is clear that the disc is vibrating out of the plane of 

rotation, just as exhibited by the disc in response to the vertical shock illustrated in Figure 3.30. 
80 

Figure 3.30. Vertical Shock and BOP Strains Recorded on Center WABTEC/SAB-WABCO 

Supplied Brake Disc During Phase 2 Testing (File 052605_18.AB2)–May 26, 2005
81 

Figure 3.31. BOP Strains Recorded During Braking on Center WABTEC/SAB-WABCO 

Supplied Brake Disc Near MP AB 158 During Phase 2 Testing (File 052605_15.AB2)–May 26, 2005

Figure 3.31 includes the wheel position signal generated by the slip ring indicating the rotational position 

of the brake discs. Figure 2.22 establishes the position of the axle with respect to this sine wave. The 

position of the brake disc indicated that a spoke experiences the largest strains when it is in the vicinity of 

the brake pads. This indicates that the calipers play a role in these high oscillations during particular 

brake applications. 

Eleven brake applications occurred during the test run conducted on May 26 that exhibited substantial 

BOP strains. Table 3.2 provides details associated with each of the events in which this behavior was 

observed. Appendix G provides additional details associated with these events. Terminology used in 

Table 3.2 is defined in Figure 3.13. 

During the southbound test conducted on May 27 with the test axle in the trailing position, no braking 

events in which high, sustained BOP strains were observed. This appeared to confirm the results 

observed during Phase 1 testing pertaining to the correlation of the high BOP strains observed during 

selected braking events when the test axle was in a leading position. No strong evidence presented itself 

to indicate that truck dynamics played a role in this phenomenon, so attention turned to the direction of 

the axle’s rotation and the disc’s interaction with the brake hardware. This was pursued in Phase 3 

testing. 
82 

Table 3.2. Summary of Measurements Collected from Center WABTEC/SAB-WABCO Supplied 

Brake Disc During Brake Applications Exhibiting Signs of High BOP Strain–May 26, 2005 

Measurements from Center Disc 

Geographic Location Duration 

(s) 

Max 

Avg-to-Pk 

Amplitude 

(με) 

Avg 

Strain 

@ Start 

(με) 

Avg 

Strain 

@ End 

(με) 

Disc 

Temp 

@ Start

 (oF) 

Disc 

Temp 

@ End

 (oF) 

Peak 

Disc 

Temp 

(oF) 

Brake 

Cyl 

Press 

(psi) 

Speed at 

Start of 

Braking 

(mph) 

1429 SW of MP AN11 16 706 701 1142 107.3 134.1 161.8 40 103 

 15 NW of MP E13 5 483 393 503 82.6 102.9 137.6 32 64 

 2139 NE of MP E17 20 665 432 869 93.2 117.8 145.9 34 77 

 195 SW of MP MN59 4 262 353 453 77.4 90.1 140.7 30 65 

 642 E of MP AB84 30 793 257 1129 74.3 126.2 162.6 35 122 

 1108 E of MP AB89 32 784 697 1617 134.1 186.8 217.2 36 89 

 1240 E of MP AB116 6 540 734 873 129.2 138.9 156.9 32 79 

1936 NE of MP AB158 44 972 391 1764 85.3 185.9 222.9 40 150 

 2230 S of MP AB179 17 796 650 1301 109.0 128.8 161.8 38 137 

1939 SW of MPAB204 5 766 409 657 80.0 83.5 96.7 33 130 

1732 NE of MP AB216 22 542 678 1124 87.5 145.5 187.3 36 84 

Regarding the extreme strains observed during Phase 2 testing, the highest strains on the center disc 

resulting from a vertical impact were observed on May 27 approximately 1000’ northeast of MP AB 224, 

near the Plains Interlocking, while traveling at 119 mph. The amplitude of the BOP strains on the center 

disc reached 2,258 microstrain, zero-to-peak, on spoke 6. The amplitude of the BOP strains on spoke 3 

was 1,785 microstrain, zero-to-peak, at the same location. This particular case was extremely high, 

especially when it is considered that the vertical shock on the left end of the axle was 102 g. High BOP 

strains resulting from impacts during Phase 2 testing were typically on the order of 1,400 microstrain, 

zero-to-peak. 

The highest amplitude of sustained BOP oscillations on the center disc during a brake application was 

observed 1936’ northeast of MP AB 158 during braking from 150 mph. In this case, BOP strain 

oscillations reached average-to-peak amplitudes of ± 972 microstrain or peak-to-peak amplitudes of 1,944 

microstrain. These oscillations were superimposed on a mean measured strain that reached 1,764 

microstrain. 

As stated earlier, the strains presented are as measured and have not been corrected for pre-load strains in 

the spoke. 

Asymmetric Vertical Shocks 

As shown in both Figure 3.9 and Figure 3.30, vertical shocks elicit large BOP strain amplitudes in the 

WABTEC/SAB-WABCO supplied brake disc that damp out within a short period of time. A common 

feature to the shocks illustrated in both of these figures is that the vertical shock on one end of the test 

axle is higher than that observed on the other end, referred to as an asymmetric shock. The vast majority 

of impacts observed during the test program were of this nature. Occasionally, impacts were measured on 

both ends of the test axle at nearly the same instant that did not elicit as large a BOP strain as some 

responses to asymmetric shocks. Figure 3.32 illustrates the response of the WABTEC/SAB-WABCO 
83 

supplied disc to shocks of similar magnitude on both ends of the axle. In this case, vertical impacts of 

88.8 g and 66.5 g were recorded on the left and right ends of the test axle, respectively, while traveling 

southbound at 60 mph approximately 1400’ northwest of MP AB 117. A lateral shock of approximately 

40 g was recorded at the same location. 

Figure 3.32. BOP Strains on WABTEC/SAB-WABCO Supplied Center Disc Resulting from 

Vertical Shocks on Both Ends of Test Axle (File 052705_08.AB2)–May 27, 2005 

The BOP strain response of the brake disc is illustrated in two separate plots, one showing the sum of the 

accelerations recorded at both ends of the axle and one showing the difference in accelerations recorded at 

both ends of the axle. One can observe that the maximum sum of the accelerations, indicated by the 

dashed line at the beginning of the second wheel revolution, reaches a value near 150 g but elicits little 

BOP strain response. The BOP strain response increases during the middle stage of the second wheel 

revolution and the third wheel revolution, which correlates well with the increase in the difference in the 

axle accelerations. 

An important aspect of the response of the WABTEC/SAB-WABCO supplied brake disc to vertical 

impacts is the orientation of the brake disc and, in particular, the position of the spoke from which strains 

are being collected. Review of the test data indicated that, for a given level of the difference in 

accelerations on both ends of the test axle (originating at either end), a variation in the amplitude of the 

BOP strain resulting from the impact was observed. An analysis was conducted on accelerations recorded 

on the May 26 test run to arrive at an explanation for this observation. 

For the analysis of asymmetric shocks, impacts were selected and the BOP strain measured on spoke 6 

was plotted against the arithmetic difference in accelerations measured on the ends of the test axle. Only 
84 

21 cases in which the acceleration difference was greater than 100 g were initially considered and only 

those cases with a single peak acceleration difference were analyzed, resulting in 17 examples included in 

this brief analysis. Figure 3.33 illustrates the results of this consideration. Any meaningful correlation 

does not appear to exist, as indicated by the low correlation factor. 

Figure 3.33. BOP Strains as a Function of Acceleration Differences Measured on May 26, 2005 

If the BOP strain on spoke 6 is plotted against the product of the acceleration difference and the 

trigonometric sine of the angular position of the spoke (as defined in Figure 2.22), however, a better 

correlation is realized. Figure 3.34 illustrates the results of this consideration. A correlation between the 

amplitude of acceleration differences and the amplitude of the response in terms of BOP strain emerges. 

Although the results illustrated in Figure 3.34 are based on a limited sample size, the relationship between 

the difference in loads measured at either end of the test axle and the BOP strains is important to the 

behavior of the brake disc. 
85 

Figure 3.34. Correlation of BOP Strain Amplitudes and Acceleration Differences 

Based on Measurements Collected on May 26, 2005 

In the development of Figures 3.33 and 3.34, it was pointed out that only those cases with a single peak 

acceleration difference were analyzed. This was an important criterion to keep in mind. Many different 

loading conditions were observed in which the amplitude of the BOP strains did not vary linearly with the 

accelerations measured on the ends of the axle. As presented here, the angular position of the 

instrumented spoke is important but so is the acceleration time history associated with the peak 

accelerations. It can be shown that several peak accelerations experienced in rapid succession can 

actually diminish the peak BOP strains observed on the spokes. Appendix G treats this subject in more 

detail. 

 

The observation that the BOP strain correlates with the difference in accelerations measured at the ends of 

the axle suggests that it is a “rotation of the axle in a vertical plane” of the axle that has the greatest effect 

on the disc. 
86 

Figure 3.35. Definition of Wheelset Acceleration Terminology 

Figure 3.35 illustrates an idealized wheelset. If one assumes that the wheelset/brake disc assembly is 

rigid and is limited to two degrees of freedom, vertical translation and rotation, then the accelerations 

measured on the ends of the axle can be used to specify the acceleration of any point on the rigid body 

through the following relationships: 

Using this formulation, it can be seen that the acceleration difference reflected in Figures 3.33 and 3.34 is 

indicative of the rotational acceleration of the axle, which is constant over the length of the axle. The 

actual measured acceleration will depend on the location of the sensor along the axle. This leads to the 

conclusion that all three brake discs on the axle will respond in a similar fashion to a given difference in 

accelerations observed at each end of the axle. This is supported by some of the observations regarding 

relative responses of the inner and outer brake discs presented in Section 3.1.3. 

Several key findings regarding strains in the WABTEC/SAB-WABCO supplied brake discs resulted from 

Phase 2 testing: 

• The vibration of the WABTEC/SAB-WABCO supplied brake disc during responses to vertical 

shocks, as well as during some brake applications, was a BOP oscillation in which the motion of the 

spokes on one side of the disc was out-of-phase with the motion of the spoke on the opposite side of 

the disc. 
87 

• When experiencing sustained oscillations during braking, the maximum BOP strains were observed to 

occur while the particular spoke was in the vicinity of the brake pad, indicating that the calipers play a 

role in these oscillations. 

• It was confirmed that high BOP strain oscillations observed during braking only occurred when the 

test axle was in the leading position of the test vehicle. Consideration of the interaction of the 

rotating brake disc with the brake mounting hardware was discussed as a possible explanation for this 

behavior. 

• Analysis of the data recorded during Phase 2 testing showed that the WABTEC/SAB-WABCO 

supplied disc’s response to asymmetric vertical impacts was more severe than that resulting from 

impacts of similar magnitude being inflicted upon the ends of the axle. The amplitude of the BOP 

strains resulting from asymmetric vertical shocks correlate well with the tilt acceleration, reflective of 

the difference between the two impacts, when it is corrected for the angular position of the 

instrumented spoke. 

3.2.3 Axle Strains 

As discussed in Section 2.3.1, several axle-mounted strain gages were employed during Phase 2 testing to 

investigate the behavior of the axle. Two gages were placed 180o apart as close to the center disc as 

possible, and two additional gages were placed 180o apart at a location approximately 1⁄4 of the distance 

between the two wheels adjacent to the S brake disc (the non-instrumented brake disc). Appendix H 

examines axle strains collected throughout the test program. This section will examine the basic 

characteristics of these signals and observations made during Phase 2 testing. 

Figure 3.36 illustrates a wheelset, without disc brakes, under equilibrium conditions. The axle strain data 

is dominated by a bending moment produced by the vehicle’s weight applied at the bearings and the 

vertical rail forces applied at the wheel rail interface. 

Figure 3.36. Bending Moments in Generic Wheelset

Figure 3.37 shows the calculated bending strain for a range of applied forces and moment arms between 

the midpoint of the bearing and the wheel/rail contact point. The published weight for the Acela coach is 

139,000 pounds. Based on the drawing for the Acela wheelset, the distance between the midpoint of the 

bearing and the wheel/rail contact point is approximately 10 inches. Based on these values, the predicted 

axle bending strains are approximately 200 microstrain. 
88 

Figure 3.37. Axle Bending Strain as a Function of Bearing Force and Moment Arm 

Figure 3.38 shows the output from a pair of axle-mounted strain gages as the test vehicle was moving 

over tangent track near MP AB 221 at 115 mph on May 26. The output from the strain gages have had 

the offset, or average reading, removed for presentation purposes. The output from the slip ring resolver 

is also plotted in this figure to illustrate the rotational rate of the axle. 

Figure 3.38. Typical Axle Bending Strains Recorded Over Tangent Track 

(File 052605_18.AB2)–May 26, 2005 
89 

The behavior of the axle strains depended on many factors. The amplitude of the strains increased in 

proportion to the lateral accelerations exerted on the axle as a result of navigating through a curve. As the 

axle rotates, this bending moment is modulated by the wheel rotation rate. Figure 3.39 illustrates the axle 

strains and spoke strains recorded during a case of sustained oscillations during a brake application from 

120 mph near MP AB 225.5 AB. In this particular case, the amplitude of the BOP oscillations is not 

considered to be high, and this case has not been identified as a case with high BOP strains, but it does 

serve to illustrate the behavior of the axle strain gages. The effects of the higher frequency content 

associated with the BOP strains on the brake disc are evident in the axle bending strains. 

Figure 3.39. Axle Bending Strains Recorded During Case of Small Oscillating BOP Strains 

During Brake Application (File 052605_18.AB2)–May 26, 2005

Figure 3.40 illustrates the strains recorded during a response to a vertical impact of 55.5 g experienced on 

the left end of the axle while traveling near the Route 128 Station at 30 mph on May 26. The effects of 

the impact can be seen on the axle. This illustration appears to indicate that the WABTEC/SAB-WABCO 

supplied brake disc responds to the impact and that the axle subsequently reacts to the motion of the disc.
90 

Figure 3.40. Axle Bending Strains Recorded During Response of WABTEC/SAB-WABCO 

Supplied Brake Disc to Vertical Impact (File 052605_18.AB2)–May 26, 2005

The frequency associated with the axle bending strain measurements are important, yet they are difficult 

to arrive at because, when the individual strain gages are at or near the neutral axis of the axle at 

particular times in the rotation cycle, the gages will see little bending and therefore very small strains. 

The frequency content of the axle strains will be considered in more detail during the discussion of Phase 

3 testing. 

3.2.4 Temperatures 

The maximum temperature measured on the back of the friction ring of the WABTEC/SAB-WABCO 

supplied brake disc located in the center of the test axle was 336 °F. This was measured following a 

series of brake applications made from high speeds between Providence, RI, and MP AB 160 on May 27 

(File 052705_05.AB2). 

As done with temperature measurements collected from the back of the center disc’s friction ring during 

Phase 1 testing, friction ring temperatures were used to estimate the state of thermal stress in the spokes 

of the disc. Based on the analysis method described in Section 3.1.5, estimates of the thermal strains in 

the spoke were derived from data collected during testing on May 26 and 27. Figures 3.41 and 3.42 

provide the results of this analysis with additional details in Appendix I. Results provided in these figures 

are similar to those determined during Phase 1 testing. These estimated thermal strains would not account 

for any existing pre-load strains in the spoke. 
91 

Figure 3.41. Brake Disc Temperature versus Initial Spoke Tensile Strain Before Brake 

Applications, Center WABTEC/SAB-WABCO Supplied Disc, Spoke 6–May 26, 2005 

Figure 3.42. Brake Disc Temperature versus Initial Spoke Tensile Strain Before Brake 

Applications, Center WABTEC/SAB-WABCO Supplied Disc, Spoke 6–May 27, 2005
92 

3.2.5 Brake Pressures 

The only brake pressure that was analyzed during Phase 2 testing was the brake cylinder. Results 

pertaining to brake cylinder pressures in Phase 2 were similar to those from Phase 1 testing–namely that 

the amplitude of the strain oscillations in episodes of sustained oscillations during braking increases with 

increasing brake pressure. Because this was demonstrated so clearly in Section 3.1.6, no additional 

results will be presented here. 

3.2.6 Sound Level Measurements 

As discussed in Section 2.3.8, sound levels under the test vehicle were recorded in order to attempt to 

capture and analyze two different noises. The first noise was the high frequency noise, or squeal, heard 

during times in which signs of saturation existed on the axle-mounted accelerometer. The second noise 

was the moaning noise heard above many of the axles of the test train during braking. 

Figure 3.43 illustrates the frequency analysis of sound measurements collected while the center 

WABTEC/SAB-WABCO supplied brake disc was exhibiting high BOP strains during a brake application 

from 137 mph near MP AB179 on May 26. In this analysis, the sound level expressed in decibels (dB) is 

plotted as a function of the frequency associated the particular component of the sound. 

 

Figure 3.43. Analysis of Sound Recorded During Sustained Oscillations While Braking 

Near MP AB 179–May 26, 2005 
93 

Within this recording, the sound on Channel 1 (the upper graph) and Channel 2 (the lower graph) has its 

strongest component of sound at a frequency of approximately 450 Hz. On both channels, a component 

of sound with a frequency of 370 Hz can be observed. In the sound recorded on Channel 2, evidence of 

contribution from a component with a frequency of 185.5 Hz exists. The 185.5 Hz frequency is close to 

the frequency associated with the sustained oscillations during braking observed in the strains. 

Analysis of sound recordings made when a high-pitched squeal was heard while navigating through 

curves did not reveal any one particular component of sound that could lend insight into the saturation of 

the axle mounted sensors. Aside from the observations cited above, no other useful information resulted 

from the noise testing conducted during Phase 2 testing. 

3.2.7 Concluding Comments

Several items were learned from the results of Phase 2 testing, including the following: 

• The load environment associated with vertical impacts was confirmed to be more severe than that 

associated with lateral impacts. It was established that vertical accelerations are typically between 2 

and 3.7 times lateral accelerations, regardless of the level of accelerations considered.

There were 276 vertical shocks above 50 g measured on the WABTEC/SAB-WABCO test axle 

during the northbound test conducted on May 26, 16 of which were above 100 g. During the 

southbound test conducted on May 27, 238 vertical shocks above 50 g were measured, 15 of which 

were over 100 g. The highest vertical acceleration observed in Phase 2 testing was 156 g, measured 

on the left end of the axle on May 26 near Union Interlocking near MP AN 20 while the test train was 

traveling at 113 mph. 

There were 44 lateral shocks above 30 g measured on the WABTEC/SAB-WABCO test axle during 

the northbound test conducted on May 26. Thirty-three lateral shocks above 30 g were measured on 

the WABTEC/SAB-WABCO test axle during the southbound test conducted on May 27. The highest 

lateral acceleration observed during Phase 2 testing was a 75.3 g shock, measured on May 26 at the 

Union Interlocking near MP AN 20 while the test train was traveling at a speed near 112 mph.

• The vibration of the WABTEC/SAB-WABCO supplied brake disc during responses to vertical 

shocks, as well as during some brake applications, was confirmed to be a BOP oscillation in which 

the motion of spokes on one side of the disc were out-of-phase with the motion of spokes on the 

opposite side of the disc. 

When experiencing sustained oscillations during braking, the maximum BOP strains were observed to 

occur while the particular spoke was in the vicinity of the brake pad, indicating that the calipers play a 

role in these oscillations. 

It was confirmed that high BOP strain oscillations observed during braking only occurred when the 

test axle was in the leading position of the test vehicle. 

The WABTEC/SAB-WABCO supplied disc’s response to asymmetric vertical impacts was more 

severe than that resulting from impacts of similar magnitude being inflicted upon both ends of the 

axle at the same time. The amplitude of the BOP strains resulting from asymmetric vertical shocks 

correlate well with a measure of these asymmetric loads when the angular position of the wheel is 

accounted for. 

The highest strains on the center disc resulting from a vertical impact reached 2,258 microstrain, zeroto-peak, on spoke 6; the amplitude of the BOP strains on spoke 3 was 1,785 microstrain, zero-topeak, at the same location. These values were extremely high. High BOP strains resulting from 

impacts during Phase 2 testing were typically on the order of 1,400 microstrain, zero-to-peak. 
94 

The highest amplitude of sustained BOP oscillations on the center disc during a brake application 

reached average-to-peak amplitudes of ± 972 microstrain, or peak-to-peak amplitudes of 1,944 

microstrain. These oscillations were superimposed on a mean strain that reached 1,764 microstrain. 

Specified strains are as measured and have not been corrected for pre-load strains in the spoke. 

• Strains recorded on the axle during Phase 2 testing exhibited the effects of the oscillatory motion of 

the center brake disc resulting from vertical impacts and selected brake applications. 

• The maximum temperature measured on the back of the friction ring of the WABTEC/SAB-WABCO 

supplied brake disc located in the center of the test axle was 336 °F. This was measured following a 

series of brake applications made from high speeds between Providence, RI, and MP AB 160 on May 

27.

Several issues associated with instrumentation were identified, including the following: 

• Noise spikes were observed on the signals from the F1 and F2 strain gages on spoke 6. 

• New accelerometers mounted to the axle to improve the collection of lateral accelerations did not 

prove effective. The signals resulting from these sensors were not used in the development of results 

presented in this document.

Following Phase 2 testing, it was decided that a sufficient amount of information pertaining to the strains 

measured on the WABTEC/SAB-WABCO supplied disc during over-the-road testing had been collected, 

and no need existed to continue testing the WABTEC/SAB-WABCO supplied brake disc in this manner 

any longer. Preparations commenced for a third phase of the test program in which the response of the 

WABTEC/SAB-WABCO supplied brake disc to over-the-road testing would be compared that of a brake 

disc of alternative design provided by Knorr. In addition to capturing the behavior of the different brake 

discs, several other goals for Phase 3 testing existed, including the following: 

• The determination of the exact value of the natural frequencies of both the WABTEC/SAB-WABCO 

supplied brake disc and the Knorr brake disc. 

• The determination of the stress levels in the spokes of the brake disc before being exposed to service 

loads. 

• The role of the brake mounting hardware, including the brake mounting tube and caliper 

links/hangers, in the behavior of the brake discs observed to date. 
95 

3.3 Phase 3 Test Results 

In Phase 3, axle 4 of Car 3534 (adjacent to Car 3413) was removed from the test trainset, and the existing 

WABTEC/SAB-WABCO supplied brake discs were removed and replaced with discs provided by Knorr. 

The newly outfitted axle and center Knorr brake disc were instrumented as specified in Figure 2.8b. 

Modal testing was conducted on the Knorr test axle, to be described in Section 3.3.2, and the axle was 

installed under Coach 3534. Figure 3.44 shows the locations of the strain gages on the spoke of the Knorr 

brake disc, recommended by Knorr. For the sake of comparison, Figure 3.45 illustrates the locations of 

the strain gages on the spokes of the WABTEC/SAB-WABCO supplied disc. 

Figure 3.44. Strain Gage Location and Cross Section Associated with Spoke of Knorr Brake Disc 

Figure 3.45. Strain Gage Location and Cross Section Associated with Spoke of 

WABTEC/SAB-WABCO Supplied Brake Disc
96 

Following a shakedown run conducted between Washington, DC, and New York, NY, a final series of 

tests was run during which direct comparisons were made between the Knorr disc and the 

WABTEC/SAB-WABCO supplied disc during a round trip between Washington and Boston. Data was 

collected in a manner that would allow decisions to be made about the potential for success of the Knorr 

brake disc design in revenue service. 

In addition to the over-the-road tests, laboratory tests were performed to investigate the natural 

frequencies of the WABTEC/SAB-WABCO supplied and Knorr brake discs, as well as the stress levels 

existing within the spokes of each type of disc before exposure to loads encountered during regular 

service. 

3.3.1 Determination of Pre-Stresses Within Spokes 

To prepare for Phase 3 of the test program, one wheelset equipped with WABTEC/SAB-WABCO 

supplied discs was removed from the test trainset and shipped to the ORX assembly plant in Tipton, PA, 

for disassembly and re-fit with the Knorr discs. During this changeover process, Mr. Mike Tomas of 

Amtrak installed strain gages on either side of each spoke of the WABTEC/SAB-WABCO supplied 

center disc at locations shown in Figure 3.45 before it was pressed off and measured the resultant strain 

when the disc was removed. The WABTEC/SAB-WABCO supplied disc was reported to have close to 

2,000 miles of service life at the time of removal, and no cracked spokes were reported. For this single 

test specimen, an average resultant compressive strain of 390 microstrain in each spoke was attributed to 

the pressing operation. Because only one disc was considered in this exercise, it is not clear from the 

results of this particular study if these stresses are typical for discs with this level of service life. Part B of 

Appendix J provides the details of this test. 

Amtrak also installed strain gages on either side of each spoke on the Knorr brake disc designated for the 

center of the axle before its installation on the axle; the location of the gages were the same as those 

shown in Figure 3.44. The spoke strains on the Knorr brake disc were measured prior to and following 

the pressing of the disc on to the axle. The installation of the Knorr disc results in an average 

compressive strain of 533 microstrain on the spokes. Part B of Appendix J also provides the details of 

this investigation. 

A test was conducted by Amtrak in which the strain-gaged spokes of the WABTEC/SAB-WABCO 

supplied brake disc removed from the center position of the axle were cut to relieve any remaining stress 

in the spokes. Strain measurements were collected prior to and following the cutting of the spokes. 

Results from this test indicated that an average compressive resultant strain of 600 microstrain in the 

spokes was present. Part B of Appendix J also provides details of this investigation. This test was not 

conducted on a brake disc of Knorr design. 

Details of the investigation conducted by Amtrak of spoke strain levels present in a WABTEC/SABWABCO supplied disc with two cracked spokes are provided in Part B of Appendix J. Measurements 

indicated that the unbroken spokes of a brake disc with some broken spokes were under more stress than 

spokes on a brake disc with no broken spokes. 

In conclusion, results from this single-sample test indicate that a pre-stress equivalent of approximately 

1,000 microstrain may be present as a residual strain in each spoke of a WABTEC/SAB-WABCO 

supplied brake disc. Appendix M discusses the sources of these existing stresses. 
97 

3.3.2 Mode Test Performed on WABTEC/SAB-WABCO Supplied and Knorr Brake Discs 

Before the commencement of over-the-road testing in Phase 3, a series of vibration tests was performed to 

identify fundamental natural frequencies of the Knorr brake disc mounted on the test axle and on a 

WABTEC/SAB-WABCO supplied brake disc on a typical axle. The WABTEC/SAB-WABCO supplied 

disc tested in this investigation was not used in the earlier phases of the test program. As described in 

Section 2.4.3, a force input was provided to each of the discs by striking them with a dead blow hammer, 

and the response of the discs was measured using accelerometers mounted at various locations around the 

discs. Each of the discs tested in this investigation were installed on axles that had been removed from 

under the coach cars. 

Table 3.3 lists identifiable modes of vibration of the WABTEC/SAB-WABCO supplied disc over a 

frequency range up to 1,200 Hz. 

Table 3.3. Vibration Frequencies Identified During Impact Tests Conducted 

on WABTEC/SAB-WABCO Supplied Brake Disc 

Frequency 

(Hz) Type of Vibration 

220 Out-of-Phase 

320 In-Phase 

360 In-Phase 

675 In-Phase 

815 In-Phase 

905 Out-of-Phase 

990 In-Phase 

The lowest identifiable frequency for the WABTEC/SAB-WABCO supplied disc was an out-of-phase 

mode corresponding to BOP mode. The frequency associated with this motion was measured as 220 Hz. 

This frequency is very close to 206 Hz, the frequency calculated in the FEA conducted at the early stage 

of the investigation using an idealized model of the WABTEC/SAB-WABCO supplied disc. 

Table 3.4 lists frequencies determined for the Knorr disc over a frequency range up to 1,200 Hz. The 

lowest identifiable frequency for the Knorr disc was an in-phase mode at approximately 420 Hz. No outof-phase mode could be identified for the Knorr disc. ENSCO did not perform any FEA to determine 

frequencies of interest in the Knorr disc. 
98 

Table 3.4. Vibration Frequencies Identified During Impact Tests Conducted 

on Knorr Brake Disc 

Frequency 

(Hz) Type of Vibration 

420 In-Phase 

675 In-Phase 

825 In-Phase 

1,020 In-Phase 

After completion of the Phase 3 over-the-road testing, a second vibration test using a slightly different 

approach to the one used in the beginning of Phase 3 was conducted on the Knorr disc to investigate 

frequencies observed in the test data. For this second vibration test, Instrumented Axle 2 still installed 

under Coach Car 3534 was used. Excitation was provided to the disc, and the response of the strain gages 

on spokes 3 and 6 were recorded and analyzed to obtain the frequency at which the spokes would vibrate. 

Excitation was provided by three hammer blows applied successively at approximately 5-second intervals 

to the friction ring at the spoke 3 location, with spoke 6 at the position within the brake pads and caliper. 

The brakes were disengaged throughout the test. Part C of Appendix J provides the details of the analysis 

of the resulting spoke strains. 

Table 3.5 lists results from this test for the Knorr disc over a frequency range up to 1,200 Hz. The lowest 

identifiable frequency identified in the spoke strain measurements for the Knorr disc was at 351 Hz. By 

comparing the strain measured simultaneously in spoke 3 with that in spoke 6, the vibration at this 

frequency corresponded to an out-of-phase mode, suggesting BOP. 

Table 3.5. Natural Frequencies Identified by Hammer/Strain Measurement Test 

for the Knorr Brake Disc

Frequency 

(Hz) 

Phase Between 

Spoke 3/6 

351 Out-of-Phase 

460 In-Phase 

680 In-Phase 

Results from over-the-road testing conducted in Phase 3 will now be presented. This section will focus 

on the results determined from data collected during the round trip between Washington and Boston. The 

only data collected during the shakedown run conducted between Washington and New York on June 16 

that will be presented will be some temperature measurements that represent the highest temperatures 

reached during the program and some basic strain data collected from some of the brake mounting 

hardware. 
99 

3.3.3 Axle Accelerations

Figure 3.46a illustrates the distribution of vertical accelerations above 50 g recorded on both the 

WABTEC/SAB-WABCO and Knorr test axles from Washington to Boston on June 17 with the test axles 

in a leading position. Figure 3.46b illustrates the corresponding information for the data collected 

between Boston and Washington on June 18, again with the test axles in a leading position. On these two 

test days, the accelerometers on the test axles exhibited effects from noise spikes and saturation from time 

to time, resulting in measurements that appeared similar to the data illustrated in Figure 3.3. 

Accelerations above 50 g were reviewed to ensure that any acceleration affected by the noise-related or 

saturation issues was disregarded. Appendix F tabulates the data illustrated in Figure 3.46. 

Figure 3.46a. Distribution of Vertical Accelerations Above 50 g Measured on Instrumented Axles 

1 and 2 in Lead Position–Washington, DC, to Boston, MA, on June 17, 2005
100 

Figure 3.46b. Distribution of Vertical Accelerations Above 50 g Measured on Instrumented Axles 

1 and 2 in Lead Position–Boston, MA, to Washington, DC, on June 18, 2005

There were 206 vertical shocks above 50 g measured on the WABTEC/SAB-WABCO test axle during 

the northbound test conducted on June 17, 96 on the left end axle box and 110 on the right end axle box, 

and 193 vertical shocks above 50 g were measured on the Knorr test axle during the same test run, 98 on 

the left end axle box and 95 on the right end axle box. Regarding the southbound test conducted on June 

18, 159 vertical shocks above 50 g were measured on the WABTEC/SAB-WABCO test axle, 78 on the 

left end axle box and 81 on the right end axle box, and 169 vertical shocks above 50 g were measured on 

the Knorr test axle, 87 on the left end axle box and 82 on the right end axle box. 

The highest vertical acceleration observed on the WABTEC/SAB-WABCO test axle during Phase 3 

testing was 189 g, measured on the left end of the axle on June 17 near MP 56 of the Metro-North Rail 

Road while the test train was traveling at 48 mph. At this same location, the Knorr test axle experienced 

its highest vertical impact on the northbound trip, 172 g. During the southbound test conducted on June 

18, the Knorr test axle experienced its highest vertical impact of the test, a 178 g impact on the left end of 

the axle in the vicinity of Transfer Interlocking near MP AB 218.5 while traveling at 131 mph. At this 

same location, the WABTEC/SAB-WABCO test axle experienced a vertical impact of 106 g on its left 

end. 

Appendix F provides the details associated with all the accelerations above 100 g in tabular form, 

including location, speed of the test train, and information regarding the measured strains at the particular 

location. 

As discussed in Section 2.3.2, ENSCO employed accelerometers provided by Vibra-Metrics to monitor 

the lateral accelerations on the test axles. On the WABTEC/SAB-WABCO test axle, ENSCO employed 

some of the original axle accelerometers, as well as the Vibra-Metrics accelerometer; the Vibra-Metrics 

accelerometer was the only accelerometer employed on the Knorr test axle. The performance of the 

Vibra-Metrics accelerometer on the WABTEC/SAB-WABCO test axle was not satisfactory, and all 
101 

results pertaining to accelerations measured by axle-mounted accelerations on the WABTEC/SABWABCO test axle will be based on the Silicon Designs +/- 200 g single-axis accelerometer employed 

during the first two phases of testing. 

Figure 3.47a illustrates the distribution of lateral accelerations above 30 g recorded on the test axle 

between Washington and Boston on June 17. Figure 3.46b illustrates the corresponding information for 

the data collected between Boston and Washington on June 18. Appendix F tabulates the data illustrated 

in Figure 3.47. On these two test days, the lateral accelerometers on the test axles exhibited effects from 

noise spikes and saturation from time to time. Accelerations above 30 g were reviewed to ensure that any 

acceleration affected by the noise-related or saturation issues was disregarded.

Figure 3.47a. Distribution of Lateral Accelerations Above 30 g Measured on Instrumented Axles 

1 and 2 in Lead Position–Washington, DC, to Boston, MA, on June 17, 2005 
102 

Figure 3.47b. Distribution of Lateral Accelerations Above 30 g Measured on Instrumented Axles 

1 and 2 in Lead Position–Boston, MA, to Washington, DC, on June 18, 2005

There were 28 lateral shocks above 30 g measured on the WABTEC/SAB-WABCO test axle during the 

northbound test conducted on June 17, 18 on the left end axle box and 10 on the right end axle box, and 

29 lateral shocks above 30 g were measured on the Knorr test axle during the same test run, 19 on the left 

end axle box and ten 10 on the right end axle box. These counts represent those shocks in which the 

measurement was deemed reliable. No lateral shocks were measured directly on either test axle during 

the June 17 test run that were considered reliable. Regarding the southbound test conducted on June 18, 

25 lateral shocks above 30 g were measured on the WABTEC/SAB-WABCO test axle, 16 on the left end 

axle box and 9 on the right end axle box, and 14 lateral shocks above 30 g were measured on the Knorr 

test axle, 6 on the left end axle box and 8 on the right end axle box. No lateral shocks were measured 

directly on either test axle during the June 18 test run that were considered reliable. 

The highest lateral acceleration observed on the WABTEC/SAB-WABCO test axle during Phase 3 testing 

was 58.5 g, measured on the left end of the axle on June 17 in the vicinity of Union Interlocking near MP 

AN 20 while traveling at 113 mph. The Knorr test axle also experienced its highest lateral impact of the 

test on the June 17 northbound test run, a 58.4 g impact on the left end of the axle near Bowie 

Interlocking near MP AP 120.8 while traveling at 125 mph. 

In summary, results from Phase 3 testing indicated that the load environment as characterized by high 

lateral and vertical impacts was similar for each of the test axles. This observation is important because it 

established the fact that each test axle is subjected to similar loads, allowing for straightforward 

comparisons of the response of the respective discs for given load conditions. 
103 

3.3.4 Spoke Strains 

Based on observations made during the earlier phases of the test program, one of the issues of interest 

entering Phase 3 testing was the response of the Knorr-designed brake disc to those events that had 

elicited high BOP strains on the spokes of the WABTEC/SAB-WABCO supplied brake disc, namely 

vertical impacts and cases in which sustained oscillations were observed in some brake applications. 

Regarding the response to vertical impacts, it was immediately evident during testing, as well as in posttest data analysis, that for comparable vertical shocks on the two test axles, the BOP strains recorded on 

the spokes of the Knorr brake disc were substantially lower than the BOP strains recorded on the spokes 

of the WABTEC/SAB-WABCO supplied brake disc. This observation is exemplified by the comparison 

of Figures 3.48 and 3.49. 

Figure 3.48 illustrates a severe vertical impact and the response of the WABTEC/SAB-WABCO supplied 

brake disc as characterized by the BOP strains measured on spoke 6. The data shown in Figure 3.48 was 

recorded on June 18 while the test train was traveling southbound in the vicinity of Lane Interlocking near 

MP AN 12 at 124 mph. As seen in the plots of vertical accelerations within the figure, the axle was 

subjected to an asymmetric vertical, or tilting, acceleration in which a positive acceleration of 147 g was 

measured on the left end axle box and a negative acceleration of -73 g was measured on the right end axle 

box. Figure 3.49 illustrates the corresponding response of the Knorr disc to the same disturbance, 

measured as a positive acceleration of 145 g on the left end axle box and a negative acceleration of 49 g 

on the right end axle box. Spoke 6 of each of the center brake discs at the time of the impacts illustrated 

in Figures 3.48 and 3.49 was approaching the straight up or 12:00 position. Spoke 6 of the Knorr disc 

was approximately 20o from the 12:00 position; spoke 6 of the WABTEC/SAB-WABCO supplied disc 

was approximately 60o from the 12:00 position. 

Figure 3.48. Vertical Shock and BOP Spoke Strain Response of WABTEC/SAB-WABCO Supplied 

Brake Disc Measured Near Lane Interlocking (File 061805_30.AB3)–June 18, 2005 
104 

Figure 3.49. Vertical Shock and BOP Spoke Strain Response of Knorr Brake Disc Measured Near 

Lane Interlocking (File 061805_30.AB3)–June 18, 2005

As seen in the bottom of Figure 3.48, the shock produces a BOP spoke strain on the WABTEC/SABWABCO supplied brake disc with an amplitude of 1,487 microstrain, zero-to-peak, that requires almost 

0.20 seconds to die out. The asymmetric vertical shock produces a BOP spoke strain with an amplitude 

of 290 microstrain, zero-to-peak, in the Knorr brake disc that requires approximately 0.03 seconds to die 

out. The frequency associated with this particular response of the WABTEC/SAB-WABCO supplied 

brake disc is 224 Hz while the frequency associated with the Knorr disc is 344 Hz. Both of these 

frequencies are close to those associated with the out-of-phase responses of the respective discs 

determined in the modal tests conducted during Phase 3 testing (see Table 3.4 for WABTEC/SABWABCO supplied disc and Table 3.5 for Knorr disc). Similar results with respect to the relative 

responses of the two discs to vertical shocks were observed throughout Phase 3 testing. The highest 

strains on the center brake disc resulting from a vertical impact only during Phase 3 testing was the event 

illustrated in Figure 3.48. 

With regards to the sustained oscillations observed during selected brake applications, 33 cases were 

observed on the WABTEC/SAB-WABCO supplied brake disc during Phase 3 testing, 24 on the 

northbound test run conducted on June 17 and 9 during the southbound test run on June 18. In none of 

these cases was any oscillation of appreciable amplitude observed on the Knorr brake disc. Table 3.6 

provides the number of events of sustained oscillations during braking for all testing on the trips between 

Washington and Boston. The test run conducted on June 17 included more brake applications than any of 

the other test runs. 
105 

Table 3.6. Summary of Observations of High BOP Strain Sustained Oscillations During Braking 

Number of Sustained Oscillation 

Events Observed on Center Disc 

Date Direction During Braking 

Position of 

Instrumented 

Axles W/S-W 

Brake Disc 

Knorr 

Brake Disc 

Number of 

Brake 

Applications 

May 16, 2005 Northbound Trailing 0 N/A 103 

May 17, 2005 Southbound Leading 8 N/A 76 

May 26, 2005 Northbound Leading 11 N/A 82 

May 27, 2005 Southbound Trailing 0 N/A 98 

June 17, 2005 Northbound Leading 24 0 147 

June 18, 2005 Southbound Leading 9 0 95 

Details associated with each of the significant events of sustained oscillations during braking are provided 

in Table 3.7 for testing conducted on June 17 and Table 3.8 for testing conducted on June 18. Appendix 

G provides additional details associated with these events. Figure 3.13 defines terminology used in 

Tables 3.7 and 3.8. A large number of the events observed on June 17 occurred on the north end of the 

corridor in relatively close proximity to one another. Many additional brake applications were made 

during June 17 testing in the New England Division to observe the sustained oscillations. This is 

reflected in the higher number of brake applications for this particular test shown in Table 3.6. The 

conditions of the brake pads on the WABTEC/SAB-WABCO supplied and Knorr brake discs were 

slightly different. The pads used with the WABTEC/SAB-WABCO supplied brake discs were installed 

before the commencement of the test program, while the pads used with the Knorr brake discs were 

installed at the same time that the Knorr discs were installed. 

 
106 

Table 3.7. Summary of Measurements Collected from Center WABTEC/SAB-WABCO Supplied 

Brake Disc During Brake Applications Exhibiting Signs of High BOP Strain–June 17, 2005 

Measurements from Center Disc 

Geographic Location Duration 

(s) 

Max 

Avg-to-Pk 

Amplitude 

(με) 

Avg 

Strain 

@ Start 

(με) 

Avg 

Strain 

@ End 

(με) 

Disc 

Temp 

@ Start

 (oF) 

Disc 

Temp 

@ End

 (oF) 

Peak 

Disc 

Temp 

(oF) 

Brake 

Cyl 

Press 

(psi) 

Speed at 

Start of 

Braking 

(mph) 

2089 NE of MP AP 4 3.5 532 1282 1348 169.2 212.2 242.1 46 85 

980 NE of MP MN 21 7 335 488 694 143.3 145.9 179.7 42 89 

1627 NE of MP MN 25 14 327 528 766 152.5 158.6 181.0 32 73 

772 E of MP MN 28 4 316 611 646 158.6 158.6 169.2 29 67 

2599 E of MP MN 45 5 186 604 677 146.8 147.2 155.6 23 69 

582 SW of MP MN 59 6 277 478 611 130.1 134.0 179.3 31 68 

1163 W of MP MN 64 9 288 470 633 136.7 138.9 156.4 28 70 

1530 SW of MP MN 68 9 314 528 704 144.6 145.9 157.8 30 74 

49 E of MP MN 69 12 395 665 861 149.0 162.6 178.4 34 72 

717 NE of MP MN 70 9 334 750 930 168.7 170.5 188.9 32 71 

1022 E of MP AB 99 11 658 756 1035 144.6 148.5 170.1 44 120 

1768 E of MP AB 102 6 290 770 922 164.8 163.0 180.6 35 85 

1525 NE of MP AB 115 14 1010 580 1038 138.4 147.6 167.9 43 81 

N/A (GPS Error) 6 526 984 1081 190.3 190.7 204.3 41 83 

N/A (GPS Error) 6 403 1172 1280 203.9 203.9 221.9 31 78 

821 SE of MP AB 125 4 256 938 1005 204.8 204.8 213.6 29 66 

639 SE of MP AB 126 6 658 1004 1176 211.4 211.4 215.3 41 71 

2129 SE of MP AB 128 6 899 1142 1486 214.9 232.0 266.3 44 94 

29 NE of MP AB 131 20 379 1218 1520 224.5 236.4 250.5 37 95 

2136 SW of MP AB 138 15 719 1015 1413 189.4 242.6 245.2 40 90 

673 SW of MP AB 140 20 749 1272 1710 221.0 276.4 278.2 51 92 

464 W of MP AB 143 13 1139 1170 1686 222.3 227.2 247.0 50 107 

2455 NE of MP AB 158 47 1372 658 2202 144.1 310.7 330.0 55 148 

2579 NE of MP AB 160 11 591 520 996 130.5 134.0 179.3 33 141 

NOTE: No oscillations of appreciable amplitude observed on center Knorr brake disc. 
107 

Table 3.8. Summary of Measurements Collected from Center WABTEC/SAB-WABCO Supplied 

Brake Disc During Brake Applications Exhibiting Signs of High BOP Strain–June 18, 2005 

Measurements from Center Disc 

Geographic Location Duration 

(s) 

Max 

Avg-to-Pk 

Amplitude 

(με) 

Avg 

Strain 

@ Start 

(με) 

Avg 

Strain 

@ End 

(με) 

Disc 

Temp 

@ Start

 (oF) 

Disc 

Temp 

@ End 

 (oF) 

Peak 

Disc 

Temp 

(oF) 

Brake 

Cyl 

Press 

(psi) 

Speed at 

Start of 

Braking 

(mph) 

2396 NE of MP AB 202 22 948 1309 1918 100.6 253.5 252.7 56 116 

2619 S of MP AB 178 21 1226 438 1483 115.6 244.3 243.4 52 130 

691 SW of MP AB 170 16 559 971 1406 169.2 198.2 225.0 35 113 

4 SW of MP AB 162 37 1200 650 2154 145.4 266.3 298.8 54 150 

1222 SW of MP AB 159 24 1132 1491 2315 249.6 301.4 327.8 54 120 

732 SW of MP AB 156 27 835 1719 2512 282.5 314.6 341.9 52 120 

784 SW of MP AN 13 15 1129 285 925 107.2 156.0 199.9 55 110 

1211 SW of MP AN 19 7 1172 643 938 143.3 148.5 179.3 51 120 

1748 SW of MP AN 55 17 1455 1466 1974 226.7 261.9 325.6 56 133 

NOTE: No oscillations of appreciable amplitude observed on center Knorr brake disc.

The highest amplitude of sustained BOP oscillations on the center WABTEC/SAB-WABCO supplied 

disc during a brake application during Phase 3 was observed on June 18 at 1748’ southwest of MP AN 55 

during braking from 133 mph. This event was subject to the effects of the sustained oscillations, the rise 

in mean strain due to heat buildup and a vertical impact experienced in the middle of the brake 

application. The highest BOP oscillations due solely to a brake application were observed near MP AB 

158.5 on June 17 during a brake application from 148 mph. In this case, a BOP strain amplitude of ± 

1,372 microstrain, average-to-peak, was observed during a prolonged application of the brakes in which 

the mean strain reached 2,202 microstrain and the temperature reached 330 °F. The strains presented are 

as measured and have not been corrected for pre-load strains in the spoke.

Figure 3.50 illustrates a comparison between the strains recorded on one side of spoke 6 of both the 

WABTEC/SAB-WABCO supplied and Knorr brake discs during a braking event. In this case, sustained 

BOP oscillations were observed on the WABTEC/SAB-WABCO supplied disc 2396’ northeast of MP 

AB 202 during braking from 116 mph. The top portion of the figure illustrates the entire event while the 

middle portion of the figure illustrates the results during a period lasting corresponding to two cycles of 

vibration of the WABTEC/SAB-WABCO supplied disc. In this case, strain oscillations reached averageto-peak amplitudes of ± 948 microstrain, or peak-to-peak amplitudes of almost 1,900 microstrain. These 

oscillations were superimposed on a mean strain that reached 1,918 microstrain due to a large temperature 

rise from 100.6 °F to 253.5 °F. The response of the Knorr disc to this braking is similar to that of the 

WABTEC/SAB-WABCO supplied disc with respect to the average tensile strain in the spoke, but it is 

clear that no BOP strain oscillations were observed on the Knorr disc. Brake cylinder pressures 

associated with the WABTEC/SAB-WABCO supplied and Knorr discs reached 56.3 psi and 54.0 psi, 

respectively, and the temperatures measured on each of the discs were similar. The lower portion of 

Figure 3.50 illustrates the frequencies associated with the motion of the two discs. The frequency 

observed in the motion of the WABTEC/SAB-WABCO supplied disc near 185 Hz is prevalent in the 

PSD, as well as other frequencies resulting from modulation by the rotational speed of the wheel. 

Clearly, no particular frequency is associated with the vibration of the Knorr disc. 
108 

Figure 3.50. Comparison of WABTEC/SAB-WABCO Supplied Disc and Knorr Disc During a 

Brake Application From 116 MPH with Sustained Oscillations (File 061805_03.AB3)–June 18, 2005

One of the factors that affects the relative responses of the spokes of the two brake discs is the cross 

sectional geometry of the spokes. Figure 3.51 compares the geometry of the spokes for the different 

brake discs in the vicinity of the strain gages. The resistance of the spokes to bending depends on the 

cross-sectional moment of inertia. Table 3.9 provides a comparison of moment of inertia values for the 

cross sections of the WABTEC/SAB-WABCO supplied and Knorr brake disc spokes. 

Figure 3.51. Comparison of Cross Sections of Spokes from WABTEC/SAB-WABCO Supplied and 

Knorr Brake Discs 
109 

Table 3.9. Properties of Cross Sections of Spokes of WABTEC/SAB-WABCO Supplied and 

Knorr Brake Discs 

Property 

WABTEC/SAB-WABCO

Supplied Brake Disc 

Spoke 

Knorr 

Brake Disc 

Spoke 

Area 1.9 in2 3.6 in2

Moment of Inertia 

(BIP) 0.44 in4

 0.85 in4

Moment of Inertia 

(BOP) 0.25 in4

 1.69 in4

The moment of inertia for BOP of the Knorr disc is over six times that of the WABTEC/SAB-WABCO 

supplied disc, providing a higher resistance to BOP vibration at the location at which the strain gages 

were installed than can be provided by that of the WABTEC/SAB-WABCO supplied disc spoke. Due to 

its higher cross-sectional area, the spoke of the Knorr disc will experience a higher stress due to thermal 

expansion than the spoke of the WABTEC/SAB-WABCO supplied disc will for a given thermal load. 

Appendix M addresses this subject. 

In summary, the Knorr disc exhibited smaller BOP strains in response to vertical shocks than those 

observed on the WABTEC/SAB-WABCO supplied brake disc. The BOP strains observed on the Knorr 

disc were not only smaller in amplitude, but they also died out in a much shorter time than the ones 

observed for the same shocks on the WABTEC/SAB-WABCO supplied disc. The BOP strains observed 

on both discs due to vertical impacts were found to have the same frequencies as those associated with 

out-of-phase vibrations on each of the brake discs observed during modal testing. The sustained 

oscillations of high BOP strains observed on the WABTEC/SAB-WABCO supplied brake disc during 

some brake applications were not observed on the Knorr disc during any brake applications. 

A subsequent section will address the subject of the BOP strains resulting from vertical impacts and 

observed in sustained oscillations during brake applications and the impact of those strains on the fatigue 

of the spokes. 

3.3.5 Axle Strains 

Section 3.2.3 discussed the measurement of axle strains. The only change in approach during Phase 3 

testing, in addition to installation of instrumentation on the Knorr test axle, was the addition of two strain 

gages on the Knorr axle near the center brake disc so that four gages were located around the 

circumference of the axle at 90° intervals. 

During a review of axle strains measured during Phase 2 testing, it was observed that axle strains could be 

affected by modulation of the strain signals by the wheel rotational rate and the effects of the lateral 

acceleration imposed on the axle as it moves through curves. This became more evident in review of data 

collected in Phase 3 testing. 
110 

To more clearly illustrate the change in axle strains with movement through a curve, the lateral 

accelerations recorded on the truck frame over a short portion of track are compared to axle strains 

recorded from both test axles in Figure 3.52. When the test vehicle moves over curved track, an increase 

or decrease in lateral acceleration will usually be recorded on the truck. A strong correlation exists 

between the presence of curves and the change in the amplitude of the axle strains. 

Figure 3.52. Comparison of Truck Frame Lateral Accelerations and Axle Strains 

(File 061805_03.AB3)–June 18, 2005 

Figure 3.53 illustrates axle strains recorded on both test axles while traveling at speeds between 0 and 150 

mph on June 18. The change in the overall amplitude of the axle strains can be seen to change as the 

speed increases; the effect of the rotational speed of the axle can be seen in the frequency spectra of both 

axles included in the lower portion of the figure. This frequency analysis was conducted for data 

collected on tangent track at relatively constant speed. 
111 

Figure 3.53. PSD of Axle Strains Collected Over Tangent Section of Track 

(File 061805_03.AB3)–June 18, 2005

Figure 3.54 provides the PSD of axle strains recorded on the test axles during the brake application 

illustrated in Figure 3.50. One can see the effect of the vibration of the WABTEC/SAB-WABCO 

supplied brake disc in the strains recorded on the WABTEC/SAB-WABCO axle. The dominant 

frequencies evident in the lower portion of Figure 3.50 can be seen in the consideration of the 

WABTEC/SAB-WABCO axle in Figure 3.54. 
112 

Figure 3.54. PSD of Axle Strains of WABTEC/SAB-WABCO Supplied Disc and Knorr Disc During a 

Brake Application from 116 MPH with Sustained Oscillations (File 061805_03.AB3)–June 18, 2005

As stated in Section 3.2.3, analysis of the axle strains requires one to be mindful of the effects of many 

factors. Appendix H discusses processing techniques to obtain more information from the modulated 

signals resulting from the axle-mounted strain gages. 

3.3.6 Temperatures 

During Phase 3 testing between Washington, DC, and Boston, MA, on June 17 and 18, the maximum 

temperature measured on the back of the friction ring of the center-mounted WABTEC/SAB-WABCO 

supplied brake disc was 342 °F. This was measured following a full-service brake application made near 

MP AB 155.5 on June 18 (File 061805_07.AB3). In a related topic, this was the incident that resulted in 

the highest mean spoke tensile strain of each of the discs, 2,668 microstrain for the WABTEC/SABWABCO supplied disc and 2,896 microstrain for the Knorr disc. The maximum temperature measured 

on the back of the friction ring of the center-mounted Knorr brake disc was 327 °F, measured following a 

suppression (i.e., less than 56 psi) brake application made near the Trenton, NJ, train station on June 18 

(File 061805_24.AB3). 

The highest brake disc temperature recorded occurred during the shakedown run made on June 16. In one 

case, the temperature on the WABTEC/SAB-WABCO supplied disc reached 361 °F and the Knorr disc 

reached 352 °F between MP 55 AN and Philadelphia, PA (File 061605_18.AB3). Figure 3.55 shows the 

temperature profiles recorded during this particular data file. 
113 

Figure 3.55. Temperature Profiles from Back of WABTEC/SAB-WABCO Supplied and 

Knorr Brake Discs During Shakedown Run (File 061605_18.AB3)–June 16, 2005 

This illustration shows that the Knorr brake disc cools down faster than the WABTEC/SAB-WABCO 

supplied disc. The thermal time constant, a measure of how quickly the brake disc dissipates heat, was 

calculated for each brake disc using the temperatures shown above. Results of this analysis, details of 

which are provided in Appendix I, show that the thermal time constant for the Knorr brake disc is 4.6 

while that for the WABTEC/SAB-WABCO supplied disc is 6.3, indicating that the Knorr disc will cool 

faster than the WABTEC/SAB-WABCO supplied disc. 

An analysis of all the brake applications made during tests conducted on June 17 and 18 was performed to 

quantify the rate in which heat builds up within the discs. This was accomplished by identifying the 

temperature change of the Knorr and WABTEC/SAB-WABCO supplied brake discs due to the braking as 

a function of the kinetic energy dissipated. Details of this analysis are presented in Appendix I and the 

results of the analysis for testing conducted on June 18 are provided in Figure 3.56. In this analysis, it is 

presumed that each brake disc of a vehicle contributes an equal amount to the braking effort of the 

vehicle, a reasonable presumption to make in this analysis. Results from this graph, as well as those 

provided for the June 17 tests, indicate that on average, for a given change in dissipated kinetic energy, 

the Knorr disc experiences a slightly higher temperature change. This finding was consistent with 

observations made during the test. 
114 

Figure 3.56. Comparison of Disc Temperature Changes versus Dissipated Kinetic Energy 

for Knorr and WABTEC/SAB-WABCO Supplied Brake Discs–June 18, 2005
115 

As done with temperature measurements collected from the back of the WABTEC/SAB-WABCO 

supplied brake disc’s friction ring during previous testing, friction ring temperatures were used to 

estimate the state of thermal stress in the spokes of each of the discs. Based on the analysis method 

described in Section 3.1.5, estimates of the thermal strains in the spoke were derived from data collected 

during testing on June 17 and 18. The results of this analysis are illustrated in Appendix I and 

summarized in Table 3.10; results from the analysis conducted using Phase 2 test data are also provided 

for the sake of comparison. The results for the WABTEC/SAB-WABCO supplied disc are similar to 

those arrived at in earlier testing. These estimated thermal strains would not account for any existing preload strains in the spokes. 

Table 3.10. Estimate of Change in Tension Per Unit Change in Temperature in Spoke 6 of 

WABTEC/SAB-WABCO Supplied and Knorr Brake Discs–June 17 and 18, 2005

Estimate of Tension Change in Spoke 6 

(microstrain/ °F) 

Test 

WABTEC/SAB-WABCO 

Supplied Brake Disc 

Knorr 

Brake Disc 

May 17 7.11 - 

May 26 7.34 - 

May 27 7.23 - 

June 17 8.14 8.86 

June 18 7.29 8.06 

Average Results 7.42 8.46 

In summary, the key findings regarding temperatures associated with the WABTEC/SAB-WABCO 

supplied and Knorr brake discs that resulted from Phase 3 testing include the following: 

• The highest temperature measured on either of the brake discs was 361 °F. This was on the order of 

what was expected by the brake suppliers. 

• Based on observations made during testing and subsequent data analysis, the Knorr brake disc heats 

up faster and cools down faster than the WABTEC/SAB-WABCO supplied brake disc. This 

difference in heating and cooling rates is considered to be small. 

• The average rise in spoke tension for the Knorr brake disc was estimated to be approximately 8.5 

microstrain/°F. The same quantity for the WABTEC/SAB-WABCO supplied brake disc was 

estimated to be approximately 7.4 microstrain/°F. 
116 

3.3.7 Brake System Hardware–Mounting Tube, Links and Calipers 

Appendix F provides information pertaining to the frequency content of acceleration data collected from 

the brake calipers, the brake mounting tube and the truck frame during periods in which sustained 

oscillations of high BOP strains during braking was observed, periods in which braking was occurring 

without sustained oscillations, and periods in which an impact resulted in high BOP strains for a short 

period of time. This section will highlight these results. 

During sustained oscillations within brake applications, accelerations measured on the brake mounting 

tube associated with the WABTEC/SAB-WABCO supplied brake disc test axle show a very strong 

harmonic characteristic. This same characteristic is not observed on the brake mounting tube above the 

Knorr brake disc test axle. Figure 3.57 provides an illustration of the time history and frequency content 

of the vertical accelerations measured on both brake mounting tubes during the period of sustained 

oscillations near MP AN 55 on June 18. Figure 3.58 shows the corresponding information for the lateral 

accelerations measured at the same locations. In both figures, a fundamental frequency of 187 Hz along 

with a number of harmonics are observed in the accelerations measured on the tube above the 

WABTEC/SAB-WABCO test axle. The amplitude of the vertical acceleration is approximately twice 

that of the lateral acceleration. 

Figure 3.57. Time History and PSD of Brake Mounting Tube Vertical Accelerations During 

Sustained Oscillations While Braking (File 061805_24.AB3)–June 18, 2005 
117 

Figure 3.58. Time History and PSD of Brake Mounting Tube Lateral Accelerations During 

Sustained Oscillations While Braking (File 061805_24.AB3)–June 18, 2005

Figures 3.59 and 3.60 provide the same type of information shown in the previous figures for the vertical 

and lateral accelerations measured near the center caliper pads over the two test axles. As seen in the 

previous figures, the same fundamental frequency of 187 Hz, along with a number of harmonics, are 

observed in the frequency distribution. On the brake pad, the lateral accelerations associated with the 

WABTEC/SAB-WABCO supplied disc reach levels as high as 30 g, while the vertical accelerations on 

the same pad are only as high as 20 g. The accelerations measured on the pad are characterized by a sawtooth shaped pattern. 
118 

Figure 3.59. Time History and PSD of Brake Caliper Pad Vertical Accelerations During Sustained 

Oscillations While Braking (File 061805_24.AB3)–June 18, 2005

Figure 3.60. Time History and PSD of Brake Caliper Pad Lateral Accelerations During Sustained 

Oscillations While Braking (File 061805_24.AB3)–June 18, 2005

The brake support links transfer the loads generated during braking from the caliper assembly to the brake 

mounting tube. Before Phase 3 testing, two spare caliper assembly support links were instrumented with 
119 

single-axis strain gages as indicated in the top of Figure 3.61. The existing links employed by the center 

caliper assembly used with the WABTEC/SAB-WABCO test axle were removed and replaced with the 

instrumented links. 

This may be related to a slip-stick behavior at the interface of the brake pad and friction ring. Appendix 

K examines this behavior. 

Figure 3.61. Illustration of Instrumented Brake Links and Locations within Truck 

Caliper 

Brake Mounting Tube 

 Brake Shoe 

Lateral 

3inches

Center 

9” 

.87” 

.57” 

D

Cross Section 
120 

Consideration of Figure 3.61 shows that the caliper support links associated with the lead axle of the truck 

will be in compression during a brake application while the links associated with the trailing axle will be 

in tension. This is likely one of the aspects of the braking process that has influenced the observation that 

sustained oscillations during selected brake applications are only observed when the test axle was in the 

lead position. The fact that the links are in compression during periods of sustained oscillations during 

braking is important. 

For the analysis of the caliper links that follows, it is assumed that the strains measured by the single gage 

on each link are a measure of the tension or compression in the link. This is a reasonable assumption 

because the links are pinned at each end. Based on a 0.5 square inch cross-sectional area, the relationship 

between strain and force carried by the link is that shown in Figure 3.62. 

 

Figure 3.62. Brake Link Strain Level–Force Relationship 

Four braking events were selected for analysis, which Table 3.11 details. During the first two events, no 

sustained oscillations of the WABTEC/SAB-WABCO supplied brake disc were observed. One of these 

segments was with the instrumented axle in the lead position and one with the instrumented axle in a 

trailing position. The other two events were selected because sustained oscillations of the 

WABTEC/SAB-WABCO supplied brake disc were observed. Appendix K provides the details of the 

analysis. 

Although data collected on June 16 was not analyzed in great detail because the test on this day was 

conducted for shakedown purposes, data recorded with the instrumented links on June 16 was considered 

in this analysis. This was because June 16 was the only day of testing that the instrumented links were in 

place with the test axle in the trail position. 
121 

Table 3.11. Details of Braking Events Selected for Further Analysis of Instrumented Caliper Links 

Date/File 

Time in 

File 

(Sec) 

Sustained 

Oscillation

Axle 

Position

Speed 

(mph)

June 16 

061605_18.AB3 375 No Trail 94 

June 18 

061805_24.AB3 310 No Lead 117 

June 18 

061805_24.AB3 580 Yes Lead 110 

June 17 

061705_25.AB3 559 Yes Lead 69 

The nominal brake cylinder pressure during all four events was 50 psi. 

The links did not evenly share the braking forces in any of the four cases considered. Table 3.12 shows 

the results for the change in strain levels in the right and left links each of the brake applications. When 

the instrumented axle is in the trailing position, the left link shows the largest change in strain. When the 

instrumented axle is in the lead position, the right link shows the largest change in strain. 

Table 3.12. Strain Changes in Right and Left Links During Selected Brake Applications 

Date/File/Time Sustained 

Oscillation 

Axle 

Position

Left 

Link 

Microstrain 

Right 

Link 

Microstrain 

June 16 

061605_18.AB3; t=375 No Trail +130 +21 

June 18 

061805_24.AB3; t=310 No Lead -32 -231 

June 18 

061805_24.AB3; t=580 Yes Lead -32 -221 

June 17 

061705_25.AB3; t=559 Yes Lead -17 -189 
122 

A major difference was observed between the strains when sustained oscillations were observed and those 

when no sustained oscillations were observed. When sustained oscillations were observed, the frequency 

spectrum of the strains recorded in the links exhibited a strong harmonic content. Figure 3.63 shows the 

frequency content of the strains recorded during the sustained oscillation of the WABTEC/SAB-WABCO 

supplied disc recorded near MP AN55 on June 18. 

Figure 3.63. Frequency Spectrum of Strains Recorded on Caliper Link During Sustained 

Oscillations While Braking (File 061805_24.AB3)–June 18, 2005

The fundamental frequency of this oscillation is 187 Hz. This frequency was observed at vehicle speeds 

of 69 and 110 mph. Four harmonic frequencies are clearly observed in the PSD. Based on the 750 Hz, 

anti-aliasing filters used during Phase 3 testing, the amplitude of the fourth harmonic is attenuated, and 

additional harmonics would be significantly attenuated. Figure 3.64 shows the time history of the strains 

recorded on the right and left links during this event. 
123 

Figure 3.64. Strains Recorded on Caliper Links During Sustained Oscillations of WABTEC/SABWABCO Supplied Brake Disc While Braking (File 061805_24.AB3)–June 18, 2005 

The 187 Hz frequency observed in the strains on the links is a characteristic of the sustained oscillation 

events. The right link strains exhibit the same distinctive saw-tooth pattern observed in the caliper pad 

accelerations in Figures 3.59 and 3.60. The strains in the left link exhibit reversals. These patterns may 

be indicative of a slip-stick behavior at the interface of the brake pad and the friction ring. Appendix K 

examines this phenomenon, but no firm conclusions are reached. 

What determines the frequency observed in the oscillations of the various components within the system 

remains unknown. 

3.3.8 Brake Pressures 

As illustrated in Section 3.1.6, brake cylinder pressure has a direct effect on the amplitude of the sustained 

oscillations of BOP strains in the brake applications in which they are observed. This trend was evident 

in data collected during the third phase of testing as well. Figure 3.65 illustrates the brake cylinder 

pressure variation and the strains on the sides of spoke 6 from each brake disc recorded on June 17 near 

MP AB 154 during a short exhibition of oscillations. 

In general, testing on June 17 and 18 provided the largest peak-to-peak sustained oscillations observed 

during testing. Figure 3.66 shows the distribution of the maximum peak-to-peak sustained oscillations 

observed during Phase 3 testing as a function of the highest brake cylinder pressure realized during the 

brake application. Figure 3.66 also provides corresponding information from results collected during the 

first two phases of testing. 
124 

Figure 3.65. Brake Cylinder Pressure and Strains Recorded on Side of WABTEC/SAB-WABCO 

Supplied and Knorr Brake Disc Spokes During Short Case of Sustained Oscillations 

(File 061705_36.AB3)–June 17, 2005 

Figure 3.66. Relationship of Peak-to-Peak Sustained Oscillations to Brake Cylinder Pressure, Data 

Collected Throughout Test Effort 
125 

3.3.9 Sound Tests 

Figure 3.67 illustrates the frequency analysis of sound measurements collected while the center 

WABTEC/SAB-WABCO supplied brake disc was exhibiting mild BOP strains during a brake application 

near MP AB 179 on June 17. Recordings were made over both the WABTEC/SAB-WABCO and Knorr 

test axles. As was the case with data presented in Section 3.2.6, the sound level expressed in decibels is 

plotted as a function of the frequency associated the particular component of the sound. 

Figure 3.67. Analysis of Sound Recorded Over Both Test Axles During Mild Case of Sustained 

Oscillations on WABTEC/SAB-WABCO Supplied Disc During Braking Near MP AB 179– 

June 17, 2005

Within this recording, it was difficult to identify any particular dominant component of sound in any of 

the channels. No frequencies that present themselves are consistent with other frequencies that have 

become evident throughout the test effort. Analysis of sound measurements collected during Phase 3 

testing did not provide any useful information. 

3.3.10 Concluding Comments

Results of Phase 3 testing provided critical information for this study. In summary: 

• Tests were conducted with a WABTEC/SAB-WABCO supplied brake disc being removed from an 

axle and a Knorr brake disc being installed on the same axle to determine pre-load stresses. Results 

from these single-sample tests indicate that an average pre-stress equivalent of approximately 1,000 

microstrain may be present as a residual strain in a spoke of a WABTEC/SAB-WABCO supplied 

brake disc, comprised of an average compressive strain of 390 microstrain attributed to the pressing 

operation and an average compressive strain of 600 microstrain in a spokes attributed to the 

manufacturing process. The installation of the Knorr disc results in an average compressive strain of 

533 microstrain on the spokes. Appendix M provides more information related to the nature of these 

existing stresses. 
126 

• During modal testing conducted on brake discs in the early stages of Phase 3 testing, a BOP mode 

was identified on the WABTEC/SAB-WABCO supplied brake disc with a frequency of 220 Hz. 

Results from tests involving the Knorr brake disc identified an out-of-phase vibration of the disc with 

a frequency of 351 Hz. These results were determined for discs installed on axles that had been 

removed from under the coach cars. 

• The load environment as characterized by high lateral and vertical impacts was similar for each of the 

test axles. This observation is important because it established that each test axle is subjected to 

similar loads, allowing for straightforward comparisons of the response of the respective discs for 

given load conditions. 

The highest vertical shock observed throughout the program was recorded during Phase 3 testing. An 

impact of 189 g was measured on the left end of the WABTEC/SAB-WABCO test axle on June 17 

near MP 56 of the Metro-North Rail Road while the test train was traveling at 48 mph. At this same 

location, the Knorr test axle experienced a vertical impact of 172 g. 

The highest lateral acceleration observed on the WABTEC/SAB-WABCO test axle during Phase 3 

testing, 58.5 g, was measured on June 17 in the vicinity of Union Interlocking near MP AN 20 while 

traveling at 113 mph. The Knorr test axle also experienced its highest lateral impact of the test on the 

June 17 northbound test run, a 58.4 g impact recorded near the Bowie Interlocking near MP AP 120.8 

while traveling at 125 mph.

• The Knorr disc exhibited smaller BOP strains in response to vertical shocks than those observed on 

the WABTEC/SAB-WABCO supplied brake disc. The BOP strains observed on the Knorr disc were 

not only smaller in amplitude, but they also died out in a much shorter time than the ones observed 

for the same shocks on the WABTEC/SAB-WABCO supplied disc. The BOP strains observed on 

both discs due to vertical impacts were found to have the same frequencies as those associated with 

out-of-phase vibrations on each of the brake discs observed during modal testing. The sustained 

oscillations of high BOP strains observed on the WABTEC/SAB-WABCO supplied brake disc during 

some brake applications were not observed on the Knorr disc during any brake applications. 

The highest amplitude of sustained BOP oscillations on the center WABTEC/SAB-WABCO supplied 

disc due solely to a brake application was ± 1,372 microstrain, average-to-peak, and was observed 

during a prolonged application of the brakes in which the mean tensile strain reached 2,202 

microstrain and a temperature of 330 °F. An event was also identified in which the WABTEC/SABWABCO supplied disc was subjected to a vertical impact during a sustained oscillation resulting from 

a brake application. 

The highest BOP spoke strain on the WABTEC/SAB-WABCO supplied disc resulting from an 

asymmetric vertical impact had an amplitude of ± 1,487 microstrain, zero-to-peak, that required 

almost 0.20 seconds to die out. The same shock produced a BOP spoke strain with an amplitude of ± 

290 microstrain, zero-to-peak, in the Knorr brake disc that required approximately 0.03 seconds to die 

out. The frequencies associated with the responses of each of the two brake discs were close to those 

associated with the out-of-phase responses of the discs determined in the modal tests. 

Strains in the preceding paragraphs are as measured and have not been corrected for pre-load strains 

in the spoke. 

The cross-sectional geometry of the spoke found on the Knorr disc offers a higher resistance to BOP 

vibration at the location at which the strain gages were installed than can be provided by that of the 

WABTEC/SAB-WABCO supplied disc spoke. Due to its higher cross-sectional area, the spoke of 

the Knorr disc will experience a higher stress because of thermal expansion than the spoke of the 

WABTEC/SAB-WABCO supplied disc will for a given thermal load. Appendix M addresses this 

subject.
127 

• The highest temperature measured on either of the brake discs was 361 °F. This was on the order of 

what was expected by the brake suppliers. 

The Knorr brake disc heats up faster and cools down faster than the WABTEC/SAB-WABCO 

supplied brake disc. This difference in heating and cooling rates is considered small. 

• The 187 Hz frequency associated with the sustained oscillation of the WABTEC/SAB-WABCO 

supplied disc during selected brake applications was observed in the accelerations and strains 

measured on the brake hardware during these periods of oscillation. 

During a given brake application, the caliper support links associated with the lead axle of the truck 

are in compression, and the links associated with the trailing axle are in tension. This is likely one of 

the aspects of the braking process that has influenced the observation that sustained oscillations 

during selected brake applications are only observed when the test axle was in the lead position. The 

fact that the links are in compression during periods of sustained oscillations during braking is 

important. 

3.4 Further Consideration of Test Results 

This section will address four topics that span across the various phases of the test effort: 

• The impact of the strains measured on the brake discs to the fatigue life of the material. 

• The sequence of events that led up to the sustained oscillations observed under certain braking 

conditions. 

• A review of the frequencies identified throughout the report. 

• The effect of cold weather on shock amplitudes. 

3.4.1 Consideration of Strains and the Impact on Fatigue

As evidenced by the test results presented to this point, it is clear that the spokes of the WABTEC/SABWABCO supplied brake discs are subjected to tensile stresses due to heat buildup during braking and to 

oscillating stresses resulting largely from vertical shocks and dynamics during braking. The oscillating 

stresses can die out quickly, as is the case with those resulting from vertical shocks, or can last for an 

extended period of time, as observed in several of the brake applications highlighted in this report. The 

combination of mean stresses and oscillating stresses could have an impact on the fatigue life of the 

material. A consideration of the data from a fatigue point-of-view is warranted. ENSCO employed an 

exercise based on the Goodman criteria to determine where in the data significant combinations of mean 

and alternating strains, and consequently stresses, occur. 
128 

The Goodman criteria is traditionally conducted in a graphical fashion in which the mean strain that the 

part in question is subjected to is represented on the horizontal axis and the amplitude of the oscillating 

strain is represented along the vertical axis. The ultimate strain of the material is marked on the 

horizontal axis, and the endurance limit strain is marked on the vertical axis; the Goodman line connects 

these two points. Combinations of average and oscillating strains are plotted on this diagram. Points on 

this diagram that fall near the Goodman line merit further consideration in a rigorous fatigue analysis.4

This exercise does not represent a rigorous fatigue analysis. Among the factors that this treatment does 

not take into account are surface conditions, the presence of stress risers, and environmental conditions. 

This analysis was used merely as a way to identify situations of concern. 

Table 3.13 shows the mechanical properties of the WABTEC/SAB-WABCO supplied brake disc used for 

this analysis. The table also provides the sources for this information. For the purpose of reference, 

Table 3.14 provides the material properties associated with the Knorr brake disc. 

The ultimate strain and the endurance limit, equal to 30 percent of the ultimate strain shown in this table, 

were used to generate a Goodman fatigue curve for alternating and mean strains measured on the 

WABTEC/SAB-WABCO supplied brake disc. An automated routine was developed to identify the 

number of spoke strains, in either braking or non-braking situations, that were near or above the Goodman 

line. Appendix G explains this method. Both the number of cycles near the Goodman line and the 

number of events near the Goodman line were identified, where an event may include many cycles. 

Table 3.15 reports the results of the analysis in terms of cycles. In all the results determined for the 

WABTEC/SAB-WABCO supplied brake disc, a pre-strain of 1,000 microstrain was used, or 1,000 

microstrain was subtracted from the measured tensile strain. 

 

4 H. O. Fuchs and R. I. Stephens, Metal Fatigue in Engineering, (New York, NY: John Wiley & Sons, 1980) 72. 
129 

Table 3.13. Key Stress and Strain Values for WABTEC/SAB-WABCO Supplied Brake Disc 

Value Stress 

(MPa) 

Stress 

(psi) 

Comparison 

to 

 Ultimate 

Stress/E Source 

Young’s Modulus (E) 210,345 30,500,000 - - Properties of Steel

Ultimate Strength 752 109,000 100% 3,574 SHTL 

Yield Strength 550 79,750 73% 2,615 SHTL 

Endurance Limit 226 32,700 30% 1,072 30% Ultimate 

Pre-Strain (Press On) 84 12,200 11% 400 see Appendix J 

Pre-Strain (As Built) 126 18,300 17% 600 see Appendix J 

Pre-Strain (Total New) 210 30,500 28% 1,000 see Appendix J 

True Fracture Stress (European) 1,232 178,689 164% 5,859 SHTL 

True Fracture Stress (USA) 1,059 153,521 141% 5,033 SHTL 

Table 3.14. Key Stress and Strain Values for Knorr Brake Disc 

Value Stress 

(MPa) 

Stress 

(psi) 

Comparison 

to 

 Ultimate 

Stress/E Source 

Young’s Modulus (E) 210,000 30,450,000 - - SWL 

Ultimate Strength 1,050 152,250 100% 5,000 N10193 

Yield Strength 900 130,500 86% 4,286 N10193 

Endurance Limit 300 43,500 29% 1,429 SWL 

Pre-Strain (Press On) 112 16,230 11% 533 see Appendix J 
130 

Table 3.15. Number of Cycles Identified in Goodman Analysis of Mean/Oscillating Spoke Strains

 WABTEC/SAB-WABCO Supplied Disc Axle 1 Knorr Disc Axle 2

 Center Disc Outer Disc Center Disc 

 Spoke 3 Spoke 6 Spoke 3 Spoke 6 Spoke 6 

 Date NB B NB B NB B NB B NB B 

16-May n/a n/a 4.5 0 n/a n/a 5.5 0 n/a n/a 

Phase 1 

17-May n/a n/a 11.5 55 n/a n/a 1 0 n/a n/a 

26-May 12 14 9 20 n/a n/a n/a n/a n/a n/a 

Phase 2 

27-May 6.5 0 10.5 0 n/a n/a n/a n/a n/a n/a 

17-Jun 2 2,754 5 3,156.5 n/a n/a n/a n/a 0 0 

Phase 3 

18-Jun 5 7,074 6 6,947 n/a n/a n/a n/a 0 0 

 

 n/a – Test plan did not include this measurement. 

 B – Braking 

 NB – Not Braking 

The table shows a very large increase in occurrences on June 17 and 18, especially those associated with 

braking. This increase was most likely due to a larger number of brake applications in rapid succession 

that were made on the last 2 days of testing than is typical in a revenue service run. No occurrences of 

mean strain/alternating strain were identified that would come close to a Goodman line observed on the 

outer disc in braking or for the Knorr disc in either braking or non-braking conditions. 

Figures 3.68 through 3.73 illustrate the occurrences identified in the analysis. Each figure contains two 

plots, one for the strains recorded on each spoke considered during a particular test. The results from a 

given test are contained within the same figure. Each plot is comprised of the alternating (BOP) strains 

plotted against the mean (tensile) strains. No plots are provided for strains recorded on the Knorr disc. 

Only those points approaching the Goodman line are provided on the graph in order to focus on the cases 

that have an implication of fatigue life of the material. Although plots provided in Figures 3.68 through 

3.73 are typically presented in terms of stress, the illustrations provided here are in terms of strain because 

strains have been referenced throughout this report. Appendix G further addresses the presentation of this 

data in terms of strain, and not stress. 

It is important to remember that the peak strain levels recorded during the test do not necessarily occur 

when either the BOP strains or mean strains are at a maximum. The peak stress level occurs when the 

sum of the BOP and mean strain is a maximum. For instance, a BOP strain of 1,700 microstrain and a 

mean strain of -500 microstrain produce a strain level of 1,200 microstrain. A similar strain can occur 

with a BOP strain level of only 1,000 microstrain but with a mean strain level of 200 microstrain.
131 

Figure 3.68. Mean Strain versus Alternating Strain, Center and Outer WABTEC/SAB-WABCO 

Supplied Brake Discs, Spoke 6–May 16, 2005

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 6 - May 16, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event

Mean Strain vs. Alternating Strain, Outer WABTEC/SAB-WABCO Disc,

Spoke 6 - May 16, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event
132 

Figure 3.69. Mean Strain versus Alternating Strain, Center and Outer WABTEC/SAB-WABCO 

Supplied Brake Discs, Spoke 6–May 17, 2005

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 6 - May 17, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event

Mean Strain vs. Alternating Strain, Outer WABTEC/SAB-WABCO Disc,

Spoke 6 - May 17, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event
133 

Figure 3.70. Mean Strain versus Alternating Strain, Center WABTEC/SAB-WABCO Supplied 

Brake Disc, Spokes 6 and 3–May 26, 2005

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 6 - May 26, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 3 - May 26, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event
134 

Figure 3.71. Mean Strain versus Alternating Strain, Center WABTEC/SAB-WABCO Supplied 

Brake Disc, Spokes 6 and 3–May 27, 2005

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 6 - May 27, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 3 - May 27, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event
135 

Figure 3.72. Mean Strain versus Alternating Strain, Center WABTEC/SAB-WABCO Supplied 

Brake Disc, Spokes 6 and 3–June 17, 2005

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 6 - June 17, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 3 - June 17, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event
136 

Figure 3.73. Mean Strain versus Alternating Strain, Center WABTEC/SAB-WABCO Supplied 

Brake Disc, Spokes 6 and 3–June 18, 2005

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 6 - June 18, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event

Mean Strain vs. Alternating Strain, Center WABTEC/SAB-WABCO Disc,

Spoke 3 - June 18, 2005

0

500

1000

1500

2000

2500

-1500 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000

Mean Strain - 1000 Micro Strain Pre-Strain

(microstrain)

BOP Strain

(microstrain)

Braking Event

Non-Braking Event
137 

The most activity is observed to have taken place during Phase 3 testing. Again, the high number of 

points on the Goodman diagrams are likely due to the number of brake applications in rapid succession 

that were made on the last 2 days of testing. Brake applications made in rapid succession resulted in the 

heating of the discs, therefore creating higher mean, or tensile, strains. Alternating strains due to 

sustained oscillations or impacts will be plotted towards the right within the Goodman diagrams, thus 

creating more points plotted on the diagrams. 

Comparison of the material properties of the two discs reveals that the Knorr brake disc has a higher 

ultimate strength and yield strength than the WABTEC/SAB-WABCO supplied brake disc does. Figure 

3.74 provides an illustration of what the Goodman line for the Knorr disc would look like compared to the 

Goodman line prepared for the WABTEC/SAB-WABCO supplied disc. This illustration indicates that 

the material comprising the Knorr disc would provide a higher degree of protection against the effects of 

fatigue than the material comprising the WABTEC/SAB-WABCO supplied disc. The materials used for 

the different brake discs were selected using a variety of considerations. Appendix M provides 

information regarding the selection of material for the WABTEC/SAB-WABCO supplied brake disc. 

Figure 3.74. Comparison of Goodman Lines for WABTEC/SAB-WABCO Supplied and 

Knorr Brake Discs 
138 

3.4.2 Sequence of Events During Sustained Oscillations While Braking 

The observations made regarding the sustained oscillations during some brake applications have 

characterized the event, but the cause and inception of the vibrations of the WABTEC/SAB-WABCO 

supplied brake disc have not been identified. To address this subject, the initial stages of four cases of 

sustained oscillations during braking recorded in Phase 3 testing were scrutinized in an attempt to identify 

the mechanism under which this motion manifests itself. 

Table 3.16 identifies the four cases considered in this section. 

Table 3.16. List of Cases of Sustained Oscillations During Braking Considered in Detail 

Measurements from Center Disc 

Figure 

No. Geographic Location 

Max 

Avg-to-Pk 

Amplitude 

(με) 

Avg 

Strain 

@ Start 

(με) 

Avg 

Strain 

@ End 

(με) 

Disc 

Temp 

@ Start

 (oF) 

Disc 

Temp 

@ End 

 (oF) 

Peak 

Disc 

Temp 

(oF) 

Brake 

Cyl 

Press 

(psi) 

Speed at 

Start of 

Braking 

(mph) 

3.75 2089 NE of MP AP 4 532 1282 1348 169.2 212.2 242.1 46 85 

3.76 464 W of MP AB 143 1139 1170 1686 222.3 227.2 247.0 50 107 

3.77 1222 SW of MP AB 159 1132 1491 2315 249.6 301.4 327.8 54 120 

3.78 1748 SW of MP AN 55 1455 1466 1974 226.7 261.9 325.6 56 133 

Strains specified in Table 3.16 are as measured and have not been corrected for pre-load strains in the 

spoke. Figure 3.13 defines terminology used in Table 3.16. 

The following parameters recorded on or near the WABTEC/SAB-WABCO test axle are illustrated in 

each of the figures: 

• The vertical accelerations recorded on the test axle 

• The truck frame accelerations 

• The accelerations recorded on the brake mounting tube 

• The strains recorded on the caliper links 

• The accelerations recorded on the caliper assembly near the pad 

• BOP strain recorded on spoke 6 

The arrangement of the illustrations allows one to follow the progression of the events from the rail, 

through the truck, and down to the brake disc. Based upon inspection of these figures, the following 

statements can be made: 

• It appears that vertical shocks from the track can play a role in the development of the sustained 

oscillations during braking. Considering the event in Figure 3.75, clearly BOP strains and caliper pad 

motion have started before t~207 seconds, but the vertical shock measured at t~207.05 seconds 

appears to coincide with an increase in the amplitude of the BOP strain oscillation that continues to 

grow. The event that is depicted in Figure 3.78 shows that the BOP strain oscillations, the caliper pad 

motion, the strains in the caliper links, and the accelerations measured on the brake mounting tube all 

begin to show increased activity at the time that a vertical shock is measured on the test axle. The 
139 

shocks in both of these particular examples are higher in amplitude on one end of the axle than on the 

other. 

• It does not appear, however, that vertical excitation from the track is necessary to start the oscillations 

of the brake disc. In Figure 3.76, one can see the effects of a disturbance in the track at t~131 

seconds on the brake disc, but this behavior appears to die out by t~131.2 seconds. The disc begins to 

display signs of increasing BOP strains at t~131.3 seconds, after the original activity has subsided. 

Inspection of Figure 3.77 shows that very little vertical activity is measured on the test axle, but the 

BOP strains in the brake disc have been initiated at t~32.2 seconds and continue to grow. 

Ultimately, this document does not offer a definitive answer on the cause for the inception of the 

sustained oscillations of the WABTEC/SAB-WABCO supplied brake disc during some brake 

applications. The effects of brake cylinder pressure, initial temperature of the disc at the start of a braking 

event, speed, and the effects of previous braking events have been examined, but no clear pattern has been 

identified. 

 
140 

Figure 3.75. Inception of 

Sustained Oscillations During 

Braking, Near MP AP 3.5 

(File 061705_10.AB3)– 

June 17, 2005 

Notes: 

Speed=85 MPH 

Initial Temp=169.2 oF 

BOP Activity Starts t~206.9 s 

Vertical Shock t~207.4 s 
141 

Figure 3.76. Inception of 

Sustained Oscillations During 

Braking, Near MP AB 143 

(File 061705_36.AB3)– 

June 17, 2005

Notes: 

Speed=107 MPH 

Initial Temp=222.3 oF 

BOP Activity Starts t~131.2 s 

Small Vertical Shock t~131 s, 

but effects appear to die out 

before t~131.2 s
142 

Figure 3.77. Inception of 

Sustained Oscillations During 

Braking, Near MP AB 159 

(File 061805_07.AB3)– 

June 18, 2005

Notes: 

Speed=120MPH 

Initial Temp=249.6 oF 

BOP Activity Starts t~32.1 s 

No Excitation from Track
143 

Figure 3.78. Inception of 

Sustained Oscillations During 

Braking, Near MP AN 55 

(File 061805_24.AB3)– 

June 18, 2005 

Notes: 

Speed=133 MPH 

Initial Temp=226.7 oF 

BOP Activity Appears to Start at 

Same Time as Vertical Shock
144 

3.4.3 Review of Frequencies

A number of different frequencies have been associated with the response of the WABTEC/SABWABCO supplied and Knorr brake discs under various conditions identified in this report. Table 3.17 

provides a compilation of these frequencies and the conditions associated with them. 

Table 3.17. Summary of Frequencies Determined During Acela Brake Disc Test Program 

Method Details W/S-W 

Brake Disc 

Knorr Brake 

Disc 

First BOP Mode 

FEA 

Only Brake Disc with Fixed Interior 

Hub Considered; Eigenvalue Analysis 

Conducted. 

206 Hz Not Measured 

Hammer/Accelerometer 

Test 

Conducted on Assembled Wheelset 

Supported by Jack Stands at Bearings; 

Frequency Analysis Conducted on 

Response to Hammer Blow. 

220 Hz Not Measured 

Hammer/Strain Test 

Conducted on Wheelset Installed in 

Truck Supported by Track; 

Frequency Analysis Conducted on 

Response to Hammer Blow. 

Not Measured 351 Hz 

Response to 

Vertical Impacts During 

Over-the-Road Test 

Frequency Analysis Conducted on 

Response to Impacts. 224 Hz-227 Hz 344 Hz 

Sustained Oscillations During Braking 

Behavior During 

Over-the-Road Test 

Frequency Analysis Conducted on 

Response of Discs. 185 Hz-187 Hz Not Observed 

3.4.4 The Effect of Cold Weather on Shock Amplitudes 

The magnitudes and the natures of shocks measured on the test axles have been discussed in great detail 

throughout this document. Previous studies conducted for Amtrak have shown that cold weather, along 

with its effect on the track structure, affects the loading experienced by brake discs. 

During the winter months of 1977, 1978, and 1979, the pins employed in the Knorr brake disc used on the 

Amfleet and Turboliner passenger vehicles operating on the NEC and in the Midwest to secure the 

friction ring to the hub of brake disc were found to loosen. These pins loosened to such a degree that they 

were often bad ordered, requiring repair. Amtrak observed a significant increase in the failure rate around 

the beginning of February. Amtrak and FRA developed a test program to investigate the high failure rates 

and evaluate alternate designs. ENSCO conducted over-the-road and laboratory testing to support the test 

program. 

The objective of the over-the-road tests was to gain an understanding of the effects of the environment 

(winter/summer) on the disc brakes by measuring the dynamics of a wheelset. Over-the-road tests 
145 

featured two accelerometers mounted to the bearing housing of an axle, one mounted in the vertical 

direction and the other in the lateral direction, and additional test instrumentation mounted to non-rotating 

components of the truck. The accelerometers signals were processed using a root mean squared technique 

based on selected time segments chosen to represent different consist speeds and track locations. 

Accelerometer data was processed to determine the percent of the time that the acceleration exceeded a 

given level. The effect of the bandwidth of the acceleration data was also investigated. 

Results from over-the-road testing demonstrated that the axle and discs are subject to higher acceleration 

levels (RMS) when the temperature is below 32 oF. In the Executive Summary of the final report, the 

investigators stated: 

The tests showed that the accelerations were more than two to one higher on 26 February when 

the roadbed was partially frozen than on the 8 March test after the temperature had been above 

freezing for a week. Even higher accelerations were observed during the shakedown run which 

was made in the middle of a severe cold weather period of the 1979 winter. 5

Amtrak maintenance records corroborated the results cited above. Records showed a sharp failure rate 

increase as the average daily temperature fell below 32 oF. This pattern was apparent in 1978 and 1979 

and was similar in 1977. The failure rate was significantly reduced 5 weeks after the average daily 

temperature exceeded 32 oF. 

Findings from the investigation cited above suggest that the shocks reported in this document, based on 

measurements made in May and June, may be lower than shocks that would be measured if the study 

were conducted in the winter due to stiffening of the ballast as a consequence of cold weather. 

 

5

 R. Scofield & R. “Avant, Amtrak/Knorr Brake Disc Study,” DOT Report No. FRA/ORD-80/62, September 1980. 

147 

4.0 OBSERVATIONS 

The following observations are made based on the test data, laboratory tests, and analysis provided in the 

appendices. Following each statement will be a reference to the material within the report where 

supporting information can be found. 

4.1 Accelerations 

1) The magnitude of measured acceleration levels was influenced by the location and orientation of the 

accelerometer, rate at which the data is recorded, and speed of the train. 

• Accelerations measured on the test axles generally increase with speed [Section 3.2.2, Figure 

3.27]. 

• Lateral accelerations measured on the axle boxes at the end of the axle are generally higher than 

lateral accelerations measured on the axle [Section 3.1.2; Section 3.2.2, Figure 3.25; Section 

3.3.3, Figure 3.47]. 

• A maximum vertical acceleration of 189 g was measured on the end of the WABTEC/SABWABCO test axle [Section 3.3.3]. 

• Vertical accelerations measured on axle bearing boxes are 2 to 3.7 times that of the lateral 

accelerations measured at the same location [Section 3.2.2, Figure 3.27, Figure 3.28]. 

2) The load environment does not change significantly when the direction of the train changes for a 

given configuration [Section 3.1.2; Section 3.2.2]. 

3) Vertical accelerations measured on axle bearing boxes exceed 50 g approximately 80-160 times per 

day [Section 3.1.2, Figure 3.6; Section 3.2.2, Figure 3.25; Section 3.3.3, Figure 3.46]. 

4) Vertical accelerations measured on axle bearing boxes exceed 100 g between 5 and 21 times per day 

[Section 3.1.2, Figure 3.6; Section 3.2.2, Figure 3.25; Section 3.3.3, Figure 3.46]. 

5) The number of high acceleration events on each test axle remains similar from day to day [Section 

3.1.2; Section 3.2.2; Section 3.3.3]. 

6) The events where vertical accelerations exceeded 100 g were generally observed at switches, bridge 

approaches, and other special track work [Appendix F]. 

7) Vertical accelerations measured on the test axles above 100 g are observed at speeds as low as 42 

mph [Appendix F]. 

8) High-amplitude vertical shocks can cause the WABTEC/SAB-WABCO supplied brake disc to exhibit 

high out-of-plane, first-mode bending strains with a frequency of 224 Hz to 227 Hz [Section 3.1.3; 

Section 3.2.3]. 

9) Vertical shocks during episodes of sustained oscillations during some brake applications can increase 

the severity of the BOP strains for a short duration [Section 3.3.4; Section 3.4.1]. 
148 

4.2 Spoke Strains 

1) For both the WABTEC/SAB-WABCO supplied and the Knorr brake discs, the spokes of the disc are 

pre-stressed (compressed) when the disc is pressed onto the axle. Based on testing conducted on one 

Knorr brake disc and two WABTEC/SAB-WABCO supplied brake discs: 

• For the WABTEC/SAB-WABCO supplied disc, the press-off operation relieved an average of 

390 microstrain per spoke [Section 3.3.1; Appendix J, Part B]. 

• Cutting all six spokes of a WABTEC/SAB-WABCO supplied brake disc relieved an average 

strain of 600 microstrain per spoke [Section 3.3.1; Appendix J, Part B]. 

• Measurements made on a retired WABTEC/SAB-WABCO supplied disc with two cracked 

spokes showed no pre-strain in the two cracked spokes and retained strain levels of 680-800 

microstrain on two of the remaining spokes [Section 3.3.1; Appendix J, Part B]. 

• For the Knorr brake disc, the pressing operating used for installation produced an average 

compressive strain of 533 microstrain per spoke; data was not taken to confirm the residual strain 

in the spoke of the Knorr disc [Section 3.3.1; Appendix J, Part B]. 

Appendix M discusses the nature of the residual compressive stresses. 

2) For Knorr brake discs and WABTEC/SAB-WABCO supplied brake discs mounted in the center and 

outer position of the axle, the observed strains in the spokes were principally due to tension resulting 

from heating during braking, BOP of the spoke and, to a lesser degree, BIP of the spoke [Section 

3.1.3]. 

3) Spoke tensile strains were mainly due to the heating of the friction rings during braking and reached 

levels near 2,700 microstrain for the WABTEC/SAB-WABCO supplied disc and 2,900 microstrain for 

the Knorr disc [Section 3.3.6]. 

4) An estimate of the average change in tensile strain as a function of temperature measured on the back of 

the friction was arrived at by determining the temperature on the back of the friction ring before brake 

application and correlating it with the corresponding tensile strain in spoke 6. Choosing to consider the 

temperatures prior to the brake applications allows for equalization of temperatures throughout the disc. 

These estimated thermal strains would not account for any existing pre-load strains in the spoke. 

On the WABTEC/SAB-WABCO supplied disc, the average tensile strain in the spokes increases by 

approximately 7.4 microstrain/ oF. For the Knorr disc, the average tensile strain in the spokes increases 

by approximately 8.5 microstrain/ oF [Section 3.1.5; Section 3.2.4; Section 3.3.6; Appendix I]. 

5) For the WABTEC/SAB-WABCO supplied disc, an out-of-phase BOP mode at a damped natural 

frequency near 227 Hz is observed in response to vertical impacts [Section 3.1.3; Section 3.2.2; 

Section 3.3.4]. 

• BOP strains resulting from vertical impacts reached an amplitude of ± 2,260 microstrain, zero-topeak [Section 3.2.2]. 

• Extreme responses to vertical impacts commonly reached values close to ± 1,500 microstrain, 

zero-to-peak [Section 3.1.3; Section 3.3.4]. 

• The WABTEC/SAB-WABCO supplied disc’s response to asymmetric vertical impacts was more 

severe than that resulting from impacts of similar magnitude being inflicted upon the ends of the 

axle [Section 3.2.2]. 
149 

• The responses of the center and outer WABTEC/SAB-WABCO supplied brake discs to vertical 

impacts are similar to each other [Section 3.1.3]. 

6) During some braking applications, sustained oscillations of the WABTEC/SAB-WABCO supplied 

brake disc in the BOP mode were observed. 

• This behavior was observed on the center and outer brake disc, but the amplitudes of the 

oscillations were higher on the center disc typically by a factor of 2.9 to 3.4 [Section 3.1.3; 

Appendix G]. 

• The frequency associated with this oscillation was typically 186 Hz [Section 3.1.3; Section 3.3.4]. 

• The magnitude of the sustained oscillations increased with increasing brake cylinder pressure 

[Section 3.1.6; Section 3.3.8]. 

• Sustained BOP oscillations during braking were only observed when the test axle was in the lead 

position or when the brake caliper links were in compression during braking [Section 3.1.3; 

Section 3.2.3; Section 3.3.4; Section 3.3.7]. 

• The duration of this sustained BOP oscillation of the WABTEC/SAB-WABCO supplied disc 

during braking was as long as 44 seconds (8,800 cycles) [Section 3.2.2, Table 3.2]. 

• Excitation from a vertical shock appears to be able to start the sustained oscillation, but it is not 

necessary for the oscillations to start [Section 3.4.2]. 

7) For the Knorr brake disc, an out-of-phase BOP mode at a damped natural frequency of 344 Hz is 

observed in response to vertical impacts [Section 3.3.4]. 

• BOP strains resulting from vertical impacts reached an amplitude of ± 290 microstrain, zero-topeak and died out much sooner than those on the WABTEC/SAB-WABCO supplied brake disc 

[Section 3.3.4]. 

8) For the Knorr disc, no sustained oscillations during braking were observed [Section 3.3.4; Section 

3.4.1]. 

9) For the WABTEC/SAB-WABCO supplied disc, BIP strains are small compared to BOP strains 

[Section 3.1.3]. 

4.3 Axle Strains 

1) Axle strains were modulated by the wheel rotational rate [Section 3.2.4]. 

2) The amplitude of the axle strains is significantly affected by the lateral acceleration imposed on the 

test axle as it is operating through a curve. Large axle strains were observed during high cant 

deficiency operations [Section 3.3.5]. 
150 

4.4 Temperature of Friction Ring 

1) For both the WABTEC/SAB-WABCO supplied and Knorr brake discs, the temperature measured on 

the back of the friction ring surface can reach a level of 360 °F [Section 3.3.6, Figure 3.55]. 

2) The Knorr brake disc heats up faster and cools down faster than the WABTEC/SAB-WABCO 

supplied brake disc. This difference in heating and cooling rates is considered to be small [Section 

3.3.6]. 

4.5 FEA Model 

1) The finite element model of the WABTEC/SAB-WABCO supplied brake disc showed good 

agreement with test results. 

• The BOP mode frequency was calculated to be 206 Hz; a value of 227 Hz was observed during 

over-the-road testing [Section 3.1.1]. 

• The model indicated that the area of highest stress in the spoke were located approximately where 

the cracks were observed [Section 3.1.1]. 

4.6 Brake Caliper Links

The following observations are based on the Phase 3 testing of two instrumented brake links used with the 

center disc of WABTEC/SAB-WABCO test axle: 

1) The links show a tensile strain change when the axle was in the trailing position and a compressive 

strain change when the axle was in the lead position [Section 3.3.7]. 

2) The right and left links did not show equal changes in the braking loads for a given brake application. 

3) The left link produced the largest strain change (by a factor of 6) when the axle was in a trailing 

position [Section 3.3.7]. 

4) The right link produced the largest strain change (by a factor of 7 to 10) when the axle was in the lead 

position [Section 3.3.7]. 

5) Instances of sustained BOP oscillations during braking of the WABTEC/SAB-WABCO supplied 

brake disc were only observed when the test axle was in the lead position or when the links were in 

compression during braking [Section 3.1.3; Section 3.2.3; Section 3.3.4; Section 3.3.7]. 

6) During episodes of braking with sustained oscillations, the links exhibited oscillations of strain at a 

frequency of 187 Hz with strong harmonic content [Section 3.3.7, Figure 3.63]. 
151 

5.0 CONCLUSIONS 

The original goals of this evaluation were to gather data from which the causes and contributing factors 

associated with the observed cracking of the original (WABTEC/SAB-WABCO supplied) disc could be 

determined. Following initial tests an axle equipped with brake discs of a different design (Knorr) was 

included in a second series of tests. The goals of the latter portion of the evaluation were to determine 

whether the Knorr disc demonstrated acceptable thermal and mechanical response of the spokes under the 

conditions tested for service with the Acela Express vehicles. 

The following key observations and conclusions are drawn from testing and analysis: 

1) A detailed metallurgical analysis of a section of a cracked brake disc conducted at the onset of the 

investigation revealed that cracks developed due to a progression of bending fatigue that originated 

along a decarburized surface layer. The report states: “Although the decarburized surface provided a 

site for fatigue initiation, the root cause of the cracking is the level of bending stresses at the crack 

sites.” 

The inspection revealed that, with the exception of the thin decarburized surface layer, the brake disc 

casting met mechanical and chemical specifications. 

2) Sustained out-of-plane bending oscillations were observed on the WABTEC/SAB-WABCO supplied 

disc during some brake applications. The oscillating strains associated with this activity were found 

to be up to ± 1,370 microstrain on a disc mounted in the center position of the axle. The magnitude of 

the sustained oscillations on the center disc was 2.9 to 3.4 times that observed on the outboard disc. 

The magnitude of the oscillating strain was observed to grow proportionally with higher brake 

pressure. During braking events, these oscillating strains were added to the mean (tension inducing) 

strains associated with the heating of the disc friction surface. Braking-induced oscillations were 

sustained through the duration of brake application, typically lasting on the order of 10 seconds, but 

lasting up to 47 seconds. The frequency of these braking related oscillations was 187 Hz. This 

activity was only observed on the WABTEC/SAB-WABCO supplied disc when the test axle was in 

the leading position of the truck. This activity was not observed at any time for the Knorr disc. 

These responses are relevant to the fatigue life evaluation. The absence of this activity in the Knorr 

disc is expected to provide a beneficial effect on the fatigue life and a reduction in the likelihood of 

crack formation. 

The sustained out-of-plane bending did not occur during all brake applications for the 

WABTEC/SAB-WABCO supplied disc. Once it was observed, it was not always repeatable during a 

given test run. 

3) Accelerations at the journal box were observed at several track locations with most peaks in the range 

of 50 to 100 g vertical and 30 to 50 g lateral. Up to 21 peaks were measured in the vertical direction 

in excess of 100 g per one-way trip between Washington and Boston, with the maximum vertical 

acceleration reaching 189 g. These were generally observed at switches, bridge approaches, and other 

special track work. Because these measurements were made in May and June, further stiffening of 

the ballast as a consequence of cold weather may result in higher loads. 

For center and outer WABTEC/SAB-WABCO supplied discs, when an asymmetric vertical shock 

occurred independent of braking, it frequently induced large out-of-plane bending oscillations of the 
152 

spokes. These oscillations were observed to be of similar amplitudes on the center and outer discs. 

For an asymmetric vertical shock of 102 g, oscillation amplitudes were observed up to ± 2,260 

microstrain and damped out in a short time. These oscillations were observed at a frequency of 227 

Hz. Laboratory tests on a wheel-axle assembly have indicated a natural out-of-plane bending 

frequency of 220 Hz for the brake disc. Since shocks of this nature are independent of braking, these 

oscillations did occur about a wide range of mean strain levels. 

A similar response to vertical shocks was observed for the Knorr disc, but the amplitudes of these 

strains were typically 40 percent of the strains observed on the WABTEC/SAB-WABCO supplied 

disc, and these strains damped quicker. 

Lateral shocks did not appear to induce similar oscillations in either of the disc designs tested. 

These responses are relevant to the fatigue life evaluation of the two disc designs. 

The reduced level of strains observed in the Knorr disc is expected to provide a beneficial effect on 

the fatigue life and a reduction in the likelihood of crack formation. 

4) Temperatures of the brake disc friction ring and strains associated with this thermal heating under test 

braking conditions were within normal ranges and on the order of what was expected by the brake 

suppliers. 

5) The installed WABTEC/SAB-WABCO supplied disc retains a compressive preload in the spokes. 

Laboratory tests on a disc that was removed from service showed compressive strain measurements in 

the range of 800-1135 microstrain. This compares with the 333-618 microstrain values determined 

by the suppliers due solely to the pressing of the brake discs on to the axle. 

6) Finite Element Analysis indicated the highest strains associated with out-of-plane bending at the 

location of the observed cracking. Discontinuities found on the surface of some spokes arising from a 

mold parting line also appeared in this location. 

In summary the following conclusion is drawn: 

The mechanical and thermal responses of the spokes of the Knorr designed disc measured during 

testing with the Acela Express vehicles were within predicted and acceptable values. Although 

experiencing slightly lower thermal strains, the WABTEC/SAB-WABCO supplied disc experienced 

significantly higher mechanical strains at times. The lower mechanical strains observed in the Knorr 

design disc, in comparison with those observed in the WABTEC/SAB-WABCO supplied disc, 

associated with the absence of sustained oscillations during braking and the response to vertical 

shocks are expected to provide a beneficial effect on the fatigue life associated with the Knorr disc 

design, as well as a reduction in the likelihood of crack formation. 

The findings discussed in this report permitted Amtrak to arrive at an acceptable alternative that allowed 

the Acela equipment to return to service. 
153 

TECHNICAL DEFINITIONS 

 Definition 

Cant Deficiency 

For a train traveling through curved track at a given speed, the cant deficiency is 

the additional height that the elevated rail would have to be raised in order to 

produce a condition in which there is no net lateral force exerted on the rail. 

Decibels A unit for expressing the ratio of two amounts of electric or acoustic signal power 

equal to 10 times the common logarithm of this ratio 

Truck/Bogie 

Swiveling carriage consisting of a frame, two pairs of wheels and a collection of 

springs used to carry and guide one end of a railroad car during navigating over 

railroad tracks 
154 

ACRONYMS AND ABBREVIATIONS 

 Acronym and Abbreviation 

Amtrak National Passenger Railroad Administration 

Axle 1 Test axle with the WABTEC/SAB-WABCO supplied discs on Coach 3413; Axle 1 

on A-end truck adjacent to Power Car 2038 

Axle 2 Test axle with the Knorr discs on Coach 3534; Axle 4 on B-end truck adjacent to 

Coach 3413 

BIP Bending of spokes in-the-plane of the disc 

BOP Bending of spokes out-of-the-plane of the disc 

DB Decibels 

°F Temperature measured in degrees Fahrenheit 

F1 Strain Gage in plane face of spoke facing Spoke 1 

F2 Strain Gage in plane face of spoke facing Spoke 5 

Faiveley Faiveley Transport 

FEA Finite element analysis 

FRA Federal Railroad Administration 

g Acceleration of gravity 

GPS Global Positioning System 

IR Infrared 

Knorr Knorr Brake Corporation 

MP&E Motive power and equipment 

MPH, mph Miles per hour 

MTI Metallurgical Technologies, Inc., P.A. 

NEC Northeast Corridor 

NECMSC Northeast Corridor Maintenance Services Company 

PSD Power Spectral Density; describes how the variance, or power, of a time series is 

distributed as a function of the different frequencies that form the signal 

PSI, psi Pounds per square inch 

R1 Strain Gage on out of plane face of spoke 6 (nut side) 

R2 Strain Gage on out of plane face of spoke 6 (opposite nut side) 

SO Sustained Oscillation 

Spoke Number Spoke naming convention 

Ε Strain 

W/S-W WABTEC/SAB-WABCO Supplied 

Wabtec Wabtec Corporation 

μE, με Microstrain - Strain times 106
155