Fleet Study of Brake Performance and 

Tire Pressure Sensors 

 July 2009

FOREWORD 

This project is one of several performed under the provisions of Section 5117 of the 

Transportation Equity Act for the 21st Century (TEA-21). The primary objective of this project 

was to explore the overall effectiveness and customer acceptance of commercial vehicle brake 

performance and tire pressure monitoring systems in a field test setting. The work performed 

under the project included: 

 Selecting brake performance and tire pressure monitoring systems that were 

representative of currently available products in the marketplace. 

 Arranging for field testing of these systems with a commercial fleet, and in a normal 

“everyday” service environment. 

 Developing data collection and analysis plans. 

 Installing systems on test vehicles. 

 Preparing for and conducting the field test. 

 Analyzing the data and observations collected during the field test. 

 Developing a report that summarizes the results of the analysis including observations 

and conclusions (this document). 

NOTICE 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for its 

contents or use thereof. 

The contents of this report reflect the views of the contractor who is responsible for the accuracy 

of the data presented herein. The contents do not necessarily reflect the official policy of the U.S. 

Department of Transportation. 

This report does not constitute a standard, specification, or regulation. 

The U.S. Government does not endorse products or manufacturers named herein. Trade or 

manufacturers’ names appear herein only because they are considered essential to the objective 

of this document. 

Technical Report Documentation Page 

1. Report No. 

FMCSA-PSV-09-001 

2. Government Accession No. 3. Recipient's Catalog No. 

5. Report Date 

July 2009 

4. Title and Subtitle 

Fleet Study of Brake Performance and Tire Pressure Sensors 

6. Performing Organization Code 

7. Author(s) 

Deborah Van Order, Douglas Skorupski, Ryan Stinebiser, Robert Kreeb 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

Booz Allen Hamilton Inc., McLean, VA 

Washington Metropolitan Area Transit Authority, Washington, DC 11. Contract or Grant No. 

DTFH61-99-C-00025, Task Order 9 

13. Type of Report and Period Covered 

Final Project Report 

May 2005–July 2008 

12. Sponsoring Agency Name and Address 

Federal Motor Carrier Safety Administration 

Office of Bus and Truck Standards and Operations (MC-PSV) 

1200 New Jersey Ave. SE 

Washington, DC 20590 

14. Sponsoring Agency Code 

FMCSA 

15. Supplementary Notes 

Contracting Officer’s Technical Representative: Deborah M. Freund 

16. Abstract 

The purpose of this project was to conduct a field study of brake performance and tire pressure monitoring systems 

on commercial heavy-duty vehicles operating under real world conditions. The study evaluated six systems in total—

three brake performance and three tire pressure monitoring systems from various suppliers participating in the 

project. Transit bus platforms were selected for this field test because of the severe urban, stop/start duty cycle under 

which transit buses operate—an environment that accelerates both brake and tire wear thus allowing the sensor 

systems to be heavily “exercised” over the study period. 

The test fleet included 12 test buses and 12 control buses. The buses were operated by the Washington Metropolitan 

Area Transit Authority in Washington, DC The test site was the Four Mile Run maintenance garage located in 

Arlington, Va. During the course of the 1-year field test, the buses averaged 129 miles per day, and the test fleet 

traveled a total of approximately 762,580 miles. 

Four sources of data were used to evaluate the brake and tire monitoring systems: visual inspections, on-board selfdiagnostic data, maintenance records, and technician interviews. 

Both the tire and brake monitoring systems were found to hold up to the rigors of an urban city environment, and to 

provide fleet managers with data that can be used to improve vehicle safety and maintenance practices. The 

information provided by such systems allowed managers to better plan and anticipate maintenance actions, and to 

initiate preventative maintenance in order to prevent potentially severe failures. Key challenges associated with 

introduction of the sensor systems included: implementation of proper training for maintenance staff; consistent and 

correct use of data obtained from the systems; and, disciplined inspections and tracking of the sensor systems 

themselves to ensure they did not add to the overall vehicle maintenance requirements. 

17. Key Words 

CMV, commercial motor vehicle, brake performance, 

tire pressure, monitoring systems, bus, inspection, 

maintenance 

18. Distribution Statement 

No restrictions 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

114 

22. Price 

N/A 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized.
SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS APPROXIMATE CONVERSION

Symbol When You Know Multiply By To Find Symbol Symbol When You Know Multiply By 

LENGTH LENGTH in inches 25.4 millimeters mm mm millimeters 0.039 ft feet 0.305 meters m m meters 3.28 yd yards 0.914 meters m m meters 1.09 mi miles 1.61 kilometers km km kilometers 0.621 AREA AREA in2 square inches 645.2 square millimeters mm2 mm2 square millimeters 0.0016 ft2 square feet 0.093 square meters m2 m2 square meters 10.764 yd2 square yards 0.836 square meters m2 m2 square meters 1.195 ac acres 0.405 hectares ha ha hectares 2.47 mi2 square miles 2.59 square kilometers km2 km2 square kilometers 0.386 VOLUME VOLUME fl oz fluid ounces 29.57 milliliters ml ml milliliters 0.034 gal gallons 3.785 liters l l liters 0.264 ft3 cubic feet 0.028 cubic meters m3 m3 cubic meters 35.71 yd3 cubic yards 0.765 cubic meters m3

* SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E38

m3 cubic meters 1.307 

MASS MASS oz ounces 28.35 grams g g grams 0.035 lb pounds 0.454 kilograms kg kg kilograms 2.202 T short tons (2000 lbs) 0.907 megagrams Mg Mg megagrams 1.103 TEMPERATURE (exact) TEMPERATURE (exact) °F Fahrenheit 5(F-32)/9 Celsius °C °C Celsius 1.8 C + 32 

temperature or (F-32)/1.8 temperature temperature 

ILLUMINATION ILLUMINATION fc foot-candles 10.76 lux lx lx lux 0.0929 fl foot-Lamberts 3.426 candela/m2 cd/m2 cd/m2 candela/m2 0.2919 FORCE and PRESSURE or STRESS FORCE and PRESSURE or STRESS lbf pound-force 4.45 newtons N N newtons 0.225 psi pound-force per square inch 6.89 kilopascals kPa kPa kilopascals 0.145 
TABLE OF CONTENTS 

LIST OF ACRONYMS .............................................................................................................. VI

EXECUTIVE SUMMARY .......................................................................................................VII

1. INTRODUCTION AND BACKGROUND.........................................................................1

1.1 COMMERCIAL VEHICLE SAFETY TECHNOLOGY DIAGNOSTICS AND 

PERFORMANCE ENHANCEMENT PROGRAM .....................................................1 

1.2 BACKGROUND AND RATIONALE FOR THIS RESEARCH EFFORT .................2 

1.3 RESEARCH OBJECTIVES ..........................................................................................3 

1.4 OVERVIEW OF APPROACH......................................................................................4 

2. DESCRIPTION OF THE FIELD TEST ............................................................................6

2.1 HOST FLEET ................................................................................................................6

2.2 VEHICLE PLATFORM ................................................................................................7 

2.3 TECHNOLOGIES UNDER EVALUATION ...............................................................8 

2.3.1 Brake Performance Monitoring Systems.......................................................... 9 

2.3.2 Tire Pressure Monitoring Systems.................................................................. 11 

2.4 FLEET TEST PLAN....................................................................................................12 

2.5 SCHEDULE.................................................................................................................13

3. DATA COLLECTION .......................................................................................................14

3.1 DATA SOURCES .......................................................................................................14 

3.1.1 Weekly Inspections......................................................................................... 14 

3.1.2 Maintenance Records...................................................................................... 16 

3.1.3 Brake Performance and Tire Pressure Monitoring System Data.................... 16 

3.1.4 Performance Based Brake Testing (PBBT) Data............................................ 17 

3.1.5 Maintenance Staff Interviews ......................................................................... 17 

3.1.6 Study Team Observations............................................................................... 17 

3.2 META DATA ..............................................................................................................18 

4. TIRE PRESSURE MONITORING SYSTEM FIELD TEST RESULTS .....................20

4.1 TPMS TEST ARTICLE DESCRIPTIONS AND INSTALLATION..........................20 

4.1.1 WABCO IVTM .............................................................................................. 20 

4.1.2 HCI Tire-SafeGuard........................................................................................ 25 

4.1.3 Michelin eTire................................................................................................. 28 

4.1.4 Control Vehicles ............................................................................................. 33 

i
4.2 TEST RESULTS..........................................................................................................33 

4.2.1 System Accuracy ............................................................................................ 34 

4.2.2 Reliability........................................................................................................ 37 

4.2.3 Maintainability................................................................................................ 40 

4.2.4 Durability ........................................................................................................ 44 

4.2.5 Maintenance Records...................................................................................... 50 

4.2.6 Isolated Events................................................................................................ 52 

4.2.7 Technician Interviews..................................................................................... 54 

4.3 OBSERVATIONS AND CONCLUSIONS ................................................................56 

5. BRAKE PERFORMANCE MONITORING SYSTEM FIELD TEST RESULTS ......58

5.1 BPMSS UNDER EVALUATION...............................................................................58 

5.1.1 MGM eStroke ................................................................................................. 58 

5.1.2 GeoDevelopment Brake Insight...................................................................... 60 

5.1.3 StrainSert Brake Performance Monitoring ..................................................... 65 

5.1.4 PBBT............................................................................................................... 69 

5.2 CONTROL VEHICLES ..............................................................................................70 

5.3 TEST RESULTS..........................................................................................................70 

5.3.1 Overall Braking System Performance and Reliability.................................... 71 

5.3.2 MGM eStroke System Analysis...................................................................... 73 

5.3.3 Brake Insight System Analysis ....................................................................... 76 

5.3.4 StrainSert System Analysis............................................................................. 78 

5.3.5 PBBT System Analysis................................................................................... 82 

5.3.6 Brake Technician Interviews .......................................................................... 85 

5.4 OBSERVATIONS AND CONCLUSIONS ................................................................87 

6. REFERENCES....................................................................................................................89

LIST OF APPENDICES 

APPENDIX A—DATA COLLECTION FORMS ....................................................................91

ii
LIST OF TABLES 

Table 1. Transit Bus Specifications .................................................................................................8 

Table 2. Bus ID and Sensor System Pairing..................................................................................13 

Table 3. Comparison of Tire Pressure Data Points........................................................................35 

Table 4. Manually Gauged Tire Pressure Inflation Results from Weekly Inspections .................35 

Table 5. Comparison of Tire Pressure Reading: TPMS versus Manually Gauged Readings........36 

Table 6. Tire Pressure Differential by System, Raw Data.............................................................36 

Table 7. Average Tire Inflation Pressure.......................................................................................37 

Table 8. Fleet Fuel Economy.........................................................................................................37 

Table 9. Incidences of Invalid Display Reading............................................................................38 

Table 10. Overall TPMS Reliability Results .................................................................................39 

Table 11. TPMS Reliability by System Type ................................................................................39 

Table 12. Sensor Condition, by TPMS ..........................................................................................40 

Table 13. Tire-Related MAXIMO Records...................................................................................50 

Table 14. Sensors Replaced ...........................................................................................................51 

Table 15. Total Bus and Wheel-End Visual Inspections by System ............................................71 

Table 16. Summary of Brake Maintenance Records on Test and Control Fleets..........................71 

Table 17. Brake System Overhauls: Test versus Control Fleets....................................................72 

Table 18. Brake System Anomalies Identified by BPMSs During Visual Inspections.................73 

Table 19. False Positive Brake Faults by System..........................................................................73 

Table 20. eStroke System Report Filtering....................................................................................75

Table 21. Brake Insight Visual Alerts............................................................................................77 

Table 22. StrainSert Visual Warnings ...........................................................................................80 

Table 23. Total PBBT Inspections.................................................................................................83 

Table 24. PBBT Measured Warnings ............................................................................................84 

Table 25. Revised PBBT Measured Warnings ..............................................................................84 

iii
LIST OF FIGURES 

Figure 1. Buses on Lifts During an Inspection ..............................................................................15 

Figure 2. Operating Area from Four-Mile Run Maintenance Garage ...........................................18 

Figure 3. WABCO IVTM Components.........................................................................................20 

Figure 4. WABCO IVTM Sensor Unit Mounting .........................................................................21 

Figure 5. WABCO IVTM System .................................................................................................22 

Figure 6. Wheel Module Installation on a Steer Tire.....................................................................22 

Figure 7. Wheel Gap and Cap Nut Removal on a Rear Wheel......................................................23 

Figure 8. WABCO ECU and Display Unit Locations ...................................................................24 

Figure 9. IVTM Diagnostic Software Screenshot..........................................................................25 

Figure 10. HCI Tire-SafeGuard Components................................................................................26 

Figure 11. HCI Tire-SafeGuard TPMS..........................................................................................27 

Figure 12. HCI Tire-SafeGuard Antennas.....................................................................................27 

Figure 13. eTire Sensor Unit..........................................................................................................28 

Figure 14. eTire SensorDock and Installation Kit .........................................................................29 

Figure 15. eTire Location Label ....................................................................................................29 

Figure 16. eTire Handheld Tag Reader..........................................................................................30 

Figure 17. eTire Gate Reader.........................................................................................................31 

Figure 18. BIBTRACK Screenshot ...............................................................................................32

Figure 19. Michelin eTire Sensor Dock Installation Process ........................................................33 

Figure 20. Individual System Reliability Charts............................................................................40 

Figure 21. Proper and Improper IVTM Module Installation .........................................................42 

Figure 22. Improper IVTM Hose Connections..............................................................................43 

Figure 23. Damaged eTire Sensors................................................................................................44 

Figure 24. Cracked eTire Sensor, Bus Number 2512 ....................................................................45 

Figure 25. Batch Number of Failed eTire Sensors ........................................................................46 

Figure 26. Damaged eTire Sensor, Bus Number 2512 ..................................................................46 

Figure 27. Cracked eTire Sensors, Bus Number 2512...................................................................47 

Figure 28. Tire-SafeGuard Band Fragments and Damaged Sensors, Bus Number 2508..............48 

Figure 29. Damaged Tire-SafeGuard Sensor, Bus Number 2506—Right Rear Inner Tire...........48 

Figure 30. Additional Damaged Tire-SafeGuard Sensors .............................................................49 

Figure 31. Additional Damaged Tire-SafeGuard Sensors .............................................................49 

Figure 32. Damaged Tires From Bus Number 2503......................................................................52 

Figure 33. IVTM Fault History for Bus Number 2503..................................................................53 

Figure 34. Damaged eTire Handheld Reader ................................................................................54 

Figure 35. MGM Brake Chamber and Display..............................................................................59 

Figure 36. eStroke System Display................................................................................................60 

Figure 37. Brake Insight Components ...........................................................................................61 

Figure 38. Brake Insight Wheel-End Sensor and Mounting Bracket ............................................62 

Figure 39. Lining Wear Sensor Installed in Brake Shoe ...............................................................63 

iv
v

Figure 40. Brake Insight ECU (S-box) ..........................................................................................63 

Figure 41. Brake Insight Display Unit...........................................................................................64 

Figure 42. Brake Insight Component Mounting Locations ...........................................................65 

Figure 43. S-Cam Brake Assembly Diagram ................................................................................66 

Figure 44. StrainSert Anchor Pin...................................................................................................66 

Figure 45. StrainSert ECU and Display Unit.................................................................................67 

Figure 46. StrainSert Component Mounting Locations.................................................................67 

Figure 47. StrainSert Calibration Software Screenshot .................................................................68 

Figure 48. PBBT and Operators Display .......................................................................................69 

Figure 49. Brake Insight Display...................................................................................................76 

Figure 50. StrainSert Display.........................................................................................................79 

Figure 51. StrainSert Anchor Pin Cable Failure ............................................................................82 

Figure 52. Failed Weld at Left Roller Assembly...........................................................................85 
LIST OF ACRONYMS 

ABS antilock braking system 

AVM Automatic Vehicle Monitoring 

BPMS brake performance monitoring system 

CAN controller area network 

CMV commercial motor vehicle 

CNG compressed natural gas 

CV commercial vehicle 

DC direct current 

ECU electronic control unit 

F Fahrenheit 

FHWA Federal Highway Administration 

FMCSA Federal Motor Carrier Safety Administration 

GVWR gross vehicle weight rating 

I/O input/output 

ID identification 

IVTM Integrated Vehicle Tire Pressure Monitoring 

LTCCS Large Truck Crash Causation Study 

MCMIS Motor Carrier Management Information System 

MCSAP Motor Carrier Safety Assistance Program 

NHTSA National Highway Traffic Safety Administration 

PBBT Performance-Based Brake Tester 

psi pounds per square inch 

RF radio frequency 

RFID radio frequency identification 

TEA-21 Transportation Equity Act for the 21st Century 

TPMS tire pressure monitoring system 

TRC Transportation Research Center 

USDOT U.S. Department of Transportation 

VDC volts direct current 

WMATA Washington Metropolitan Transit Authority 

vi
EXECUTIVE SUMMARY 

PROJECT FUNDING 

Under the provisions of Section 5117 of the Transportation Equity Act for the 21st Century of 

1998 (TEA-21), Congress authorized the U.S. Department of Transportation (USDOT) to 

“... conduct research on the deployment of a system of advanced sensors and signal processors 

in trucks and tractor trailers to determine axle and wheel alignment, monitor collision alarm, 

check tire pressure and tire balance conditions, measure and detect load distribution in the 

vehicle, and adjust automatic braking systems.” The research program responding to this 

directive is called the Commercial Vehicle Safety Technology Diagnostics and Performance 

Enhancement Program (CV Sensor Study). This study was completed as a Task Order under the 

CV Sensor Study. 

PROJECT SUMMARY 

The Report summarizes the findings from a field test of various brake and tire condition 

monitoring systems installed on heavy duty commercial motor vehicles (CMVs). Specifically, 

three different brake performance and three tire pressure monitoring systems were installed on 

12 heavy duty commercial vehicles and operated for one year (one brake sensor system and one 

tire pressure monitoring system were installed on each test vehicle). Transit buses were chosen 

for the study because of the rather severe environment in which they operate relative to brake 

and tire wear conditions. The costs, benefits and overall performance of the brake and tire 

systems were compared with a similarly operated control fleet. The information from this project 

can be used by fleet managers (as well as by truck original equipment manufacturers and sensor 

system developers) to make important decisions related to the application, design, and use of 

such systems. 

BACKGROUND AND RATIONALE FOR STUDYING BRAKE AND TIRE 

MONITORING SYSTEMS 

Brake System Monitoring 

The number of CMVs with defective brakes that are operating on highways is a situation that has 

plagued the industry for years. Commercial vehicle inspection data shows that about 19 percent 

of all inspected vehicles—nearly one in five—were found to have one or more brake defects.*

 In 

2008, the Commercial Vehicle Safety Alliance Roadcheck 2008 conducted a 72-hour inspection 

of 67,931 vehicles in Canada, Mexico, and the United States and placed 23.9 percent of them out 

 

*

 The 1998 Motor Carrier Management Information System (MCMIS) Inspection File is operated and maintained by the Federal Motor Carrier 

Safety Administration (FMCSA). The MCMIS Inspection File contains the results of all driver-vehicle safety inspections of interstate 

commercial motor vehicles performed by states participating in the Motor Carrier Safety Assistance Program (MCSAP). In 1998, 1.9 million 

inspections of interstate vehicles were conducted. By 2008, the number of inspections had grown to more than 3 million. 

vii
of service because of various vehicle related defects and violations. Brake-related issues 

accounted for 52.6 percent of these vehicles being placed out of service.†

 

It is well understood that commercial vehicle braking system design and operation is directly 

linked to stopping distance and handling—and thus, to overall safety. Properly maintained and 

performing brakes are critical in preventing and mitigating crash situations. Although vehicle 

defects on large trucks are not commonly pinpointed as the causative factors in crashes, when 

defects are found, they frequently involve malfunctioning or defective brakes. Deficient or out of 

adjustment brakes were a factor in 29.4 percent of the fatal crashes investigated in the Federal 

Motor Carriers Safety Administration (FMCSA) 2006 Large Truck Crash Causation Study 

(LTCCS).(1) Further, the study ranked brakes as the number one equipment-related causal factor 

associated in the investigated crashes. Optimally adjusted braking systems could help prevent or 

mitigate crashes even when the braking system itself is not the initial cause of the crash. 

Tire Pressure Monitoring 

Lack of proper tire pressure maintenance also impacts safety as well as fuel efficiency in CMVs. 

Research sponsored by the FMCSA indicates that a significant portion of fleet operators do not 

regularly perform tire pressure maintenance recommended by tire manufacturers.(2)

 For example, 

FMCSA research has shown: 

 Approximately 7 percent of all CMV tires are under-inflated by 20 pounds per square 

inch (psi) or more. Only 44 percent (approximately) of all tires are within 5 psi of 

their target pressure. 

 Tire-related costs are the single largest maintenance expense for CMV fleet operators. 

National average tire-related costs per tractor-trailer are about 2 cents per mile, or 

about $2,500 for an annual 125,000-mile operation. 

 Improper inflation increases total tire-related costs by approximately $600 to $800 

annually per tractor-trailer combination. 

 For the average fleet operator in the United States, improper tire inflation increases the 

annual procurement costs for both new and retreaded tires by about 10 to 13 percent. 

 On average (and for a tractor-trailer), improper tire inflation reduces fuel economy by 

about 0.6 percent. 

 Weakened and worn tires that result from improper inflation is likely responsible for 

about one road call per year per tractor-trailer combination. 

Tire inflation and condition directly link to vehicle stability and handling—and thus, to overall 

safety. Properly maintained and performing tires aid drivers in preventing and mitigating crash 

situations, while also improving fuel efficiency. 

Technologies that could substantially reduce the cost and effort associated with tire and brake 

maintenance, and improve the ability to detect problems early, would likely lead to a reduction in 

the number and severity of commercial-vehicle-related crashes. Such technologies would also 

 

†

 Roadcheck 2008 (conducted June 3–5, 2008) inspected 67,931 CMVs at roadside inspection sites across Canada, Mexico, and the United 

States using the Commercial Vehicle Safety Alliance inspection procedures (web site: www.cvsa.org). 

viii
reduce costs and downtime associated with brake and tire systems. To address these issues, 

numerous types of brake performance monitoring and tire pressure monitoring systems have

emerged in the marketplace over the last decade. However, the market penetration for these 

systems has remained comparatively low. 

(3, 4) Previous FMCSA-sponsored research documented the performance, accuracy, and overall 

f 

STUDY OBJECTIVE 

Following the work of previous FMCSA-sponsored studies that assessed the performance of 

brake condition and tire pressure management systems in a controlled test-track environment, the 

primary objective of this research project was to document the performance, reliability, 

durability, and maintainability of leading-edge commercial vehicle brake performance and tire 

pressure monitoring systems in “real world” service. This study focused on examining the ability 

of brake performance monitoring systems (BPMSs) to detect abnormalities, defects, and/or 

misadjustments of the brake system, and for tire pressure monitoring systems (TPMSs) to detect 

tire pressure, temperature and air loss, in routine operational service environments. 

The project examines the performance of these systems and their ability to withstand the rigors 

of CMV operation. The project also included a review of fleet maintenance managers acceptance 

of the systems, and if and how they influenced on-going maintenance practices. The purpose of 

the project was to assess the relative ability of different types of brake and tire pressure 

monitoring systems to perform as promised, rather than to compare the performance of various 

manufacturers’ products with each other. 

Following are the specific objectives of the study: 

PMS. 

 vehicle fuel economy. 

S and BPMS 

nd tire pressure monitoring 

ovide sufficient information for 

ufficient self-diagnostic 

adequacy of these systems in a controlled, test track environment. However, the performance o

these systems operating in real-world service has not been well documented. Because fleet 

operators rely heavily on such data, this lack of credible information has likely limited the 

widespread deployment of such systems. 

 Assess the benefits of using an onboard B

 Assess the benefits of using a TPMS. 

 Determine whether TPMS can improve

 Determine the maintainability, durability, and reliability of TPM

operating in a non-controlled service environment. 

 Determine the degree to which brake performance a

systems influence fleet maintenance practices. 

 Determine whether audio and visual displays pr

maintenance personnel to diagnose tire and brake problems. 

 Determine whether the various sensors systems incorporate s

features to allow for efficient maintenance (i.e., so that they do not create more 

problems than they solve). 

ix
An ancillary goal of the project was to assess the benefits of a performance-based brake tester 

(PBBT). 

OVERVIEW OF PROGRAM APPROACH 

The study team’s approach to evaluating the brake and tire sensor systems focused on 

completing the following tasks: 

 Identify a commercial fleet operator (or “host” fleet) with characteristics that would 

allow for effective and fair evaluation of the monitoring systems, including: 

– an operating environment and duty cycle that could be considered severe for brake 

and tire wear (so that the sensor systems could be adequately “exercised” during the 

observation period). 

– homogeneity of the fleet in terms of vehicle type, make and model (so that sensor 

systems could be installed on like-vehicles and eliminate the potential for the vehicles 

themselves to influence the evaluation). 

– consistency of operations within the fleet relative to: driver assignments, routes, 

mileage accumulation, and maintenance operations (so that these external factors 

would not influence sensor system evaluation, thus allowing for a valid comparison 

with each other, and with the performance of a “control” fleet). 

– perhaps most importantly, the host operator should be interested in evaluating tire and 

brake sensor systems for possible widespread application on their own fleet, and 

should be generally supportive of the field study. 

 Select candidate brake and tire sensor systems based on previous test track experience 

and commercial availability. 

 Develop a test plan focused on evaluating the reliability, maintainability, accuracy, 

costs, and benefits of the various systems. 

 Install candidate sensor systems. 

 Conduct the field test. 

 Analyze the data to determine maintainability, reliability, and durability of the systems, 

as well as fuel economy and safety benefits. 

 Document results in a final report. 

The host fleet selected was the Washington Metropolitan Area Transit Authority (WMATA). 

WMATA operates approximately 1,500 buses in Washington, DC, and the surrounding 

metropolitan area. Transit bus platforms were selected for this field test because their severe 

urban, stop/start duty cycle leads to accelerated brake and tire wear (thus “exercising” the sensor 

systems). In addition, the fundamental brake and tire designs for transit buses are very similar to 

a conventional tractor, thus allowing the results of this study to be extended to heavy-duty 

(class 8) trucks. 

The study team evaluated three brake performance and three tire pressure monitoring systems on 

12 heavy-duty urban transit buses in revenue service for a period of one year. A control fleet of 

x
12 identical buses were operated in a similar manner and used for comparison. A maintenance 

garage located in Arlington, VA, was selected as the test site based on its availability of buses of

a consistent age and operating environment, and because of the low turnover and experience of 

the maintenance staff. With the assistance of WMATA and system vendors, the study team 

retrofitted the candidate systems on the buses at the bus garage. The buses operated in an area 

covering approximately 300 square-miles south and west of Washington, DC. The majority o

miles were accumulated in an urban environment with minimal high-speed highway travel. The

buses averaged 16 miles per hour in revenue service and were driven an average of 129 miles p

day. 

Over t

f 

er 

he course of the evaluation period, the sensor systems were inspected weekly. Also, the 

brake monitoring systems were capable of storing various brake system failures and 

, brake 

ed all 

d to 

s. 

, 

ance monitoring systems from different manufacturers were 

evaluated under this program, as were three different tire pressure monitoring systems. The 

y 

abnormalities (as fault codes)—and this data was downloaded as part of the test program. 

Additional data were collected in conjunction with WMATA’s various maintenance 

inspections, which included a safety inspection every 3,000 miles and a comprehensive 

preventative maintenance inspection every 6,000 miles. In addition to the inspections

system performance was evaluated once per month using a PBBT. WMATA staff record

maintenance and fueling activities and entered the data into a maintenance management 

database. This information was made available to the study team to assist with the evaluation. 

At the conclusion of the test, maintenance staff were interviewed about their experience 

operating and maintaining the systems. Other than the standard data-recording capabilities of 

the candidate systems, no additional (or special-purpose) data-logging devices were adde

the vehicles. The sensor system displays were located out of the drivers’ view per the request 

of WMATA fleet managers. The study team and WMATA technicians were responsible for 

monitoring the systems’ display status. This was done to limit driver distraction as well as 

reduce the incidence of operators ceasing bus service because of information from the display

Note: limiting vehicle related information to the bus driver (such as diagnostic information

dash warning lights, and even fuel gauge readings) is common in the transit industry. 

SYSTEMS EVALUATED 

Three different brake perform

BPMSs and TPMSs selected represent a range of technological approaches. Further, the 

characteristics of these specific systems were assessed under controlled test track conditions

through prior research efforts completed under the CV Sensors Study project sponsored b

FMCSA. The manufacturers and systems that participated in the field study are as follows. 

BPMS 

 MGM eStroke is an electronic brake stroke monitoring system that provides brake 

stroke status readings for pushrod-operated air brakes. A Hall-effect sensor mounted 

within the brake chamber is used measure brake stroke travel. 

 GeoDevelopment Brake Insight is a brake stroke monitoring system similar to the 

MGM system in that it uses a Hall-effect sensor. It differs in that the sensor and related 

hardware are mounted outside the brake chamber. 

xi
 StrainSert Brake Performance Monitoring system uses strain gauges affixed to th

anchor pins on the S-cam drum brake assembly to measure shear stresses that develop 

during a braking event. Earlier studies have confirm

e 

ed that these shear stresses 

E

se its, or 

from system also 

ce 

correlate closely with actual brake force. The anchor pin shear stress values are 

monitored on each brake assembly and readings can be compared among all 

assemblies to detect brake force imbalance and other abnormalities. 

ach of these systems includes an electronic control unit (ECU) which receives data from 

nsors (either the Hall-effect sensor in the cases of the MGM and GeoDevelopment un

 stain gauges in the Strainsert system) and analyzes the information. Each 

includes display units that provide diagnostic data to the vehicle operator and/or maintenan

personnel. Detailed information on these systems can be found in Section 5. 

TPMS 

 WABCO Integrated Vehicle Tire Pressure Monitoring (IVTM) was developed in 

conjunction with Michelin and launched in the CMV industry in 2003. Each tire and 

wheel assembly is equipped with a sensor that attaches to the valve stem and is secured 

to the rim by two lug nuts. A pneumatic hose runs between the sensor and valve stem. 

Tire inflation pressure and temperature data is wirelessly transmitted to an ECU on 

board the vehicle. 

 HCI Tire-SafeGuard consists of pressure and temperature sensors, a transmitter, and 

a driver’s display. The measurement sensor is strapped to the wheel hub inside the tire. 

Data is transmitted in a way similar to the WABCO system. 

 Michelin eTire for commercial vehicles was introduced by Michelin North America in 

October 2002. The system includes a radio frequency (RF) transmitter, pressure and 

temperature sensors, and an antenna, all of which are encased in impact- and heatresistant plastic. The passive pressure and temperature sensors (which do not require 

batteries) receive power via RF transmissions from an external reader device. The 

eTire unit mounts to the inside sidewall of the tire via a molded rubber dock that 

chemically cures to the tire’s sidewall. The system is designed to work with a tire 

pressure gate reader, but may also be used with a handheld reader. 

Detailed information on the TPMS evaluated in the study may be found in Section 4. 

TEST FLEET AND PLATFORM SELECTION 

WMATA was selected as the host fleet based on fleet size, age of vehicles, operating 

environment, proximity to the project sponsor and study team, maintenance facilities and 

practices, experience with the candidate technology under evaluation, and willingness to 

participate. 

The test platform was a 2005 Orion VII 40-foot, low-floor bus with a gross vehicle weight rating 

of 42,500 pounds. Although a transit fleet was selected for this field test, it is important to note 

that the brakes and tires used on buses and trucks are similar. Both use S-cam brakes 

manufactured by the same suppliers. The diameters of the brakes are the same, but the width of a 

xii
bus drum brake is slightly wider for improved heat dissipation. The tires used on both platforms 

are radial ply in construction, operate at similar pressures, and are similar in size. Bus tires are 

tailored for durability, while truck tires are tailored for fuel efficiency. The TPMSs an

evaluated under this program were standard production units available in the CMV market. 

SUMMARY OF RESULTS 

This study should prove useful to fleet operators in evaluating the capabilities and limitation

d BPMSs 

s of 

alternative BPMS and TPMS technological approaches, and help fleets in developing 

cations of BPMS for future vehicle purchases. The information 

should also be useful to suppliers and commercial vehicle manufacturers that are developing new 

’s 

formation had a significant impact on inspection practices, including 

enhancing the overall efficiency of operations. While no firm procurement commitments were 

easured 

TPMS sensors were found to be consistent and reliable reporting tire inflation 

e more frequent issue was “no reads” resulting from missing 

f 

tem awareness and operation. 

ce 

f an 

le 

re 

performance/functional specifi

BPMSs and TPMSs. 

Overall, both the onboard TPMSs and BPMSs provided information on the condition of the bus

brakes and tires that was useful for improving maintenance practices and detecting tire and brake 

abnormalities. This in

made, at the end of the field study, WMATA maintenance managers indicated they will consider 

the adoption of one or more monitoring technologies for new vehicle procurements and/or the 

retrofit of existing buses. 

Key observations related to the on-board monitoring systems are as follows: 

TPMS 

 TPMSs provided accurate tire inflation pressure data when compared to m

(tire-gauged) values. 



pressures. On average, the systems reported false positives (a false low-pressure 

reading) 6 percent of the time or false negatives (a missed low-pressure reading) 2 

percent of the time. Th

sensors and sensors in the wrong wheel location. 

 Keeping track of individual wheel/tire sensor units themselves was a significant 

challenge during the field test. This logistical challenge arose because of the high 

frequency of tire changes; the fact that sensor mounting locations were out of view o

technicians; and, lack of fleet-wide training on sys

Training was limited to the host depot, but occasionally tire and brake maintenan

would occur at other depots. Training across the entire system would be required to

prevent technicians from misplacing and inadvertently discarding sensors. 

 The durability of two TPMS sensor designs was initially challenged by operating in

transit service. Failures occurred with the wheel-mounted sensors two months into 

testing. Excessive heat build-up caused the sensor housing to become brittle, crack, 

and fail. The sensor manufacturer provided a design change that consisted o

isolating pad on the bottom side of the sensor that contacted the wheel rim. This simp

solution prevented further failures. Failures also occurred with the sensors that adhe

to the tire sidewall. Specifically, the plastic casings on these sensors were found to 

xiii
crack within the first few months of operation. The cracked sensor casings were 

determined to be caused by a manufacturing batch defect. Replacement sensors w

found to be significantly more durable. 

 An improvement in adherence to targeted tire pressures was not found on the test fleet 

compared to the control fleet. (i.e., average observed tire pressures between the cont

and test fleets were roughly the same). This may be because WMATA takes a proactive role in maintaining target inflation pressures to comply with the tire vendor’s 

warranty. Average tire inflation pressure

ere 

rol 

 was 111 pounds per square inch (psi) for both 

on 

s 

-mounted sensors difficult to connect to the 

se the 

the test and control fleets (target: 115 psi). Tire life and fuel economy were also similar

in both fleets. This was likely the case because, as noted earlier, the real time display 

of tire pressure readings was purposefully not made available to drivers, but only to

maintenance personnel and technicians. Drivers could therefore not act immediately 

such information to correct any tire pressure problems that may have been detected. In 

most CMV fleets, such real time information would be provided to drivers, and drivers 

would have the opportunity to act on the information as needed (e.g., adding air to tire

at the next convenient time if low pressure was detected), thereby improving the 

average tire life and fuel economy of the fleet. 

 Two tire blowouts could have been prevented during the course of the field test had the 

TPMS displays been visible to the bus operators. To maximize safety and operational 

benefits, system data needs to be available to the driver in real time (as well as being 

made available to maintenance staff). 

 Technicians preferred the wheel-mount tire pressure sensors for two reasons: 

– They were easier to install. 

– Tires could be changed without removing or disconnecting the system. 

Conversely, the technicians found valve-stem

inner tires on a set of dual-tire assembly. This issue may be unique to buses becau

wh els and tires are surrounded e by the structure of the vehicle. Tire-mounted sensors 

required more time to install (versus wheel or valve-stem mounted systems) because of 

the required tire surface preparations. 

BPMSs 

 Onboard BPMS were found to influence WMATA’s inspection practices favorably. 

WMATA inspects its buses every 3,000 and 6,000 miles. With more than 200 buses 

operating out of each maintenance facility, these inspections require a significant 

amount of time. For the 3,000-mile inspection, WMATA has begun relying on the 

BPMS to assess the brakes. This reduces inspection times and allows more buses to be 

inspected within a given period. 

 The durability of BPMS sensors was excellent in a rigorous urban transit operating 

environment. A single sensor failure occurred during the roughly 1 million miles 

traveled by the 12 test buses. Most maintenance actions were few, and were limited to 

broken wires, loose connectors, and sensor adjustments. 

 In transit service, information from onboard monitoring systems needs to reach 

maintenance personnel in a timely fashion to be useful. WMATA’s buses are equipped 

xiv
with a serial databus and WiFi transmitter capable of wirelessly transmitting various

diagnostic data from the bus to a server housed at the maintenance garage. Each ti

the bus returns to the garage, this data is off-loaded and e-mailed to maintenance 

supervisors. The eStroke systems evaluated under this pro

me 

gram were integrated into 

buses 

. 

sue 

e withheld from service. 

gs in 

ing wear sensor embedded in the shoe lining. 

 to 

ation. For 

he axle 

this wireless data transfer system on-board the buses. The study team found that 

with eStroke alarms were inspected and problems corrected in a timely fashion (on the 

same or the next day). On buses with monitoring systems that only communicated via 

in-vehicle display, a week or more could elapse before brake problems were addressed

 MGM’s eStroke system proved useful in the early detection of a manufacturing is

in the alignment between the brake chamber and slack adjuster on the test buses. This

issue was corrected by the vehicle and brake manufacturers under the terms of their 

warranties. 

 Based on interviews, the alerts from the BPMSs provided technicians with useful 

information to quantify driver complaints and reduce their frequency. Complaints with 

brakes are time consuming to troubleshoot because they require performing an 

inspection on a lift. Technicians commented that BPMSs reduced the number of driver

complaints and provided real-time information they could use to decide whether the 

bus should b

 The BPMSs evaluated under this program were not able to detect worn brake linin

need of replacement. All but one of the test buses underwent a rear brake overhaul at 

roughly 70,000 to 80,000 miles into the field test. In the weeks and days leading up to 

the rear brake overhauls, none of the monitoring systems triggered an alarm indicating 

poor brake performance or excessive stroke travel. It should be noted that the Brake 

Insight system featured a wire-loop lin

Unfortunately, the sensor embedded into the lining was placed at a depth lower than 

the minimum thickness used by WMATA to replace shoe linings. 

 Onboard BPMSs provided a new source of information enabling technicians to identify

and address brake issues. As with any new data source, a learning period is required to 

understand, interpret, and build confidence with the data generated from these 

monitoring systems. WMATA experienced this learning process with the systems 

evaluated under this program. WMATA and the study team worked with system 

vendors to tailor algorithms (and warning thresholds) for WMATA’s transit vehicles

minimize false positives and improve the overall reliability of the inform

example, overstroke alarms were not always attributed to a defective brake. A majority 

of cases were found to be caused by either high operating brake temperatures, or 

because of the particular brake setup on the Orion VII test buses. Specifically, t

design on the Orion VII buses is such that the 0.50-inch reserve stroke is utilized in

normal brake operation—and because the brake stroke was operating at its limit in

normal operations, numerous “false positive” overstroke alarms were detected. 

 Based on the result of the field study, WMATA has confidence in brake performance 

monitoring systems and plans to specify their use in all of the buses they purchase in 

the future. 

xv
xvi

ROADMAP TO REPORT 

Section 1 provides background information on FMCSA’s Commercial Vehicle Safety 

Technology Diagnostics and Performance Enhancement Program (the program under which this 

project was sponsored). It also provides details on the rationale for this research effort, its 

objectives, and the approach used to evaluate TPMSs and BPMSs. 

Section 2 provides a description of the field test. This section includes information on the host 

fleet, the vehicle platform, and the technologies evaluated. 

Section 3 describes the sources of data used to evaluate the technologies. These sources include 

system data, maintenance records, vehicle inspections, and maintenance interviews. 

Section 4 focuses on the activities related to the evaluation of the TPMSs. It details the work 

involved in installing the systems, presents analyses of data collected during the field test, and 

provides key observations and conclusions of the study. 

Section 5 focuses on the activities related to the evaluation of the BPMSs. It details the work 

involved in installing the systems, presents analyses of data collected during the field test, and 

provides key observations and conclusions of the study 
1. INTRODUCTION AND BACKGROUND 

1.1 COMMERCIAL VEHICLE SAFETY TECHNOLOGY DIAGNOSTICS AND 

PERFORMANCE ENHANCEMENT PROGRAM 

The purpose of the Commercial Vehicle Safety Technology Diagnostics and Performance 

Enhancement Program (CV Sensor Study) is to “define performance requirements, assess 

benefits, and accelerate deployment of driver and vehicle assistance products and systems and, in 

particular, advanced sensor and signal processors in trucks and tractor trailers with an emphasis 

on onboard diagnostic and improved safety-related products.” 

The study emphasizes soliciting input from key industry stakeholders (fleet operators, 

manufacturers, and suppliers) on potential research initiatives, testing and demonstration 

procedures, equipment specifications, and data collection and reporting methodologies. The 

study focuses on conducting research that complements (rather than duplicates) efforts by private 

industry. Objectives of the research include evaluating the probable impact of selected vehicle 

technologies on improving overall commercial motor vehicle (CMV) safety and assessing their 

cost savings potential and/or operational benefits, thus helping to create market demand and 

encourage commercialization. 

To help identify possible research areas, the following tasks were completed: 

 An extensive literature search of relevant technical journals and databases. 

 Individual interviews and discussions with representatives from truck and trailer 

manufacturers, fleet operators, owner operators, and industry suppliers, as well as staff 

at the National Highway Traffic Safety Administration (NHTSA), Federal Motor 

Carrier Safety Administration (FMCSA), and Federal Highway Administration 

(FHWA) who are involved in commercial vehicle safety research. 

 A meeting of key industry stakeholders to review candidate research areas and make 

suggestions regarding future work under the CV Sensor Study. 

As a result of this background research and interviews, the following candidate areas of research 

were identified: 

 Brakes and related controls. 

 Tire inflation and condition monitoring systems. 

 Truck and tractor alignment (“dynamic alignment”). 

 Testing and analysis of high-speed data bus networks (J1939 CAN). 

 Cost, benefits, and implementation issues associated with event data recorders. 

 Active suspensions systems. 

 Advanced vehicle diagnostic and prognostic tools. 

 Issues related to implementation of “Smart Copilot” onboard systems. 

1
The focus of this report is on the first, second and seventh research areas on the list—brakes and 

related controls, tire inflation and condition monitoring systems, and advanced vehicle diagnostic 

and prognostic tools. 

In prior tasks under the CV Sensors Study contract, FMCSA completed comprehensive 

evaluations of brake performance and tire pressure monitoring systems on a closed test track. 

The primary objective of that work (Task Order #3, completed in November 2003, Report 

FMCSA-PSV-04-002; and Task Order #8, completed in July 2006, Report FMCSA-PSV-07-

001) was to document the performance and operational characteristics of leading-edge 

commercial vehicle brake performance and tire pressure monitoring systems. Baseline 

performance characterization of the systems were performed according to “scripted” test 

matrices and under controlled-test-track environments. In contrast, this study focused on a more 

naturalistic field study of brake performance and tire pressure monitoring systems. 

1.2 BACKGROUND AND RATIONALE FOR THIS RESEARCH EFFORT 

Brake and tire deficiencies are the two most common vehicle-related deficiencies cited in 

commercial vehicle roadside inspections. It is well understood that an improperly adjusted or 

maintained braking system can compromise stopping distance, handling, and overall safety. 

Likewise, worn and improperly maintained tires can also impair vehicle handling and braking—

particularly during evasive or emergency type of maneuvers. Sensor systems that could provide 

an early warning to drivers and/or maintenance personnel of possible problems with brakes or 

tires would help reduce the incidence of such deficiencies and lead to an overall improvement in 

commercial vehicle safety. 

As noted, previous task orders under this contract, specifically Task Orders 3(3)

 and 8(4), provided 

comprehensive baseline assessments of a variety of state-of-the-art brake and tire pressure 

monitoring systems and tire pressure management systems. However, the maintenance, 

reliability and overall customer acceptance of these systems was not evaluated—and hence, a 

more real world testing of the systems was deemed appropriate. 

An urban transit bus fleet was chosen to evaluate the durability and reliability of the brake 

performance and tire pressure monitoring systems. The vehicles in an urban bus fleet are 

subjected to some of the most severe service conditions encountered by CMVs. Inter-city transit 

buses generally operate at moderate-to-low speeds, have high axle weights, and make frequent 

stops. These factors effectively accelerate wear and tear on both brakes and tires—as well as on 

the sensor systems involved in this field study. 

It is also important to note that the foundation brakes and tires used on these vehicles are similar 

to those used on many long-haul trucks and motor coaches—and therefore the results of this field 

study should be useful to mangers of these fleets as well. 

Additional factors favoring the use of a transit fleet for evaluating the various sensor systems 

was consistency of operating and maintenance practices, tight control and tracking of driver 

assignments, as well having a sizeable fleet of essentially identical vehicles. These conditions 

minimized the influence of such “externalities” on the durability, reliability and performance of 

the sensors systems. 

2
1.3 RESEARCH OBJECTIVES 

The primary objective of this research study was to document the performance, reliability, 

durability, and maintainability of leading-edge commercial vehicle brake performance and tire 

pressure monitoring systems in real-world service. Three different brake performance monitoring 

systems from different manufacturers were evaluated under this program, as were three different 

tire pressure monitoring systems. The brake performance monitoring system (BPMS) and tire 

pressure monitoring system (TPMS) selected represent a range of technological approaches. The 

study focused on examining the ability of brake performance monitoring system sensors to detect 

abnormalities, defects, and/or misadjustments of the brake system and for the TPMSs to detect 

tire pressure, temperature, and air leakage rates in a non-controlled service environment. 

The study team examined the performance of these systems and their ability to withstand the 

rigors of revenue-service operation. The study also included a review of fleet maintenance 

personnel acceptance of the systems and if/how the systems influenced brake and tire 

maintenance practices. 

This study should prove useful to fleet operators in evaluating the capabilities and limitations of 

alternative BPMS and TPMS approaches and in determining specifications for future truck 

purchases. The information should also be useful to suppliers and commercial vehicle 

manufacturers that are developing new BPMSs and TPMSs. 

Following are the specific objectives of the study: 

 Assess the benefits of using an onboard BPMS. 

 Assess the benefits of using a TPMS. 

 Determine whether TPMS can improve vehicle fuel economy. 

 Determine the maintainability, durability, and reliability of TPMS operating in a noncontrolled service environment. 

 Determine the maintainability, durability, and reliability of BPMS operating in a noncontrolled service environment. 

 Determine the degree to which brake performance and tire pressure monitoring 

systems influence fleet maintenance practices. 

 Determine whether audio and visual displays provide sufficient information for 

maintenance personnel to diagnose tire and brake problems. 

 Determine whether the various sensors systems incorporate sufficient self-diagnostic 

features to allow for efficient maintenance (i.e. so that they do not create more 

problems than they solve). 

An ancillary goal of the project was to assess the benefits of a performance-based brake tester. 

3
1.4 OVERVIEW OF APPROACH 

The study team’s approach to evaluating the brake and tire sensor systems focused on 

completing the following tasks: 

 Identify a commercial fleet operator (or “host” fleet) with characteristics that would 

allow for effective and fair evaluation of systems and technologies, including: 

– an operating environment and duty cycle that could be considered severe for brake 

and tire wear (so that the sensor systems could be adequately “exercised” during the 

observation period). 

– homogeneity of the fleet in term of vehicle type, make and model (so that sensor 

systems could be installed on like-vehicles and eliminate the potential for the vehicles 

themselves to influence the evaluation). 

– consistency of operations within the fleet relative to: driver assignments; routes, 

mileage accumulation, and maintenance operations (so that these externalities would 

not influence sensor system evaluation, thus allowing for a valid comparison with 

each other, and with the performance of a “control” fleet). 

– perhaps most importantly, the host operator should be interested in evaluating tire and 

brake sensor systems for possible widespread application on their own fleet, and 

should be generally supportive of the field study. 

 Select candidate brake and tire sensor systems based on previous test track experience 

and commercial availability. 

 Develop a test plan focused on evaluating the reliability, maintainability, accuracy, 

costs, and benefits of the various systems. 

 Install candidate sensor systems. 

 Conduct the field test. 

 Analyze the data to determine maintainability, reliability, and durability of the systems, 

as well as fuel economy and safety benefits. 

 Document results in a final report. 

The host fleet selected was the Washington Metropolitan Area Transit Authority (WMATA). 

WMATA operates approximately 1,500 buses in Washington, DC. and the surrounding 

metropolitan area. Transit bus platforms were selected for this field test because their severe 

urban, stop/start duty cycle leads to accelerated brake and tire wear (thus challenging the sensor 

systems.) In addition, the fundamental brake and tire designs are very similar to a conventional 

tractor, thus allowing the results of this study to be extended to heavy-duty (class 8) trucks. 

The study team evaluated three brake performance and three tire pressure monitoring systems on 

12 heavy-duty urban transit buses in revenue service for a period of one year. A control fleet of 

12 identical buses were operated in a similar manner and used for comparison. A maintenance 

garage located in Arlington, VA, was selected as the test site based on its availability of buses of 

a consistent age and operating environment, and because of the low turnover and experience of 

the maintenance staff. With the assistance of WMATA and system vendors, the study team 

retrofitted the candidate systems on the buses at the garage. The buses operated in an area 

4
covering approximately 300 square-miles south and west of Washington, DC. The majority

miles were accumulated in an urban environment with minimal high-speed highway travel. The

buses averaged 16 miles per hour in revenue service and were driven an average of 129 miles per

day. 

 of 

Over the course of the evaluation period, the systems were inspected weekly, and system data 

was downloaded as part of the test program. Additional data were collected in conjunction with 

WMATA’s various maintenance inspections, which included a safety inspection every 3,000 

miles and a comprehensive preventative maintenance inspection every 6,000 miles. In addition to 

the inspections, brake system performance was evaluated once per month using a performancebased brake tester. WMATA staff recorded all maintenance and fueling activities and entered the 

data into a maintenance management database. This information was made available to the study 

team to assist with the evaluation. At the conclusion of the test, maintenance staff were 

interviewed about their experience operating and maintaining the systems. Other than the 

standard data-recording capabilities of the candidate systems, no additional (or special-purpose) 

data-logging devices were added to the vehicles. The system status displays were located out of 

the drivers’ view per the request of WMATA fleet managers. The study team and WMATA 

technicians were responsible for monitoring the systems’ display status. This was done to limit 

driver distraction as well as reduce the incidence of operators ceasing bus service because of 

information from the displays. Note: limiting vehicle related information to the bus driver, (such 

as diagnostic information, dash warning lights and even fuel gauge readings) is common in the 

transit industry. 

5
2. DESCRIPTION OF THE FIELD TEST 

2.1 HOST FLEET 

A very critical step in conducting the field test was to identify a fleet suitable for field testing 

BPMSs and TPMSs. Several potential host fleet operators were evaluated. The key factors 

considered in selecting an appropriate test fleet included the following: 

 Service Environment/duty cycle. An environment which accelerates the “wear and 

tear” on the sensor systems was favored in order to maximize the potential for 

uncovering (or amplifying) operational pros and cons of the various systems. Further, 

for practical considerations related to data collection, a fleet that operated within a 

limited geographic area and utilized a single maintenance facility was preferred. Urban 

pickup/delivery, vocational, and transit fleets were therefore targeted. These types of 

fleets would simplify data collection and provide a severe operating environment for 

brakes and tires. 

 Vehicle Type. Because this study focused on sensor systems applicable to commercial 

vehicles, a fleet of heavy-duty trucks such as class 7/8 tractor trailers, heavy-duty 

vocational trucks, or urban transit buses were considered. 

 Fleet Location. A fleet operating in the Washington, DC, area, close to the study team 

and sponsor (FMCSA) was preferred in order to minimize program costs. 

 Vehicle Age. All else being equal, a fleet with newer vehicles was preferred so as to 

minimize the potential for breakdowns and maintenance problems with other vehicle 

systems (which would limit mileage accumulation during the study period). 

 Maintenance Facilities and Practices. Candidate fleets were evaluated to determine 

whether their facilities and maintenance systems were sufficient for supporting the 

program. A fleet with a modern and reliable electronic maintenance management 

system was required so that maintenance actions could be recorded (in particular, labor 

hours and parts replacement related to the brake and tire systems). 

 Experience with Candidate Technologies. Previous experience with the systems was 

seen as an asset for planning, training, and conducting the test. In considering 

candidate fleets, maintenance employees were asked about their familiarity with the 

sensor systems under evaluation. 

 New Technology Test Experience. Fleets whose employees had previous experience 

with pilot testing new technologies were sought. 

 Willing Participant. A fleet’s commitment to participate in the program was critical 

to program success. 

The search identified several potential host fleets and included package delivery, beverage 

delivery, refuse collection, and transit fleets operating in the greater Washington DC 

metropolitan area. WMATA was selected on the basis of the fleet criteria defined above. 

6
WMATA operates approximately 1,500 buses out of 10 maintenance depots in and around 

Washington, DC, Maryland, and Virginia. Each bus averages roughly 30,000 miles annually, or 

about 120 miles per day. The bus fleet is primarily made up of standard 40-foot urban transit 

buses with a 12-year service life. The fleet included buses manufactured by Orion, New Flyer, 

NABI, and Flxible. In addition to the standard buses, WMATA also operates a smaller number 

of 30-foot and 60-foot articulated models. Currently, 70 percent of the fleet is fueled with diesel, 

but the remaining buses operate on compressed natural gas (CNG). WMATA also operates about 

50 hybrid electric buses. The average age of WMATA’s fleet is 7.6 years. All preventative 

maintenance and repairs are performed in-house. 

2.2 VEHICLE PLATFORM 

The test fleet consisted of 12 Orion VII series, 2005 model year, urban transit buses. The buses 

are a “low floor” design, 40 feet long and 102 inches wide, and operate on CNG. All 12 buses 

are operated out of WMATA’s Four-Mile Run maintenance garage. The buses were selected 

because they were new, operated in a combination of urban and suburban service, and had CNG 

propulsion/fueling systems. The selection of CNG-fueled vehicles reduced the chance that they 

could be reassigned to another garage since only 2 of the 10 garages can accommodate the 

maintenance and fueling requirements of CNG buses. In addition, the new CNG buses were 

already equipped with one of the brake performance monitoring systems under evaluation—

specifically, the MGM eStroke. After a series of pilot tests in 2007, WMATA began ordering 

new buses with the MGM’s eStroke system. Further, using a system manufactured by Clever 

Devices,(5)

 brake performance data (from the eStroke system) as well as engine and transmission 

fault codes, are offloaded wirelessly to a central server located at the maintenance garage. 

Each bus’s gross vehicle weight rating (GVWR) is 42,540 pounds. The passenger capacity is 41 

seated and 36 standing passengers for a total of 77 passengers. The curb weight of the buses is 

30,990 pounds. The 16,500-pound front and 28,600-pound rear axles are manufactured by 

Rockwell. Four S-cam Meritor brake assemblies are mounted on each wheel end. Front brakes 

measured 16-1/2 inches by 6 inches, and the rear brakes measure approximately 16-1/2 inches by 

8-5/8 inches. Table 1 provides a full vehicle specification. 

7
Table 1. Transit Bus Specifications 

General 

Bus Model Orion VII 

Serial Number 4V4NC9JH91N317953 

Model Year 2005 

Engine Cummins C8.3 Gas Plus 

Transmission Voith D864.3E 

Front Suspension Air 

Rear Suspension Air 

Wheelbase (inches) 286 inches 

ABS WABCO 4S4M 

GVWR (pounds) 42,540 

Brakes 

Front Rear 

Manufacturer ArvinMeritor ArvinMeritor 

Type S-Cam Drum S-Cam Drum 

Size (inches) 16.5 x 6 16.5 x 8.63 

Lining Meritor A3222F2296 Meritor A3222F2294 

Slack Adjusters Haldex, 5-Notch Adjustment Haldex, 5-Notch Adjustment 

Chamber Type MGM E-Stroke Type 24 Long MGM E-Stroke Type 30 Long 

Drum Dayton-Walther 

85123561002 

Webb 

64051B 

Tires 

Front Rear

Manufacturer Goodyear Goodyear 

Make/Type City Tire City Tire 

Size 305/70 R22.5 305/70 R22.5 

Pressure (psi) 115 115 

Weight Distribution 

Front Axle Rear Axle

Curb Weight, 

30,990 pounds 

11,000 19,990 

GVWR, 

42,540 pounds 

14,780 27,760 

2.3 TECHNOLOGIES UNDER EVALUATION 

The brake performance and tire pressure monitoring systems selected for this study were chosen 

on the basis of two main criteria: documented successful performance under controlled testing, 

and commercial availability. Previous research sponsored by the FMCSA (Task Orders 3(3) and 

8(4)

 under the “Sensors Study” program) assessed the accuracy and reliability of BPMSs and 

TPMSs respectively under controlled conditions, and essentially provided important baseline 

characterization data about the products prior to conducting the field study. A brief review of the 

work completed under these task orders is presented in the following sections. 

8
2.3.1 Brake Performance Monitoring Systems 

Task Order 3, Onboard Sensors for Determining Brake System Performance,

(3)

 documented the 

performance of leading-edge technological approaches to monitoring commercial vehicle 

braking systems. The study focused on various sensors and their ability to detect abnormalities, 

defects, and misadjustments of the braking system. 

Brake monitoring systems were tested on a closed test track at Transportation Research Center 

(TRC) in East Liberty, Ohio, in the 2001–2002 time frame. The brake monitoring systems were 

installed on a conventional tractor-trailer and included the following technologies.‡

 Brake chamber stroke sensors. 

 Anchor pin strain gauges. 

 Wheel-speed sensors. 

 Brake shoe thermocouples. 

The systems were evaluated under a variety of braking maneuvers including low-to-high 

deceleration rates on dry and wet pavement, on level and graded (sloped and super-elevated) 

surfaces, and under no-load and fully loaded conditions. Each brake assembly was optimally 

adjusted to establish baseline performance criteria. The test matrix included deliberately “staged” 

faults or defects on brake assemblies. The sensor outputs were examined to determine their 

ability to detect brake defects under various braking conditions. 

Brake Chamber Stroke sensors. Brake stroke monitoring systems examined included the MGM 

eStroke and Spectra Brake Inspector. Both systems were able to detect brake deficiencies. 

Following are the key observations and conclusions on these systems: 

 System accuracy varies depending on the load, deceleration rate, and type of brake 

deficiency. The systems had the most difficulty detecting brake deficiencies when the 

trailer was unloaded and when braking was at low deceleration rates; however, the 

manufacturers noted that these systems are designed to primarily detect overstroke 

conditions during hard-braking applications (rather than precise stroke measurement 

during light decelerations). 

 In-cab displays featured indicator lights for each brake assembly and provided the 

driver with valuable real-time data on the overall condition of the vehicle’s braking 

system. 

 Brake stroke monitoring cannot differentiate between out-of-adjustment brakes and 

oil-soaked shoe linings. During testing, the linear potentiometers recorded oil-soaked 

shoe linings as an overstroke condition. 

 The resolution and accuracy of stroke sensors make them well-suited for detecting 

brake maintenance needs and potential brake safety issues. 

 

‡

 An additional brake performance monitoring system from NorCorp Systems was also briefly evaluated. This system attempted to correlate 

brake system control pressure with vehicle deceleration rate in order to determine if the brakes were working properly. Software problems 

however prevented complete testing of this system. 

9
Based largely on the observations above, two brake stroke monitoring systems were selected for 

the fleet test—MGM eStroke and GeoDevelopment Brake Insight. The MGM eStroke brake 

monitoring system featuring its brake chamber mounted sensor was selected for the fleet test 

because of its performance at TRC and its commercial availability. The Brake Insight system 

was selected because WMATA had previously evaluated the Spectra system and was interested 

in comparing it with a similar product from GeoDevelopment. Both the GeoDevelopment and 

Spectra systems have a similar design and use a sensor that mounts externally to the brake 

chamber. Furthermore, the GeoDevelopment system offered a shoe lining wear sensor, which 

WMATA was interested in evaluating. 

Anchor Pin Strain Gauges. Task Order 3 also examined instrumented drum brake anchor pins 

manufactured by StrainSert. Each anchor pin is fitted with two strain gauges oriented 90 

degrees apart. One of the strain gauges is mounted normal to the direction of rotation, and is 

intended to predominantly measure the mechanical non-friction force exerted by the movement 

of the brake shoe as it moves against the drum. The other strain gauge is offset 90 degrees from 

normal and is intended to primarily measure the rotational friction force between the drum and 

the lining. The StrainSert anchor pins can be continuously monitored by measuring the 

electrical signal (voltage) generated by the strain gauges internal to the pins. The anchor pin 

strain gauges performed well during track testing and were considered for field testing for the 

following reasons:

 Track testing demonstrated a highly predictable relationship between force data 

generated by the strain-gauged anchor pins and the vehicle’s deceleration rate. 

Instrumented anchor pin force is therefore an accurate measure of a vehicle’s braking 

performance. 

 Instrumented anchor pins can accurately detect brake deficiencies in specific 

(individual) wheel assemblies, including out-of-adjustment brakes, disconnected 

brakes, and/or oil-soaked shoe linings. They can also measure the effect of an out-ofadjustment brake on the other (properly adjusted) brakes on a vehicle. This capability 

lends itself to application on advanced brake balancing control schemes that may be 

possible with electronic controlled braking systems. 

 Instrumented anchor pins can accurately detect even low brake forces. By resolving the 

resultant force into the “X” (friction force) and “Y” (normal force) directions, the 

instrumented anchor pins can differentiate between an out-of-adjustment brake and a 

brake with oil-soaked shoe linings. 

 The instrumented anchor pins performed reliably throughout the testing. 

At that time of testing (2002), StrainSert had only recently developed the instrumented anchor 

pins and had not yet incorporated them into a complete and commercially available brake 

monitoring system. Therefore, the anchor pins were connected directly to a data acquisition 

system, and their outputs were later analyzed using a custom database tool. StrainSert has since 

made progress in developing a complete brake monitoring package. The system includes the 

anchor pins, electronic control unit (ECU), display unit, and software package with useradjustable brake force and alarm thresholds. 

10
Wheel speed sensing. Wheel-speed sensors are a standard component of anti-lock braking 

systems (ABSs) used on heavy-duty trucks and buses. ABS wheel-speed sensors can be used to 

measure individual wheel slip by comparing the calculated speed of each wheel to the calculated 

average for all wheels—or to some other “actual” speed reference such as a transmission signal. 

During braking (particularly hard braking) individual wheel/tire assemblies actually experience a 

very minor degree of skidding which causes the truck to slow down. The working hypothesis 

was that if a particular brake assembly was not functioning properly (due to out of adjustment 

brakes, worn brakes, or any other defect), then that wheel would be rotating slightly faster than 

the other wheel/tire assemblies which were braking normally (and therefore experiencing a slight 

degree of skidding). These differences in individual wheel speeds (during a braking event) could 

then be compared to determine if one or more brake assemblies was malfunctioning. 

Unfortunately, the wheel-speed sensor system tested during Task Order 3 was sufficiently 

accurate to only detect grossly out-of-adjustment and/or disconnected brakes. Wheel-speed 

sensors do not provide sufficient accuracy to detect brakes that are marginally out of adjustment 

(i.e., 2 1/8 inches or less). Additionally wheel-speed sensors cannot differentiate between out-ofadjustment brakes and oil-soaked linings. 

Brake Shoe Thermocouples. As part of the Task Order 3 work, thermocouples were mounted at 

varying depths within the shoe linings to test their sensitivity for determining brake deficiencies. 

These tests showed that because of the unpredictable variations in initial brake temperature, the 

comparatively slow response time of thermocouples, and the general inaccuracies inherent with 

thermocouples, their ability to detect and differentiate brake deficiencies during discrete braking 

events was found to be very limited. 

Of the four brake monitoring technologies tested, brake chamber stroke sensors and anchor pin 

strain gauges proved to be the most viable candidates for continued testing in the field. 

Specifically, the following brake performance monitoring systems were made part of the field 

study at WMATA: 

 The eStroke system, by MGM Brakes.(6)

 The Brake Insight, by GeoDevelopment.(7)

 The StrainSert Anchor Pin System.(8)

2.3.2 Tire Pressure Monitoring Systems 

Task Order 8, Tire Pressure Monitoring and Maintenance Systems Performance,

(4)

 documented 

the performance, accuracy, and operational characteristics of leading-edge approaches to 

commercial vehicle tire pressure monitoring and maintenance systems. The study focused on the 

various TPMS sensors and their ability to provide accurate tire pressure readings, detect slow 

and rapid changes in tire pressure, and maintain tire pressure under adverse conditions. 

Task Order #8 evaluated a variety of tire pressure monitoring and automatic inflation systems 

under controlled conditions on a closed test track at Transportation Research Center, Inc. (TRC) 

in East Liberty, Ohio, in the 2005 to 2006 time frame. The TPMSs were installed on a 

conventional tractor-trailer combination and on a motor coach, and they included the following 

types of systems: 

11
 Tire equalizers to balance pressures between tires in a dual installation. 

 Tire pressure monitors to keep track of the pressure in each tire. 

 Tire pressure maintenance systems to maintain tire pressure at desired levels. 

The tire pressure monitoring and maintenance systems were evaluated under a variety of staged 

test conditions such as slow leaks, fast leaks, single tire incidents and multi-tire incidents. These 

tests were completed to determine threshold warning levels, failure modes, and general system 

operation. Only tire pressure monitoring systems, not tire pressure maintenance systems, were 

viewed as viable candidates for the fleet field test—primarily because pressure maintenance 

systems would need to tap into the secondary air supply, which WMATA was not interested in 

doing because the buses were new and under warranty. 

Based on the results from testing under Task Order #8, and on consultation with WMATA bus 

operations and maintenance staff, it was determined to evaluate the following types of tire 

pressure monitoring systems as part of the field study: 

 A valve-stem-mounted system. 

 A wheel-mounted system. 

 A tire-mounted TPMS. 

Specifically, the three systems selected for the field test were manufactured by WABCO, HCI, 

and Michelin and are described as follows: 

WABCO IVTM System. WABCO’s IVTM system, developed in conjunction with Michelin, 

was launched in the CMV industry in 2003. Each tire and wheel assembly is equipped with a 

sensor that attaches to the valve stem and is secured to the rim by two lug nuts. A pneumatic 

hose runs between the sensor and valve stem.(9)

HCI Tire-SafeGuard. HCI Corporation manufactures the Tire-SafeGuard TPMS. The system 

consists of pressure and temperature sensors, a transmitter, and a driver’s display. The 

measurement sensor is strapped to the wheel inside the tire.(10)

 

Michelin eTire. Michelin North America introduced its eTire system for commercial vehicles in 

October 2002. The sensor includes a radio frequency (RF) transmitter, pressure and temperature 

sensors, and an antenna, which are encased in impact- and heat-resistant plastic. The sensor 

mounts to the inside sidewall of the tire via a molded rubber dock that chemically cures to the 

tire’s sidewall. It should be noted that the system is designed to be used with a tire pressure gate 

reader. For the purpose of this test, a Michelin-supplied handheld tire pressure reader was 

used.(11)

2.4 FLEET TEST PLAN 

Based on the project’s overall objectives, and on available resources, 12 buses were selected for 

testing the candidate systems during a 1-year period. Because there were three unique brake 

performance systems, and three unique tire pressure monitoring systems to be evaluated, the test 

12
buses were divided into three groups of four buses each, with each group receiving a different 

combination of a particular tire pressure monitoring and brake performance monitoring system. 

Twelve identical buses operating from the same facility were used as a control group. Table 2

provides information on sensor system pairings and bus identification (ID) numbers. It should be 

noted that the test buses came pre-equipped from the factory with MGM’s eStroke system. 

Table 2. Bus ID and Sensor System Pairing 

Bus Numbers BPMS TPMS 

2501–2504 (4 buses) MGM eStroke WABCO IVTM 

2505–2508 (4 buses) GeoDevelopment Brake Insight HCI Tire-SafeGuard 

2509–2512 (4 buses) StrainSert Brake Performance Monitoring Michelin eTire 

2513–2524 (12 buses) Control Group Control Group 

During the 1-year test period, WMATA operated the test buses as part of its normal scheduled 

fleet. WMATA also recorded maintenance data such as fuel usage, accumulated mileage, and 

preventive maintenance inspection results. The data collection methods and data sources are 

discussed further in Section 3. 

2.5 SCHEDULE 

Program planning started in fall 2005. During this time, the study team secured the participation 

of the host fleet and the vendors of the sensor systems. In April 2006, the installations of the 

candidate systems began. The study team worked closely with WMATA maintenance 

technicians and system vendors to ensure that each system was installed properly and that it 

would not interfere with other systems on the bus. The installations were staggered throughout 

the summer of 2006 because of scheduling challenges with vendor representatives and 

component availability. 

The field test data collection effort covered the period from November 2006 through November 

2007. During this time, the vehicles were inspected weekly. In addition, a Performance Based 

Brake Tester (PBBT), supplied by Link Radlinski,(12)

 was used to test each bus once a month to 

verify braking system performance and operation. At the conclusion of the test phase, all testing 

hardware was removed from the vehicles. 

13
3. DATA COLLECTION 

3.1 DATA SOURCES 

Six sources of data were used to perform an assessment of the reliability, durability, and 

maintainability of the BPMSs and TPMSs under evaluation: 

 Weekly inspections. 

 Maintenance records. 

 BPMS and TPMS data. 

 PBBT data. 

 Maintenance staff interviews. 

 Study team observations. 

3.1.1 Weekly Inspections 

Buses were withheld from service once per week to inspect the sensor systems, brakes, and tires. 

WMATA technicians performed the inspections with the assistance of the study team. The study 

team developed data collection forms to assist with the inspections and capture the findings. 

While weekly inspection of the test buses was completed very consistently, bus availability and 

technician time constraints limited the amount of inspections performed on the control buses. 

The following information was recorded weekly from the test buses, (the weekly brake and tire 

data collection forms are provided in Appendix A): 

 Bus mileage. 

 Tire temperature at the time of inspection (hot or cold). 

 Ambient temperature. 

 Weather conditions. 

 Tire pressures as they appear on display. 

 Tire pressures measured with gauge. 

 Tire condition (damaged, scuffed, and so forth). 

nsors. 

Buses were driven to an area that WMATA had designated for study activities and inspected 

individually. Figure 1 shows several test buses undergoing weekly program inspections. 

 Brake monitoring display status. 

 Mounting/physical condition of se

14
Figure 1. Buses on Lifts During an Inspection 

The technician first recorded the bus’s ID number and mileage. The technician would then use 

the display/control units of each system, to record the tire pressures as well as any alarms, 

warnings, or faults generated by either the TPMS or the brake monitoring system. Once the 

system displays were checked and system status recorded, the bus was elevated approximately 

three feet to conduct the manual tire pressure measurements and the brake system inspections. 

Tire pressure in each of the bus’s tires with a checked using a manual tire pressure gauge, and 

readings were recorded on the tire data collection form (see Appendix A). While measuring tire 

pressures, the technician also visually inspected the tire for damage or unusual wear. 

It should be noted that all but two of the brake and tire monitoring system display units were 

mounted in the input/output (I/O) cabinet just behind the driver’s seat, per WMATA’s request 

The MGM e-Stroke display was also mounted directly behind the driver’s seat but outside the 

I/O cabinet (eTire’s system did not have an on-board display). This was done to purposely limit 

the availability of vehicle diagnostic information to the operator, thereby preventing the potential 

for drivers to misinterpret the information and issue unwarranted road-calls. The eTire system 

used a handheld radio frequency identification (RFID) reader to obtain the pressure readings 

from the sensor tags located in each tire. 

Next, the bus lift was raised higher so that a technician could walk under the bus to inspect the 

brake lining thickness at each brake assembly, and also measure the pushrod stroke at the brake 

chambers. To measure the brake stroke, one technician sat on the driver’s seat inside the bus 

while a second technician held a tape measure up to the brake chamber near the pushrod. The 

technician inside the bus stepped on the brake pedal, and the technician outside the bus measured 

the brake stroke travel (in inches) and recorded the information on the data collection form (see 

Appendix A). 

15
In addition to the inspection data above, the technician also recorded whether the system 

accurately detected a low-pressure warning or a malfunctioning brake, and whether the system 

reported a legitimate fault or a false alarm. 

The study team collected the completed forms from the supervisor on a weekly basis and entered 

the data into a database created specifically for the study. 

3.1.2 Maintenance Records 

WMATA, like most large fleets, maintains a database to track and monitor vehicle maintenance. 

WMATA uses the MAXIMO(13) database to track vehicle mileage, fuel consumption, and all 

maintenance activities such as brake and tire replacements. 

The MAXIMO database was used to gather a variety of information related to the test and 

control buses including: fuel usage, mileage accumulation, failures with the BPMS and TPMS, 

and maintenance actions related to brakes and tires (see Sections 4.2 and 5.3 for more details and 

data analyses results). 

3.1.3 Brake Performance and Tire Pressure Monitoring System Data 

All of the brake performance monitoring systems involved in the field test incorporated the 

capability to record various diagnostic and sensor data within the ECUs of each system. The tire 

pressure monitoring systems on the other hand did not include data recording capability. The 

WABCO IVTM system featured a J1939 connection, however the system was not integrated 

with the automatic vehicle monitoring (AVM) system on the bus because of program funding 

and time constraints. Tire Safeguard did not offer a downloading feature, and the eTire system 

did not have an onboard vehicle storage or display. 

The Brake Insight brake monitoring system was capable of storing brake alarms, warnings, and 

faults. These alarms were recorded along with a date and time stamp and a description of the 

problem. The system reported on problems with the brakes or with the system itself. For 

example, brake problem descriptions included brake overstroke, dragging brake, brake out of 

adjustment, high brake temperature, and worn shoe lining. An example of a system problem 

includes sensor out of alignment, communication failure, failed sensor, low voltage reading, and 

missing sensor. The system was capable of storing 2,700 lines of data. The data could be 

exported via a RS-232 connection located on the faceplate of the display unit. 

MGM’s eStroke brake monitoring system was also capable of recording data. The type of data 

recorded was similar to the information recorded by the Brake Insight system minus the high 

temperature and worn shoe lining information. The eStroke system was integrated into the AVM 

system on the bus and capable of wirelessly off loading data to a central server located at the bus 

garage. The server would automatically send emails to the maintenance personnel and study 

team on the status of the bus’s brakes. The work required to integrate the eStroke system with the 

AVM was completed by MGM and the AVM system manufacturer, Clever Devices, as part of 

the bus procurement contract between WMATA and Orion. 

The StrainSert system also recorded brake alarms, warnings, and faults, however, it did not 

provide a fault description. The system identified the faulty brake and provided anchor pin force 

versus time data to assist maintenance personnel with determining the fault. The system was 

16
capable of storing 100 heavy braking events. StrainSert defined heavy brake events as those 

exceeding 1,800 pounds of force on the S-cam anchor pin. The events are downloadable via an 

Ethernet cable. 

3.1.4 Performance Based Brake Testing (PBBT) Data 

The Link Radlinski Model 20200 Portable Roller Brake Tester, also known as a PBBT, was 

incorporated into the program to help evaluate the performance of the BPMS and to assess the 

benefit of off-board brake systems relative to onboard systems. PBBTs are well regarded in the 

industry for their ability to detecting brake deficiencies and are used in manufacturing plant, fleet 

maintenance garages, and vehicle inspection facilities. A detailed description of the PBBT used 

for this study can be found in Section 5.1.4. 

Buses were tested on the PBBT roughly once per month. The baseline brake testing of the 24 

buses, performed in October 2006, took several days and was spread over a 3-week period. 

WMATA and the study team became more efficient in coordinating the logistics involved in 

testing the program buses over time, ultimately being able to test 24 buses in a single day. 

The results obtained from PBBT testing were analyzed and compared with the data collected by 

the sensors from each bus. This data was used for evaluating the durability and accuracy of the 

systems. It was also used for assessing whether the current maintenance intervals are appropriate 

or if new maintenance practices could reduce operating costs (See Section 5.2). 

3.1.5 Maintenance Staff Interviews 

The maintenance technicians and supervisors were interviewed informally throughout the test 

period and formally at test completion. The intent of the interviews was to gather information 

about their experiences with maintaining the systems, the ease of the data presentation, and the 

usefulness of the information (See Sections 4.2.7 and, 5.2.6 for more details and data analyses 

results). 

3.1.6 Study Team Observations 

The study team worked closely with WMATA maintenance technicians to accurately document 

and diagnose malfunctions and failures related to the BPMSs and TPMSs under evaluation. The 

study team recorded in a journal the issues related to the test systems throughout the fleet test. 

The journal was a valuable resource for analyzing the durability, reliability, and maintainability 

of the brake and tire sensor systems. This analysis is discussed further in Section 4.2 and 5.3. 

17
3.2 META DATA 

This section provides general information on the data collected during the field test, including 

where the buses were operated, total miles traveled, and total maintenance actions performed. 

Specifically, the section provides information on the following: 

 Vehicle operation, including vehicle miles traveled. 

 Total data forms collected. 

 Total maintenance actions recorded. 

 Total equipment failures. 

The test buses operated in an area covering approximately 300 square miles south and west of 

Washington, DC (see Figure 2). The majority of miles were accumulated in an urban 

environment with minimal highway driving. The buses averaged 16 miles per hour in revenue 

service and approximately 129 miles per day. The test fleet accumulated 762,580 miles during 

the course of the field test. 

Figure 2. Operating Area from Four-Mile Run Maintenance Garage 

5 mi

4-Mile Run 

Garage

Data was collected approximately once a week during the data collection phase of the program, 

resulting in a total of 1,210 tire and brake data forms—619 for tire data and 591 for brake data. 

In addition to the information collected on the weekly forms, the control and test vehicles were 

tested on the PBBT about once per month, for a total of 200 PBBT records. 

Maintenance records were gathered for all brake and tire maintenance inspections from 

November 2006 to November 2007. A total of 248 maintenance records were identified during 

this period. Of the maintenance actions identified, 124 were tire-related items. Of the tire-related 

18
incidents, 24 records were generated for changing tires based on accumulated mileage, and 12 

records were generated for replacing tires that had been penetrated by a foreign object. The 

remaining actions, 124 items, were brake related. As part of the brake actions, 25 brake 

overhauls were completed. 

During the test, 10 road calls required support equipment to respond. Nine of the road calls were 

tire related; two of those were because of extensive tire damage and are discussed in more detail 

in Section 4. The tenth road call was in response to accident damage. A refuse truck hit bus 

number 2504 at the front-right corner. The accident was unrelated to the field test equipment 

under evaluation and was instead the result of refuse truck driver error. 

Sections 4 and 5 provide detailed results on the TPMSs and BPMSs evaluated under this field 

test. 

19
4. TIRE PRESSURE MONITORING SYSTEM 

FIELD TEST RESULTS 

4.1 TPMS TEST ARTICLE DESCRIPTIONS AND INSTALLATION 

Installation of the three TPMSs equipment began in April 2006. All installations were performed 

at WMATA’s Four-Mile Run bus maintenance facility in Arlington, VA. The study team worked 

closely with WMATA maintenance technicians and system vendors to ensure that each system 

was installed properly on the buses and that it would not interfere with other systems on the bus. 

The installations were staggered throughout summer 2006 because of scheduling challenges with 

vendor representatives and component availability. The IVTM equipment was the first system 

installed. The eTire installation occurred in June 2006, and the Tire-SafeGuard installation 

occurred in August 2006. The display units for the IVTM and Tire-SafeGuard systems were 

installed inside the I/O cabinet located behind the driver’s seat and out of view of the driver (The 

eTire system was not designed to interact with the driver, and does not include an onboard 

display unit). 

4.1.1 WABCO IVTM 

4.1.1.1 General Description 

WABCO’s IVTM system,(9)

 developed in conjunction with Michelin, was launched in the CMV 

industry in 2003. The IVTM system consists of the following three main components, which are 

shown in Figure 3: 

 Tire pressure sensor wheel module (left). 

 Display unit (center). 

 ECU (right). 

Figure 3. WABCO IVTM Components 

Each tire and wheel assembly is equipped with a sensor unit which contains a pressure 

transducer and a transmitter. Power for the sensor unit is supplied from a built-in lithium battery 

with a 5-year service life. The sensor unit is secured to the wheel rim at a location adjacent to the 

valve stem using the two closest lug nuts. A pneumatic hose runs between the sensor and valve 

stem (Figure 4). 

20
Figure 4. WABCO IVTM Sensor Unit Mounting 

The valve-stem mounted sensors continually check tire pressure and transmit a pressure reading 

to an ECU (i.e., a receiver/processor) via an RF signal every 15 minutes. The ECU is generally 

mounted on the tractor (or vehicle) chassis halfway between the front and rear axles. It contains a 

built-in antenna to receive the pressure data from the tire pressure sensor units. If the sensor 

detects a loss of pressure (below a pre-determined level), the system increases the transmission 

frequency to once every 30 seconds. The radio frequency signal is 433 MHz. The ECU is 

hardwired into the vehicle’s 24V power supply circuit. 

The centralized location of the ECU among the tires being monitored (roughly equidistant 

between the 4 wheel locations on the tractor) eliminates the need for additional antennas housed 

within the sensors. If the vehicle is equipped with a trailer, a second (separate) ECU is located at 

a centralized point between the tires on the trailer chassis and is used to collect inflation pressure 

data from the trailer tires. This data is then re-transmitted to the “main” ECU on the tractor. 

The main (tractor-based) ECU is hardwired to a display unit which is designed to sit on the 

dashboard, (although it could be located elsewhere—and in fact was located out of view of the 

bus driver as noted earlier). 

The dashboard-mounted display unit warns the driver visually and acoustically of low tire 

pressures. A yellow lamp indicates a slow rate of pressure loss or slight decrease in pressure; a 

red lamp indicates extremely low pressure. The yellow and red lights are located at the lower 

edge of the display. Using an on-screen menu, the position of any instrumented tire can be 

selected by the driver and its inflation pressure can be queried. If all tires are within an 

appropriate inflation pressure range, the display is blank. If any of the tires have low pressure, 

the display screen shows the location of the tire and its pressure. Figure 5 shows (for example) a 

low pressure reading of 5.9 bar (85 psi) on the right-rear tag axle tire. 

21
Figure 5. WABCO IVTM System 

Tire 

location 

indicator 

4.1.1.2 Installation 

The WABCO IVTM TPMS was installed on bus numbers 2501, 2502, 2503, and 2504. WABCO 

provided a service engineer to lead the installation. 

The buses used in this field test had six tires—single tires on the steer axle and dual tires on the 

drive axle—thus requiring a total of six sensor units to be installed. 

To install the sensor unit, the pneumatic hose should first be connected to the sensor module and 

then connected to the tire’s valve stem. With the hose already connected to the valve stem, the 

sensor unit can be slipped over the wheel lugs and secured in place with the cap nuts. Attaching 

the sensor unit to the valve stem with the short hose before securing it to the wheel proved to be 

the quickest way to mount the wheel module to the steer tires. Figure 6 illustrates the wheel 

module installation process on a steer tire. 

Figure 6. Wheel Module Installation on a Steer Tire 

Step 1: Attach 

hose to module

Step 2: Secure 

module to wheel 

with cap nuts

On the rear wheels, the procedure varies slightly from that used on the front wheels. Connecting 

the hose to the inner tire posed some challenges on the rear wheels; on transit buses, these tires 

are set back inside a wheel well, which limits access to the inner dual tire. Also, the gap between 

the dual tires is insufficient to place a hand between them to connect the hose to the inner tire’s 

22
valve stem. (Note: WMATA did not use valve-stem extensions on its buses’ inner dual tires, 

which would have simplified the wheel module installation on the rear axle.) 

To create access to the inner tire’s valve stem, WABCO recommends loosening all 10 cap nuts 

on the outer rear wheel and removing only the two cap nuts nearest the valve stem on each tire. 

Loosening all of the cap nuts allowed the outer wheel to separate from the inner wheel. This 

creates a gap wide enough to reach a hand in-between the inner and outer rear tires without 

having to remove a wheel from the bus. Figure 7 shows the tight gap between dual tires and the 

cap nuts being loosened and removed from the rear wheel lugs. 

Figure 7. Wheel Gap and Cap Nut Removal on a Rear Wheel 

Notice in Figure 7 that the engineer removed the cap nuts from the 3 o’clock and the 9 o’clock 

positions on the rear wheel. The valve stems on this dual tire assembly are positioned 180 

degrees out of phase with each other. It is necessary to mount the wheel modules 180 degrees out 

of phase to avoid affecting the balance of the tires. 

The IVTM ECU and display were both installed inside the bus’s passenger compartment and 

hidden from view of the driver and passengers. Figure 8 illustrates the mounting locations for the 

ECU and the display unit. The ECU was mounted to a support beam above the lighting track on 

the street side of the bus. The ECU mounting location is hidden behind an access panel. It was 

important to install the system’s ECU in a location near the center of the bus to enable the best 

possible signal reception broadcast from the sensor modules at the wheel ends. The ECU was 

electrically protected by a 5-ampere circuit breaker and hardwired to a terminal strip in the I/O 

cabinet located behind the driver’s seat. 

23
Figure 8. WABCO ECU and Display Unit Locations 

After all of the IVTM hardware components were installed, the system was programmed in 

accordance with the manufacturer’s instructions. The IVTM ECU was equipped with a 

diagnostic data port that could be accessed with a laptop, so that an engineer could communicate 

with the ECU via WABCO’s IVTM diagnostic software. Figure 9 depicts the main diagnostic 

screen. 

The diagnostic screen shows tire pressure readings and system messages. Each wheel module 

was hard-coded with a unique ID number which represented a specific wheel positions on the 

bus. The screen is currently indicating low tire pressure readings for the curbside rear dual tires, 

(red colored icons). 

The WABCO IVTM installation required 2-1/2 days to equip all four buses with the system. 

(Note: The WABCO IVTM system typically can be installed on a six-wheeled vehicle in 1.5 

hours by a trained professional.) The WABCO field engineer, a study team engineer, and a 

WMATA maintenance technician performed the installations. 

24
Figure 9. IVTM Diagnostic Software Screenshot 

4.1.2 HCI Tire-SafeGuard 

4.1.2.1 General Description 

HCI Corporation(10) manufactures the Tire-SafeGuard TPMS which consists of the following 

components: 

 Wheel module (the tire condition sensing unit that is affixed to the wheel hub.) 

 Relay antennas (which relay RF signals from the wheel modules to the ECU). 

 ECU (which processes data transmitted from the wheel modules). 

 Driver display unit (which displays tire condition information to the driver). 

Figure 10 shows two wheel modules (top left), a wheel module secured to a wheel by a hose 

clamp (top right), the ECU and display unit (bottom left), and one of three antennas (bottom 

right). 

25
Figure 10. HCI Tire-SafeGuard Components 

The wheel module includes an integrated pressure-temperature transducer, an RF transmitter, 

and internal battery (with an estimated life of more than 5 years) to supply power for the unit. 

The wheel module (or sensor unit) is strapped to the hub of the wheel inside the tire. The 

sensor’s transmitter automatically switches on when the vehicle is moving faster than 15 miles 

per hour. It transmits RF signals (which include pressure, temperature, and wheel position data) 

to a nearby antenna unit, which is hardwired (using coaxial cable) to the system’s ECU/receiver 

unit. The data is processed by the ECU and the tire condition information (and/or warnings as 

appropriate) are then sent to the driver’s display unit. 

The system monitors tire pressure continuously even when the vehicle is parked. The system 

alerts drivers of low-pressure situations by providing the location, temperature, and pressure 

readings via display icons and an audible signal. Figure 11 shows the wheel-mounted sensor 

(right) and driver’s display (left). The display pictured is showing a right-front tire pressure of 

112 psi and temperature of 89 degrees Fahrenheit (F). The pressure warning threshold is 

adjustable by the user and can be set from 18 to 130 psi. The display can be connected to any 12 

volts DC (VDC) power source on the vehicle. It is 3-1/2 inches by 1-1/8 inches by 5/8 inches in 

size and weighs 1.5 ounces. The system has been on the market since 1991. 

26
Figure 11. HCI Tire-SafeGuard TPMS 

Shows the tire position 

corresponding to current 

readings being displayed. 

Valve stem port

4.1.2.2 Installation 

The Tire-SafeGuard TPMS was installed on bus numbers 2505, 2506, 2507, and 2508. 

A study team engineer and a WMATA maintenance technician installed the Tire-SafeGuard 

system by following the instruction manual provided with each system. A wheel module sensor 

was installed on every wheel of the bus. With the tire removed from the wheel, the sensor is 

strapped to the wheel at the lowest point of the drop center well using a hose clamp. Once the 

sensor is securely strapped to the wheel, the tire is remounted to the wheel and inflated. The top 

right photo in shows a tire pressure sensor strapped to a wheel. Notice that the wheel sensor is 

mounted next to the wheel’s valve stem port. The choice of the mounting location near the valve 

stem port helps prevent damage to the sensor unit when changing the tire. During previous test 

track work at TRC, it was discovered that the paddle arm on tire-changing machines frequently 

contacted and damaged sensors. With the wheel sensor mounted near the valve-stem port on 

each wheel, the tire changer knows the approximate location of the sensor inside of the tire and 

can avoid contacting it with the paddle arm. 

The Tire-SafeGuard system requires three antennas—one at the front of the bus (to pick up RF 

signals from the steer tires) and one near each of the left and right rear dual tire assemblies. 

Figure 12 shows the two styles of antennas that were installed. These antennas are connected to 

the ECU via coaxial cable. 

Figure 12. HCI Tire-SafeGuard Antennas 

27
Once all of the hardware was installed on the bus, the system was programmed and calibrated 

using the display unit. The display unit has two buttons that can be pressed and held to activate 

the system’s setup modes. The engineer performing the installation first entered the “warning 

threshold setup” and set the warning threshold tire pressure at 103 psi (~10 percent below the 

115 psi set tire pressure). The engineer then entered the manual setup screen of the “tire setup 

select mode.” The tire setup select mode allowed the engineer to “train” the tire pressure sensors 

to specific wheel locations. While in manual mode, the engineer trained one sensor at a time to a 

specific wheel position, beginning with the right steer tire. The engineer released 3 pounds of 

pressure from the right steer tire until the display screen indicated that the system recognized the 

sensor located in the right steer tire. The engineer then repeated this process on the remaining 

tires in the following order, as specified by the operator’s manual—right rear outer tire, right rear 

inner tire, left rear inner tire, left rear outer tire, and left steer tire. 

The Tire-SafeGuard installation required 2-1/2 days to equip four buses with the system. A study 

team engineer and a WMATA maintenance technician performed the installation. 

4.1.3 Michelin eTire 

4.1.3.1 General Description 

Michelin North America(11)

 introduced its eTire system for commercial vehicles in October 

2002. The eTire system consists of the following components: 

 Tire condition sensor unit (an integrated transducer unit with multiple sensors, unique 

ID code, and RF transmitter). 

 Tire sensor unit docking fixture (SensorDock). 

 eTire location label. 

 Vehicle ID tag (similar in design to the tire condition sensor unit, but does not 

incorporate any sensors). 

 Portable handheld tag reader (a fixed position gate reader system can also be used). 

 BIBTRACK (an internet based software application, hosted by Michelin, for assisting 

fleets with tire maintenance and tracking needs). 

eTire Sensor Unit. The “heart” of the system is the eTire sensor. This sensor unit contains 

pressure and temperature sensors (pressure readings are temperature compensated), an RF 

transmitter, and an antenna—all encased in impact- and heat-resistant plastic. The unit measures 

4 inches by 1.5 inches and weighs less than an ounce (see Figure 13). 

Figure 13. eTire Sensor Unit 

28
The eTire sensor unit inside the tire is differentiated from the HCI or WABCO units in that it 

does not require a battery to supply power for transmitting the tire condition data. Rather, the 

sensor unit receives just enough power via RF signals from the reader so that it can reflect a 

signal back to the reader containing tire condition data. Specifically, the eTire sensor reports the 

current tire temperature, pressure, and a factory programmed sensor ID which can be correlated 

to a particular tire position. 

SensorDock. The sensor unit attaches to the truck tire inner sidewall using a docking type station 

called the SensorDock, (Figure 14, left). The SensorDock, which itself is molded rubber, 

attached to the inside sidewall through a chemical curing process similar to the process used to 

repair tires. The eTire kit includes a variety of tools and adhesives to attach the SensorDock to 

the tire, (Figure 14, right). 

Figure 14. eTire SensorDock and Installation Kit 

To install the sensor, the unit is slipped over a knob on the rubber dock and locked into place. 

Once installed, the sensors remain attached to the tire throughout the entire life of the tire 

including retread processes. At the end of the tire’s useful life, the sensor can be removed and 

reattached to a new truck tire with a new SensorDock. 

eTire Location Label. The eTire sensors are read (or queried) using external transceivers (see 

next section). The handheld reader requires the operator to wave the device over the exact 

location of the sensor. Michelin has made finding the location of the sensor easier by providing 

an eTire label that is installed on the outside of the tire at the location of the eTire sensor, (see 

Figure 15) 

Figure 15. eTire Location Label 

29
Vehicle ID Tag. The eTire system also includes a RFID vehicle ID tag which is similar in design 

to a tire sensor unit expect that it does not contain any sensors. The vehicle ID tag is coded with 

a unique identifier that is assigned to a particular vehicle. It can be mounted in a variety of 

locations, but on the test buses, was mounted inside an access panel on the front of the bus. 

Tag Readers. Two options are available to read the sensors—a handheld reader, or a gate reader. 

The handheld sensor tag reader uses RFID technology to read and record the tire pressure in each 

tire (see Figure 16). 

Figure 16. eTire Handheld Tag Reader 

In order to read and store tire pressures using the handheld reader, the handheld reader first must 

be “waved” passed the vehicle ID tag located at the front of the bus. The vehicle ID tag enables 

the eTire system to associate individual tires to a particular vehicle. Once the handheld reader 

has identified the vehicle, the reader is then used to detect tire pressures and is waved passed the 

eTire location labels on each tire. The reader must be waved as close to the surface of the tire as 

possible until a sensor is detected and its associated tire pressure is displayed on the screen. A 

technician could use the handheld reader to record tire condition data from multiple vehicles. 

The tire condition data (sensor ID, pressure) and associated vehicle ID is then downloaded from 

the handheld reader to a laptop computer via a printer cable connected to a docking station. 

Michelin supplied the necessary software with the eTire system to install and run the 

BIBTRACKTM application, a software program which allows tire condition data to be transferred 

from the handheld reader to the BIBTRACK online database. 

The eTire sensor units affixed to each tire can also be read using a gate reader in a more time 

efficient manner—and indeed this method is preferred if a location and other operational 

logistics support such an arrangement. A gate reader, as installed at a refuse facility near 

Indianapolis, IN, is shown in Figure 17. 

30
Figure 17. eTire Gate Reader 

The gate reader requires the vehicle to be traveling less than 5 miles per hour to receive the 

information from all of the tires, including the inside duals. The gate reader is equipped with 

wired and wireless options for connecting to the internet. If the gate reader is used, it can be 

connected directly to the BIBTRACK application and data is automatically transferred from the 

gate reader to the BIBTRACK system. 

BIBTRACK Software Application. The eTire system is not designed to provide tire condition 

data directly to drivers, and in fact does not include a driver’s display option. Rather, the system 

is designed to collect tire condition data in a comprehensive fashion from vehicles in a fleet, and 

then make the data available for viewing and analysis on a Michelin hosted website application. 

BIBTRACK is the Michelin website that tracks tires and provides recommended actions for 

problem tires. The website stores up-to-date information on a fleet’s tires, tracks tire costs, and 

monitors inventories. The screenshot in Figure 18 shows the “Mounted Tire Report” for bus 

number 2509 from the BIBTRACK system. This report provides the user with a snapshot of tire 

data, which includes sensor/tire position, the date the tire was mounted, the date of the last tire 

pressure scan, tire mileage, tire pressures, and the number of low-pressure readings. 

31
Figure 18. BIBTRACK Screenshot 

Access to BIBTRACK is included with the purchase of the eTire system. A fleet manager must 

create a username and password to access his fleet’s tire data in BIBTRACK—this only takes a 

few minutes to set up. Once logged in, the fleet manager can access tire reports and modify his 

fleet’s profile (number of vehicles, tire type, and tire inventory). In addition to BIBTRACK’s 

secure website, the handheld and gate readers have ID numbers that are assigned to a particular 

facility which ensures that fleet managers can only view data from tire sensors scanned at their 

own facilities. 

4.1.3.2 Installation 

The Michelin eTire system was installed on WMATA bus numbers 2509, 2510, 2511, and 2512. 

Michelin provided a sales engineer and a service engineer to lead the installation. 

The eTire installation at WMATA’s Four-Mile Run bus maintenance facility began with 

training the study team and technicians on installing a sensor dock to the inside sidewall of a 

tire. The procedure includes scraping the inside wall of the tire to remove the ridges left behind 

by the tire mold process, buffing the inside wall smooth using a grinder, adhering the sensor 

dock to the tire wall using vulcanized paste, and installing the sensor onto the sensor dock 

using a soapy-water mix to reduce friction between the two surfaces. The Michelin engineer 

demonstrated the installation procedure on one tire and then observed the tire technician and 

the study team engineer as they took turns installing the sensor dock on other tires. Figure 19

shows the tire technician smoothing a small area of the inside sidewall of the tire (left), 

applying vulcanizing paste to the sensor dock (middle), and installing the eTire sensor over the 

sensor dock (right). The sensors were installed near the valve stem to make them easier to 

locate with the handheld reader. 

32
Figure 19. Michelin eTire Sensor Dock Installation Process 

After the initial sensor dock installation training, the WMATA tire technician installed sensor 

docks and sensors in new tires that would eventually be mounted on the test buses. The sensors 

were installed in 24 new tires in a little more than 8 hours. The sensor-equipped tires were 

marked with chalk to identify them, and they were set aside in the garage’s tire bay until they 

could be mounted onto the test buses. 

Following the eTire sensor installation training, the Michelin engineers trained the study team 

engineer to use the handheld reader and BIBTRACK software. 

After all of the eTire-equipped tires were installed on the test buses, the study team engineer 

used the handheld reader to scan each tire into inventory and to assign the tire to a specific wheel 

position on the bus. This data was then uploaded to BIBTRACK. 

Because the study focused on a small number of buses, and because of physical space limitations 

at WMATA, the installation of a drive-by gate reader was not practical. To this extent, the eTire 

system was not used to its full potential. According to Michelin, much of the cost savings 

potential of the eTire system is gained by eliminating the driver or maintenance technician labor 

hours needed to physically check tire pressures. 

4.1.4 Control Vehicles 

To facilitate the evaluation of the TPMSs, data related to tire condition, maintenance and cost 

was also monitored for a group of buses that did not utilize any type of tire monitoring system 

(i.e., standard buses). These buses were the same make and model as the test buses and included 

bus numbers: 2513–2524. The buses were operated out of the Four Mile Run facility, and used in 

a similar service environment as the test fleet. 

4.2 TEST RESULTS 

Data was collected from the test fleet and control vehicles over a 14-month period, from 

September 2006 to November 2007. During the first months, the vehicles and their newly 

installed equipment were validated and confirmed to be working properly (this period essentially 

represented a “shakedown” period for the equipment as well as for data collection procedures). 

Full data collection began in November 2006. Data was collected approximately once a week 

during the test period. Throughout the course of the test, a total of 619 tire data forms were 

collected. Data collected for both the test and control buses included tire-related maintenance 

records and manually measured tire pressures. The data collected from the test buses also 

33
included TPMS measurements of inflation pressures, system display messages, system data 

downloads, and visual tire inspections. 

A total of 124 tire-related maintenance records were generated from the 24 test and control buses 

during the field test. To gain additional insight regarding “normal” tire related maintenance and 

cost, a total of 185 tire maintenance records were gathered and analyzed for the 24 test and 

control buses from March 2006 through January 2008 and provide a comparison of pre-test, test, 

and post test tire-related maintenance records. Sixty of the 124 records were generated for tires 

changed due to normal tire wear because of accumulated mileage, and 22 records were generated 

for tires changed due to penetration by a foreign object. A total of 257 TPMS sensor issues, 

which were not tire issues, were recorded during the test period. The issues were categorized as: 

missing sensor, no sensor output, disconnected sensor, equipment failure, and incorrect 

installation. Many of the TPMS sensor issues were addressed and resolved by the TPMS 

manufacturers and are discussed in Sections 4.2.3 and 4.2.4. 

Detailed test results are presented in this section and are organized as follows: 

 Performance. 

 Reliability. 

 Maintainability. 

 Durability. 

 Functionality. 

 Maintenance analysis. 

 Technician interviews. 

4.2.1 System Accuracy 

The accuracy of TPMSs is defined as the ability of the system to provide a true assessment of the 

tire pressure as compared to a manually gauged assessment. 

To evaluate the performance (or accuracy) of the TPMSs, manual tire pressure measurement 

readings were taken once a week on each bus and then compared with pressures as reported by 

the TPMSs at the time of the manual measurement. Each bus was inspected approximately 56 

times throughout the course of the test, resulting in a total of 3,714 tires inspected. Occasionally, 

certain buses were unavailable for a weekly inspection because a test bus was mistakenly put 

into service or a bus was removed from service and awaiting maintenance. Not all of the 

inspections yielded valid comparisons because of problems with the TPMSs themselves, or in a 

few instances, because of problems associated with manually checking the tires. 

Of the total inspections completed, 77 percent provided valid numeric tire pressure readings for 

both the manual and the system readings. Table 3 shows the overall results of weekly tire 

pressure inspections in terms of valid readings as well as reasons for invalid manual or system 

tire pressure readings. 

34
Table 3. Comparison of Tire Pressure Data Points 

Data Points Validity N % 

Successfully Recorded Valid 2,850 76.7% 

TPMS Problem Invalid 654 17.6% 

Low Pressure, No Numeric Value Invalid 71 1.9% 

Tire/ Valve stem issues Invalid 139 3.8% 

Total Data Points All 3,714 100.0% 

The sensors themselves presented the largest challenge in recording tire pressure readings. In 

17.6 percent of the inspections, the sensors could not be read because they failed, were missing, 

or were mounted to tire positions other than those to which they were assigned. The issue of 

sensor failures was addressed early in the fleet test, but missing and improperly assigned sensors 

continued to be an issue throughout the test period. In 1.9 percent of the inspections, the system 

recorded a “Low” pressure reading, which by design of a particular display unit prevents the 

measured pressure reading from being displayed. 

The final 3.8 percent of tire inspection “problems” were due to a flat tire, slow leak, or valve 

stem related issue. These valve stem issues include problems with the valve stem itself, access to 

the valve stem, and human error. A majority of the valve stem related issues were noted with the 

inner dual tire on the rear axle. The data collection forms stated the valve stem was “out of 

reach,” and a tire pressure measurement could not be obtained. The reason for this was traced to 

a particular stick pressure gauge that one technician used to measure tire inflation pressures. This 

gauge had an angled chuck that did not line up properly with the valve stem on the rear inner 

tires. The angled chuck pressure gauge was replaced with a straight foot chuck pressure gauge 

that included a 30-degree reverse angle, so that the inflation pressure could be measured at all 

tire positions. 

Table 4 presents the results of the manually checked tire pressure readings. A total of 3,534 

manual tire pressure readings were obtained (Note: 41 manual tire pressure readings were left 

blank on some inspection forms, however, those data points were invalid due to TPMS problem). 

As shown, 80 percent of tires measured were within 5 psi of, or greater than, the 115 psi target 

inflation pressure. 

Table 4. Manually Gauged Tire Pressure Inflation Results 

from Weekly Inspections 

Tire Pressure (psi) 

# of 

Measurements

Percent 

(%) 

>115 1,342 38% 

115–110 1,496 42% 

109–105 517 15% 

104–100 136 3% 

99–95 19 1% 

< 95 24 1% 

Total # of Valid Measurements 3,534 100% 

35
Table 5 compares the tire pressure readings from the TPMSs with the manually measured 

readings. Technicians recorded manual tire pressure readings using standard-issue pressure 

gauges. Although these were high-quality gauges, they were not specifically calibrated for the 

purposes of this field test. The gauges were capable of reading pressures from 10 to 160 psi in 2 

psi increments. As shown in Table 5, about 91 percent of the time, the physically measured and 

sensor system readings were within 5 psi of each other—and 97 percent of the time, they were 

within 10 psi. 

Table 5. Comparison of Tire Pressure Reading: 

TPMS versus Manually Gauged Readings 

Tire Pressure Difference Number 

Percent 

of Total 

No difference 367 12.9% 

< 5 psi 2,230 78.2% 

> 5 psi 253 8.9% 

> 10 psi 87 - 

> 15 psi 45 - 

> 20 psi 36 - 

> 25 psi 29 - 

> 100 psi 11 - 

Total # of Measurements 3,058 100.0% 

Table 6 presents the tire pressure differential data for each TPMS participating in the field study. 

As shown, the percentage of readings showing “no difference” between the manually measured 

and the TPMS measured pressures ranged from 9.1 percent (eTire) to 16.9 percent (WABCO). 

The TPMSs were within 5 psi of the manually measured readings: 94.5 percent of the time 

(WABCO); 87 percent of the time (Tire SafeGuard); and 93 percent of the time (eTire). 

Table 6. Tire Pressure Differential by System, Raw Data 

Pressure Difference Total 

# 

Total

% 

WABCO 

IVTM 

# 

WABCO 

IVTM 

% 

TireSafe 

Guard

# 

Tire 

Safe 

Guard 

% 

eTire

# 

eTire

% 

No difference 367 12.9% 175 16.9% 132 11.4% 60 9.1% 

< 5 psi 2,230 78.2% 803 77.6% 874 75.6% 553 83.9% 

Within 5 psi 91.1% 94.5% 87% 93% 

Between 5 psi and 10 psi 166 5.8% 32 3.1% 90 7.8% 44 6.7% 

Within 10 psi 96.9% 97.6% 94.8% 99.7% 

Between 10 psi and 15 psi 42 1.5% 11 1.1% 30 2.6% 1 0.2% 

Between 15 psi and 20 psi 9 0.3% 3 0.3% 6 0.5% 0 0.0% 

Between 20 psi and 25 psi 7 0.2% 3 0.3% 4 0.3% 0 0.0% 

Between 25 psi and 100 psi 18 0.6% 6 0.6% 12 1.0% 0 0.0% 

> 100 psi 11 0.4% 2 0.2% 8 0.7% 1 0.2% 

To assess the ability of TPMS improve average tire inflation pressures (and thus reduce tire wear 

and improve fuel economy), the tire inflation data collected from the test buses was compared to 

36
the data collected from the control buses. It should be noted that the bus maintenance facility had 

limited resources and only assigned two technicians to inspect buses during one shift each week, 

which limited the number of buses that were inspected. A priority was placed on inspecting all 

12 of the test buses and as many of the control buses as possible in the time allotted. Control fleet 

tire pressure data was augmented with data collected and recorded by the Goodyear 

representative. Table 7 shows the average tire pressures between the test and control fleets. 

Table 7. Average Tire Inflation Pressure 

Fleet Front 

Left 

Front 

Right 

Rear 

Inside 

Left 

Rear 

Inside 

Right 

Rear 

Outside 

Left 

Rear 

Outside 

Right 

Test Fleet 112.8 112.9 111.7 111.4 110.1 110.2 

Control Fleet 112.0 111.3 111.3 111.1 110.0 111.7 

As shown in Table 7, the comparison of inflation pressures for the test fleet and the control fleet 

indicated a minimal difference between them. The overall averages for the test and control fleets 

were 111.5 psi and 111.2 psi, respectively. The study team found that the similarity of inflation 

pressures between the two fleets was influenced by WMATA’s active role in maintaining tire 

pressure as part of its tire contract with Goodyear. Goodyear is under contract to measure the 

inflation pressure of each tire once per month and add air when the measured pressure is below 

111 psi (at the time the field test began, WMATA technicians were responsible for maintaining 

tire inflation levels). Another factor influencing test results is the location of the system displays. 

The TPMS display was not visible to the driver, so corrective action was not taken daily as part 

of a pre-trip inspection as would be the case with a typical for-hire trucking fleet. 

Because differences in average tire inflation pressures between the test and control fleets were 

minimal, and since the test and control fleets operate similar routes, differences in fuel economy 

between the fleets were also minimal. As shown in Table 8, the fuel economy of the control fleet 

was slightly better than the test fleet prior to initiating the field test, and this relationship 

remained stable throughout the field test (test fleet fuel economy did not improve). For reasons 

unrelated to tire pressures, both the test and control fleets experienced a slight increase in fuel 

economy during the field test period. 

Table 8. Fleet Fuel Economy 

Test 

Fleet 

Period 

Test 

Fleet 

Mileage 

Test Fleet 

Fuel 

Consumption 

(diesel equivalent)

Test 

Fleet 

MPG 

Control 

Fleet 

Period 

Control 

Fleet 

Mileage 

Control Fleet 

Fuel 

Consumption 

(diesel equivalent)

Control 

Fleet 

MPG 

Pre-Test 343,865 110,223 3.12 Pre-Test 342,751 107,773 3.18 

Test 375,939 111,761 3.36 Test 391,041 111,604 3.50 

Total 719,804 221,984 3.24 Total 733,792 219,377 3.34 

4.2.2 Reliability 

For fleet operators, technicians, and supervisors to rely on TPMSs for tire inflation maintenance, 

they must have confidence in the system and the information it presents. In short, the information 

presented by the display units must be reliable, and system reliability is essential for encouraging 

37
the use of TPMS equipment in commercial vehicles. For TPMSs, this means that the system 

correctly and consistently reports whenever a tire is under (or over) inflated; and that the system 

does not report an over/under inflation problem when in fact inflation pressure is within targeted 

limits. If the TPMS does not detect/report an improper inflation condition (when such a 

condition does indeed exist), it is termed a “false negative.” If the TPMS incorrectly reports a tire 

inflation problem when none exist then it is called a “false positive.” 

The reliability of the TPMSs was determined by comparing the information presented on 

driver/operator displays (i.e., warnings, or lack thereof) to the measured (manually gauged) tire 

pressures. A TPMS display status was identified as valid (or correct) if: 

 The TPMS did not display an alert for a low-pressure condition and the manual tire 

pressure reading showed actual pressure to indeed be within 10 percent of target (target 

pressure was 115 psi); or, 

 The system did warn the operator of a low-tire condition and the manual reading did 

indeed show that the tires were under-inflated by 10 percent or more. 

Alternatively, invalid (or incorrect) display status readings were divided into three categories: 

 False Positive. The TPMS reported a low tire pressure (≤ 103 psi), when the actual 

condition observed was a normal tire pressure (> 103 psi). 

 False Negative. The TPMS reported a normal tire pressure (> 103 psi), when the 

actual condition observed was a low tire pressure (≤ 103 psi). 

 No Read. The TPMS reported no reading of tire pressure. The cause of the condition 

could be related to issues such as a missing sensor, an improper sensor threshold 

setting, or a special condition (e.g., flat tire). 

Excessive false-positive readings may cause the user to discount true (or real) tire warnings. A 

false-negative reading results in an undetected unsafe condition, which can lead to loss of vehicle 

control, damage, and injury. Similar to false positives, excessive no-read conditions can diminish 

confidence in the performance of the TPMS. 

An analysis of all three system in total showed that the TPMS displays were valid 75 percent of 

the time (i.e., for 75 percent of the inspections, the display status matched actual conditions). As 

shown in Table 9, no-read conditions (17 percent) accounted for the largest percentage of invalid 

display status readings. False-positive and false-negative conditions accounted for less than 10 

percent of the total invalid display status readings. 

Table 9. Incidences of Invalid Display Reading 

Type of Reading Data Points Percentage 

False Positive 227 6% 

False Negative 61 2% 

No Read 635 17% 

Total 953 25% 

38
System reliability varied considerably among the three TPMSs under test. An assessment of the 

individual systems found the Tire-SafeGuard system had the most reliable performance, with a 

valid display status reading in 90.9 percent of the tire inspections, as shown in Table 10. The 

poorest performing system, eTire, provided valid tire pressure display status in about 53% of the 

tire inspections. 

Table 10. Overall TPMS Reliability Results 

System Valid Invalid 

WABCO IVTM 82.0% 18.0% 

Tire-SafeGuard 90.9% 9.1% 

eTire 53.2% 46.8% 

The eTire system was found to have an extensive number of no-read conditions during the tire 

inspections. As shown in Table 11, the eTire system could not report the tire pressure in 44 

percent of the tire inspections. The majority of these failures were traced to a manufacturing 

defect, with the sensor causing the casing to be brittle and fail prematurely upon impact. The 

failure conditions are further explained in Section 4.2.4. 

Table 11. TPMS Reliability by System Type 

System 

False 

Positive 

False 

Negative No Read 

Total 

Percent 

WABCO IVTM 10.4% 1.5% 6.1% 18.0% 

Tire-SafeGuard 6.8% 2.3% 0.0% 9.1% 

eTire 1.3% 1.1% 44.4% 46.8% 

In Figure 20, the distribution of valid and invalid tire assessments is shown for each system. 

Discounting the high failure rate of the eTire equipment, the largest contributor to invalid display 

status readings for each system was the presence of false-positive conditions. 

Another measure of reliability of TPMSs was whether the sensor systems were deemed to be 

“operational” during visual inspections and system checks by WMATA technicians and/or study 

team staff. The status of the sensor was identified as “operational” if a tire pressure reading was 

obtained at the control unit. If a sensor could not read the tire pressure, it was deemed “nonoperational.” A sensor may be non-operational for several reasons, including communication 

failures, disconnected components, missing components, and broken components. An analysis of 

the entire field test showed the sensors were operational during 83.6 percent of the visual 

inspections. 

39
Figure 20. Individual System Reliability Charts 

Operational reliability (sometimes referred to as system availability) varied among the three 

installed systems. Tire-SafeGuard had the most reliable sensors. As shown in Table 12, the 

sensors were operational during 98.2 percent of the visual inspections. The poorest performing 

system, eTire, was fully operational in only 64.1 percent of the visual inspections. A 

manufacturing flaw, discussed later in Section 4.2.4, was identified in the eTire sensors, resulting 

in low system reliability. Other sensor failures were caused by component batch manufacturing 

issues, improper sensor installation, and sensor material fatigue. 

Table 12. Sensor Condition, by TPMS 

Sensor Condition 

WABCO 

IVTM 

TireSafeGuard eTire 

Operational 89.0% 98.2% 64.1% 

Non-Operational 11.0% 1.8% 35.9% 

4.2.3 Maintainability 

Maintainability pertains to the maintenance efforts necessary to keep the TPMSs in working 

order. This section focuses on the human interaction with the TPMSs and the steps taken to 

ensure that all sensors and system hardware were fully functional. Maintainability issues 

contributed to nearly one-half of the system malfunctions or failures during the field test. These 

40
maintenance shortfalls include missing sensors, improper sensor tracking, improper sensor 

installation, and improper sensor handling. 

4.2.3.1 Sensor Tracking 

A major challenge during the field test was keeping track of TPMS sensors. The eTire system 

sensors presented a particular challenge because of the location of the sensor. Unlike the other 

systems evaluated, the eTire sensor was located on the inside of the tire’s sidewall out of the 

technician’s view—even after the tire itself was removed from the wheel rim. The study team 

found that if the technician was unaware of the presence of the sensor, there was a good chance 

that it would be discarded during tire changes. This was a problem throughout the test period 

because of technician reassignments, in-field (emergency) tire repairs, and miscommunications. 

The problem persisted even after increasing the training provided to technicians. 

As an example, during one weekly inspection, test engineers learned of an initiative to replace 

the steer tires on the bus fleet at the Four-Mile Run facility because of a warranty issue with 

premature wear. During weekly inspections, two test buses (number 2511 and number 2512) 

were found to have missing eTire sensors in the front steer tires. The eTire-equipped tires had 

been removed from the bus and discarded. 

In another example, the rear tires on bus number 2512 were replaced because of damage, and the 

sensors were not transferred to the new tires. This time, however, the oversight was discovered, 

and the sensors were recovered before the tires were discarded. 

Keeping track of the eTire sensors was a constant challenge for both the study team engineers 

and WMATA technicians throughout the field test. By the end of the test period (after just 14 

months of testing), more than half of the eTire sensors on the four test buses had been misplaced. 

It was recognized that transit bus service presented unique challenges to maintaining a TPMS. 

These challenges include the fact that numerous technicians may service any particular bus, and 

that buses experience a relatively high frequency of tire changes per year compared to over the 

road tractor-trailer combinations. At times, the Four-Mile Run garage had five overlapping 

maintenance shifts, and occasionally technicians would be reassigned to work at a different bus 

garage; therefore, it was difficult to ensure that all technicians at the Four-Mile Run garage had 

working knowledge of the TPMS installed on the 12 test buses. 

The HCI’s Tire-SafeGuard system had a similar, but less severe problem with tracking sensors. 

The sensor tracking problem was mostly attributed to the sensor module‘s mounted location 

inside the tire which was not clearly visible to maintenance technicians, (although unlike the 

eTire sensor, the Tire-SafeGuard sensor was plainly visible once a tire was removed from the 

wheel rim). Again, not all technicians were aware that they were dismounting tires from a bus 

equipped with TPMS. 

Tires missing their sensors were sometimes discovered during the weekly system inspection. 

Most of Tire-SafeGuard tire pressure sensors were eventually recovered from tires removed from 

the test buses once the tire was removed from the rim (thereby exposing the sensor). 

The Tire-SafeGuard sensors, once remounted on a bus, needed to be retrained to match their 

wheel location in the ECU. This was not necessary if the wheel and sensor was removed from a 

41
bus, fitted with a new tire, and remounted to the same wheel location on the bus. However, the 

remounting of a wheel sensor to exactly the same position on the same bus was rare. Often, the 

new sensor module would remain unassigned (not recognized by the ECU) until the replaced tire 

was identified during the weekly inspections. To retrain new sensors to specific wheel positions, 

a technician must first hold down the setup button on the system display unit until the system 

enters “Tire Setup Mode.” Once in setup mode, the technician must release 3 psi of air out of the 

right front steer tire. Releasing 3 psi from the tire triggers the sensor module to transmit a signal 

which the ECU receives and associates with the tire position. Once the ECU recognizes a tire 

position, the display cues the technician to move to another tire position and release air from that 

tire. The steps are repeated for the remaining tire positions until the system recognizes all of the 

sensor modules on the vehicle. Once the retraining is complete, the technician refills the tires to 

the proper inflation pressure. 

In contrast, WABCO’s IVTM system had just three recorded instances of missing sensors. In all 

three instances, the tires failed in service and were replaced by field technicians during a road 

call. Unfortunately, the sensors that were removed from the damaged tires were not remounted to 

the new tires and were lost. The sensors were lost because the sensors are retained by lug nuts, 

which must be removed along with the sensor before changing the tire. 

4.2.3.2 Sensor Replacement 

Another challenge with maintaining TPMS during the fleet study was ensuring that tire pressure 

sensors were re-installed properly when tires were replaced. The WABCO IVTM wheel module 

was improperly installed (or installed in the wrong wheel positions) on test buses several times 

during the fleet study. 

For example, proper and improper installations of WABCO’s IVTM wheel module are shown in 

Figure 21. For proper installation (left side of figure), the module is installed between the wheel 

lug nuts that are to the immediate left and right of the valve stem. A hose attaches the wheel 

module to the tire’s valve stem. The hose on the wheel module end has a Schrader valve, which 

allows a technician to manually check the tire pressure or add air pressure to the tire without 

disconnecting the hose from the valve stem. 

Figure 21. Proper and Improper IVTM Module Installation 

Valve stem port

Proper Installation Improper Installation

42
The improper installation (right side) shows the wheel module attached to the two lugs to the left 

of the valve stem, rather than directly below it. As a result, the hose required to connect the 

module to the valve stem was too short, rendering the system useless. 

Another example of an improper installation is shown in Figure 22. The hose that connected the 

wheel module to the tire valve stem was improperly connected on two different tires on a 

particular bus. On both tires, one end of the hose was connected to the Schrader valve at the 

hose’s other end, instead of the tire’s valve stem. 

Figure 22. Improper IVTM Hose Connections 

In some instances, problems with the IVTM system occurred because a sensor unit had been reinstalled on a different wheel during routine tire changes. 

One example occurred on bus number 2503. On this bus, the IVTM display denoted that a wheel 

module was missing on the right-rear inner tire. Upon inspection, a technician noticed that a 

wheel module was mounted and properly connected to the right-rear inner tire. However, there 

was no wheel module mounted on the right-steer wheel. The technician compared the serial 

number written on the wheel module on the right-rear inner tire to the serial numbers 

programmed in the ECU, which revealed that the module installed at the right-rear inner tire 

belonged at the right-steer tire location. 

In another example, the same bus as in the previous example (number 2503) experienced a flat 

tire at the right-rear inner location. The technicians replaced the tires and reinstalled the modules. 

In the process, the two wheel modules that were programmed to the right-rear tires were mixed 

up and installed with the sensor programmed to the outer tire attached to the inner tire and vise 

versa. The error was identified the following morning, and the IVTM ECU was reprogrammed to 

reflect the change. 

4.2.3.3 Maintenance Damage 

Additional TPMS maintainability issues were noted during routine tire maintenance. In one 

example, the hose that connects the IVTM sensor unit to the tire’s valve-stem was damaged 

during a tire change, as was the tire’s valve stem. It was unclear what specific actions/events 

caused this damage. 

43
In another situation, an IVTM wheel module was removed from service because of a reported 

leak at the right-rear inner tire. The technician reported a hissing noise at the valve stem. By 

removing the IVTM connecting hose, the noise was eliminated. The module was tested by 

connecting the hose to a different tire, but the failure could not be replicated. The module was 

reinstalled on the tire with no further failure reports. Another challenge with the IVTM system is 

properly installing the sensor on the inner dual tire. The location of the inner tire valve stems 

makes it difficult to ensure the module has been properly installed. 

4.2.4 Durability 

Durability pertains to the ability of the TPMS components to withstand the wear and tear 

experienced in normal operating service. Most durability concerns identified during the fleet 

study were attributed to manufacturing defects or minor design deficiency. Manufacturing 

defects led to a need for numerous sensor replacements throughout the fleet test. Durability 

issues were communicated to the manufacturers, and damaged or defective parts were shipped to 

manufacturers for analysis. The manufacturers addressed their systems’ durability issues and 

provided design improvements or new installation instructions to combat future failures. 

4.2.4.1 eTire Durability Issues 

The eTire system had the most prevalent durability issue among the TPMSs tested during the 

fleet test. Damaged eTire sensors were initially identified in November 2006. Maintenance 

technicians noticed that the eTire system’s handheld sensor reader did not detect sensors in 

several tires on bus numbers 2509, 2510, 2511, and 2512. A closer look revealed that 14 tires 

contained sensors that did not communicate with the handheld reader. The tires were removed 

for inspection. In each tire, the sensor unit had separated from the SensorDock mounted onto the 

inside sidewall of the tire. In only one case did the SensorDock separate from the tire wall itself. 

Figure 23 shows one of the sensors discovered in the inspection. 

Figure 23. Damaged eTire Sensors 

As shown in Figure 23, the sensors were found broken into numerous pieces. The failed 

components were returned to the manufacturer for analysis. To continue the field test, new 

sensors were installed in each of the 14 tires. To secure the sensors to the sensor dock until a 

failure analysis could determine the cause of the separation, an inner liner sealant was used to 

lubricate the sensor before it was installed on the sensor dock as recommended by the 

manufacturer. When dry, the sealant helps secure the sensor to the dock. 

44
In January 2007 an eTire representative visited the maintenance facility to determine the cause of 

the sensors cracking and detaching from the sensor dock. During the inspection of the four eTireequipped buses, a broken sensor was located in the left steer tire of bus number 2512. The sensor 

appeared to have broken free of the SensorDock, which was still mounted to the tire sidewall. 

The failure mode of the sensor appeared to mimic the findings in November 2006. Under further 

inspection, the right steer tire was removed. In this instance, the sensor was attached to the 

SensorDock, but it appeared to have a small crack in the casing (Figure 24). The crack was 

located on the thin plastic casing in direct contact with the mushroom-shaped mounting knob on 

the sensor dock. While interviewing the technicians directly responsible for installing and 

maintaining the systems, it was noted that several other sensors had exhibited similar failure 

modes. The failed components were returned to Michelin for a full analysis. 

Figure 24. Cracked eTire Sensor, Bus Number 2512 

The manufacturer initially believed the sensors failed due to improper installation of the 

SensorDock and sensor tag. However, while at the maintenance facility, the eTire representative 

was able to determine that the eTire installation methods were followed properly by observing 

the tire technician install a new SensorDock and sensor unit in a new tire. When comparing the 

failed sensor units found in the left steer tire to those found in the right steer, identical surface 

cracks were identified in each sensor casing. Other damaged sensors that had been collected up 

to this point exhibited identical failure modes. All of the damaged sensors were collected and 

inspected by Michelin. 

Michelin determined that each of the damaged sensors had the same batch number printed on 

them. The discovery led to a determination that a manufacturing flaw existed with these sensors. 

Figure 25 shows an eTire sensor with the batch number written in the upper right portion of the 

sensor. The original sensors installed at the beginning of the test had a batch number of 

0-122691. 

The eTire sensors that replaced the faulty sensors came from batch number 0-122831 and did not 

exhibit failures during future inspections. The sensors from the batch number exhibiting the 

manufacturing defect were segregated from inventory and removed from the test site. 

45
Figure 25. Batch Number of Failed eTire Sensors 

In May 2007 additional damaged sensors were identified during the replacement of the rear tires 

of bus number 2512 (see Figure 26 and Figure 27). Each of the four sensors exhibited similar 

patterns of cracking and damage as the other eTire sensors. Despite the visible damage to the 

sensor casing, three of the four sensors were still functional. These four sensors were found to be 

from the same defective manufacturing batch identified earlier by the eTire representative. 

Figure 26. Damaged eTire Sensor, Bus Number 2512 

46
Figure 27. Cracked eTire Sensors, Bus Number 2512 

Throughout the test, eTire sensors with batch number 0-122691 continued to experience a high 

failure rate. All of the failed sensors identified up to this point in the test were part of the first 

batch of eTire sensors. Of the 48 sensors ordered as part of the initial installation, an estimated 

75 percent failed because of the identified manufacturing defect. 

In response, Michelin provided an additional 24 sensors that were used to complete a fleet-wide 

replacement of the sensors on eTire-equipped buses. After the new batch of sensors was 

installed, the sensor failure rate reduced significantly. Only a handful of failed sensors were 

identified after the fleet-wide sensor replacement, and their batch numbers did not match those of 

the faulty manufacturing batch. The durability of the eTire sensors improved after the fleet-wide 

sensor replacement; however, eTire sensors continued to be lost during tire replacements because 

of challenges associated with keeping technicians trained on the system—and with other logistics 

issues discussed previously. 

4.2.4.2 Tire-SafeGuard Durability Issues 

Two durability failures occurred with the Tire-SafeGuard system. The first failure was identified 

in September 2006 when two sensors installed on bus number 2508 could not be detected. 

According to the system display, the steer tire sensors were experiencing significantly different 

temperature and pressure readings compared to the rear tires. To test the system, air was 

manually released from each of the steer tires, but no tire pressure change was indicated on the 

display. To establish communication with these sensors, a new antenna was tested in place of the 

existing antenna, and an effort was made to reprogram the sensors. These attempts failed to 

result in communication with the sensors. 

Because of the failed troubleshooting attempts, the steer tires of bus number 2508 were removed 

and inspected. Upon removing the tires from the wheel rim, several small fragments of metal 

were discovered in each tire. The sensors removed from the tires exhibited unusual wear and 

abrasion on the outer casing. Upon further investigation, it was determined that the excess metal 

strapping used to retain the sensor was not removed before the wheel was reinstalled. The excess 

47
metal appeared to have broken free of the metal band while the bus was in service, fracturing 

into smaller pieces while moving freely within the tire and ultimately damaging the sensors. As 

Figure 28 shows, the metal fragments were part of the metal strap used to retain the sensors to 

the wheel. 

Figure 28. Tire-SafeGuard Band Fragments and Damaged Sensors, Bus Number 2508 

The second failure was identified in January 2007. During an analysis of tire data on bus number 

2506, it was noted that the readings of the right rear inner tire had retained a consistent value of 

128 psi and 111 degrees Fahrenheit over several weeks of data collection. These values were 

significantly higher than other tire readings on the same bus, which averaged 110 psi and 55 

degrees Fahrenheit. The right rear wheels were removed from bus number 2506 to determine the 

condition of the suspect sensors. Upon removal of the tire from the wheel rim, the sensor was no 

longer attached to the rim. The sensor appeared to have broken free of the metal band intended to 

secure it to the rim (see Figure 29). 

Figure 29. Damaged Tire-SafeGuard Sensor, Bus Number 2506—Right Rear Inner Tire 

Missing Edge on 

Damaged Sensor

Module

48
The sensor module experienced significant damage to its underside. A large piece of plastic had 

broken away from the bottom and one side of the sensor. The broken plastic had originally 

assisted in forming the open slots on the underside of the sensor used to secure it to the wheel 

rim. Initial investigations identified improper installation, heat-induced plastic fatigue, or heatinduced metal expansion as possible failure modes. 

In July 2007 five additional damaged sensors were identified. Four of the damaged sensors were 

removed from bus number 2507. Unlike the earlier failure, the damaged sensors were still 

secured to the wheel rim by the metal strap. As Figure 30 and Figure 31 show, the sensors 

exhibited casing cracks and casing failures (i.e., plastic casing was broken and missing). In each 

of the failed sensors, the failure location was identified at the underside of the sensors. Large 

portions of the plastic casing were missing from the underside of the sensor. In addition, two of 

the sensor casings had split open completely. The sensors were replaced, and the manufacturer 

conducted an analysis to determine the cause of the problem. 

Figure 30. Additional Damaged Tire-SafeGuard Sensors 

Figure 31. Additional Damaged Tire-SafeGuard Sensors 

49
At the end of July 2007, Tire-SafeGuard issued a product improvement notice to the study team, 

along with a shipment of new wheel sensors. The revised wheel module sensor units included 

“foot pads” that would adhere to the underside of the sensor. Each sensor was affixed with two 

of the foot pads to provide improved heat distribution for the plastic sensor housing. The product 

improvement was intended to prevent the sensor from splitting or cracking while in service. 

After this product improvement, no further failures were identified during the field test. 

4.2.4.3 WABCO IVTM Durability Issues 

The WABCO IVTM tire pressure monitoring system did not experience any durability issues 

during the fleet study. There were reported cases of missing sensors or improper hose 

connections as noted in Section 4.2.3, but its valve-stem mounted wheel modules and other 

hardware components completed the field test without failure. 

4.2.5 Maintenance Records 

Tire-related maintenance actions were gathered from the fleet’s maintenance database 

(MAXIMO). The records collected identified tire-replacement actions, TPMS hardware 

replacements, and bus recovery actions caused by a tire failure while in service. The review of 

the MAXIMO data aimed to identify repeated TPMS and tire issues, reduced road calls resulting 

from system installation, and extended tire life resulting from system installation. 

Overall, 124 tire failures were recorded in MAXIMO during the field test (see Table 13). The 

failures were divided into seven categories—flat tire, foreign object, low pressure, mileagebased, road call, TPMS failure, and worn tires. The most records were generated for “mileagebased” tire changes, with 60 records total. The vehicles’ tires were replaced because they reached 

the scheduled replacement mileage. The “worn tires” category (17 total records) designated tire 

changes that were necessary because of an observed worn condition rather than having reached 

their expected mileage life. These tires may have experienced increased wear for reasons other 

than normal wear, and had to be changed before the scheduled maintenance task. 

Table 13. Tire-Related MAXIMO Records 

Tire Failure 

Category Test Fleet Control Fleet Total 

Flat Tire 6 1 7 

Foreign Object 12 10 22 

Low Pressure 3 1 4 

Mileage Based 24 36 60 

Road Call 6 3 9 

TPMS Failure 5 0 5 

Worn Tire 7 10 17 

Total 63 61 124 

The category with the second most maintenance records, “foreign object,” identified 

maintenance actions caused by a foreign object piercing the tire. Under these records, the tires 

were replaced as part of an unscheduled maintenance service. The “flat tire” category had seven 

records. A review of the database for these failures did not note the cause of the flat tire. The 

50
failures were likely caused by unreported foreign objects or slow leaks at the rim or valve stem. 

Clarification was not provided in the maintenance record. 

The remaining categories accounted for less than 15 percent of the maintenance records: 

 Low Pressure (2.5 percent). A tire was reported to have a low pressure—much lower 

compared to the other five bus tires. Of the four instances, only a single instance was 

reported as a failure. In this instance, a leak was identified at the valve stem. The 

remaining records had no maintenance actions. 

 Road Call (7 percent). The maintenance records identified nine road calls during the 

field test. For test vehicles, the TPMS was not queried to verify that it identified the 

fault causing the road call. Two of these instances occurred on Interstate 395 and 

caused excessive tire damage. Section 4.2.6 provides further information. 

 TPMS Failure (5 percent). A TPMS failure was identified as any record generated by 

the TPMS itself. In general, the faults were generated because of faulty tire sensors or 

incorrectly installed hardware. In two instances, an inspection by the mechanic 

identified a faulty sensor (one eTire and one Tire-SafeGuard) that had to be replaced. 

In the remaining three instances, the IVTM air hose was either missing or improperly 

installed but did not result in road calls. Section 4.2.3 further discusses these failures 

and the resulting corrective action. 

As the analysis of the MAXIMO data shows, the test and control fleets experienced a similar 

number of tire failures throughout the course of the test—and it is inconclusive (or unlikely) that 

the TPMS systems helped reduced these catastrophic type tire failures. 

In addition to the five reported TPMS failures by WMATA, the study team kept records on the 

number of sensors replaced during the course of the field test. As shown in Table 14, 73 spare 

sensors were used to support the program. 

Table 14. Sensors Replaced 

Sensor Type 

Number of 

Sensors Initially 

Installed 

Number of 

Sensors Replaced 

IVTM 24 3 

Tire-SafeGuard 24 18 

eTire 24 52 

Total 72 73 

Three IVTM sensors were replaced during the test period. One sensor developed a leak. It was 

removed from the bus and returned to WABCO for analysis. The other two sensors were 

misplaced during a roadside tire replacement. 

Eighteen Tire-SafeGuard system sensors were replaced during the test period. Two sensors were 

damaged as a result of faulty installation and six sensors suffered heat damage. HCI Corporation 

51
equipped the replacement sensors with an insulator pad. The remaining 10 sensors were 

misplaced during tire changes. 

Fifty-two eTire sensors were replaced. Forty sensors were identified as having a manufacturing 

defect as discussed previously. The remaining 12 sensors were misplaced during tire 

replacements. 

4.2.6 Isolated Events 

Throughout the course of the field test, several isolated tire and TPMS equipment failures were 

apparent. However, they were not directly attributable to a TPMS failure, technician failure, or 

system functionality issue. Each of these failures offered important insights to the operation of 

the TPMSs. 

4.2.6.1 Tire Blowout 

One incident occurred on July 18, 2007. While traveling on I-395 in Virginia, both right rear 

tires on bus number 2503 went flat. The outer tire, shown on the right in Figure 32, seemed to 

experience a small puncture from a foreign object, causing it to lose air. The inner tire, shown on 

the left in Figure 32, appeared to fail shortly after the outer tire failure and sustained the most 

extensive damage. The photograph indicates that the failure likely occurred at the 2 o’clock 

position on the outer tire. At this position, a piece of rubber was identified as missing from the 

tire, possibly after contacting a piece of construction debris. In the interviews, technicians 

identified construction debris, such as concrete rebar, as a common cause of tire failures on buses 

maintained at the Four-Mile Run facility. 

Figure 32. Damaged Tires From Bus Number 2503 

Bus number 2503 was equipped with the IVTM system, which can store tire fault information. 

After connecting a laptop to the IVTM ECU, six separate faults were identified. The system 

detected low and critically low tire pressure at both right rear tire locations, (see Figure 33). The 

system detected that the right rear tires were extremely under-inflated and indicated that the tire 

pressures were at a “critical under-pressure” level. If the display had been in the driver’s clear 

view instead of in an inaccessible cabinet, one can reasonably assume that (1) the system would 

52
have notified the driver of the dangerous tire conditions before the tire blowout occurred and (2) 

the driver would have been able to pull to the side of the road safely. It should be noted that the 

IVTM system does not store date or time stamps (however, the latest IVTM software does now 

include date and time stamp information on system messages). Therefore, the low and critically 

low tire pressure warnings detected by the system cannot be associated with certainty to the July 

18 tire failure. 

Figure 33. IVTM Fault History for Bus Number 2503 

4.2.6.2 Sensor Module Air Leak 

Another unique failure involved an IVTM wheel module. A technician reported a leak at the 

right rear outer tire’s valve stem. Removing the wheel module eliminated the source of the leak. 

An effort was made to troubleshoot the source of the air leak. First, a new hose was attached to 

the module and valve stem, but the leak was still present. During troubleshooting, it was 

determined that the wheel module itself was causing the leak. The wheel module was removed 

from service and returned to the manufacturer. 

4.2.6.3 Cracked Antenna on eTire Handheld Tag Reader 

A routine inspection on June 10, 2007, revealed a damaged eTire handheld sensor reader. The 

reader was unable to locate or read sensor tags inside the tires of bus number 2509, nor was it 

able to detect or read the vehicle ID sensor tag located at the front of the vehicle. A technician 

attempted to troubleshoot the handheld reader. First, the reader was returned to the cradle dock to 

53
ensure upload of the latest software to the reader. The software update did not correct the issue. 

Inspecting the case of the reader revealed a crack near the reader’s antenna. As shown in Figure 

34, the crack in the casing appeared to damage the internal antenna. Through the casing crack, 

the internal antenna coil was severed from the reader. The damaged reader was returned to 

Michelin’s local service representative and a new reader was shipped to the Four-Mile Run 

facility on June 27, 2007. Therefore, system tire pressures on the eTire equipped buses could not 

be recorded during the weekly inspections on June 10, June 17, and June 24, 2007. The new 

handheld reader was placed into service the week of July 1, 2007. 

Figure 34. Damaged eTire Handheld Reader 

4.2.7 Technician Interviews 

Informal discussions took place with technicians and mechanics throughout the study period, and 

formal interviews were conducted at the end of the field test. The participants believed that, 

overall, TPMSs could benefit the fleet. They stressed the need for accurate tire pressure 

information and their preference for tire pressure monitoring through communication with an 

AVM system (i.e., the Clever Devices WiFi based diagnostic system). In an ideal situation, the 

AVM system would alert the maintenance staff of under inflated tires in the fleet, reducing the 

need for manually measuring tire pressures. The technicians’ comments related to the individual 

TPMSs are as follows. 

4.2.7.1 WABCO IVTM 

Overall, the participants were satisfied with the operation of the IVTM. They identified three 

main areas of concern—sensor hose condition, propensity for misplacing sensors, and sensor 

installation. 

During the inspection of the sensor units, the technicians said they found that the hose connected 

to the units was, on occasion, either damaged or leaking air. The failure was noted most often at 

54
the right front steer tire of the bus. The buses at WMATA encounter street curbs and the guide 

rail in the bus wash rack more frequently at this wheel location than other wheel locations. As a 

result, any equipment installed external to the wheel at this wheel location is prone to damage. 

The failure of the equipment was resolved by disconnecting the hose from the valve stem if a 

leak was identified. The action was taken to eliminate the risk of a complete loss of pressure 

(although such a solution eliminated the ability to sense tire pressure at the right front steer tire. 

In several cases, sensor units were found missing from tires after road call incidents to replace a 

flat tire. Shop supervisors speculated that the technicians responsible for the road call might have 

forgotten to reinstall the sensor units when replacing the tire, leaving the sensor units on the 

roadside. Alternatively, the particular technician responding to such a road call may not have 

been properly trained and/or been aware of the IVTM sensors. 

Technicians commented that installing the wheel modules at the rear dual wheel assemblies was 

difficult because of the wheel well orientation and the small gap between the rear dual wheels. 

The limited accessibility made it difficult to attach the wheel module’s hose to the inner valve 

stem. To increase the accessibility of the valve stem, the technicians had to increase the gap 

between the dual tires and access the valve stem from under the bus while the bus was slightly 

elevated on a post lift. The first action required the technicians to loosen or remove all of the lugs 

on the wheel assembly to increase the gap between the wheels. With the lug nuts loosened, the 

technician then lay on his side on the floor and reached his arm under the wheel well and into the 

gap between the tires to attach the hose to the inner tire’s valve stem. 

4.2.7.2 HCI Tire-SafeGuard 

Although each of the three TPMSs tested in the study had pros and cons, the maintenance 

technicians preferred the HCI Tire-SafeGuard system. The technicians noted the ease of 

installation and the detailed installation procedures. The wheel-mounted tire pressure sensor 

required only a screwdriver to mount the sensor to the wheel rim. To protect the wheel sensor 

from damage during tire replacement, the installation procedures advised mounting the sensor 

near the valve stem port on the wheel. The procedure provided the technicians with the 

approximate mounting location so they could avoid damaging the sensor with the tire removal 

tools. 

However, because the Tire-SafeGuard tire pressure sensors are located inside the tire, their 

presence and status were difficult to track. A technician could not tell that a tire was equipped 

with the sensor simply by looking at the tire. As a result, the sensors could be lost during tire 

replacement. In some instances, technicians unknowingly replaced Tire-SafeGuard-equipped tire 

and wheel combinations with standard tire and wheel combinations. The buses returned to 

service and the missing sensors went undetected until the next weekly system inspection (by 

study team staff). Technicians commented that if a sensor was absent from a tire location, the 

system display only indicated a loss of sensor communication at that location. Tracking the 

sensors and sensor-equipped tires increased the technicians’ workload. 

55
4.2.7.3 Michelin eTire 

Technicians found the most problems with the eTire TPMS during the fleet study. The problems 

reflect issues with the durability of the eTire sensors (see section 4.2.4), sensor tracking, and use 

of the handheld sensor reader. 

Technicians commented that they preferred sensors that attach to the wheel instead of the tire. 

They mentioned that the high frequency of tire changes on buses increased the chances of 

misplacing sensors—particularly with a system such as eTire where the sensor is attached to the 

tire rather than the wheel rim (the implication being that tires are wear items that are discarded, 

but rims are not). Technicians also discussed the challenges of mounting the sensor in a 

consistent location so that it could be easily read using a handheld unit. Early in the test program, 

technicians sometimes mounted sensors on the wrong side wall. Procedures were developed to 

ensure a consistent mounting location. These were not issues encountered with the other systems. 

The technicians found the eTire handheld reader difficult and time consuming to use. To read the 

eTire sensor tags mounted to the inside sidewall of the tire, technicians had to hold the handheld 

reader within inches of the tire sidewall and move the reader slowly until it detected a sensor. 

Moving the reader between the dual tires on the rear axle was especially difficult because of the 

wheel well and close proximity of the dual tires. Also, the reader would often “time out” if it did 

not locate a sensor within 10 to 15 seconds. 

In addition, the handheld reader needed to be returned to its docking station to ensure it was 

fully charged for its next use. In should be noted that the handheld unit is designed for “spot 

checks” of tire pressure condition, and that Michelin recommends the use of a drive-by gate 

reader as the preferred method to read a vehicle’s tire pressure. In this scenario, a bus would 

drive between two gates that automatically detect and download tire pressures at each of the 

tires. When presented with this option, technicians believed the gate reader would be a great 

improvement over the handheld reader. The gate reader was not used in this study because of 

funding constraints and the infrastructure modifications required to properly mount and power 

the gate reader. 

Tires equipped with the eTire sensors were also difficult to track since sensor units are not easily 

visible. Technicians noted that the eTire sensors would be easier to track if the entire bus fleet 

utilized the tire pressure monitoring system. 

4.3 OBSERVATIONS AND CONCLUSIONS 

TPMS sensors were found to be consistent and reliable in reporting tire inflation pressures when 

compared to measured (tire-gauged) values. On average, the systems reported false positives (a 

false low-pressure reading) 6 percent of the time or false negatives (a missed, or undetected lowpressure reading) 2 percent of the time. The more frequent issue was “no reads” resulting from 

missing sensors, failed sensors, or sensors in the wrong wheel locations. 

Keeping track of individual wheel/tire sensor units themselves was a significant challenge during 

the field test. This logistical challenge arose because of the high frequency of tire changes; the 

fact that sensor mounting locations were out of view of technicians; and lack of fleet-wide 

56
training on system awareness and operation. Training was limited to the host depot, but 

occasionally tire and brake maintenance would occur at other depots. It is recommended that 

training across the entire system be required to prevent technicians from misplacing and 

inadvertently discarding sensors. 

The durability of two TPMS sensor designs was initially challenged by the operating conditions 

experienced in transit service. Failures occurred with the wheel-mounted sensors two months 

into testing. Excessive heat build-up caused the sensor housing to become brittle, crack, and fail. 

The sensor manufacturer provided a design change that consisted of an isolating pad on the 

bottom side of the sensor that contacted the wheel rim. This simple solution prevented further 

failures from occurring. Failures also occurred with the sensors that adhere to the tire sidewall 

early in the test program. The plastic casings on these sensors developed cracks during testing. 

The cracked sensor casings were determined to be caused by a manufacturing batch defect. 

Replacement sensors were found to be significantly more durable. 

An improvement in adherence to targeted tire pressures was not found on the test fleet compared 

to the control fleet (i.e., average observed tire pressures between the control and test fleets was 

roughly the same). This may be because WMATA takes a pro-active role in maintaining target 

inflation pressures to comply with the tire vendor’s warranty. Average tire inflation pressure was 

111 psi for both the test and control fleets (target: 115 psi). Tire life and fuel economy were also 

similar in both fleets. This was likely the case because, as noted earlier, the real time display of 

tire pressure readings was purposefully not made available to drivers, but only to maintenance 

personnel and technicians. Drivers could therefore not act immediately on such information to 

correct any tire pressure problems that may have been detected. In most commercial truck fleets, 

such real time information would be provided to drivers, and drivers would have the opportunity 

to act on the information as needed (for example, adding air to tires at the next convenient time if 

low pressure was detected), thereby improving the average tire life and fuel economy of the fleet. 

Two tire blowouts could have been prevented during the course of the field test had the TPMS 

displays been visible to the bus operators. To maximize safety and operational benefits, it is 

recommended that system data be made available to the driver in real time (as well as being 

made available to maintenance staff). 

Technicians preferred the wheel-mount tire pressure sensors for two reasons: they were easier to 

install and tires could be changed without removing or disconnecting the system. Conversely, the 

technicians found valve-stem-mounted sensors difficult to connect to the inner tires on a set of 

duals. This issue may be unique to buses because the wheels and tires are surrounded by the 

structure of the vehicle. Tire-mounted sensors required more time to install (versus wheel or 

valve-stem mounted systems) because of the required tire surface preparations. 

57
5. BRAKE PERFORMANCE MONITORING SYSTEM FIELD 

TEST RESULTS 

5.1 BPMSS UNDER EVALUATION 

The systems selected for this field evaluation program were chosen because they represented a 

range of technological approaches for assessing brake performance, and because each was 

examined under controlled test track conditions in prior studies sponsored by the FMCSA. The 

systems participating in the field trial included:

 MGM eStroke. 

 GeoDevelopment Brake Insight. 

 StrainSert Brake Performance Monitoring. 

 Link Radlinski Model 20200 Portable Roller Brake Tester. 

5.1.1 MGM eStroke 

5.1.1.1 General Description 

The eStroke, by MGM Brakes, is an electronic brake stroke monitoring system that provides 

brake stroke status readings for pushrod-operated air brakes. The eStroke system consists of a 

Hall-effect sensor that moves in parallel with the brake actuator piston rod to induce a voltage 

that is proportional to linear movement. The output from the Hall-effect sensor (i.e., the voltage 

change) is sent to an integrated ECU/display unit where it is correlated to brake stroke position. 

By monitoring brake stroke travel and position, the system can detect overstroke, dragging brake, 

and/or a nonfunctioning brake actuator. The system must be calibrated during installation for 

brake systems having different maximum actuator travel in order to properly determine what 

degree of travel constitutes “overstroke,” dragging brakes, etc. 

The Hall-effect sensor itself is contained within the air brake chamber thereby eliminating 

packaging interference with chassis and suspension components. This “internal chamber” design 

also serves to protect the sensor components from exposure to the harsh environmental 

conditions in the wheel end area. Figure 35 shows the MGM brake chamber. 

58
Figure 35. MGM Brake Chamber and Display 

The ECU/display unit was located in a control box directly behind the driver’s seat. The display, 

shown in Figure 36, consists of five lights. One light identifies the functional status of the overall 

system, (if the light is red, the system is not operating properly). The four remaining lights 

identify the status of the brake assembly at each wheel-end. Unless a green light is present at a 

particular wheel-end, one of the following conditions had occurred: 

 Overstroke. If the light at a particular wheel-end location rapidly blinks red. 

 Dragging Brake. If the light at a particular wheel-end location slowly blinks red. 

 Brake Actuator Not Functioning. If the light at a particular wheel-end location 

alternates between red and green. 

 Sensor Fault. If the light at a particular wheel-end location blinks orange. 

As noted in Section 3, the eStroke brake monitoring system is capable of storing brake alarms, 

warnings, and faults. These events are recorded by the ECU/display unit along with a date and 

time stamp and a description of the fault(s) detected. The system reports on problems with the 

brakes or with the system itself. For example, brake problem descriptions (fault codes) include 

brake overstroke, dragging brake, or brake actuator not working. Examples of system fault codes 

include: sensor out of alignment; communication failure; failed sensor; low voltage reading; and 

missing sensor. The system is capable of storing over 2000 separate incidents (or fault codes). 

The data can be exported onto the vehicle’s SAE J1587 or J1939 electronic databus which can 

then be displayed on “smart” dashboards or transmitted to off-board vehicle monitoring devices. 

59
Figure 36. eStroke System Display

For this field test, the eStroke system data output (i.e., fault code reports) was linked to the AVM 

system on the bus (supplied by Clever Devices). The AVM system was capable of wirelessly off 

loading data to a central server located at the bus garage. The server would then automatically 

send emails to the maintenance personnel and study team on the status of the bus’s brakes as 

reported by the eStroke system. 

5.1.1.2 Installation 

The MGM eStroke system was factory installed at Orion Bus Industries manufacturing facilities 

in Mississauga, Ontario, Canada. The work required to integrate the eStroke system with the 

AVM was completed by MGM and the AVM system manufacturer, Clever Devices, as part of 

the bus procurement contract between WMATA and Orion.

5.1.2 GeoDevelopment Brake Insight 

5.1.2.1 General Description 

The Brake Insight system, by GeoDevelopment,§ is a brake stroke monitoring system that is 

similar to the MGM eStroke in that it uses a Hall-effect sensor to measure brake stroke travel and 

positioning. The difference is that the sensor and related hardware are mounted outside (or 

external to) the brake chamber. The Brake Insight system consists of the following main 

components: 

 Wheel-end sensors (to electronically monitor brake stroke travel). 

 Thermocouples and lining wear sensors. 

 S-box control unit (ECU). 

 Display unit. 

 

§

 GeoDevelopment, distributed by Truck Trailer Transit, 1601 Theodore Street, Detroit, MI 48211; www.tttonline.com. 

60
Wheel-end sensors. The wheel-end sensors (shown in Figure 37), consist of (A) a Hall-effect 

sensor, (B) a magnetic clevis pin (which links the push rod from the brake chamber to the S-cam 

brake lever assembly), and (C) visual indictors of brake stroke. Similar to the eStroke system, the 

Hall-effect sensor for the Brake Insight system has a magnet (i.e., the clevis pin) that moves in 

parallel with the actuator piston rod to induce a voltage change in the sensor. Brake Insight has 

an electronic control module (called the S-box) that processes the voltage change and determines 

the travel and position of the push rod. The Brake Insight system is capable of detecting normal 

stroke, overstroke, dragging brake, and a nonfunctioning brake actuator based on brake stroke 

travel. 

The outside casing of the Hall-effect sensor unit is also scribed with precisely placed markings 

(visual indicators) which can be used to determine the relative position of the pushrod, (“C” in 

Figure 37). The markings are calibrated in a manner that would allow a technician to quickly 

determine an over or understroke condition during a conventional (manual) brake actuation test. 

Using the magnetic clevis pin as a pointer, the indicators provide a visual means to check the 

brakes in the event of a power or display failure. 

Figure 37. Brake Insight Components 

61
Each brake stroke actuator sensor is mounted on a bracket that allows the unit to be adjusted for 

various brake chamber designs and stroke lengths (from 1.75 inches to 3.0 inches). Figure 38

shows a wheel-end sensor before mounting on a brake chamber. The two bolts that secure the 

sensor to the bracket can be loosened, and the sensor can slide along the bracket to line up with 

the magnet on the pushrod. 

Figure 38. Brake Insight Wheel-End Sensor and Mounting Bracket 

62

Lining Wear and Brake Temperature Sensors. Unique to the Brake Insight system is the 

application of shoe lining wear and temperature sensors (Figure 39). These sensors are integrated 

into the system and help signal normal maintenance requirements (prognostics), as well as to 

detect potential brake system problems. The information from these sensors can augment the 

information from the brake stroke sensor to diagnose potential brake system issues. 

The shoe lining wear sensor is simply a metal plug with a set of electrical contacts affixed on 

the surface. The plug is then embedded in the shoe lining at a depth indicative of end-of-life 

wear. When the lining wears to this depth, the metal contacts on the plug become shortcircuited, sending a signal to the ECU indicating the shoe linings have reach the end of their 

useful life. A thermocouple is also integrated into the sensor plug, and the output is 

continuously monitored by the ECU to detect excessively high brake temperature. For the 

WMATA application, when a temperature of 685 degrees F is reached, the ECU sends a high 

temperature warning signal to the Display unit. 

The left photo (Figure 39) shows the wear side of the sensor, and the center photo shows the 

mounting hardware where the thermocouple connects to the wear sensor and shoe. A single cable 

is connected to the sensor unit (right photo) to route information to the ECU (S-box). This cable 

contains three leads (one for the thermocouple and two for the lining wear sensor). 
Figure 39. Lining Wear Sensor Installed in Brake Shoe 

Figure 40. Brake Insight ECU (S-box) 

S-box ECU. The brake stroke actuator sensors and the lining wear/temperature sensor units at 

each wheel-end were hardwired to the S-box control unit (or ECU, see Figure 40). The Brake 

Insight monitoring system is designed to store fault codes associated with various brake system 

abnormalities and events, as well as fault codes associated with the system itself (self diagnostic 

codes). The various fault codes are recorded along with a date and time stamp and a description 

of the problem or issue. For example, brake problem descriptions included brake overstroke, 

dragging brake, brake out of adjustment, brake out of service, high brake temperature, and worn 

shoe lining. There are unique fault codes to identify each wheel-end (i.e. for each sensor unit 

installed on the vehicle) and for each type of occurrence. Examples of a “system” problem 

include sensor out of alignment, communication failure, failed sensor, low voltage reading, 

and/or missing sensor. The system is capable of storing 2,700 fault codes. The data could be 

exported via a RS-232 connection located on the faceplate of the display unit. 

Display Unit. The S-box is hardwired to the Display unit (Figure 41) which was located in the 

I/O box behind the driver’s seat. The Display can be programmed to operate in “Driver Mode,” 

or in “Maintenance Mode.” In Driver mode the software will display only critical faults to the 

driver as defined by the customer. In maintenance mode all fault conditions are displayed. 

63
Figure 41. Brake Insight Display Unit 

5.1.2.2 Installation

The GeoDevelopment Brake Insight system was installed on WMATA bus numbers 2505, 2506, 

2507, and 2508. Truck Trailer Transit Inc., a distributor of Brake Insight, provided three service 

technicians to lead the installation. The factory technicians began by installing the magnetic 

clevis pins on the pushrods extending from the brake chambers. Normally, the original clevis 

pins that link the pushrod to the slack adjuster are removed and replaced with the magnetic pins. 

However, the clevis pins used with the MGM brake chambers on the WMATA buses were 

welded in place and could not be removed. To overcome this issue, the technician installed a 

pipe clamp with a magnetic end on the pushrod near the existing clevis pin. Once the magnets 

were installed on the pushrods, the technicians mounted the wheel-end sensors to the brake 

chambers using the supplied brackets. 

Cabling was routed from the sensors mounted at each brake chamber along the underside of the 

bus to the S-box located just forward of the front driver’s side wheel well. The S-box was 

mounted to the underside of the bus, near the air reservoir, and protected from road debris. 

Figure 42 shows the location of the S-box, display unit, and wheel-end sensors. 

64
Figure 42. Brake Insight Component Mounting Locations 

To install the lining wear/brake temperature sensor units, all of the wheels and brake drums were 

first removed from the test bus. Next, the four S-cam drum brakes were disassembled as if 

performing a scheduled brake job. Using a drill press, a guide hole was drilled through the brake 

shoe from the metal side of the shoe. Then, the technician flipped the shoe over and drilled 

through the brake lining and shoe with a 5/8-inch hole saw. The lining wear/temperature sensor 

unit was then mounted in the hole in the brake shoe. 

5.1.3 StrainSert Brake Performance Monitoring 

5.1.3.1 General Description. 

The StrainSert brake monitoring system consists of the following main components: 

 Instrumented brake anchor pins. 

 ECU. 

 Display unit. 

Instrumented Anchor Pins. The StrainSert system uses strain gauges affixed to the anchor 

pins on the S-cam drum brake assembly to measure shear stresses that develop during a 

braking event. Earlier studies have confirmed that these shear stresses correlate closely with 

actual brake force. 

Each anchor pin is fitted with two strain gauges oriented 90 degrees apart. One of the strain 

gauges is mounted normal to the direction of rotation, and is intended to predominantly measure 

the mechanical non-friction force exerted by the movement of the brake shoe as it moves against 

65
the drum. The other strain gauge is offset 90 degrees from normal and is intended to primarily 

measure the rotational friction force between the drum and the lining. 

The system consists of four anchor pins, (one for each brake assembly) which are mounted on 

the primary shoe, (Figure 43). The output from the sensors are routed to an electronic control 

unit. The anchor pin shear stress values are monitored and compared with the values of the other 

brakes and with empirical data. Values exceeding preset thresholds (either absolute values, or 

differential values among sensors) are flagged as “faults”, and an appropriate alert signal is sent 

to a display unit. 

Figure 43. S-Cam Brake Assembly Diagram 

Primary 

Anchor 

Pin

S-Cam 

shaft 

Street Side

Rear Brake 

Assembly

Primary 

Anchor 

Pin

S-Cam 

shaft 

Street Side

Rear Brake 

Assembly

Figure 44 shows an instrumented anchor pin uninstalled (left and center photos), and installed in 

a brake assembly (right photo). 

Figure 44. StrainSert Anchor Pin 

Notice the grease fitting on the anchor pin in the top left photo. The grease fitting on the original 

anchor pin was relocated from the center of the pin to the connector end to accommodate the 

addition of the strain gauges. 

ECU and Display. Sensor units are hardwired to an ECU, which is then connected to a Display 

Unit (Figure 45). The various threshold warning parameters and fault code algorithms are 

customized for specific applications by an authorized technician (see Installation, next section.) 

The system is designed to detect out-of-adjustment brakes, dragging brakes, oil-soaked linings, 

and disconnected shoe linings. 

66
Figure 45. StrainSert ECU and Display Unit 

5.1.3.2 Installation

The StrainSert Brake Monitoring System was installed on WMATA bus numbers 2509, 2510, 

2511, and 2512. StrainSert provided three service engineers to lead the installation and configure 

the systems.

StrainSert custom manufactured the brake anchor pins to replace those used in the Orion VII 

buses. To install the instrumented anchor pins all of the wheels and brake drums were removed 

from the test bus. Next, the four S-cam drum brakes were dissembled as if performing a 

scheduled brake job. The stock anchor pins were replaced with the StrainSert units and the 

brakes were then reassembled. 

Figure 46 illustrates the mounting locations for the ECU, display unit, and brake anchor pins. 

The ECU was mounted to a support beam above the lighting track on the street side of the bus. 

Figure 46. StrainSert Component Mounting Locations 

The display unit was installed inside the I/O cabinet located behind the driver’s seat and out of 

view of the driver. 

67
An eight-conductor shielded cable connected each instrumented anchor pin to the ECU. The 

shielded cables were routed from each brake assembly along the underside of the bus to an 

electrical compartment below the driver’s side window. The cables were secured to the bus’s 

wire harnesses and hoses with zip ties. The cables were then pulled into the I/O cabinet behind 

the driver’s seat and into the cable tray behind the lighting track in which the ECU was mounted. 

Once all of the system hardware was installed on the bus, the system was calibrated. A StrainSert 

engineer connected a laptop to the Ethernet port on the front of the display unit. The laptop 

provided a user interface to communicate directly with the ECU. The engineer set trigger 

parameters and warning thresholds for the system using the programming menu depicted in 

Figure 47. 

The “trigger parameters” shown at the bottom left portion of Figure 47 represent the brake force 

values at which the system would start and stop monitoring a braking event. In this example, the 

“start trigger level” was set at 2,500 foot pounds to ensure the system would begin monitoring 

only if the brakes were applied with intent to stop the vehicle—not if the driver was simply 

riding the brakes. In this example, the system captures brake force readings in 1-second intervals 

and stops monitoring braking force when the force drops below 2,000 foot pounds. 

Figure 47. StrainSert Calibration Software Screenshot 

The brake balance parameter (not shown in Figure 47) establishes the threshold at which a brake 

warning or alarm would trigger. The brake balance parameter compares the braking force of the 

left brake and the right brake on each axle. If the difference is greater than 30 percent, a 

“warning” is logged by the system. If the difference is greater than 40 percent, the system logs an 

68
“alarm.” An alert message is sent to the Display unit only after the system generates a minimum 

number of warnings or alarms. A user-defined “alarm count limits” parameter is used to set the 

number of warnings or alarms required for the system to generate an alert. For the screen shot 

shown in Figure 47, the Alarm Count Limits parameter was set to send an alert to the display 

unit after generating either 10 system warnings or 10 system alarms. 

5.1.4 PBBT 

The Link Radlinski Model 20200 portable Performance Based Brake Tester (Figure 48), also 

known as a PBBT, was incorporated into the program to serve as a reference measurement tool 

for evaluating the accuracy of the on-board brake performance monitoring systems, and more 

generally to assess the pros and cons of off-board brake system testing relative to integrated 

onboard BPMS. This roller dynamometer-based system, with two 11 kW three-phase drive 

motors with mechanical brakes, can dynamically measure the rolling resistance; brake threshold 

pressure; service brake force; parking brake force; and antilock braking systems (ABS) sensors, 

valves, and wiring. The PBBT provides an industry-accepted reference measurement of brake 

performance. Basically, the vehicle’s front or rear wheels are positioned on top of the roller 

dynamometer. An electronic alpha-numeric message board is positioned in front of the bus and 

provides the operator with instructions for executing various braking maneuvers. The rolling 

resistance of the dynamometer is pre-programmed to simulate the weight and rolling inertia of 

the bus itself. 

The large wall-mounted display, shown in the right photo of Figure 48, was provided with 

analog dials to indicate: left and right brake forces; percent brake force differences; digital 

displays of brake system control air pressure; axle weight; and overall deceleration rates. 

Figure 48. PBBT and Operators Display 

5.1.4.1 Installation 

The offboard PBBT system was installed at the Four-Mile Run maintenance facility. The 

offboard system monitored the rolling resistance and service brake force for the test fleet and the 

control fleet.

69
5.2 CONTROL VEHICLES 

To facilitate the evaluation of the BPMSs, data related to brake condition, maintenance and cost 

was also monitored for a group of buses that did not utilize any type of brake performance 

monitoring system (i.e., standard buses). These buses were the same make and model as the test 

buses and included bus numbers: 2513–2524. The buses were operated out of the Four Mile Run 

facility, and used in a similar service environment as the test fleet. 

5.3 TEST RESULTS 

This section of the report presents the results of an evaluation of the brake performance monitoring 

systems based on analyses of data and information collected throughout the field test. 

The analysis of the data and information collected focused on assessing: 

 Reliability, durability, and maintainability of the systems. 

 Whether BPMSs can be used by fleet and maintenance mangers to improve the 

efficiency of brake system maintenance by reducing the need for visual inspections. 

Data collected for the test vehicles included: 

 Weekly visual inspections of brake systems (brake chambers; drums, linings, etc). 

 Maintenance records for the brake system as well as the BPMSs. 

 Weekly visual inspections of the brake monitoring systems, including recording the 

status of the display units (presence of warnings, alarms, etc.). 

 Periodic downloads of the brake monitoring system’s fault code logs. 

The visual inspections were completed over a 14-month period, from September 2006 to 

November 2007. During the first 2 months, the test vehicles and their newly installed equipment 

were validated and confirmed to be working properly. Full data collection began in November 

2006. Visual inspections were performed approximately once a week by a study team engineer 

and included brake assemblies, pad wear, brake stroke, and system status. Information was 

recorded on brake data forms as shown in Appendix A.

Each bus was inspected approximately 50 times throughout the course of the test, resulting in a 

total of 590 bus inspections and 2,360 separate wheel-end inspections (see Table 15). In addition, 

electronic reports listing all recorded fault codes were downloaded from the systems’ ECUs 

approximately once per month, depending on the system. 

70
Table 15. Total Bus and Wheel-End Visual 

Inspections by System 

System 

Bus 

Inspections 

Wheel-End 

Inspections 

eStroke 190 760 

Brake Insight 199 796 

StrainSert 201 804 

Total 590 2,360 

The 24 buses were also tested on the PBBT approximately once per month to record the “actual” 

performance of the braking system, for a total of approximately 200 PBBT records. 

5.3.1 Overall Braking System Performance and Reliability 

A total of 124 brake maintenance records were generated and identified during the test period. 

These maintenance records included scheduled maintenance actions—specifically brake 

overhauls; and, unscheduled maintenance actions which included brake fault, brake overhaul, 

dragging brake, hardware failure, over-stroke, pulling brakes, sensor fault, and slack adjuster. 

Table 16. Summary of Brake Maintenance Records on Test and Control Fleets 

Brake Maintenance 

Issue 

Test Vehicle 

Maintenance 

Records 

Control Vehicles 

Maintenance 

Records 

Total 

Maintenance 

Records 

Brake Fault 10 6 16 

Brake Overhaul 8 11 19 

Dragging Brake 8 11 19 

Hardware Failure 15 6 21 

Overstroke 9 8 17 

Pulling Brakes 6 2 8 

Sensor Fault 9 4 13 

Slack Adjuster 10 1 11 

Total 75 49 124 

The eight maintenance record categories in the table above were pre-defined by WMATA’s 

maintenance management system (MAXIMO). Most category titles such as “brake overhaul”, 

“dragging brakes”, and “overstroke” define a specific abnormality or maintenance activity. In 

contrast, the “Brake Fault” category is a generic label for a variety of miscellaneous brake related 

maintenance actions. Many maintenance records labeled as a “Brake Fault” included descriptions 

such as brake noise, hot brakes, brake lock-up, and brake chamber leak. The maintenance records 

labeled as “Sensor Fault” were all associated with the eStroke system’s wheel-end sensors 

(eStroke was factory installed on both the test and control buses). The most common cause for 

the sensor faults was a damaged wire harness near the MGM brake chamber that impaired 

communications between the eStroke sensor and ECU. During the test period, other than eStroke 

sensor faults, the BPMS only caused a single failure, which was due to a cable failure. 

71
The brakes on the Orion VII buses were found to be quite reliable. Throughout the course of the 

test, the maintenance records did not highlight any significant brake failures. As shown in Table 

16, the number of maintenance records on the 24 test and control buses were rather evenly 

distributed across the eight maintenance record categories (No one type of abnormality occurred 

with significantly greater frequency than the other seven types). In addition, the Orion VII buses’ 

original equipment manufacturer-installed brake linings accumulated roughly 2.5 times the 

average mileage of the aftermarket brake linings used by WMATA on the general fleet. The first 

brake overhauls for the test and control buses were conducted after the buses had accumulated, 

on average, 62,000 miles. In comparison, WMATA’s general bus fleet, equipped with 

aftermarket brake linings, required a brake overhaul after approximately 25,000 miles. 

The durability of the factory brakes were so good that there were five buses that did not require a 

brake overhaul at all during the field test—and this group accumulated an average of 75,929 

miles by the end of the test, (see Table 17) 

Table 17. Brake System Overhauls: Test versus Control Fleets 

Test 

Fleet 

Bus # 

Test Fleet 

# of Brake 

Overhauls 

Test Fleet 

Total Test 

Mileage 

Test Fleet

Mileage at 

Overhaul 

Control 

Fleet 

Bus # 

Control 

Fleet 

# of Brake 

Overhauls 

Control 

Fleet 

Total Test 

Mileage 

Control 

Fleet 

Mileage at 

Overhaul 

2501 0 70,396 N/A 2513 1 84,263 58,146 

2502 0 77,102 N/A 2514 0 80,616 N/A 

2503 1 56,901 56,901 2515 1 84,042 60,327 

2504 0 69,269 N/A 2516 1 84,691 68,874 

2505 1 71,912 55,864 2517 1 65,290 63,070 

2506 0 82,263 N/A 2518 1 76,400 65,607 

2507 1 79,521 65,607 2519 1 76,936 75,112 

2508 1 78,859 67,192 2520 1 77,898 64,773 

2509 1 76,320 63,238 2521 1 72,524 61,259 

2510 1 71,584 67,110 2522 1 71,748 69,359 

2511 1 77,173 57,849 2523 1 74,021 59,247 

2512 1 56,300 47,119 2524 1 73,489 65,160 

The weekly visual inspections of the BPMSs identified a total of 69 brake “anomalies” as 

defined by the presence of a visual alert or fault signal at the display unit (see Table 18). Brake 

anomalies occurred in only 11.7 percent of the total weekly visual inspections. 

Of the 69 “unsafe” brake conditions reported by the BPMSs, 50 were confirmed to be actual 

brake system faults (see Table 19). The remaining 19 brake faults were identified as false 

positives. A brake fault was categorized as a false positive when the reported unsafe brake 

conditions could not be visually validated through an inspection of the brake assemblies. The 

false positives accounted for 27.5 percent of the total brake inspections. 

72
Table 18. Brake System Anomalies Identified by 

BPMSs During Visual Inspections 

System Yes No Total Entries 

eStroke 2 188 190 

Brake Insight 48 151 199 

StrainSert 19 182 201 

Total 69 521 590 

Table 19. False Positive Brake Faults by System 

System Total Reports Valid Reports 

False Positive 

Reports 

eStroke 2 2 0 

Brake Insight 48 40 8 

StrainSert 19 8 11 

Total 69 50 19 

The following sections are an analysis of the findings for each BPMS: 

5.3.2 MGM eStroke System Analysis 

The eStroke system has two methods of reporting brake system fault messages. The first method 

is through the use of the visual display on-board the vehicle. The second method was through the 

wireless transmittal of eStroke alerts (fault code logs) to a server residing at the maintenance 

garage, (this was accomplished by utilizing the vehicle’s AVM system). A review of information 

collected by these two methods is reviewed in the following sections. 

5.3.2.1 System Display Warnings 

Shortly after the first of WMATA’s Orion VII buses entered into service, the eStroke system 

generated numerous alerts for a dragging brake condition on each of the buses in service. WMATA 

technicians inspected the brake assemblies on several buses and verified the dragging brake 

warnings reported by eStroke. The WMATA maintenance staff determined that the buses were 

delivered from the manufacturer with an alignment problem between the brake chamber pushrod 

and the slack adjuster. The manufacturer had selected a camshaft retention method that allowed the 

removal of the automatic brake adjuster without removal of the wheel-end. The retention method 

necessitated use of a shorter S-cam shaft, which was not delivered with the brake assemblies. As a 

result, the pushrod and slack adjuster were misaligned, slowing the pushrod’s travel and causing a 

dragging brake. An interim fix corrected the pushrod alignment by installing hardened spacers. The 

permanent retrofit replaced the original camshaft with a shorter version. The misaligned brakes 

could have gone undetected for months if eStroke had not alerted maintenance of the defect, 

proving the effectiveness of the system to the maintenance staff. 

During the course of the field test, 190 visual inspections were completed on the eStroke test 

buses (bus numbers 2501 to 2504). The eStroke display alerted the inspector to brake system 

issues during two of the inspections—one a sensor fault and the other a dragging brake 

condition. The sensor fault was identified on the left rear brake assembly of bus number 2502 

73
during an inspection in June, 2007. A physical inspection of the equipment revealed that the 

cable connecting the sensor to the system was disconnected (and therefore the eStroke correctly 

identified the issue). During the previous week, a maintenance record had been generated 

because of a cracked sensor at the same location that was replaced (the cracked sensor was most 

likely caused by the previously described slack adjuster alignment issue). It is assumed that the 

cable was not properly attached after the sensor was replaced. 

5.3.2.2 eStroke System Brake Status Reports 

As noted, the eStroke fault code logs (stored on the system’s ECU) are wirelessly transmitted 

from the vehicle to server at the bus garage. The server generates automatic system reports and 

distributes them to WMATA daily via email. Fault code reports were tracked for test-group bus 

numbers 2501, 2502, 2503, and 2504. WMATA packaged the daily reports together and emailed 

them to the study team once per week. 

A review of the weekly fault code logs identified a need to add a logic filter to eStroke’s 

reporting method. The system reports a fault code for any and all “abnormalities”, as well as a 

“cleared message” code when the fault is cleared as defined by the SAE 1708 protocol. During 

the course of the test, the eStroke system generated 4,230 messages. Many of the messages were 

related to overstroke conditions caused by the axle design, dragging brakes and cracked sensors 

associated with the early brake system misalignment issues. Compounding the problems with the 

brake system itself, was an overly sensitive dragging brakes threshold setting within MGM’s 

eStroke software. WMATA stated that within 2 months of putting the eStroke-equipped buses 

into service, it recognized the need to establish a filtering method for fault codes in order to 

differentiate between “real” brake problems requiring maintenance attention—versus fault codes 

associated with lesser (or temporary) problems such as hot brakes, or the inherent sensitivity of 

the Orion bus brake system to dragging brakes (i.e. the alignment issue). Hence, WMATA 

monitored the daily eStroke reports to determine the number of messages that had to be 

generated before an “actionable” failure could be identified. In the end, WMATA only inspected 

buses that generated more than 30 alerts in a single day. Ideally, the brake system on the bus 

would have been be reconfigured to create a shorter brake stroke. Reducing the normal operating 

baseline stroke travel from 2.5 inches to 2.25 inches would significantly reduce the number of 

overstroke conditions detected by the eStroke system. In the current configuration, the pushrod 

stroke is within the acceptable FMVSS stroke range when the brakes are cold. However, when 

the current brake configuration becomes hot, the pushrod stroke can exceed 2.5 inches which is 

then logged as an overstroke condition. Such design changes however would not only require 

extensive modifications to the brake system but also changes to the orientation of the axle, 

changes Orion was unwilling to perform. 

Similarly, the study team also developed a filtering method that could be applied to the weekly 

fault code reports to better identify those instances in which a real brake system failure likely 

existed. This was accomplished by comparing the eStroke fault code messages to actual brake 

maintenance actions performed. As shown in Table 20, the 4,230 alerts generated during the 

field test could only be correlated to 24 actual maintenance records where brake repairs were 

performed, (resulting in a reliability factor of less than 1 percent). To achieve a reliability factor 

higher than 90 percent, the eStroke system had to generate at least 60 fault codes in one week at 

a single wheel location. By flagging these instances, a reliability factor of 92 percent was 

74
achieved, with 12 eStroke alerts and 11 corresponding brake system maintenance records. A 

review of maintenance records showed that system “alerts” (as defined by 60 or more fault codes 

in one week) generally corresponded to maintenance actions such as: dragging brakes; brake 

overhauls; broken eStroke hardware; and overstroke conditions. 

Table 20. eStroke System Report Filtering 

Filter 

(Alerts Per Week) 

Total Alerts Number of Related 

Maintenance Actions 

Reliability 

(%) 

No filter 4,230 24 0.5 

Greater than 20 39 15 38 

Greater than 30 27 13 48 

Greater than 40 16 12 75 

Greater than 50 13 11 85 

Greater than 60 12 11 92 

The remaining alerts from the weekly eStroke system report were identified as false-positive 

reports. Some of these alerts were the result of a software problem which was identified during 

the preliminary testing phase. Specifically, in December 2006 the eStroke monitoring system 

detected intermittent dragging brake conditions but the brakes were inspected and found to be 

operating properly. The cause of the false (or improper) dragging brake condition alert was 

identified as a signal lag between the time the sensor signal was generated and the time the 

eStroke ECU received the signal. The signals were originally sent via the vehicles’ network to 

the ECU through a J1708/J1587 connection. The lag time between signal generation and signal 

processing at the ECU was significant enough to generate inaccurate brake condition readings. 

The system was upgraded to accept and process signals directly from the signal source. As a 

result, the number of false alarms declined. 

Other alerts identified in Table 20 were likely minor faults generated as a result of a momentary 

communication loss; or because of high (but within performance limits) brake temperatures. 

Although WMATA (and the study team) have generated tailored filters to eliminate an excessive 

number of false-positive alerts, the weekly fault code report does not provide a definitive 

evaluation of the condition of the brake assembly. However, the weekly fault code reports do 

provide sufficient information such that maintenance staff can interpret the data and decide 

whether to pull the bus from service and complete a more thorough inspection. 

5.3.2.3 Hardware Failures 

During the first few months of field test, failures related to the eStroke wiring harness (that runs 

between the sensor units and the ECU) as well as the sensor units themselves, were identified as 

significant problems on the entire fleet of eStroke-equipped buses. (Note: all 24 buses that were 

the focus of this field test (12 in the “test” fleet and 12 in the “control” fleet) were equipped with 

the eStroke system since that system was ordered as standard equipment on all buses.) 

Of the 124 brake system related maintenance records collected on the test and control fleet, 34 

were generated as a result of a eStroke hardware malfunctions (“Hardware Failure” and “Sensor 

Fault” maintenance record categories, Table 16). These malfunctions (or failures) included 

chaffed wiring harnesses, pinched harnesses, cracked sensors, and loose sensors. Put another 

75
way, over the 12 month test period, every bus in the field test program experienced about 1.3 

hardware failures on average, (34 hardware failures divided by 24 buses). Over time, the 

frequency of the failures decreased for a variety of reasons including: 

 Replacement of the sensors and rerouting of the wire harnesses. 

 Correction of the misaligned brake chambers installed by the vehicle manufacturer 

 Engineering changes to MGM’s sensor harnesses and wiring that include the addition 

of a protective loom around all cables with exposed single conductive wires. 

5.3.2.4 Summary Observations (eStroke) 

Overall, WMATA’s confidence in the eStroke system, (developed through this field test 

program), led it to revise its brake maintenance intervals to eliminate a visual brake inspection 

during the “A” inspection, which occurs at the 3,000-mile interval. Instead, the technicians rely 

exclusively on the eStroke system log to identify any brake abnormalities. A thorough brake 

inspection is performed at the “B” inspection, which occurs at the 6,000-mile interval. At the 

B inspection, the technicians measure brake lining wear, measure brake stroke, verify slack 

adjuster operation, and perform other brake diagnostic procedures. 

5.3.3 Brake Insight System Analysis 

The weekly inspections of the Brake Insight-equipped buses included recording the presence of any 

alerts at the system display. The display, shown in Figure 49, consists of 14 lights. The system is 

operated by querying the status of each wheel-end. If a particular alarm was present at that location, a 

red indicator was illuminated, along with a separate indicator to identify the type of fault. 

Figure 49. Brake Insight Display 

76
During the field test, the Brake Insight system issued 48 alerts indicating dragging brakes, sensor 

faults, high temperature, worn shoe linings, and/or brake overstroke. Table 21 lists the total 

number of alerts by type. 

Table 21. Brake Insight Visual Alerts 

Bus # Overstroke 

Dragging 

Brake 

High 

Temp 

Sensor 

Fault 

Lining 

Wear Total 

2505 1 1 7 4 0 14 

2506 9 0 0 5 0 14 

2507 5 3 2 3 0 13 

2508 2 0 4 2 0 8 

Total 17 4 13 14 0 48 

5.3.3.1 Overstroke 

The Brake Insight system recorded 17 overstroke alerts on the four test buses during the field 

test. This represents 35 percent of the total alarms. In 3 of the 17 cases, there is a corresponding 

service record in WMATA’s maintenance database. Two of these records show that slack 

adjusters were serviced; the third record notes that the bus pulls to the left. For another nine of 

the overstroke alarms, the measured stroke was 2.5 inches, which is just at the adjustment limit 

for the rear brake assemblies. The alarms were cleared after the inspections. The cause of these 

alarms is assumed to be related to the original equipment manufacturer’s brake design. During 

testing, the WMATA technicians noted that design of the brakes for the Orion VII buses 

provided little margin before an overstroke condition developed. As a result, the brakes were 

found to overstroke frequently during service as the brake drum heated and expanded. After the 

buses returned to the maintenance facility and the brakes cooled, the brake stroke was within the 

allowable stroke limits. Due to this heating and cooling effect, a large number of overstroke 

conditions reported by the Brake Insight system could not be validated during a visual 

inspection. (Note: this same phenomena occurred with the eStroke system). 

The remaining five alarms could not be validated. A visual inspection found the brake stroke to 

be well within the adjustment limit. A review of the maintenance records showed that the 

WMATA technicians did not perform any brake adjustments or inspections on the identified 

buses. It is assumed these five alarms were simply false positives. 

5.3.3.2 Dragging Brakes 

The Brake Insight system detected four occurrences of dragging brakes representing 8 percent of 

the total alarms. All four alarms occurred within the first 2 months of the test program. The 

weekly inspections did not reveal any indication that the brakes were dragging. The dragging 

brake alerts were attributed to a software problem identified in January 2007. Brake Insight uses 

an algorithm to monitor and compare the current and previous positions of each stroke sensor. 

The software program’s sampling rate monitored the stroke sensors at each brake chamber too 

frequently, causing the warning thresholds for a dragging brake condition to be too sensitive. 

Brake Insight installed a software upgrade to reduce the sampling rate of the stroke sensors on 

March 9, 2007. As a result, the detected number of dragging brakes was reduced to zero. 

77
5.3.3.3 High Temperature 

The Brake Insight system recorded 13 “high brake pad temperature” alarms, representing 27 

percent of the alarms. A high brake pad temperature was detected when the system detected a 

pad temperature in excess of 685 degrees Fahrenheit, as set by the system vendor. The weekly 

inspections and maintenance records from the alarm period did not reveal any brake-related 

issues. 

Two factors could have caused the high brake pad temperature alarms. First, all of the alarms 

occurred between May and August 2007. Because of the Washington, DC, summer climate, high 

brake pad temperatures caused by the stop-and-go operating service of a transit bus are not 

uncommon. The brake pad temperatures may have reached the 685 degrees threshold during 

normal operation without causing any abnormal (or unsafe) brake conditions. After the bus 

returned to the maintenance facility or reached the end of its route and remained idle for an 

extended period of time, the brake pad temperatures would have returned to normal. Second, the 

placement of the temperature sensor in the shoe lining may have induced false pad temperature 

alarms. In fact, 7 of the 13 brake pad temperature alarms occurred after a rear brake reline 

procedure which may have resulted in incorrectly installed temperature sensors; these 7 alarms 

also occurred in conjunction with a sensor fault. After the alarm was cleared and the sensor fault 

corrected, no further alarms were generated. 

5.3.3.4 Sensor Fault 

Fourteen wheel-end sensor faults occurred during the test program, representing 30 percent of 

the total alarms. The majority of the failures were identified as communication issues, but four 

alarms were traced to wiring issues. For example, on March 9, 2007, the display on bus number 

2507 indicated a loss of communication between the S-Box (ECU) and the sensor at the right 

front wheel-end. An inspection of the hardware revealed a broken wire in the wire harness where 

the wheel-end cable was connected to the main trunk line leading to the S-Box. The wire harness 

was replaced, and the sensor fault alarm was extinguished. The cause of the broken wire was not 

identified. No actual sensor units were replaced or found to be faulty during the field test. 

5.3.3.5 Lining Wear 

As mentioned earlier, the Brake Insight system features a wear sensor imbedded in the shoe 

lining. As the lining wears out, the brake drum contacts and cuts the wire loop sensor, causing an 

open circuit and generating a lining wear alarm. No lining wear alarms were generated during the 

field test; however, on three different occasions, technicians recorded shoe lining thickness as 

having failed (minimum thickness requirement not met) the weekly inspections. All three 

failures resulted in the rear brakes being overhauled. Upon inspection, the lining wear sensors 

were installed deeper in the lining than the WMATA’s acceptable wear limit, resulting in the 

brake shoes being replaced before the sensor could be activated. 

5.3.4 StrainSert System Analysis 

The StrainSert system is designed to identify out-of-adjustment brakes, dragging brakes, oil 

soaked linings, and/or disconnected brakes. The weekly visual inspection of the StrainSertequipped buses included identifying and logging the presence of any alerts or warnings at the 

system display. The display, shown in Figure 50 consists of two counters and a scrolling display. 

78
The two counters identify the number of alerts and warnings generated at each wheel location. 

The scrolling display lists the potential brake problems at specific wheel locations. The 

StrainSert system categorizes brake problems as follows: 

 Warning. The warning message is triggered if the difference in brake force between 

brakes on the same axle is greater than or equal to 30 percent. The occurrences are 

tracked per wheel location on the system display. 

 Alarm. The alarm indicator is triggered if the system detects a difference greater than 

or equal to 40 percent in brake force between brakes on the same axle. (The out-ofservice brake criteria used by the Commercial Vehicle Safety Alliance defines the 

alarm criteria). The system’s algorithm is able to identify the specific wheel location 

that is likely to have a brake problem by comparing brake forces between axels (as 

well as between wheel-ends on the same axle), and determining which wheel-end (on 

each axle) deviates the most from the average of all wheel-ends. In Figure 50, the 

display shows that the system detected two instances of the brake force differential 

exceeding 40 percent between the left and right rear brake assemblies, with the brake 

problem likely in the left rear brake assembly. 

 Service Required. The “service required” message indicates that a wheel-end has 

exceeded a pre-determined threshold limit for the number of alarms or warnings within 

a given timeframe. For the WMATA field test, a warning or alarm must have occurred 

10 times at a single location during the same trip before the “service required” message 

appears on the display. The message indicates the wheel-end of the assembly requires 

immediate servicing. 

Figure 50. StrainSert Display 

The StrainSert-equipped buses generated 19 total alerts during the field test, indicating either a 

system warning, system alarm, or required brake service. Table 22 shows the alert distribution. 

The alerts were distributed among bus numbers 2509, 2510, and 2511. During the field test, 

79
StrainSert requested to use one of the installed systems in its labs to refine the software 

algorithms and thresholds. The system installed on bus number 2512 was removed and provided 

to StrainSert for lab testing. As a result, bus number 2512 did not record any failures. 

Table 22. StrainSert Visual Warnings 

Bus # Warning Alarm 

Service 

Required Total 

2509 1 3 5 9 

2510 1 1 2 4 

2511 4 0 2 6 

2512 0 0 0 0 

Total 6 4 9 19 

5.3.4.1 System Software 

During the course of the test, StrainSert issued one software update and two threshold updates to 

improve the performance and accuracy of the system. The software update, issued December 13, 

2006, upgraded the display unit software to provide wheel-end specific data. The original 

software tracked the alarms and warnings at the vehicle level but did not specifically identify the 

wheel-end that triggered the alert. The revised system software tracks the alarms and warnings 

for specific wheel-ends, incrementing the specific counter each time the ECU detects an alert. 

As noted, the StrainSert system compares brake forces among wheel-ends and generates alerts 

if/when the differential readings exceed a pre-determined threshold. However, during early 

development, StrainSert found that comparing brake forces during light (or momentary) braking 

events can be problematic as the braking forces can vary significantly between wheel-ends even 

though the barking system is properly adjusted and in good working condition. The reason 

appears to be that no matter how perfectly balanced brake actuation systems are, contact between 

the brake pad and brake drum at each wheel end will inevitably occur at a different instant (in 

time) and with slightly different force. These difference are of no consequence as brake control 

pressure increases, but during mild or momentary braking, the brake forces at each wheel-end 

could be significantly different—if only for a moment, and until control pressure increases. 

As a result, throughout the field test, StrainSert refined the “threshold braking force” value which 

determines the point at which the system begins comparing braking force between the left and 

right brake assemblies. When the StrainSert system was first installed, the system compared the 

braking force between the right and left brake assemblies if the braking force threshold at either 

wheel end exceeded 250 pounds in resultant force. During early testing, the StrainSert system 

produced an excessive number of alarms and warnings even though subsequent investigations 

confirmed the braking systems were functioning properly. During mild braking maneuvers the 

overall braking force at the wheel ends is correspondingly low—and the absolute differences in 

braking forces between wheel ends is relatively small. However, the relative (percentage) 

difference in brake forces at each wheel-end (on the same axle) can be significant—and this 

percentage difference is what was causing the excessive alerts on the StrainSert system. 

To improve the accuracy of the StrainSert system, the brake force threshold was raised from 250 

pounds to 2,500 pounds to ensure that the system only compared brake forces during a heavy 

80
application of the brakes. The threshold increase effectively reduced the number of false-positive 

alarms, and in fact the system did not report any unbalanced braking conditions between 

December 2006 and May 2007. StrainSert was concerned that the system was now too 

insensitive, and they reduced the brake force threshold to 1,800 pounds in June 2007. As a result, 

19 system alerts were generated between June 2007 and the end of the field test. 

5.3.4.2 Brake Overhauls 

During the field test, three brake overhauls were performed on the StrainSert-equipped buses. 

The StrainSert system gave advance notice of the need for one of the three brake overhauls. On 

September 30, 2007, the StrainSert system on bus # 2510 issued a “service required” warning on 

the right rear brake assembly. At the subsequent inspection, the rear brake linings were identified 

as worn, and the rear brake stroke was measured as 2.5 inches. The shoe lining was worn to the 

point that its performance was diminished, which reduced the force on the anchor pin at a set 

brake application pressure relative to the other three brake assemblies. Following the brake 

overhaul, the system issued no further alarms, the brake lining met the wear criteria, and the 

brake stroke was measured as 2.25 inches. 

The remaining two brake overhauls occurred on bus numbers 2509 and 2511. On these two 

buses, the shoe linings were replaced prior to the point at which the brake would have degraded 

sufficiently to cause an alert. 

5.3.4.3 Overstroke and Maintenance Actions 

On two separate occasions the StrainSert system correctly identified overstroke conditions. In the 

first instance, bus number 2509 had three “service required” alerts at the left front brake 

assembly between September 29, 2007, and October 28, 2007. During the visual inspections, the 

brake stroke at the front left brake was measured as 2.0 inches, just at the adjustment limit for the 

front brake assemblies. 

In the second instance, the StrainSert system issued an alert because of intermittent rear brake 

force differentials in excess of 30 percent for the left rear brake assembly of bus number 2510. 

The force recorded on the left rear brake was 30 percent higher than on the right rear brake. The 

alert coincided with a maintenance record reporting that a driver had experienced a brake 

shimmy while in service. A visual inspection measured the brake stroke as 2.5 inches, the 

maximum allowable stroke measurement. As a result, the left rear brake assembly was adjusted. 

After the corrective action was completed and the system was reset, the StrainSert system did not 

generate any further warnings. 

5.3.4.4 StrainSert Hardware Failures 

There was one equipment anomaly that occurred on the StrainSert-equipped buses during the 

field test. During a weekly inspection, the StrainSert system measured a large, constant force 

load of 21,000 lbs on one of the StrainSert anchor pins of bus number 2509. Because the bus was 

stationary, the StrainSert system should not have been recording a load force at any of the anchor 

pins. After troubleshooting the left front brake equipment and the anchor pin, the cable to the 

anchor pin was inspected and a gash was identified approximately 3 inches from the anchor pin 

connector (see Figure 51). The gash caused a short between the exposed metal shielding and the 

copper wiring. After installing a new cable, the force load on the left front anchor pin was 

81
measured at 0 pounds, as expected. No similar failures were identified on other vehicles during 

the course of the test. 

Figure 51. StrainSert Anchor Pin Cable Failure 

5.3.4.5 Unsubstantiated System Alerts 

The StrainSert system generated eight false-positive alerts. Bus number 2511 generated five 

warnings at the right front brake assembly. The five alerts spanned nine visual inspections, 

between June 24, 2007, and August 19, 2007. In most cases, the brake overstroke was measured 

at or below 1.5 inches. No maintenance actions or visual brake inspections could provide a basis 

for the warning. A sixth warning was generated on bus number 2511 on November 4, 2007. A 

warning was generated at the left front brake assembly. Again, no failures were identified during 

the visual inspection, and no maintenance records were generated during this time. After the 

alarm was cleared, no further warnings were generated during the test. 

The final two false-positive alerts were generated on bus number 2509 at the left front brake 

assembly. The alerts occurred during two consecutive visual inspections. Before the false 

alarms, a brake overstroke condition had been identified at the same location. A maintenance 

record was generated to adjust the brake stroke. It is assumed that the two false reports were a 

direct result of the system not being reset after the overstroke condition was corrected. If this 

assumption is correct, the StrainSert system only generated six false-positive alarms during the 

year-long field test. 

5.3.5 PBBT System Analysis 

The PBBT is a brake monitoring system installed at the WMATA maintenance garage that 

served as a reference check of actual braking performance (see Section 5.1.4 for additional 

information on the PBBT used for this field test). This system was used to document (or 

baseline) the performance of the vehicle’s braking system as needed throughout the field test. 

Along with providing a check of total vehicle brake performance, the PBBT also was used to 

monitor the force differential between the left and right brake assemblies, as well as the 

differences in total brake force between the front and rear wheels. The PBBT indicated a possible 

82
brake defect if it detected a difference between the braking force at the left and right wheel ends 

greater than 30 percent. 

Originally, the test and control vehicles were to be tested once a month on the PBBT, and the 

results were to be compared to the findings of the onboard BPMSs. However, the test plan 

required modifications after it became apparent that PBBT system data could not be compared 

with the onboard BPMS data. The condition of the vehicle’s braking system could well have 

been be in a different state of repair at the time of the PBBT test versus when various alerts were 

recorded by BPMSs. In other words, the temporal difference between data that was collected by 

the PBBT versus the BPMSs effectively clouded the analyses. 

The PBBT performed 179 bus inspections during the field test. Table 23shows the distribution of 

the inspections by bus number for both the control and test fleets. Each inspection recorded the 

rolling resistance and service brake force for both the front and rear axles, providing 716 

readings of brake force differential (between the left and right wheels on each axle). 

Table 23. Total PBBT Inspections 

Test 

Fleet 

Bus # 

Test Fleet 

# of 

Inspections

Control 

Fleet 

Bus # 

Control Fleet 

# of 

Inspections 

2501 9 2513 7 

2502 9 2514 8 

2503 8 2515 6 

2504 7 2516 6 

2505 8 2517 6 

2506 7 2518 7 

2507 7 2519 6 

2508 8 2520 8 

2509 9 2521 6 

2510 8 2522 8 

2511 6 2523 8 

2512 9 2524 8 

Of the 716 brake force differential tests completed, 74 (10 percent) were identified with a brake 

force differential of greater than 30 percent, which is defined as a fault or failure. During the test 

period a mechanical failure of the PBBT was identified that had an effect on the test data. The 

data shown in Table 24 includes warnings generated as a result of PBBT hardware failure. 

Table 25 shows the revised data and identifies only those warnings that occurred when the test 

stand was operating properly (49 fault/failures, or 6.8 percent of total tests). 

83
Table 24. PBBT Measured Warnings**

Test 

Fleet 

Bus # 

Test Fleet 

# of Faults 

Control 

Fleet 

Bus # 

Control 

Fleet 

# of Faults 

2501 6* 2513 2* 

2502 1 2514 3* 

2503 6* 2515 2 

2504 3* 2516 0 

2505 3 2517 2 

2506 2 2518 1 

2507 1 2519 4* 

2508 4* 2520 5* 

2509 3 2521 1 

2510 6* 2522 6* 

2511 2 2523 6* 

2512 4 2524 1 

An asterisk (*) identifies the warning totals affected by the 

hardware failure, deeming that data inaccurate. 

Table 25. Revised PBBT Measured Warnings 

Test 

Fleet 

Bus # 

Test Fleet 

# of Faults 

Control 

Fleet 

Bus # 

Control 

Fleet 

# of Faults 

2501 4 2513 1 

2502 1 2514 2 

2503 4 2515 2 

2504 2 2516 0 

2505 3 2517 2 

2506 2 2518 1 

2507 1 2519 2 

2508 1 2520 2 

2509 2 2521 1 

2510 3 2522 4 

2511 2 2523 4 

2512 2 2524 1 

5.3.5.1 PBBT Hardware Failure 

During the inspections in October 2007, a hardware failure was identified on the PBBT. The left 

roller assembly was exhibiting a rolling resistance that was two times the rolling resistance of the 

right roller assembly. During testing of bus number 2510, the PBBT’s left roller assembly locked 

up prematurely, almost immediately upon brake application. The incident repeated itself while 

testing two additional buses. An inspection of the equipment revealed no visual problems with 

 

** As explained in the “PBBT Hardware Failure” section, the data in Table 24 was found to be inaccurate because the PBBT test stand failed. 

Table 25 excludes the readings taken during the PBBT failure. 

84
the left roller assembly, motor, or chain. The manufacturer was contacted, and a date was 

scheduled to troubleshoot and repair the PBBT. 

On December 12, 2007, the manufacturer determined that the weld securing the roller to the 

roller bearing on the PBBT’s left roller assembly was broken, as indicated by the arrow in the 

left side of Figure 52. As a result, the disc that had been secured within the roller became 

exposed. When functioning properly, the disc is secured within the roller, as shown in the right 

side of Figure 52. It is responsible for holding the roller in position, centered between the PBBT 

frame, and turning the roller during a test. With the failed weld, the roller assembly was not 

centered within the frame and had actually shifted far enough to the left to contact the PBBT 

frame. By contacting the PBBT frame, the rolling resistance for the left roller assembly had 

increased significantly, as compared to the right roller assembly. The manufacturer stated that 

the weld failure in this location was a known design flaw that it had identified and resolved on 

the new PBBT assemblies. A new roller was installed on December 19, 2007. After the new 

roller was installed, the PBBT operated properly. 

Figure 52. Failed Weld at Left Roller Assembly 

After the questionable brake performance measurement data (i.e. data taken while the PBBT was 

not functioning properly) were removed from the overall data set, the remaining data revealed 

that two of the test buses experienced the highest incidence of brake force differential. On 

December 19, 2006, the rear axle of bus number 2502 had a rolling resistance differential of 51 

percent and a service brake force differential of 4 percent. Neither the brake performance 

monitoring system (eStroke) nor the maintenance records logged a system fault during this time. 

On June 21, 2007, the rear axle of bus number 2503 had a rolling resistance differential of 51 

percent and a service brake force differential of 7 percent. Again, neither the on-vehicle braking 

system (eStroke) nor the maintenance records logged a system fault during this time. The cause 

of the failures could not be identified. 

5.3.6 Brake Technician Interviews 

The interview process with brake technicians was an open discussion about the fleet study, brake 

maintenance practices, and the BPMSs. The responses related to the individual BPMSs are 

presented below. 

85
5.3.6.1 eStroke 

Of the three BPMSs tested in the fleet study, the participants were most familiar with the eStroke 

brake monitoring system. The entire fleet of 40-foot transit buses operating out of the Four-Mile 

Run maintenance facility is equipped with the factory-installed eStroke system. 

The eStroke system experienced few failures during the field test. The most common failure was 

the result of a short circuit in the wire harness at the wheel-end, resulting in a complete 

replacement of the wiring harness. The stroke-sensing hardware mounted inside the brake 

chamber experienced minimal failures. The reliability of this hardware was attributed to the 

equipment’s protection from the environment. 

The eStroke system successfully detected the two most common brake problems—dragging 

brakes and slack brakes (overstroke). Initially, the buses were delivered from the manufacturer 

with the brakes shimmed incorrectly, resulting in a dragging brake condition. As discussed 

earlier, the eStroke system detected the dragging brake condition, alerted the maintenance staff 

of the problem, and ultimately increased brake life. Also, the early detection of slack brakes 

identifies an improperly functioning slack adjuster and reduces driver complaints concerning 

hot brakes. 

WMATA’s confidence in the eStroke system resulted in the elimination of the measurement of 

the pushrod stroke during the “A” preventative maintenance inspection (every 3,000 miles). 

Originally, the brakes were inspected on each bus during the A inspection and the B inspection 

(at 6,000 miles). The eStroke system is now used to monitor the brake system prior to the more 

complete B inspection. During the B inspection, the brake system is inspected thoroughly, 

including brake stroke travel. The B inspection has also been updated to inspect the eStroke 

cabling for any defects and to verify proper functioning of the eStroke sensors. 

5.3.6.2 Brake Insight 

Unlike the eStroke system, which integrates sensor apparatus within the brake chamber, the 

Brake Insight equipment is mounted outside the brake chamber. Technician raised concerns 

about the vulnerability of the externally mounted system hardware to damage from road debris. 

The use of lining wear sensors was deemed ineffective by the technicians. As noted earlier, the 

wear sensors never flagged a brake lining for replacement because the sensor was placed too 

deep in the brake shoe. Also, current maintenance procedures require the inspection of brake 

assemblies every 6,000 miles, which results in brake shoe replacement before the wear sensor 

flags the need for replacement. 

The ability to measure brake temperature through the use of a thermocouple was identified as 

beneficial. The thermocouples helped determine whether the brake had properly burnished after 

a brake overhaul. Also, it was suggested by the technicians that the thermocouples could possibly 

reduce the number of driver complaints caused by “hot brakes.” 

86
5.3.6.3 StrainSert 

The StrainSert system was cited by the technicians as an effective tool for detecting brake 

problems. Unlike stroke-sensing brake monitoring systems, StrainSert can detect brake 

performance changes caused by oil-soaked brake linings, which warn of a leaking wheel seal. 

Technicians noted that when performing brake overhauls, the brake anchor pins, springs, and 

brake shoes are discarded and replaced with new hardware. As a result, care is not taken in 

removing the old brake hardware. For example, an original anchor pin is often stuck in the preoverhaul brake assembly. To free the anchor pin from the brake assembly, the technician can 

break the pin free by using a hammer. This overhaul procedure is not appropriate for StrainSert’s 

instrumented anchor pins. Pounding a StrainSert’s anchor pin to remove it from the assembly 

could damage the internal strain gauges. As a result, the damaged StrainSert anchor pins could 

negatively affect the system’s ability to accurately detect brake problems. 

The technicians also provided suggestions for future system improvements. One suggestion was 

to further develop the StrainSert software to communicate with an AVM. 

5.4 OBSERVATIONS AND CONCLUSIONS 

The team evaluated three different BPMSs during the year-long field test of 12 urban transit 

buses operated by WMATA. Two of the systems monitored brake performance by monitoring 

pushrod stroke, and the third system measured the strain exerted on the brake anchor pins. Below 

are the observations regarding the performance of the systems evaluated under this program. 

Onboard BPMSs were found to favorably affect WMATA’s inspection practices. WMATA 

inspects buses at 3,000- and 6,000-mile intervals. With more than 200 buses operating out of a 

maintenance facility, these inspections require a significant amount of time. For the A inspection 

(3,000 miles), WMATA has begun relying on the BPMS to assess the brakes. This is reducing 

inspection times and allowing more buses to be inspected in a given period. 

The BPMSs were found to withstand the rigors of the urban transit operating environment. A 

single sensor failure occurred during the roughly 1 million miles traveled by the 12 test buses 

during the test program. The maintenance actions performed on the buses were limited to broken 

wires, loose connectors, and sensor adjustments. 

In transit service, information from onboard monitoring systems must reach maintenance 

personnel in a timely fashion to be useful. WMATA’s buses are equipped with a controller area 

network (CAN) databus and WiFi transmitter capable of wirelessly transmitting alarms from a 

bus onboard monitoring system to a server housed at the maintenance garage. This data is 

offloaded each time the bus returns to the garage and emailed to maintenance supervisors. The 

eStroke systems evaluated under this program were integrated into this CAN network. The study 

team found that buses with eStroke alarms were inspected and problems corrected in a timely 

fashion (same day or next day), whereas 1 week or more could elapse before brake problems 

were addressed on buses with monitoring systems that only communicated via an in-vehicle 

display. Of note, transit managers limit the amount of prognostic maintenance data operators 

receive because of union work rules. 

87
88

MGM’s eStroke system aided in identifying a manufacturing issue with the alignment between 

the brake chamber and slack adjuster on the new Orion VII low-floor buses. The foundation 

brake manufacturer selected a camshaft retention method for the Orion VII buses that made it 

possible to remove the slack adjuster without removing the wheel-end. This retention method 

required a shorter S-cam shaft and a specific installation method. This caused an alignment issue 

that in-turn caused the rear brakes to drag, which the eStroke system identified. Inspections 

performed as a result of the eStroke alerts revealed the cause of the dragging brake to be this 

alignment issue. The vehicle and brake manufacturer corrected the issue under warranty. 

Based on interviews, the prognostic alerts from the BPMSs provided technicians with useful 

information to quantify driver complaints and reduce complaint frequency. Complaints with 

brakes are time consuming to troubleshoot because they require an inspection performed on a 

lift. Technicians commented that BPMSs reduce the number of driver complaints and provide 

real-time information used to decide whether to hold the bus from service. 

The BPMSs evaluated in this program did not indicate brake linings in need of replacement. All 

but one of the test buses underwent a rear brake overhaul at roughly 70,000 to 80,000 miles. 

None of the systems evaluated triggered an alarm before these rebuilds. Of note, the Brake 

Insight system featured a wire-loop lining wear sensor embedded in the shoe lining. 

Unfortunately, the sensor embedded in the lining was lower than the minimum thickness criteria 

used by WMATA to replace shoe linings. 

Onboard BPMSs provide a new source of information for technicians to use to identify and 

address brake issues. As with any new data source, time is required to understand, interpret, and 

obtain confidence in the data generated by these monitoring systems. WMATA and the study 

team went through this process with the systems evaluated in this program. They worked with 

the system vendors to tailor algorithms to minimize false positives and improve system 

reliability. The algorithms were modified to function more effectively with a foundation brake 

setup that operated close to adjustment limits, and in an operating environment that cycled the 

brakes frequently causing hot brake conditions. Based on previous independent testing and 

participation in this program, WMATA has confidence in BPMSs and plans to equip all future 

buses with a BPMS system. 
6. REFERENCES 

1. Federal Motor Carriers Safety Administration (FMCSA). (2006). Large Truck Crash 

Causation Study (LTCCS). FMCSA; Washington, DC. Accessed online June 25, 2009: 

http://ai.fmcsa.dot.gov/ltccs/default.asp 

2. Kreeb, R. M., Nicosia, B. T., & Fisher, P. J. (2003). Commercial Vehicle Tire Condition 

Sensors Task Order 5 (FMCSA Report No. FMCSA-PSV-04-002). FMCSA; Washington, 

DC. Accessed online June 25, 2009: http://www.fmcsa.dot.gov/facts-research/art-publicreports.asp 

3. Kreeb, R., Nicosia, B., Skorupski, D., & Radlinski, R. (2003). On-board Sensors for 

Determining Brake System Performance Task Order 3 (FMCSA Report No. FMCSA-PSV04-001). FMCSA; Washington, DC. Accessed online: July 1, 2009: 

http://www.fmcsa.dot.gov/facts-research/art-public-reports.asp 

4. Brady, S., Fisher, P., Kreeb, R., & Nicosia B. (2007). Tire Pressure Monitoring and 

Maintenance Systems Performance. Task Order 8 (FMCSA Report No. FMCSA-PSV-07-

001). FMCSA; Washington, DC. Accessed online June 25, 2009: 

http://www.fmcsa.dot.gov/facts-research/art-public-reports.asp 

5. Clever Devices, Plainview, NY; www.cleverdevices.com 

6. MGM Brakes, Charlotte, NC; www.mgmbrakes.com 

7. GeoDevelopment, distributed by Truck Trailer Transit, Detroit, MI; www.tttonline.com 

8. StrainSert, West Conshohocken, PA; www.strainsert.com 

9. WABCO, Troy, MI; www.wabco-auto.com 

10. HCI Corporation, Cerritos, CA; www.tiresafeguard.com 

11. Michelin North America, Greenville, SC; www.michelintruck.com 

12. Link Radlinski, East Liberty, OH; www.linkeng.com 

13. Maintenance Management Systems, Inc., Chicago, IL; www.mmgts.com 

89

APPENDIX A—DATA COLLECTION FORMS 

91
92