NBSIR 77-1213

Skid Resistance Measurement

Tests of New FHWA Reference

Systems at the Eastern Field

Test Center

R. W. Kearns and J. F. Ward

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

February - April 1976

Final

Issued March 1 977

Prepared for Federal Highway Administration

Office of Development

Washington, D.C. 20590
(

•

4 -

. I
*««

NBSIR 77-1213

SKID RESISTANCE MEASUREMENT

TESTS OF NEW FHWA REFERENCE

SYSTEMS AT THE EASTERN FIELD

TEST CENTER

R. W. Kearns and J. F. Ward

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards.

National Bureau of Standards

Washington, D.C. 20234

February - April 1976

Final

Issued March 1 977

Prepared for Federal Highway Administration

Office of Development

Washington, D.C. 20590

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Acting Director
-

k

.
Federal Highway Administration Policy

The opinions, findings, and conclusions expressed in this publication

are those of the authors, and not necessarily those of the Federal

Highway Administration.

CONTENTS

Page

1. INTRODUCTION 2

PREPARATION, MEASUREMENT, AND ADJUSTMENT OF SUBSYSTEMS 3 2.1. Test Preparations 3 2.1.1. Auxilliary Test Equipment 3 2.1.2. Test Tires 4 2.2. Skid Trailer Weight Distribution 4 2.3. Tow Vehicle Displacements 6

2.4. Speed Measuring Subsystem 6

2.5. Pavement Wetting Subsystem 10

2.5.1. Flow Rate Measurement 10

2.5.2. Water Distribution 15

2.5.3. Wetted Width Measurement 15

2.6. Force Measuring Subsystem 27

2.6.1. Orientation of the Transducer 27

2.6.2. Vertical Load Channel Calibration 2 7 2.6.3. Traction Channel Calibration 30

2.6.4. On-Board Calibration Signal Equivalent Values 30

2.6.5. Experimental Determination of the Effective

Unloading Constant 32

2.7. Suspension Subsystem 32

2.7.1. Trailer Displacements due to Static Traction 33

a. Displacement of the Test Wheel Hub 33

b. Displacement of the Test Tire Interface

c. Displacement of the Trailer Body through

33

Roll Axis Rotation 35

d. Displacement of the Trailer Body through

Pitch Axis Rotation 35

e. Displacement of the Trailer Body through

Yaw Axis Rotation 38

2.7.2. Suspension Characteristics under Transient

Loading 38

THE DYNAMIC SKID RESISTANCE MEASUREMENT OF SELECTED SURFACES

AND ARS-IRS CORRELATIONS 42

3.1. Surfaces selected 42

3.2. Tests Planned 42

3.3. The Test Matrix 44

3.4. Tests Accomplished 64

3.5. Control of Operating Variables 64

3.6. Measurement of Uncontrolled Variables 71

3.7. The Correlation Test Plan 71

3.8. Test Matrix Results 72

3.9. Ranking of the Sources of Dispersion 77
Page

3.10. Deviation from Desired Test Speeds 86

3.11. Skid Resistance Speed Gradients 92

3.12. Supplementary Comparisons between Systems 94

3.13. The Correlation Equations 101

3.14. Use of the Correlation Equations 101

4. THE IRS GROUND STATION 105

5. RECOMMENDATIONS 107

6. REFERENCES 110

Appendix A. Flow Turbine Calibration Results 111

Appendix B. The Test Matrix 121

ii
ILLUSTRATIONS

Page

1. Test tire radial spring rate test set-up 5 2. Change in tow vehicle hitch height with load 8

3. Change in tow vehicle rear wheel radius with load 9

4. Radar check of vehicle speed 11

5. Simultaneous speed data recorded by the stationary radar

and the ARS 1 and IRS tow vehicles in motion (fifth

wheels) 12

6. Tandem arrangement for comparison of flow rate

measurements 14

7. ARS 1 static water distribution at 40 mph (64 km/h) 16

8 . ARS 2 static water distribution at 40 mph (64 km/h) 17

9. ARS 3 static water distribution at 40 mph (64 km/h) 18

10. IRS static water distribution at 40 mph (64 km/h) 19

11. Measurement of the stream width from an ARS nozzle at the

level of impact 20

12. Impact on the pavement of the stream from a stationary

ARS nozzle 21

13. Measurement of water trace width after the trailer

has passed 24

14. Wetted width by delayed measurement on pavement 25

15. ARS 1 pavement wetting at 40 mph (64 km/h) 26

16. A simulated traction force being applied to a locked

wheel through an air-bearing force plate 28

17. Displacement of the test wheel hub due to static traction. . 34

18. Roll displacement of the trailer body due to static

traction 36

19. Pitch displacement of the trailer body due to static

traction 37

20. Yaw displacement of the trailer body due to static

traction 39

21. Effect of different damping factors on second-order

linear systems 41

22. ARS 1 axle-to-surface transient response 43

23. Weather conditions - dynamic tests, March 30 - April 8 , 1976. . 65

24. Weather conditions - dynamic tests, April 19 - April 29, 1976 66

25. Test matrix results, surface 2 23

26. Test matrix results, surface 3 74

27. Deviations from the mean skid number observed for tires

of groups 1 and 3, from skid resistance measurements on surface 2 78

28. Deviations from the mean skid number observed for tires

of groups 2 and 4, from skid resistance measurements on surface 3 79

iii
Page

29. Deviations from the mean skid number observed for the

lanes of surfaces 2 and 3 80

30. Average deviations from nominal speed observed during skids

at 18, 20, and 22 mph (29.0, 32.2, and 35.4 km/h) gg

31. Average deviations from nominal speed observed during skids

at 38, 40, and 42 mph (61.1, 64.4, and 67.6 km/h) 89

32. Average deviations from nominal speed observed during skids

at 58, 60, and 62 mph (93.3, 96.5, and 99.8 km/h) 90

33. Skid resistance as a function of speed, calculated for

surfaces 2 and 3 at EFTC 9 3 34. Skid resistance speed gradients calculated from measurements on surface 2 at EFTC 95

35. Skid resistance speed gradients calculated from measurements on surface 3 at EFTC 96

36. Direct comparisons between systems on the last day of

testing 100

37. Use of the correlation equations to predict IRS values. . . . 104

38. Uncertainty increase in a two-stage correlation 106

IV
TABLES

Page

1. Skid trailer static load distributions 7 2. Wetted width measurements 22

3. Hitch heights for optimum transducer orientation 29

4. On-board calibration values 31

5. Test plan sequence 45

6. Supplementary test plan sequence 67

7. Summary of direct comparisons between skid measurement

systems 69

8. Test matrix results, surface 2 75

9. Test matrix results, surface 3 76

10. Ranking of possible sources of dispersion in ARS 1 results 81

11. Ranking of possible sources of dispersion in ARS 2 results 82

12. Ranking of possible sources of dispersion in ARS 3 results 83

13. Ranking of possible sources of dispersion in IRS results . . 84

14. Ranking of possible sources of dispersion in the

overall results 85

15. Dispersion sample standard deviations 87

16. Mean deviation from nominal speed 91

17. Measured and calculated skid resistance speed gradients. . . 97

18. Difference between two sets of average SN measurements ... 99

19. Correlation summary 102

20. Correlation equations 103

v

SKID RESISTANCE MEASUREMENT TESTS

OF

NEW FHWA REFERENCE SYSTEMS

AT THE

EASTERN FIELD TEST CENTER

February - April, 1976

ABSTRACT

The Federal Highway Administration

(FHWA) is developing a program to improve

the method of measuring wet weather pavement skid resistance (SN) and to reduce the

variation in results. At the national level, an interim reference system (IRS) is maintained and operated by the National Bureau

of Standards. At the regional level, an area reference system (ARS) is maintained and

operated at each FHWA Field Test Center.

Intercomparisons between these reference

systems and the highway measuring systems at

the state level provide measurement assurance.

In this report, the first correlations

between three identical, newly manufactured

systems (ARS 1, 2, 3) and the IRS are given.

Computed standard deviations of mid-range

predicted SN values are typically less than

0.1 SN. SN is given as a function of test

speed for each system, on two test surfaces.

Speed gradients of SN are found to be characteristic of the surface.

The test program is explained, from

preparations and calibrations of subsystems

through dynamic measurement on the surfaces.

Controlled and uncontrolled variables are identified, discussed, and in some cases,

investigated experimentally. A ranking of

the sources of dispersion is given.

A ground station for improved SN calculation precision and on-site statistical

analysis, operated in parallel with traditional SN data acquisition methods, is found

to perform reliably.
Key Words: Accident reduction, skidding;

correlation, skid resistance; highway

safety; measurement, skid resistance;

pavement, skid resistance; pavement wetting

system; skid resistance, measurement;

tire-pavement interface forces; wet pavement skid resistance.

1. INTRODUCTION

The measurement of the skid resistance of highways, under wet

weather conditions, is part of the Federal Highway Administration (FHWA)

skid accident reduction program. Currently, the majority of state

highway departments measure pavement skid resistance with locked-wheel

skid measurement systems, which are reported to be in substantial com- pliance with the requirements of ASTM Designation E 274-70 [1].* To

reduce the variability between measurement results from different designs of systems meeting these requirements and to improve the precision

of the measurements made by a given system, FHWA is developing a hierarchical system for the measurement of pavement skid resistance [2].

At the national level, the FHWA program provides for an interim reference system (IRS) maintained by the National Bureau of Standards (NBS)

At the regional level, the program provides for area reference systems

(ARS) maintained at FHWA Field Test Centers (FTC). At the state level,

skid resistance measuring systems are maintained to inventory and rank

the condition of the highways in accordance with their skid resistance.

The plan to assure measurement consistancy between the states includes

the monitoring and development of test procedures and equipment by NBS

to relate the measurements made within the hierarchical system to the

national measurement system.

The measurement assurance program for FTC and ARS includes the

periodic evaluation of test facilities and procedures, determination of

the performance characteristics of the ARS and its subsystems, measurement of selected reference surfaces maintained at the FTC with the IRS,

and correlation of the measurements made by the ARS and IRS. The program

also includes the evaluation of new test equipment and procedures, and

experimental studies for the development of numerical data, where appropriate, to identify and quantify sources of difference in the skid

number measurements. As a result, improvements in procedure and equipment are expected as the program develops with the increased measurement

capabilities being transferred to the highway measurement systems at the

FTC.

This report describes tests conducted under the direction of NBS

personnel at the FHWA Eastern FTC (EFTC), East Liberty, Ohio, in the

spring of 1976. At that time, three new skid measurement systems were

* Numerals in brackets denote the literature references cited at the

end of this report.

2
tested along with the IRS. The new systems were designed and manufactured for use as area reference systems. Subsequently, FHWA has assigned

the unit described herein as ARS 1 to the Central/Western FTC (C/W FTC),

College Station, Texas, and has assigned the unit described as ARS 3 to

the EFTC.

2. PREPARATION, MEASUREMENT, AND

ADJUSTMENT OF SUBSYSTEMS

Previous results from skid trailer test programs indicated the

measurements lacked precision, agreement among trailers using selfwatering systems was poor, and the causes for the differences between

trailer measurements were difficult to define [3]. To identify sources

of these differences, certain preparations were made and auxiliary

equipment and experiments were designed to measure, and intercompare

with the IRS, the performance characteristics of the components and

subsystems. When conditions were found to be outside the ASTM re- quirements or conditions were found that would adversely affect the

total performance of the ARS, adjustments and modifications were made or recommended

.

2.1. Test Preparations

Auxiliary test equipment was prepared at NBS to supplement the

facilities of the Field Test Center. Test tires were obtained and prepared by examination and break-in.

2.1.1. Auxiliary Test Equipment

Skid resistance is reported in terms of skid number (SN) , which is

determined from the force required to slide a locked test tire at a stated speed in the presence of a water layer applied to the pavement

ahead of the test tire, divided by the effective wheel load and multiplied by 100. The speed, the thickness of the water layer, and forces

at the tire-pavement interface are the principal quantities to be measured. To relate these measurements to the national measurement system,

auxiliary test equipment was designed, using component types which are routinely calibrated in various NBS laboratories.

A differential radar speed meter, whose time base was calibrated

against a standard frequency and whose radiated frequency is specified

to be well within limits required for its 0.1 miles per hour (mph) (0.16

kilometers per hour (km/h)) resolution was used to record the speed of

the system during a skid resistance measurement (sec. 2.4).
A turbine flow rate meter was calibrated to a 0.6 gallons per

minute (gpm) (2 liter per minute (1/min)) uncertainty in the Fluid

Meters Section. Calibrations were obtained for the turbine both alone

and as part of a digital differential flow rate system having a 0.1 gpm

(0.4 1/min) resolution. The system is part of the on-board IRS instrumentation, but is adaptable to measuring the flow rate in the ARS

(sec. 2.5).

Two load cells, calibrated to a 0.25 pounds-force (lbf) (1.1 newton

(N) ) uncertainty in the Engineering Mechanics Section, were utilized in

a test fixture to apply simultaneously known traction and load forces to

a force plate during its calibration [4], The force plate was used

subsequently to calibrate the force transducers of the trailers (sec.

2 . 6 ).

Other auxiliary test equipment included a voltmeter, calibrated to

1 millivolt uncertainty by the Electrical Instruments Section; an inflation pressure gage, calibrated to 0.05 pounds-force per square inch

(psi) (345 pascals (Pa)) uncertainty by the Pressure and Vacuum Section;

and an automatic sequential displacement recording system (sec. 2.7).

2.1.2. Test Tires

In preparation for the testing at EFTC, each of the ASTM E 501 test

tires [5] to be used was mounted on a wheel, balanced and broken in with

a drive of approximately 200 miles (320 km). In addition, a representative sample of the tires was examined at NBS to verify the nominal

radial spring rate of 1280 lbf /in (224 kN/m) at 24 psi (165 Pa) inflation pressure under 1085 lbf (4800 N) load after break-in. The value

was determined from radial load versus deflection data obtained with the

test setup shown in figure 1.

2.2. Skid Trailer Weight Distribution

To measure the static weight distribution of the test trailer, a vertical load cell was placed under the hitch and a force plate was placed under each tire of the trailer. E 501 test tires were used. The

fifth wheel, in the ARS trailer design, bears part of the load when in

operation and was placed in operating position during the measurement.

The piping of the left nozzle was filled with water, the air-bearings of

the force plates were inflated, and the trailer was oriented. Load

readings were then taken simultaneously and weight adjustments were made

to bring the readings into the range of desired values.

The ARS trailers were not provided with adjustable trim weights or fastenings for weights. Each measurement was interrupted while EFTC

personnel fabricated the weights needed. To minimize delay, other

testing proceeded after the desired weights had been temporarily taped

in place. Later the weights were fastened more securely but time did

not permit a re-check of the static loads.

4
Figure 1. Test tire radial spring rate test set-up.

5
It is believed that all equipment contributing to the weight of the

new ARS trailers had been installed by the time of the load measurements. However, they had only a prime coat of paint. An undetermined

weight due to finish coats was added to ARS 1 after the static testing

period. ARS 2 and ARS 3 went through dynamic testing with the prime

coat only.

The measured load distributions are shown in table 1.

2.3. Tow Vehicle Displacements

Changes in the tow vehicle hitch height and rear wheel radius were measured as a function of crew weight and water load. Changes in hitch

height affect the skid number measurement in two ways; the vertical load

reduction factor is altered and the transducer is rotated such that a component of the vertical load may be measured as traction (sec. 2.6).

Changes in tow vehicle rear wheel radius change the water pump rate.

Figure 2 shows the results for hitch height. For ARS and IRS, a rise on the order of 0.2 in (0.5 cm) occurred with removal of 800 lbf

(3560 N) water load. The hitch was more sensitive to water load changes

than to equal crew load changes.

Figure 3 indicates rear wheel radius changes. Data are shown for

the left wheels; right wheel behaviour was similar. The radius change

of the ARS units, which have dual rear wheels, amounted to 0.7 percent

of the nominal 15.5 in (39. A cm) radius over the range of water load.

The ARS radius was less sensitive to crew load changes than to equal

water load changes.

The single rear wheels of the IRS changed by 1.9 percent of the

nominal 14. A in (36.6 cm) radius over the range of water load. IRS

wheel radius was roughly as sensitive to crew load changes as to equal

water load changes.

2. A. Speed Measuring Subsystem

Each ARS employs a commercial fifth wheel in a modified mounting

within the trailer frame. The wheel is instrumented for speed readout.

The speed signal is recorded as a step function on the ARS strip chart

recorder

One speed recording range was operational in the ARS: 0 to 100 mph

(0 to 161 km/h) in 50 scale divisions, each 0.8 mm wide. Each division

represented 2 mph (3.2 km/h). Apparent 0.2 mph (0.3 km/h) signal steps

could be discerned on the chart with the aid of a 7X hand lens. However, because the width of the line drawn by the pen was equivalent to

about 0.8 mph (1.3 km/h), a more sensitive recording range would have

been appropriate.

6
Table 1. Skid trailer static load distributions

Trailer

Left wheel

weight

Right wheel

weight

Hitch

weight

lbf

a lbf lbf

ARS 1 1085 1085 146

ARS 2 1085 1085 141

ARS 3 1085 1085 148

IRS 1086 1085 149

3

1 lbf = 4.448 N.

7
HITCH

RISE

N

0 4000 8000

Ibf

LOAD REMOVED

Figure 2. Change in tow vehicle hitch height with load.

8
INCREASE

IN

WHEEL

RADIUS

N

4000 8000

i n

01

00

T

LOAD CHANGES SHOWN ARE DUE TO WATER WEIGHT

EXCEPT AS CREW WEIGHT IS REMOVED FROM!

I. . . DRIVER SEAT

2. . . OPERATOR SEAT

3. . . PASSE NGER SEAT w

ARS 1

c m

0 2

- 0.0

LOAD REMOVED

0.2

0.0

0.6

0.4

02

0.0

Figure 3. Change in tow vehicle rear wheel radius with load.

9
Speed subsystems of ARS 1, ARS 2, and the IRS were adjusted on the

measured mile and then checked against the IRS stationary radar system

(fig. 4). The tests showed agreement between the radar and the vehicle

speed subsystems within 0.5 mph (0.8 km/h) at speeds of 20, 40, and 60

mph (32, 64, and 97 km/h). The results of a check at about 40 mph (64

km/h) are shown in figure 5.

A compatibility of measured mile calibrations and radar was thus

confirmed. After the initial adjustment of each vehicle, further use of

the measured mile was left to the discretion of the vehicle operators.

The EFTC measured mile facility is on a 7.5 mile (12 km) one-way

oval track. Because measured mile procedures on this track were difficult and time-consuming, use of the radar for speed checks saved many

man-hours. The saving allowed speed checks to be made frequently, which

helped to assure accurate readout on board.

During dynamic correlation, checks were made in the same speed

range as the subsequent skid tests. Each vehicle’s speed subsystem was compared to the radar before each morning or afternoon session and whenever the speed range changed during a session. In addition, the radar was used to independently record the speed of each vehicle during each

skid test.

2.5. Pavement Wetting Subsystem

The pavement wetting subsystem was tested for water flow rate,

uniformity of water distribution, and width of the water trace on the

pavement

.

2.5.1. Flow Rate Measurement

The new ARS trailers are fitted with turbine flow sensors of the

type used in the IRS. These provide on-board capability for measuring

flow rate during skid tests. The ARS sensors were shipped to NBS in

advance and were calibrated by the Fluid Meters Section. The calibrations relate the electrical pulse rate of the sensors to water flow.

The results were provided to EFTC before the trailers were completed.

The final turbine calibration reports are shown in Appendix A. ARS

instrumentation converts the sensor pulse train to an analog signal for

display on a channel of the ARS strip chart recorder.

The IRS sensor combined with its readout instrumentation has been

calibrated as a complete flowmeter in the Fluid Meters Section. This

flowmeter uses digital signal processing which is intrinsically free

from the offset and gain drifts associated with analog equipment. It

provides numerical readout with resolution of 0.1 gpm (0.4 1/min)

.

10
Figure h. Radar cheek of vehicle speed.

11
radar

records

:

00

_c o

CO6

o

'£> o

o

o

o

Q33d$

o

Q£

<

0

CO

1 z

O

CJ33dS

E o

oo

Q33dS

12

Figure

5.

Simultaneous

speed

data

recorded

by

the

stationary

radar

and

the

ARS

1

and

IRS

tow

vehicles

in

motion

(fifth

wheels).
The offset and gain adjustments of the ARS flow instrumentation can

be checked and set at any time by comparing the analog readout with the sensor pulse rate. Had this been done by EFTC personnel, their settings

would have been checked by direct comparison with the IRS flowmeter.

Unfortunately, the ARS circuit had only one connector so that either the

readout or a pulse rate counter could be connected to the sensor, but

not both simultaneously. It is recommended that this circuit be modified

to permit such simultaneous measurements so that full use can be made of

the calibration data.

Since the needed simultaneous measurements could not be made,

comparisons between the readouts of the IRS and the ARS were used to

make the primary adjustment. The pipe section containing the IRS flow

sensor was connected by flexible hose in tandem with the ARS sensor as shown in figure 6. Simultaneous readings were taken at various flow

rates. Adjustments were made to bring the ARS readout into agreement

with the IRS flowmeter.

It would have been desirable to bring the readout of each ARS into

agreement with the reference flowmeter at two flow rates within the

upper and lower limits of the useful range. The flow sensor manufacturer specifies linear operation between 3.7 and 60 gpm (14 and 230

1/min) for this model, and the desired operating range is approximately

10 to 50 gpm (40 to 190 1/min). However, EFTC personnel chose to use zero flow as one set point. The positive value was chosen in the upper

half of the operating range.

The ARS recording range was 0 to 50 gpm (0 to 190 1/min) in 50

scale divisions, each 0.8 mm wide. The width of the line drawn by the

pen was equivalent to about 0.5 gpm (2 1/min). EFTC personnel used a 7X

lens to read the record with 0.1 gpm (0.4 1/min) precision.

The comparison results, in this case, were satisfactory after

adjustments to the ARS. Calibration signals generated by the ARS instrumentation were determined to have these equivalent values:

ARS 1, 46.5 gpm (179 1/min);

ARS 2, 46.3 gpm (175 1/min);

ARS 3, 46.5 gpm (176 1/min).

These values were provided to the ARS crews for the purpose of readout

scaling during the skid tests. The crews used the readouts to trim and

monitor flow rates.

13
1 = IRS FLOW SENSOR

2 = ARS FLOW SENSOR

Figure 6. Tandem arrangement for comparison of flow

rate measurements.

14
2.5.2. Water Distribution

Distributions measured on the static distribution gage (SDG) have

been shown previously [6, sec. 2. 5] [7, sec. 2.5], both for the type of

nozzle used on the new ARS systems and for the flow channel of the IRS.

During this testing period, SDG distributions were measured for ARS 1 and the IRS. Later, distribution measurements were also obtained for

ARS 2 and ARS 3. Representative examples of distributions at 40 mph (64

km/h) flow rates are shown in figures 7, 8, 9, and 10.

2.5.3. Wetted Width Measurement

To lay the desired water film thickness, the flow rate must be

adjusted as a function of the width of the water trace at the leading

edge of the tire-pavement interface. A satisfactory method for direct

measurement of the width under dynamic conditions is not available.

Techniques used during these tests included measurement of stream width

pumped while the vehicle remained stationary, measurement of the trace

left on the pavement after the vehicle passed, and measurement from

photographs taken under dynamic conditions.

ARS pavement wetting nozzles were of the "Penn State" diverging

stream type. The orifices were typically centered 3.25 inches (8.3 cm)

above the pavement with a 25° angle between the pavement and the top

nozzle surface. ARS wetted widths were measured for these conditions.

In the method favored by EFTC, a ruler was used to measure stream

width from the stationary vehicle at the level where the stream normally

impacts on the pavement. For ARS 1, this measurement was performed over pavement at the approximate flow rate for 40 mph (64 km/h) (fig. 11) and

EFTC personnel considered the result to agree with characteristic values

for this type of nozzle. They estimated the characteristic value for 20

mph (32 km/h) to be 6-1/4 to 6-1/2 inches (15.9 to 16.5 cm) and for 60

mph (97 km/h) to be 7-3/4 to 7-7/8 inches (19.7 to 20.0 cm). Later, for

ARS 2, they performed the measurement over the SDG basin at flow rates

corresponding to 20, 40, and 60 mph (32, 64, and 97 km/h). The water

passed into the vanes of the basin without actual impact. The method

neglects the effects of impact and divergence (shown for the static case

in fig. 12) which affect the stream width encountered by the tire.

The measured widths at the level of impact are included in table 2.

The results of similar measurements for the non-divergent IRS flow

channel are also included, where the level of impact is taken as 1/4

inch (0.6 cm) below the lip of the channel.

15
Figure 7- ARS 1 static water distribution at 70 mph (6k km/h).

16
Figure 8. ARS 2 static water distribution at ^0 mph (6^ km/h).

17
Figure 9. ARS 3 static water distribution at ^0 mph (6k km/h).

18
IRS l 4 40 RUN 4 4 1 76

Figure 10. IRS static water distribution at Ho mph ( 6U km/h).

19
Figuie it. Measurement df the stiream Width from an ARS riozzle

at the level df impact. The flow rate corresponds

tb HO (fob fetfi/H) :
:|#11

Impact on the pavement of the stream from a stationary

ARS nozzle. The flow rate corresponds to 40 mph

(6h km/h). The trailer wheel has been extended to

avoid interference with the stream.

Figure 12.

21
Table 2. Wetted width measurements

Speed Unit Flow rate Method Wetted width

mph

3 b

gpm in

C

20 ARS 2 13.5 Level of impact 5.8

20 ARS 1 13 Pavement 9.4

20 ARS 1 13 Photo d

10 to 13

20 IRS 14.7 Level of impact 7.8

20 IRS 14 Pavement 8.9

20 IRS 14 Photo 8

40 ARS 1 — Level of impact 7.2

40 ARS 2 25.8 Level of impact 7.2

40 ARS 1 26 Pavement 9.8

40 ARS 1 26 Photo

d

13 to 14

40 IRS 28.9 Level of impact 7.9

40 IRS 29 Pavement 10.4

40 IRS 29 Photo 14

60 ARS 2 39.0 Level of impact 8.2

60 ARS 1 39 Pavement

d

20 to 37

60 ARS 1 39 Photo 13

60 IRS 44.1 Level of impact 8.1

60 IRS 44 Pavement

d 11.6 to 16

60 IRS 44 Photo 17

a

1 mph = 1.609 km/h.

b ,1 gpm = 3.8 1/min.

C

1 in == 2.54 cm.

d While most water is concentrated within the smaller width. spray can be

seen iextending to the larger width.

22
The adjusted flow rate values set by the ARS crews during skid

testing were calculated by EFTC personnel from the ASTM E 274-70 quantity specification using the widths they considered characteristic of

this method. IRS flow rate settings were based on E 274-70 and the flow

channel orifice width, 8 inches (20 cm)

.

Trace widths left on the pavement by the streams under dynamic

conditions were measured for ARS 1 and the IRS. The test wheels of the

trailers were extended beyond the water trace (fig. 13) . The pavement

wetting outlets remained in their normal position. This arrangement was chosen to minimize the change of the aerodynamic disturbance of the

stream and the disturbance due to the tire running in the trace. The

brake was not applied. Several measurements were made of each trace

with increasing delay times to allow extrapolation back to the moment of

passage. The data are plotted in figure 14. The lines for 60 mph (97

km/h) relate to the wettest part of the trace rather than the total

width. At this speed, spray enlarged the total width to about 37 inches

(94 cm) for the ARS 1 trace and about 16 inches (41 cm) for the IRS

trace.

Generally the water trace on the pavement spreads in the first

minute after the trailer passes. In the case of the ARS, the 60 mph (97

km/h) trace width decreased. Apparently the water was so thin that

evaporation overcame spreading. Uncertainties in the use of this method

include measurement between the indistinct boundaries of the trace and

the validity of linear extrapolation.

Downward views of the water laid by ARS 1 and the IRS were photographed by a high speed camera from mountings on the test trailers.

Again, the test wheels were extended. After processing, the photographic images were measured to obtain stream widths at the normal

position of the tire-pavement interface. The photographs also give some information about the coupling of water to the pavement: streaks indicate water in contact with the pavement, while the remainder of the

stream is in the air and moving almost as fast as the trailer. A wave

or pulse effect prominent in ARS 1 streams, is visible in figure 15 as lateral bands. Note also the inboard drift of the stream.

A comparison of the results of these methods, all listed in table

2, illustrates the difficulty in assigning a "wetted width" with confidence.

23
Figure 13. Measurement of water trace width after the trailer has

passed. A 20 mph (32 km/h) run by ARS 1 is shown.
WIDTH

WIDTH

b) IRS!

cm

35

30

25

20

Figure lU. Wetted width by delayed measurement on pavement.

25
Figure 15. APS 1 pavement wetting at Uo mph (6h km/h).

26
2.6. Force Measuring Subsystem

The transducer of the force measuring subsystem is located inboard

on the trailer axle, remote from the force it is to measure at the tirepavement interface. The force at the interface is equivalent to a moment and force acting along vectors in three-dimensional space with

respect to the centroid of the transducer. Displacements occurring

within the system may alter these vectors. Many effects due to these

displacements are minimized by using the tow vehicle and trailer system

as a fixture for calibrating the transducer "in situ". Known forces are applied at the tire-pavement interface by means of an air-bearing force

plate and calibration box. A locked test wheel is shown on a force

plate in figure 16. However, some effects are not accounted for, such

as the lateral force at the tire-pavement interface and the yawing

orientation of the trailer usually present during skid testing. Displacements of the interface are larger when the tire is "hot" as in a running condition. The tests described here were conducted with the

tires at room temperature. However, the tires were previously broken in

with approximately 200 miles (320 km) of use, which provides some additional flexibility.

2.6.1. Orientation of the Transducer

The transducer measures a load and traction component of force. To

reduce the effects of any cross-axis misalinement, the hitch height was changed until the removal and application of the static vertical load

had a minimal effect on the traction channel output. The hitch height

found to satisfy this condition is given for each system in table 3. A

change of 0.1 in (0.2 cm) in this test is discernible with the on-board

tracion instrumentation. This orientation was specified to be maintained

during the dynamic measurement program and should be maintained during

future trailer use.

2.6.2. Vertical Load Channel Calibration

For wheel transducer vertical calibration, the skid trailer was positioned with the left (test) wheel supported by the IRS air-bearing

force plate. Tire inflation pressures were set to 24 psi (165 kPa) , and

the hitch height was adjusted to the optimum value (sec. 2.6.1).

Force was applied vertically to the trailer body such that the

vertical load at the test wheel varied over a range of at least + 230

lbf (1020 N) to - 330 lbf (- 1470 N) from the normal static load (table

1) . A force plate test was conducted, using the appropriate on-board

amplifier of the system under test for the transducer vertical load

signal and an IRS amplifier for the force plate vertical load signal.

The outputs were recorded point-by-point, both graphically and numerically.

27
Figure 1 6 . A simulated traction force being applied to a locked test wheel through an air-bearing force plate.

28
Table 3. Hitch heights for optimum

transducer orientation

System Hitch height 3

ARS 1

. b

in

14

ARS 2 12-13/16

ARS 3 14-3/4

IRS 13-7/8

Height is measured from level pavement to center of hitch ball.

1 in = 2.54 cm.

29
For all systems, the graphical results verified linear operation of

the vertical transducer channel, free from hysteresis. No data point

varied by more than 5 lbf (22 N) from the best straight line. Equivalent force values of the on-board calibration signal outputs were calculated from the numerical data by the method of least squares.

2.6.3. Traction Channel Calibration

For traction calibration, the skid trailer was hitched in line with

its tow vehicle. The tow vehicle and trailer were put in normal operating condition. Specifically, tire inflation pressures were set, hitch

height was adjusted to the optimum value, and crew weights were simulated

in the tow vehicle. A 150 to 175 lb (70 to 80 kg) deadweight was placed

in each seat. The left (test) wheel of the trailerwas supported by the

IRS air-bearing force plate.

Traction force was applied to the test tire through the force plate

over the range from zero to 800 lbf (3560 N) . A force plate test was conducted, using the appropriate on-board amplifier of the system under

test for the transducer traction signal and an IRS amplifier for the

force plate traction signal. The outputs were recorded point-by-point,

both graphically and numerically.

For all systems, the graphical results verified linear operation of

the traction transducer channel. No data point varied by more than 5 lbf (22 N) from the best straight line. For ARS 1, hysteresis between

increasing and decreasing traction values resulted in a difference of 10

lbf (44 N) or less. For ARS 2 and ARS 3, the difference was 5 lbf (22

N) or less. The decreasing values were the lower. The IRS transducer

traction signal was free of hysteresis. Equivalent force values of the

on-board calibration signal outputs were calculated from the numerical

data (including both increasing and decreasing values) by the method of

least squares.

2.6.4. On-Board Calibration

System Equivalent Values

To adjust the gain and offset of the transducer instrumentation,

two known values are required for each channel. In each ARS, means were

provided to generate one reference output signal for each channel. Zero

load served as the other value. A series of signals was available for

each channel in the IRS.

The equivalent force values represented by these signals, as calculated from the numerical calibration data, are given in table 4.

30
Table 4. On-board calibration values

System

Transducer

1 serial number Force channel Value

lbf

b

ARS 1 80 Vertical 338

Traction 831

ARS 2 77 Vertical 339

Traction 910

ARS 3 76 Vertical 337

Traction 828

IRS — Vertical

:

400 396

300 296

200 196

100 98

-100 -100

-200 -199

-300 -298

-400 -397

Traction:

100 99

200 200

300 301

400 401

500 501

600 600

700 700

800 802

900 903

Installed in the left side of the trailer.

b

1 lbf = 4.448N.

31
2.6.5. Experimental Determination

of the Effective Unloading Constant

The vertical force on the test wheel of a towed two-wheeled skid

trailer is reduced as the traction force at the tire-pavement interface

is increased. This reduction in vertical force can be described in

terms of an effective unloading constant [5] which can then be experimentally determined. The reduction in vertical force is given by:

F = F F

WL WL WL

Z Z X

o where Frrr = the total vertical force on the left (test) wheel acting

WL

z at the tire-pavement interface, F^ = the vertical force on the left

z wheel acting at the tire-pavement interface when the applied traction is

zero, FTTT = the traction force on the left wheel, and — = the effective

WL L vertical force unloading constant where H is the hitch height and L is

the length from the hitch to the center line of the trailer axle.

There is an uncertainty in the definitions of H and L because of • the finite size and shape of the hitch ball.

The determination of the effective vertical force unloading constant

for each system was made by using the same set-up of trailer and tow

vehicle that was used for traction channel calibration. Vertical and

traction outputs were recorded point-by-point, both graphically and

numerically, while the traction was varied over the range from 0 to 800

lbf (3560 N). Graphical results confirmed the linear reduction of

vertical force with increasing traction for each system. No data point

varied by more than 5 lbf (22 N) from the best straight line. Hysteresis

differences between rising and falling values did not exceed 5 lbf (22

N). The method of least squares was applied to the numerical results to

calculate the effective unloading constant for each system. The values

were

ARS 1; H/L = 0.117,

ARS 2; H/L = 0.108,

ARS 3; H/L = 0.126,

IRS; H/L = 0.125.

2.7. Suspension Subsystem

The measurement of pavement skid resistance at normal traffic

speeds is a dynamic test. While data reduction is to occur during a period of steady state friction, the fact is that transient oscillations

dominate the entire trace. Trailer motions introduce transient dynamic

forces into the system and vice versa. The effective dynamic characteristics of the suspension subsystems need to be measured and reported

32
in analytical form in order that dynamic analyses of the system can be

performed for comparison with experimental results and for use in design

parameter studies. Further, measurements made by systems of similar or even the same design differ. Consequently, in an attempt to document

the dynamic characteristics that might affect measuring performance and

account for these differences, the effective spring rates, natural

frequencies. Coulomb and viscous friction of the suspension systems were

determined. Measurements of certain static displacements were also made

as the traction force at the tire-pavement interface was increased to

approximately 800 lbf (3560 N) and then decreased to zero.

The dynamic characteristics of the trailer suspensions were obtained while the trailers were connected to a ball hitch on a stationary

fixture. The displacements of the trailer under static traction load were measured with the trailer alined and connected to the tow vehicle

with the tow vehicle drive wheels and trailer test wheel locked. Consequently, part of the motion occurring under traction force is due to

motion between the hitch point and the tow vehicle tire-pavement interface.

Dynamic characteristics were measured for the IRS, ARS 1, and ARS 2,

but time permitted static traction tests on ARS 2 only. Its static performance can be compared to previous tests of IRS and other units [6, sec . 2 . 7 ] [ 7 , sec 2.7].

2.7.1. Trailer Displacements due to

Static Traction

a. Displacement of the Test Wheel Hub

The test wheel hub moves back due to static traction loading. This

displacement is the resultant of motions throughout the system. Tow

vehicle tire and suspension deflections, trailer suspension deflections,

and trailer yaw contribute.

The component of hub displacement along the direction of the initial

alinement of the ARS 2 tow vehicle and trailer was measured with respect

to the floor. The measured displacement is plotted in figure 17. At

800 lbf (3560 N) the ARS hub was displaced 0.32 in (0.81 cm) to the rear. At the same loading, the axle was observed to shift 0.25 in (0.64

cm) to the right, and to yaw to the left by 13 minutes of arc.

b. Displacement of the Test Tire Interface

As the tire of the locked test wheel distorts under static traction

loading, the interface of the tire and the force plate moves with respect

to the test wheel hub.

33
HUB

DISPLACEMENT

TO

THE

REAR

0 1000

N

2000 3000

cm

0.8

06

04

02

0.0

0 200 400 600 800

Ibt

STATIC TRACTION

Figure 17. Displacement of the test wheel hub due to static traction.

34
The rearward displacement of the force plate was measured with

respect to the ARS hub. No slippage of the tire on the force plate

friction surface was noted. Displacement at 800 lbf (3560 N) was 0.32

in (0.81 cm) to the rear.

c. Displacement of the Trailer Body through Roll Axis Rotation

The body of a two wheeled skid trailer may roll about a longitudinal axis during skid testing. The suspension and tires of the

trailer respond to asymmetric forces developed by test wheel traction

loading

.

Roll displacements of the ARS 2 trailer are shown in figure 18.

The displacements were measured with respect to the floor by two sensors applied vertically to the rear corners of the trailer body. Lateral

separation of the sensors was 48.5 in (123 cm).

At 800 lbf (3560 N) static traction loading, the ARS trailer body

fell 0.10 in (0.25 cm) on the left (test wheel) side and fell 0.02 in

(0.05 cm) on the right side, corresponding to a roll axis rotation of 6

minutes of arc. Roll continued to increase as unloading began, peaking

at 10 minutes of arc as traction fell to 500 lbf (2220 N) . The static

friction of the suspension held the ARS body near its peak roll position

after the traction loading was removed.

d. Displacement of the Trailer Body through Pitch Axis Rotation

The average of the displacements measured by the two trailer body

roll sensors represents the vertical displacement of the center of the

trailer body at the rear with respect to the floor. This, combined with

the vertical displacement measured by a third sensor at the trailer

hitch, yields trailer body pitch.

The pitch displacements for ARS 2 are plotted in figure 19. The

longitudinal separation of the front and rear sensors was 156 in (396

cm) .

At 800 lbf (3560 N) static traction loading, the ARS trailer body

remained unmoved at the hitch and fell 0.09 in (0.23 cm) at the rear corresponding to a pitch axis rotation of 2 minutes of arc. Pitch

continued to increase as unloading began, peaking at 3 minutes of arc at

500 lbf (2220 N). The static friction of the suspension held the ARS

body near the peak pitch angle after traction loading was removed.

35
REAR

CORNER

DISPLACEMENT

N

,1

LUCO

CtL

I

0 1000 2000 3000

cm

0.2

0.0

0.2

0.4

Figure l8. Roll displacement of the trailer body due to static traction.

36
DISPLACEMENT

1000

N

2000 3000

cm

0.2

0.0

0.2

0.4

Figure 19. Pitch displacement of the trailer body due to static traction.

37
e. Displacement of the Trailer Body through Yaw Axis Rotation

Yaw displacements of the ARS 2 trailer are shown in figure 20. The

displacements were measured with respect to the floor by two sensors applied horizontally at the hitch and at a rear corner of the trailer

body. Longitudinal separation of the sensors was 160 in (406 cm)

At 800 lbf (3560 N) static traction loading, the hitch moved 0.07

in (0.18 cm) to the left and the rear of the body moved 0.39 in (0.99

cm) to the right, corresponding to yaw axis rotation of 10 minutes of arc to the left. Yaw increased slightly as traction was reduced to 750

lbf (3340 N), but diminished to less than 2 minutes of arc when traction was removed. The nearly equal yaw of axle (sec. 2. 7.1. a) and body show

that the major yaw displacement originated within the right trailer tire

rather than the suspension of the ARS.

2.7.2. Suspension Characteristics under

Transient Loading

The effective dynamic characteristics of the trailer suspension can

be measured in terms of natural frequencies, Coulomb (sliding) friction,

spring rates, and damping factors.

There are three natural frequencies of interest. One, arising from

flexing of the trailer tires, can be seen under conditions of small

variations in vertical load where there is no relative movement within

the suspension subsystem. The axle and the body are free to rotate in

a vertical plane about the hitch within a small range of displacement of

the tires. The other two natural frequencies occur under conditions of

larger variations in vertical load where there is relative movement

within the suspension subsystem. In this case, there are two degrees of

freedom:

a) the body can rotate in a vertical plane about the

hitch, while

b) the axle can rotate in a vertical plane about the

trailing arm hinge (in designs of the ARS and IRS

type)

There is a mode of operation associated with each degree of freedom.

Each mode may be produced independently by starting the system oscillation under the proper initial conditions. A dynamic analysis of the

trailing arm suspension design of the IRS revealed two dynamic tests

which are easy to conduct and which approximate the independent modes

such that the measurement of each natural frequency is enhanced. The

enhancement is more evident under the condition that the shock absorbers

are deactivated.

38
DISPLACEMENT

1000

N

2000 3000

cm

0.4

00

04

0.8

Figure 20. Yaw displacement of the trailer body due to static traction.

39
The first test is to raise the trailer by a point on the body,

allowing it to pivot about the hitch, and to drop it abruptly. With

this initial condition, an oscillation occurs where the angular displacements of the body and axle trailing arms are out of phase. In this

mode, the vibration which has the lower frequency is enhanced. The

second test is to raise the trailer at the axle and drop it abruptly.

With this initial condition, an oscillation occurs where the angular

displacements of the body and axle trailing arms are in phase. In this

mode, the higher frequency is enhanced. After the transient decays such

that there is no further relative movement in the suspension, the oscillation of the trailer on the tires persists because there is little

damping in the tires.

Trailer response to these tests was measured by an air-bearing

force plate under the left trailer wheel and by linear potentiometer

displacement sensors operating between the pavement surface, the trailer

axle, and the trailer body. The force and displacement signals were plotted versus time, using X - Y recorders. Tests were conducted on ARS

1, ARS 2, and the IRS.

The frequencies found using tests of the first type were 3 Hz for

both ARS 1 and ARS 2, and 2 Hz for the IRS. The second type of test

gave frequencies of 13 Hz for ARS 1, 12 Hz for ARS 2, and 13 Hz for the

IRS. For all systems, a value of about 4 Hz was observed for the small

amplitude tire oscillation.

Temporary disconnection of the shock absorbers, which are the major

source of viscous damping, aids in determining the amount of Coulomb

friction in the suspension. With viscous damping eliminated, the amplitude of the vertical force transient exhibits a linear decay which is a measure of the Coulomb friction. The value of Coulomb friction in the

left side of the trailer suspension was determined by this method to be

5 lbf (22 N) for ARS 1, 7 lbf (31 N) for ARS 2, and 10 lbf (44 N) for

the IRS.

The change in force and displacement from their equilibrium values

is a measure of spring rate. Simultaneously recorded force and displacement data gave values for the effective spring rate between the

trailer body and the pavement surface of 1500 lbf /in (260 kN/m) for ARS

1, 1000 lbf /in (175 kN/m) for ARS 2, and 260 lbf /in (45 kN/m) for the

IRS.

When the shock absorbers are connected, the transient is damped by

both Coulomb and viscous friction, and the decay is non-linear. A non- dimensional family of curves which can be used to estimate the amount of

damping in systems represented by a second degree differential equation

is shown in figure 21 [8]. The recorded transient decay can be compared

to the curves to estimate the effective damping factor, 4. The curves

40
Figure 21. Effect of different damping factors

on second-order linear systems.

41
also indicate that a system with a damping factor of 0.8 nears equilibrium before a system with a damping factor of either 0.6 or 1.0. A

more optimum value (not shown) is 0.7. A measuring system which is

subject to disturbing transients will measure the near-equilibrium value

more often with a damping factor of 0.7.

The effective damping factors observed for ARS 1 and ARS 2 were

0.4. The IRS, which has been provided with extra shock absorbers,

exhibited an effective damping factor of 0.6.

There is evidence that the shock absorbers used provide effective

damping in one direction only. From figure 22, it can be seen that

upward deflection of the trailer axle in undamped. It may be possible

to increase the damping factor simply by installation of double-acting

shock absorbers.

3. THE DYNAMIC SKID RESISTANCE MEASUREMENT OF

SELECTED SURFACES AND ARS- IRS CORRELATIONS

The purpose of the dynamic skid resistance measurement test program

was

:

a) to measure selected surfaces at the EFTC, and to compare

the results with those from a previous program,

b) to measure the consistency and precision in the measure

ments of ARS 1, ARS 2, ARS 3 and the IRS, and

c) to interrelate and correlate the degree of match in the

measurements between the IRS and the three ARS.

3.1. Surfaces Selected

The surfaces selected were a 100 ft (30.5 m) longitudinal section

of lanes 1 through 7 of surfaces identified as 2 and 3 at the EFTC. So

that the reduced data would all be within the same 100 ft (30.5 m)

section at all test speeds, pylons were set out where the operator would

start the automatic skid test sequence for each test speed. These are

the same sections of surfaces 2 and 3 measured in a previous program [6].

3.2. Tests Planned

The test program is designed to yield the following information:

a) skid resistance, SN, of a surface as a function of test

speed

,

b) skid resistance - test speed gradient, 0, as a function of

test speed,

c) skid resistance - test tire inflation pressure gradient

at 40 mph (64 km/h)

,

42
n cm

Figure 22. ARS 1 axle-to-surface transient response. Damping

is evident only on downward motion.

43
d) skid resistance - water depth gradient at 40 mph

(64 km/h)

,

e) variability in SN with choice of test tire,

f) variability in SN with choice of lane within the

surface, and

g) variation in SN with drying time between tests.

The test plan includes the control of the following operating

variables

a) hitch height,

b) inflation pressure of the tow vehicle rear tires,

c) inflation pressure of the IRS air spring shock absorbers,

d) surface cleanliness,

e) surface minimum degree-of-dryness

,

f) test speed, and

g) water flow rate.

The test plan includes the measurement of the following uncontrolled variables:

a) weather,

b) surface temperature,

c) test tire inflation pressure,

d) test tire lock-up position, and

e) IRS test tire tread depth.

The planned test program is contained in table 5. It consists of

two main exercises. In the first, occupying time periods 1 through 6,

each skid resistance measuring system measures the two surfaces at three

nominal speeds. Variation of + 2 mph (+ 3.2 km/h) about the nominal

speeds are introduced to measure the local speed gradients. The second

exercise, time periods 7 and 8, re-measures the two surfaces to detect

variation of SN over the term of the test program. Also, variations are introduced about the normal test tire inflation pressure and about the

normal pavement wetting flow rates to determine the effect of changes in

these variables on the SN measurement.

3.3. The Test Matrix

The test program consists of a number of measurements of the steady

state friction force on a locked test wheel as it is dragged over a wetted pavement surface under known load and at a constant speed. The

test is used by others to measure the difference in skid resistance

between various designs of tires. The test is used by a majority of

state highway departments to inventory and rank the condition of the

highways according to skid resistance. In the latter case, a standardized, but special purpose, full-scale automotive tire is mounted on the

44
Table 5. Test plan sequence

Time Period //I First Day

Measurement

System Surface Lane

Test

Speed Tire

IRS 1 2 3

X 2 2 40 A1

X 2 3 40 B1

X 2 4 42 A1

X 2 5 42 B1

X 2 1 38 A1

X 2 2 38 B1

X 3 2 40 A2

X 3 3 40 B4

X 3 4 42 A2

X 3 5 42 B4

X 3 1 38 A2

X 3 2 38 B4

X 2 4 38 D1

X 2 3 38 Cl

X 2 7 42 D1

X 2 6 42 Cl

X 2 5 40 D1

X 2 4 40 Cl

X 3 4 38 D4

X 3 3 38 C2

X 3 7 42 D4

X 3 6 42 C2

X 3 5 40 D4

X 3 4 40 C2

X 2 6 40 El

X 2 7 40 FI

X 2 5 38 El

X 2 2 42 FI

X 2 1 42 El

X 2 6 38 FI

X 3 6 40 E2

X 3 7 40 F4

X 3 5 38 E2

X 3 2 42 F4

X 3 1 42 E2

X 3 6 38 F4

X 2 7 38 G1

X 2 3 42 G1

X 2 1 40 G1

X 3 7 38 G2

X 3 3 42 G2

X 3 1 40 G2

45
Table 5. Test plan sequence - continued

Time Period //

1

Second Day

Measurement

System Surface

1 IRS 3 2

x 2 x 2 x 2 x 2 x 2 x 2 x 3 x 3 x 3 x 3 x 3 x 3

x 2 x 2 x 2 x 2 x 2 x 2 x 3 x 3 x 3 x 3 x 3 x 3

x 2 x 2 x 2 x 2 x 2 x 2 x 3 x 3 x 3 x 3 x 3 x 3

x 2 x 2 x 2 x 3 x 3 x 3

Lane

Test

Speed Tire

2 40 A1

3 40 B1

4 42 A1

5 42 B1

1 38 A1

2 38 B1

2 40 A4

3 40 B2

4 42 A4

5 42 B2

1 38 A4

2 38 B2

4 38 D1

3 38 Cl

7 42 Dl

6 42 Cl

5 40 Dl

4 40 Cl

4 38 D2

3 38 C4

7 42 D2

6 42 C4

5 40 D2

4 40 C4

6 40 El

7 40 FI

5 38 El

2 42 FI

1 42 El

6 38 FI

6 40 E4

7 40 F2

5 38 E4

2 42 F2

1 42 E4

6 38 F2

7 38 G1

3 42 G1

1 40 G1

7 38 G4

3 42 G4

1 40 G4

46
Table 5. Test plan sequence - continued

Time Period #2

First Day

Measurement Test

System Surface Lane Speed Tire

IRS 1 2 3

X 3 2 40 A4

X 3 3 40 B2

X 3 4 42 A4

X 3 5 42 B2

X 3 1 38 A4

X 3 2 38 B2

X 2 2 40 A3

X 2 3 40 B3

X 2 4 42 A3

X 2 5 42 B3

X 2 1 38 A3

X 2 2 38 B3

X 3 4 38 D2

X 3 3 38 C4

X 3 7 42 D2

X 3 6 42 C4

X 3 5 40 D2

X 3 4 40 C4

X 2 4 38 D3

X 2 3 38 C3

X 2 7 42 D3

X 2 6 42 C3

X 2 5 40 D3

X 2 4 40 C3

X 3 6 40 E4

X 3 7 40 F2

X 3 5 38 E4

X 3 2 42 F2

X 3 1 42 E4

X 3 6 38 F2

X 2 6 40 E3

X 2 7 40 F3

X 2 5 38 E3

X 2 2 42 F3

X 2 1 42 E3

X 2 6 38 F3

X 3 7 38 G4

X 3 3 42 G4

X 3 1 40 G4

X 2 7 38 G3

X 2 3 42 G3

X 2 1 40 G3

4 7
Table 5. Test plan sequence - continued

Time Period #2

Second Day

System Surface Lane Speed Tire

1 IRS 3 2

X

3 2 40 A2

X 3 3 40 B4

X

3 4 42 A2

X 3 5 42 B4

X

3 1 38 A2

X 3 2 38 B4

X 2 2 40 A3

X 2 3 40 B3

X 2 4 42 A3

X 2 5 42 B3

X 2 1 38 A3

X 2 2 38 B3

X 3 4 38 D4

X 3 3 38 C2

X 3 7 42 D4

X 3 6 42 C2

X 3 5 40 D4

X 3 4 40 C2

X 2 4 38 D3

X 2 3 38 C3

X 2 7 42 D3

X 2 6 42 C3

X 2 5 40 D3

X 2 4 40 C3

X 3 6 40 E2

X 3 7 40 F4

X 3 5 38 E2

X 3 2 42 F4

X 3 1 42 E2

X 3 6 38 F4

X 2 6 40 E3

X 2 7 40 F3

X 2 5 38 E3

X 2 2 42 F3

X 2 1 42 E3

X 2 6 38 F3

X 3 7 38 G2

X 3 3 42 G2

X 3 1 40 G2

X 2 7 38 G3

X 2 3 42 G3

X 2 1 40 G3

48
Table 5 . Test plan sequence - continued

Time Period //

3

First Day

Measurement Test

System Surface Lane Speed T ire

IRS 1 2 3

X 2 2 20 A1

X 2 3 20 B1

X 2 4 22 A1

X 2 5 22 B1

X 2 1 18 A1

X 2 2 18 B1

X 3 2 20 A2

X 3 3 20 B4

X 3 4 18 A2

X 3 5 22 B4

X 3 1 18 A2

X 3 2 18 B4

X 2 4 18 Dl

X 2 3 18 Cl

X 2 7 22 Dl

X 2 6 22 Cl

X 2 5 20 Dl

X 2 4 20 Cl

X 3 4 18 D4

X 3 3 18 C2

X 3 7 22 D4

X 3 6 . 22 C2

X 3 5 20 D4

X 3 4 20 C2

X 2 6 20 El

X 2 7 20 FI

X 2 5 18 El

X 2 2 22 FI

X 2 1 22 El

X 2 6 18 FI

X 3 6 20 E2

X 3 7 20 F4

X 3 5 18 E2

X 3 2 22 F4

X 3 1 22 E2

X 3 6 18 F4

X 2 7 18 G1

X 2 3 22 G1

X 2 1 20 G1

X 3 7 18 G2

X 3 3 22 G2

X 3 1 20 G2

49
Table 5. Test plan sequence - continued

Time Period #3

Second Day

Measurement Test

System Surface Lane Speed Tire

1 IRS 3 2

X 2 2 20 A1

X 2 3 20 B1

X 2 4 22 A1

X 2 5 22 B1

X 2 1 18 A1

X 2 2 18 B1

X 3 2 20 A4

X 3 3 20 B2

X 3 4 22 A4

X 3 5 22 B2

X 3 1 18 A4

X 3 2 18 B2

X 2 4 18 D1

X 2 3 18 Cl

X 2 7 22 D1

X 2 6 22 Cl

X 2 5 20 D1

X 2 4 20 Cl

X 3 4 13 D2

X 3 3 18 C4

X 3 7 22 D2

X 3 6 22 C4

X 3 5 20 D2

X 3 4 20 C4

X 2 6 20 El

X 2 7 20 FI

X 2 5 18 El

X 2 2 22 FI

X 2 1 22 El

X 2 6 18 FI

X 3 6 20 E4

X 3 7 20 F2

X 3 5 18 E4

X 3 2 22 F2

X 3 1 22 E4

X 3 6 18 F2

X 2 7 18 G1

X 2 3 22 G1

X 2 1 20 G1

X 3 7 18 G4

X 3 3 22 G4

X 3 1 20 G4

50
Table 5. Test plan sequence - continued

Time Period #4

First Day

Measurement

System Surface Lane

Test

Speed Tire

IRS 1 2 3

X 3 2 20 A4

X 3 3 20 B2

X 3 4 22 A4

X 3 5 22 B2

X 3 1 18 A4

X 3 2 18 B2

X 2 2 20 A3

X 2 3 20 B3

X 2 4 22 A3

X 2 5 22 B3

X 2 1 18 A3

X 2 2 18 B3

X 3 4 18 D2

X 3 3 18 C4

X 3 7 22 D2

X 3 6 22 C4

X 3 5 20 D2

X 3 4 20 C4

X 2 4 18 D3

X 2 3 18 C3

X 2 7 22 D3

X 2 6 22 C3

X 2 5 20 D3

X 2 4 20 C3

X 3 6 20 E4

X 3 7 20 F2

X 3 5 18 E4

X 3 2 22 F2

X 3 1 22 E4

X 3 6 18 F2

X 2 6 20 E3

X 2 7 20 F3

X 2 5 18 E3

X 2 2 22 F3

X 2 1 22 E3

X 2 6 18 F3

X 3 7 18 G4

X 3 3 22 G4

X 3 1 20 G4

X 2 7 18 G3

X 2 3 22 G3

X 2 1 20 G3

51
Table 5. Test plan sequence - continued

Time Period //A Second Day

Measurement Test

Surface Lane Tire

1 IRS 3 2

X

3 2 20 A2

X 3 3 20 BA

X 3 A 22 A2

X 3 5 22 BA

X 3 1 18 A2

X 3 2 18 BA

X 2 2 20 A3

X 2 3 20 B3

X 2 A 22 A3

X 2 5 22 B3

X 2 1 18 A3

X 2 2 18 B3

X 3 A 18 DA

X 3 3 18 C2

X 3 7 22 DA

X 3 6 22 C2

X 3 5 20 DA

X 3 A 20 C2

X 2 A 18 D3

X 2 3 18 C3

X 2 7 22 D3

X 2 6 22 C3

X 2 5 20 D3

X 2 A 20 C3

X 3 6 20 E2

X 3 7 20 FA

X 3 5 18 E2

X 3 2 22 FA

X 3 1 22 E2

X 3 6 18 FA

X 2 6 20 E3

X 2 7 20 F3

X 2 5 18 E3

X 2 2 22 F3

X 2 1 22 E3

X 2 6 13 F3

X 3 7 18 G2

X 3 3 22 G2

X 3 1 20 G2

X 2 7 18 G3

X 2 3 22 G3

X 2 1 20 G3

52
Table 5. Test plan sequence - continued

Time Period #5

First Day

Measurement

System Surface Lane

Test

Speed Tire

IRS 1 2 3

X 2 2 60 A1

X 2 3 60 B1

X 2 4 62 A1

X 2 5 62 B1

X 2 1 58 A1

X 2 2 58 B1

X 3 2 60 A2

X 3 3 60 B4

X 3 4 62 A2

X 3 5 62 B4

X 3 1 58 A2

X 3 2 58 B4

X 2 4 58 D1

X 2 3 58 Cl

X 2 7 62 D1

X 2 6 62 Cl

X 2 5 60 D1

X 2 4 60 Cl

X 3 4 58 D4

X 3 3 58 C2

X 3 7 62 D4

X 3 6 62 C2

X 3 5 60 D4

X 3 4 60 C2

X 2 6 60 El

X 2 7 60 FI

X 2 5 58 El

X 2 2 62 FI

X 2 1 62 El

X 2 6 58 FI

X 3 6 60 E2

X 3 7 60 F4

X 3 5 58 E2

X 3 2 62 F4

X 3 1 62 E2

X 3 6 58 F4

X 2 7 58 G1

X 2 3 62 G1

X 2 1 60 G1

X 3 7 58 G2

X 3 3 62 G2

X 3 1 60 G2

53
Table 5. Test plan sequence - continued

Time Period it 5 Second Day

Measurement

System Surface

1 IRS 3 2 x 2 x 2 x 2 x 2 x 2 x 2 x 3 x 3 x 3 x 3 x 3 x 3

x 2 x 2 x 2 x 2 x 2 x 2 x 3 x 3 x 3 x 3 x 3 x 3

x 2 x 2 x 2 x 2 x 2 x 2 x 3 x 3 x 3 x 3 x 3 x 3

x 2 x 2 x 2 x 3 x 3 x 3

Lane

Test

Speed Tire

2 60 A1

3 60 B1

4 62 A1

5 62 B1

1 58 A1

2 58 B1

2 60 A4

3 60 B2

4 62 A4

5 62 B2

1 58 A4

2 58 B2

4 58 D1

3 58 Cl

7 62 D1

6 62 Cl

5 60 D1

4 60 Cl

4 58 D2

3 58 C4

7 62 D2

6 62 C4

5 60 D2

4 60 C4

6 60 El

7 60 FI

5 58 El

2 62 FI

1 62 El

6 58 FI

6 60 E4

7 60 F2

5 58 E4

2 62 F2

1 62 E4

6 58 F2

7 58 G1

3 62 G1

1 60 G1

7 58 G4

3 62 G4

1 60 G4

54
Table 5. Test plan sequence - continued

Time Period #6

First Day

Measurement

System Surface Lane

Test

Speed Tire

IRS 1 2 3

1

2 60 A4

3 60 B2

4 62 A4

5 62 B2

1 58 A4

2 58 B2

2 60 A3

3 60 B3

4 62 A3

5 62 B3

1 58 A3

2 58 B3

4 58 D2

3 58 C4

7 62 D2

6 62 C4

5 60 D2

4 60 C4

4 58 D3

3 58 C3

7 62 D3

6 62 C3

5 60 D3

4 60 C3

6 60 E4

7 60 F2

5 58 E4

2 62 F2

1 62 E4

6 58 F2

6 60 E3

7 60 F3

5 58 E3

2 62 F3

1 62 E3

6 58 F3

7 5S G4

3 62 G4

1 60 G4

7 58 G3

3 62 G3

1 60 G3
Table 5. Test plan sequence - continued

Time Period #6

Second Day

Measurement

System Surface

1 IRS 3 2

x 3 x 3 x 3 x 3 x 3 x 3 x 2 x 2 x 2 x 2 x 2 x 2

x 3 x 3 x 3 x 3 x 3 x 3 x 2 x 2 x 2 x 2 x 2 x 2

x 3 x 3 x 3 x 3 x 3 x 3 x 2 x 2 x 2 x 2 x 2 x 2

x 2 x 2 x 2

Lane

Test

Speed Tire

2 60 A2

3 60 B4

4 62 A2

5 62 B4

1 58 A2

2 58 B4

2 60 A3

3 60 B3

4 62 A3

5 62 B3

1 58 A3

2 58 B3

4 58 D4

3 58 C2

7 62 D4

6 62 C2

5 60 D4

4 60 C2

4 58 D3

3 58 C3

7 62 D3

6 62 C3

5 60 D3

4 6 Q C3

6 60 E2

7 60 F4

5 58 E2

2 62 F4

1 62 E2

6 58 F4

6 60 E3

7 60 F3

5 58 E3

2 62 F3

1 62 E3

6 58 F3

7 58 G2

3 62 G2

1 60 G2

7 58 G3

3 62 G3

1 60 G3

56
Table 5. Test plan

Time Period #7

First Day

sequence - continued

A 15%

Water Tire

Measurement Test Flow Inflation

System Surface Lane Speed Rate Pressure Tire

IRS 1 2 3

X 2 1 40 - 24 A1

X 2 2 40 - 24 Bl

X 2 4 40 + 24 A1

X 2 5 40 + 24 Bl

X 3 1 40 - 24 A2

X 3 2 40 - 24 B4

X 3 4 40 + 24 A2

X 3 5 40 + 24 B4

X 2 1 40 Normal 20 A1

X 2 2 40 Normal 20 Bl

X 2 4 40 Normal 28 A1

X 2 5 40 Normal 28 Bl

X 3 1 40 Normal 20 A2

X 3 2 40 Normal 20 B4

X 3 4 40 Norma 1 28 A2

X 3 5 40 Normal 28 B4

X 2 3 40 - 24 Cl

X 2 4 40 - 24 D1

X 2 6 40 + 24 Cl

X 2 7 40 + 24 D1

X 3 3 40 - 24 C2

X 3 4 40 - 24 D4

X 3 6 40 + 24 C2

X 3 7 40 + 24 D4

X 2 3 40 Normal 20 Cl

X 2 4 40 Normal 20 D1

X 2 6 40 Normal 28 Cl

X 2 7 40 Normal 28 D1

X 3 3 40 Normal 20 C2

X 3 4 40 Normal 20 D4

X 3 6 40 Normal 28 C2

X 3 7 40 Normal 28 D4

X 2 5 40 — 24 El

X 2 2 40 + 24 FI

X 2 1 40 + 24 El

X 2 6 40 - 24 FI

X 3 5 40 - 24 E2

X 3 2 40 + 24 F4

X 3 1 40 + 24 E2

X 3 6 40 - 24 F4

57
Table 5. Test plan sequence - continued

Time Period \\1

Second Day

A15%

Measurement

System Surface Lane

Test

Speed

Water

Flow

Rate

Tire

Inflation

Pressure Tire

IRS 1 2 3

2 5 40 Normal 20 El x 2 2 40 Normal 28 FI

x 2 1 40 Normal 28 El

x 2 6 40 Normal 20 FI

X 3 5 40 Normal 20 E2

X 3 2 40 Normal 28 F4

X 3 1 40 Normal 28 E2

X 3 6 40 Normal 20 F4

x 2 7 40 — Normal G1

x 2 3 40 + Normal G1

X 3 7 40 - Normal G2

X 3 3 40 + Normal G2

X 2 7 40 Normal 20 G1

X 2 3 40 Normal 28 G1

X 3 7 40 Normal 20 G2

X 3 3 40 Normal 28 G2

X 3 7 40 - Normal G4

X 3 3 40 + Normal G4

X 2 7 40 - Normal G1

X 2 3 40 + Normal G1

X 3 7 40 Normal 20 G4

X 3 3 40 Normal 28 G4

X 2 7 40 Normal 20 G1

X 2 3 40 Normal 28 G1

X 2 1 40 — Normal A1

X 2 2 40 - Normal B1

X 2 4 40 + Normal A1

X 2 5 40 + Normal B1

X 3 1 40 - Normal A4

X 3 2 40 - Normal B2

X 3 4 40 + Normal A4

X 3 5 40 + Normal B2

X 2 1 40 Normal 20 A1

X 2 2 40 Normal 20 B1

X 2 4 40 Normal 28 A1

X 2 5 40 Normal 28 B1

X 3 1 40 Normal 20 A4

X 3 2 40 Normal 20 B2

X 3 4 40 Normal 28 A4

X 3 5 40 Normal 28 B2

58
Table 5. Test plan sequence continued

Time Period #

7

Third Day

A15 %

Water Tire

Measurement Test Flow Inflation

System Surface Lane Speed Rate Pressure Tire

IRS 1 2 3

X 2 3 40 - Normal Cl

X 2 4 40 - Normal D1

X 2 6 40 + Normal Cl

X 2 7 40 + Normal Dl

X 3 3 40 - Normal C4

X 3 4 40 - Normal D2

X 3 6 40 + Normal C4

X 3 7 40 + Normal D2

X 2 3 40 Normal 20 Cl

X 2 4 40 Normal 20 Dl

X 2 6 40 Normal 28 Cl

X 2 7 40 Normal 28 Dl

X 3 3 40 Normal 20 C4

X 3 4 40 Normal 20 D2

X 3 6 40 Normal 28 C4

X 3 7 40 Normal 28 D2

X 2 5 40 — Normal El

X 2 2 40 + Normal FI

X 2 1 40 + Normal El

X 2 6 40 - Normal FI

X 3 5 40 - Normal E4

X 3 2 40 + Normal F2

X 3 1 40 + Normal E4

X 3 6 40 - Normal F2

X 2 5 40 Normal 20 El

X 2 2 40 Normal 28 FI

X 2 1 40 Normal 28 El

X 2 6 40 Normal 20 FI

X 3 5 40 Normal 20 E4

X 3 2 40 Normal 28 F2

X 3 1 40 Normal 28 E4

X 3 6 40 Normal 20 F2

59
Table 5. Test plan sequence cont inued

Time Period #8

First Day

A15%

Measurement

System Surface Lane

Test

Speed

Water

Flow

Rate

Tire

Inflation

Pressure Tin

IRS 1 2 3 X 3 1 40 - Normal A4

X 3 2 40 - Normal B2

X 3 4 40 + Normal A4

X 3 5 40 + Normal B2

X 2 1 40 - Normal A3

X 2 2 40 - Normal B3

X 2 4 40 + Normal A3

X 2 5 40 + Normal B3

X 3 1 40 Normal 20 A4

X 3 2 40 Normal 20 B2

X 3 4 40 Normal 28 A4

X 3 5 40 Normal 28 B2

X 2 1 40 Normal 20 A3

X 2 2 40 Normal 20 B3

X 2 4 40 Normal 28 A3

X 2 5 40 Normal 28 B3

X 3 3 40 - Normal C4

X 3 4 40 - Normal D2

X 3 6 40 + Normal C4

X 3 7 40 + Normal D2

X 2 3 40 - Normal C3

X 2 4 40 - Normal D3

X 2 6 40 + Normal C3

X 2 7 40 + Normal D3

X 3 3 40 Normal 20 C4

X 3 4 40 Normal 20 D2

X 3 6 40 Normal 28 C4

X 3 7 40 Normal 28 D2

X 2 3 40 Normal 20 C3

X 2 4 40 Normal 20 D3

X 2 6 40 Normal 28 C3

X 2 7 40 Normal 28 D3

X 3 5 40 _ Normal E4

X 3 2 40 + Normal F2

X 3 1 40 + Normal E4

X 3 6 40 - Normal F2

X 2 5 40 - Normal E3

X 2 2 40 + Normal F3

X 2 1 40 + Normal E3

X 2 6 40 - No rmal F3

60
Table 5. Test plan sequence - continued

Time Period #8

Second Day

A15%

Measurement

System Surface Lane

Test

Speed

Water

Flow

Rate

Tire

Inflation

Pressure Tire

IRS 1 2 3 X 3 5 40 Normal 20 E4

X 3 2 40 Normal 28 F2

X 3 1 40 Normal 28 E4

X 3 6 40 Normal 20 F2

X 2 5 40 Normal 20 E3

X 2 2 40 Normal 28 F3

X 2 1 40 Normal 28 E3

X 2 6 40 Normal 20 F3

X 3 7 40 — Normal G4

X

3 3 40 + Normal G4

X 2 7 40 - Normal G3

X 2 3 40 + Normal G3

X 3 7 40 Normal 20 G4

X 3 3 40 Normal 28 G4

X 2 7 40 Normal 20 G3

X 2 3 40 Normal 28 G3

X 3 7 40 - Normal G2

X 3 3 40 + Normal G2

X 2 7 40 - Normal G3

X 2 3 40 + Normal G3

X 3 7 40 Normal 20 G2

X 3 3 40 Normal 28 G2

X 2 7 40 Normal 20 G3

X 2 3 40 Normal 28 G3

X 3 1 40 — Normal A2

X 3 2 40 - Normal B4

X 3 4 40 + Normal A2

X 3 5 40 + Normal B4

X 2 1 40 - Normal B3

X 2 2 40 - Normal A3

X 2 4 40 + Normal B3

X 2 5 40 + Normal A3

X 3 1 40 Normal 20 A2

X 3 2 40 Normal 20 B4

X 3 4 40 Normal 28 A2

X 3 5 40 Normal 28 B4

X 2 1 40 Normal 20 B3

X 2 2 40 Normal 20 A3

X 2 4 40 Normal 28 B3

X 2 5 40 Normal 28 A3

61
Table 5. Test plan sequence continued

Time Period #8

Third Day

A15 %

Water Tire

Measurement Test Flow Inflation

System Surface Lane Speed Rate Pressure Tire

IRS 1 2 3

X 3 3 40 - Normal C2

X 3 4 40 - Normal D4

X 3 6 40 + Normal C2

X 3 7 40 + Normal D4

X 2 3 40 - Normal C3

X 2 4 40 - Normal D3

X 2 6 40 + Normal C3

X 2 7 40 + Normal D3

X 3 3 40 Normal 20 C2

X 3 4 40 Normal 20 D4

X 3 6 40 Normal 28 C2

X 3 7 40 Normal 28 D4

X 2 3 40 Normal 20 C3

X 2 4 40 Normal 20 D3

X 2 6 40 Normal 28 C3

X 2 7 40 Normal 28 D3

X 3 5 40 - Normal E2

X 3 2 40 + Normal F4

X 3 1 40 + Normal E2

X 3 6 40 - Normal F4

X 2 5 40 - Normal E3

X 2 2 40 + Normal F3

X 2 1 40 + Normal E3

X 2 6 40 - Normal F3

X 3 5 40 Normal 20 E2

X 3 2 40 Normal 28 F4

X 3 1 40 Normal 28 E2

X 3 6 40 Normal 20 F4

X 2 5 40 Normal 20 E3

X 2 2 40 Normal 28 F3

X 2 1 40 Normal 28 E3

X 2 6 40 Normal 20 F3

6 2
test wheel to enhance the sensitivity of the measurement to differences

in surface characteristics. One purpose of this program is to correlate

the measurement performance of different systems. However, variations

in the test results occur which are not due to differences in the measuring system. The test tire is subject to wear and the surface over which it is dragged has nonuniform skid resistance characteristics. To

measure the effect of the different operating variables and to account

for some of the disagreement between measuring systems, the nonuniformities of standard tires and test surfaces need to be identified.

Accordingly, the test program employs, as a building block, a test

matrix which allows a measure of the difference between test tires and

the difference between the lanes comprising the test surface to be

determined while the significance of a given variable is being tested.

A sample test matrix is shown below:

Time Period

Surface

Tires

Speed A B C D E F G

S-AS 1 2 3 4 5 6 7

S 23 4 5671 S + AS 4 5 6 7 1 2 3

Letters A thru G designate 7 tires while numbers 1 through 7 identify

the 7 lanes comprising the surface [7]. The nominal test speed is S,

while AS is an incremental change in the speed. Since AS is small, the

slope over the range 2AS approaches the speed gradient at S.

Twenty-one skid resistance measurements are made in one matrix,

using seven tires, over seven lanes of the surface, with three values of

the test variable being studied. The mean of the twenty-one test values

represents the skid resistance of the surface measured during the time

period using the "average" tire, over the "average" lane at the "average"

value of the test variable. The novelty of the test matrix is that it

yields a skid resistance- test variable gradient for the surface as well

as an indication of the dispersion due to choice of tire or lane. The

operating variables planned for study using test matrices were test

speed, water depth, and test tire inflation pressure. The test matrix

is explained further in Appendix B.

63
Twenty-eight test tires were used by the four systems in the conduct of the test program.

3.4. Tests Accomplished

The accomplishment of the full program requires eighteen days of

dynamic testing. Since some unsuitable weather was expected, the tests

which would allow comparison of results with those of the previous program were scheduled to be conducted first. The weather conditions at

the test site are given in figure 23 for the March 30 to April 8, 1976

period and in figure 24 for the April 19 to April 29 period. Correlation testing did not occur on March 31, April 1, 2, 3, 4, 19, 21, or 25.

By April 28, the first six time periods of two days each had been com- pleted. With one more day available for testing a supplemental period 7 was designed (table 6) for additional direct comparisons between the skid

measuring systems. These comparisons did not include all the combinations

of tires and lanes used in the earlier tests. Table 7 summarizes the

comparisons between systems.

3.5. Control of Operating Variables

Previous experiments with the IRS had shown that the operating

variables listed in section 3.2 required control.

Variations in hitch height, as expected, change the unloading

constant of the test wheel vertical load. Variations in hitch height

which also rotate the force transducer cause a shift in the zero of the

traction channel and change the unloading constant, as measured by the

vertical load channel, from the expected value. Two-axis force transducers are used on the IRS and the three ARS units. The static tests

(sec. 2.3) had demonstrated the changes of hitch height resulting from

variation of water load.

Variations in the inflation pressure of the tow vehicle rear tires

change the water laying system pump rate with test speed. The test crews controlled the inflation pressure of the tow vehicle rear tires.

The IRS trailer suspension system utilizes air-adjustable shock

absorbers. Variations in shock absorber inflation pressure result in a shift in trailer body orientation which changes the static vertical test

wheel load. The IRS shock absorber pressure was controlled by the IRS crew.

In a previous IRS - Western (WARS) test program it was noted that

the average skid number values measured, in each of seven two-day pairs

of test conducted with both systems, on the second day were lower than

the average skid numbers measured on the first day [7, sec. 3.10]. It

64
Vaanivagdwgi

x

15o

o x 5 9

3NN

AdOOD N

AliSOW

Sdooio %oc mnn

ANNOS MN

MNN

N

ANNOS

ANNOS N

MN

MN

ANNOS MN

M

AIMOIS ONIAdd SSOVddOS M

ANNOS MS

MONS

Niva iHon isvoasAO

ANNOS MSS

ANNOS 3SS aiOD

13M S3DV3dOS lsvoasAO

AAV3H

Niva 'S MONS

Niva

SJ.SOO

ANNOS Alison

a3iVM ONIONViS

sxavwsa

MS

woad 033dS

ON I

M

3 0 3a01Va3dW31

65

Figure

23.

Weather

conditions

during

dynamic

tests,

March

30

-

April

8,

1976.
D '3aniVd3dW31

avaiD

ANNOS

dV31D ANNOS

%00l

ISVOaSAO

N 9-f %98 A0nO10 N %92Aanoio

9-0 ANN ns N 9 -e MN 21 %S6 AanoiON 01-9 %oot 1 SV383AO N %06Aan01D

01-9 %00 l AANN 01 !SVOa 3 AO %00l lSVOa3 AO N W- 2 L

301

Niva

Niva

N\ Ol-O Aanoio

ANNf.S S 9 3 9-0 S 01-0 ANNOS 3 9-0 13M W1V0

Niva

Aanoio

ANNHS A\N 01-0

Niva

ANNOS

M

01-9 Aanoio 9 9-0 ANNnS 3 9

swyoj.sy3aN0Hi oo o& ooo o o o o O oCO

X»—Zo£

an

Q_<

Hdw 30'aaoiva3dw3i

WOdd 033dS

SXdVW3d QNIM

66

Figure

24.

Weather

conditions

during

dynamic

tests,

April

19-

April

29,

1976.
Table 6. Supplementary test plan sequence

Time Period #7

One Day

Measurement

System Surface Lane

Test

Speed Tire

IRS 1 2 3

X 2 2 20 D2

X 2 3 20 B2

X 2 4 40 D2

X 2 5 40 32

X 2 1 60 D2

X 2 2 60 B2

X 3 2 20 F2

X 3 3 20 A2

X 3 4 40 F2

X 3 5 40 A2

X 3 1 60 F2

X 3 2 60 A2

X 2 4 20 F2

X 2 3 20 A2

X 2 7 40 F2

X 2 6 40 A2

X 2 5 60 F2

X 2 4 60 A2

X 3 4 20 B2

X 3 3 20 D2

X 3 7 40 B2

X 3 6 40 D2

X 3 5 60 B2

X 3 4 60 D2

X 2 6 20 F2

X 2 7 20 A2

X 2 5 40 F2

X 2 2 40 A2

X 2 1 60 F2

X 2 6 60 A2

X 3 6 20 A2

X 3 7 20 F2

X 3 5 40 A2

X 3 2 40 F2

X 3 1 60 A2

X 3 6 60 F2

67
Table 6. Supplementary test plan sequence - continued

Time Period //7 One Day (continued)

Measurement Test

System Surface Lane Speed Tire

1 2 3

X 2 2 20 D2

X 2 3 20 B2

X 2 4 40 D2

X 2 5 40 B2

X 2 1 60 D2

X 2 2 60 B2'

X 3 2 20 D2

X 3 3 20 B2

X 3 4 40 D2

X 3 5 40 B2

X 3 1 60 D2

X 3 2 60 B2

68
Table 7. Summary of direct comparisons between skid measurement

systems

Speed Time period Surface 2 Surface 3

mph

3

20

|

3

b

IRS /ARS 1 ARS 2/ARS 3 4 ARS 2/ARS 3 IRS /ARS 1

40

|

1 IRS /ARS 1 ARS 2/ARS 3

2 ARS 2/ARS 3 IRS /ARS 1

60

j

5 IRS /ARS 1 ARS 2/ARS 3 6 ARS 2/ARS 3 IRS /ARS 1

IRS /ARS 2 ARS 1/ARS 3

20, 40, 60 | IRS /ARS 3 IRS /ARS 2/ \1 ARS 1/ARS 3 IRS /ARS 1 ARS 1/ARS 2 ARS 2/ARS 3

3

1 mph = 1.609 km/h.

This notation means that the two systems were tested alternately

on lanes of the same surface.

69
was suspected that the power-broom sweeping of the surfaces at the

beginning of each test day was not adequate to remove the rubber deposited on the surfaces by tire wear. In this test program the surfaces were flooded with pressurized water from a tank truck the evening before

a test day.

The degree-of-dryness of the test surfaces was controlled such that

any wetness was not visible in the lane immediately before a skid test.

The test plan sequence was ordered to facilitate control of the degreeof-dryness. Testing was also delayed as necessary to allow a longer

time for the lane to dry.

Test speed on-board recorded values and driver visual aids were

checked against the IRS stationary differential radar system at least once each day. The resolution of the system is 0.1 mph (0.2 km/h).

Variations from the planned speed occurring during a test run were re- corded by the radar system. These recorded values were used in the data

reduction for all systems.

Water flow rate was recorded in all systems. The records were used

to trim the flow rate to predetermined values corresponding to the

different test speeds.

In all previous test programs in which the IRS participated, both

an instrumentation operator and a driver were in the tow vehicle. The

operator would zero the instrumentation as necessary immediately before

each skid test. The operator was not made available for this test

program. The instrumentation was zeroed, however, at the beginning of

the morning and afternoon test periods. IRS traction force, vertical

load, and flow rate signals were recorded on board in both digital and

analog form. The digital data was transferred to a ground station where

it was scaled into engineering units and filed on a magnetic tape. In

order to utilize the available time most effectively, only minimal data

reduction was carried out while the tests were being conducted. Only

two or three tests were processed by the ground station each day. As a result, it was not until the third time period that drift in the vertical

load signal of the IRS was detected by inspection of the ground station

plots. Experiments then verified a vertical signal drift due to temperature changes at the IRS wheel transducer. Immediate steps were taken

to control this effect by re-zeroing the force signals of the IRS just

prior to each test. The problem would have been avoided had a full-time

instrumentation operator been available to carry out the normal procedures.

70
3.6. Measurement of Uncontrolled Variables

The weather conditions around the test site and the surface temperature of surface 2 have been given in figures 23 and 24. The surface

temperature of the base material on which surface 2 was constructed was

the same within + 4 °F (2 °C) . The temperature was measured with a sensor located in the northwest corner of the surface. The temperature

sensitive area of the sensor was placed in contact with the top of the

aggregate. A conductive grease was used to provide thermal contact

between the sensor, the sides of the protruding aggregate, and the

binder of the surface. The surface temperatures ranged from 35 °F to

110 °F (2 °C to 43 °C) during the testing period.

Wind speeds were measured at the site, approximately 5 feet (1.5 m)

above the surface. It was noted that the wind speed dropped by approximately half within a foot (0.3 m) of the surface.

Test tire inflation pressure was measured after a five mile warm-up

drive and before each skid test.

The test wheel lock-up position was noted by an observer during

each test. When mounted on the IRS, the tread depth at each groove across the width of the tire at the lock-up position was measured and

recorded after the skid. The tire tread surface temperature, higher

where the skid occurred, was a further guide in locating the lock-up

position.

The type of tire wear measured on these surfaces was reported

previously [6, sec. 3.6].

3.7. The Correlation Test Plan

A purpose of the dynamic skid test program listed previously is to

correlate the degree to which the measurement results from the three ARS

units and the IRS match. The correlation is degraded by three sources

of dispersion:

a) different times at which the measurements are made,

b) any nonuniformity between test tires, and

c) any nonuniformity between lanes of the test surface.

The correlation test plan is designed to measure these sources of

dispersion. If their existance can be proven and quantified, their

effect on the results can be taken into account to yield a more precise

correlation. Knowledge of the bounds of these sources of dispersion

can be used to establish an uncertainty band around the correlation

result

.

71
Statistical confidence in the results can be improved by running a sufficiently large number of tests. Confidence in the correlation of

equipments is enhanced by finding consistency both in the magnitude of

the results at a point, and in the first derivative of the results with

respect to a test variable through that point. Confidence in the measurement results in enhanced when the data points are found to be within

small intervals of variation.

The number of tests planned was based on the degree of precision

previously reached by the IRS and anticipated for the ARS units, under

controlled test procedures. Tests of the sensitivity of SN to water

flow rate and test tire inflation pressure were planned for 40 mph (64

km/h) since this is the more usual test speed. The test plan contained

182 skid measurement tests for each participating skid measurement

system.

3.8. Test Matrix Results

The skid resistance measurement results are given in terms of a mean value at the nominal test speed, the measured skid resistance- test

speed gradient through that mean value, and a 99 percent confidence

interval about the mean value.

As noted in section 3.5, drift affected the dynamic vertical load

measurements made by the IRS during the first three time periods. Consequently, for consistency, all SN results have been calculated using

test wheel load reductions derived from the experimentally determined

effective unloading constants. Preliminary comparisons were made between SN results calculated this way and those calculated on the basis

of dynamically measured vertical loads (excluding the drifted data).

The comparisons showed agreement between the two methods with 2 SN. The

unloading constant method yielded higher values for two systems (ARS 1

and the IRS) and lower values for the other two systems, on both surfaces.

Each curve (SN ) shown by dashed lines in figures 25 and 26 represents a least squares fit to all the measurements made by one skid

measurement system on one surface during the first six time periods.

The measurements were clustered about the three nominal test speeds;

V = 20, 40, and 60 mph (32, 64, and 97 km/h). The slope of the curve obtained from all the measurements (0 , the calculated skid resistancetest speed gradient) has been used to adjust the value of each measurement to correspond exactly to its nominal speed. The means of the

adjusted values are shown (SN^g, SN^g, and SN^g) and the 99 percent

confidence interval (L) is shown for each mean. A straight line segment

is shown through each mean, representing the speed gradients (^o’ ®40’

and Qgg) measured using the incremental speeds of the test plan. The

numerical values for the plots of figures 25 and 26 are given in tables

8 and 9

.

72
70

60

50

70

60

50

70

60

50

70

60

50

30

T

ARS I

km/h

60 90 120

“T

ARS 2

IRS '

_J i i I 20 40 60 80

mph

SPEED

Figure 25. Test matrix results, surface 2.

7 3
70

60

50

70

60

50

70

60

50

70

60

50

30

~T

ARS I

km/h

60

~l

90

~r

120 ~

ARS 2

ARS 3

_L_

20

I L

40 60

mph

SPEED

Figure 26. Test matrix results, surface 3.

74
Table 8. Test matrix results, surface 2

{ sn 20 = 61.36 02 o — ~ 0 • 30 L = ±1.5

SN40 = 57.61 O4 0 = -0.05 L = ±1.6

ARS 1 sn 6 0 = 58.04 06O = +0.39 L = ±1.4

SN = 69.66 -- 0.53 V + 0.006 V

2 0

C = - 0.53 + 0.011 V

sn20 = 59.56 ii i o o CO o

CM CD L = ±1.4

sn 40 = 55.21 O40 = +0 . 04 L = ±1.3

ARS 2

' sn60 = 55.63 0 6O = -0.54 L — ±2.1

SN = 67.96 -- 0.54 V + 0.006 V

2 0

C

c - 0.54 + 0.011 V

sn20

— 63.63 02 o = — 0 • 20 L

— ±1.6

sn40 = 57.83 O 40 = -0.29 L = ±1.0

ARS 3

1 SN60 = 58.54 0 6O = +0.02 L = ±1.8

1 SN = 75.46 - 0.75 V + 0.008 V

2

1 Q

cc = - 0.75 + 0.015 V

SN20 = 67.82 0 2O = -0.58 L ±1.4

sn 40 = 63.72 04 o = +0.34 L = ±1.1

IRS

sn60 = 61.43 06 0 = +0.59 L = ±1.6

SN = 74.95 - 0.42 V + 0.003 V

2 Q

C = - 0.42 + 0.007 V

75
Table 9. Test matrix results, surface 3

sn20 = 66.20 II

Csl O © -0.12 L = ±1.8

sn40 = 57.87 0 4O “ -0.51 L = ±1.3

ARS 1 < sn60 = 50.94 °60 = -0.27 L = ±1.7

SN = 76.27 - 0.55 V + 0.002 V

2 e

c

c

= - 0.55 + 0.004 V

sn20

- 65.38 02O = -0.59 L

— ±1.5

sn40 = 57.60 0 4O = -0.43 L = ±1.0

ARS 2 sn6 o = 48.87 06 0 = +0.28 L = ±2.0

SN = 73.54 - 0.41 V + 0.0001 V

2 0

Cc ~ - 0.41 + 0.0001 V

- sn20

- 67.81 0 2O •= -0.46 L

— ±1.2

sn 40 = 60.95 0 4 0 = -0.36 L = ±1.3

ARS 3 sn60 53.03 0 6 0 = -0.30 L = ±1.4

SN = 73.89 - 0.28 V - 0.001 V

2 0°

c = - 0.28 - 0.002 V

/ sn 20 — 69.11 02O = -0.88 L

_ ±1.9

sn40 = 64.14 04 0 = -0.14 L = ±1.6

IRS

SN60 = 55.86 06 0 = -0.25 L = ±1.6

SN = 72.37 - 0.09 V - 0.003 V

2

| 0

C = - 0.09 - 0.006 V

c

76
According to the test plan, each skid measuring system made a matrix of SN measurements on a given surface, at each nominal speed,

using a group of seven tires. The test matrix was used to calculate the

deviation of the SN measurements made by each tire from the mean of all

the measurements in the matrix. In the first six time periods, each

tire group was included in six matrices (two systems measuring one surface at three nominal speeds). The exceptions were tire C of group

1, with five matrices completed, and all the tires of group 2, with five

matrices completed. The deviations for each tire, as calculated from

the several matrices, were then averaged to obtain the overall measurement deviation for each tire. These overall deviations are shown in

figure 27 for the tire groups used to measure surface 2 and in figure 28

for the tire groups used to measure surface 3.

Six of the twenty-eight tires (identified by asterisks in the

figures) made SN measurements that deviated significantly from the

measurements made by the "average tire" of their particular group.

The SN measurements also form a matrix for each system on a given

surface, at each nominal speed, for seven lanes of the surface. The

matrix was used to calculate the deviation of the SN measurements

made on each lane from the mean of all the measurements in the matrix.

In the first six time periods, the lanes of surfaces 2 and 3 were included in twelve matrices (four systems measuring each surface at three

nominal speeds) . The twelve calculated deviations were then averaged to

obtain the overall measurement deviation for each lane. These overall

deviations are shown in figure 29.

The outside lanes 1 and 7 of surface 2 were found to yield a significantly higher SN than the surface average, while center lanes 4 and

5 measured significantly lower. On surface 3, lane 2 measured significantly higher than the average.

The bounds of the deviations due to tires and lanes were approximately + 2 SN. No significant deviations attributable to time of day,

surface temperature or variations of test tire inflation were detected.

3.9. Ranking of the Sources of Dispersion

The calculated deviations have been used to rank the sources of

dispersion in the skid number measurement. The ranking of SN dispersions

attributed to tires and surfaces is shown for the four measuring systems

separately in tables 10, 11, 12, and 13.

When the deviations are pooled without regard to system, the overall ranking is obtained, as shown in table 14. Tire dispersions are slightly higher than surface dispersions.

77
DEVIATION,

SN

DEVIATION,

SN

*

TEST TIRES

Figure 27 . Deviations from the mean skid number observed for tires of

groups 1 and 3, from skid resistance measurements on surface 2. Deviations marked with an asterisk are considered

large enough to be statistically significant.

78
DEVIATION,

SN

DEVIATION

,

SN

Figure 28. Deviations from the mean skid number observed for tires of

groups 2 and 4, from skid resistance measurements on surface 3- Deviations marked with an asterisk are considered

large enough to be statistically significant.

79
DEVIATION,

SN

DEVlATION,SN

-1-2 i

na

1 2 3 4 5 6 7

LANES, SURFACE 2

LANES , SUR FACE 3

Figure 29. Deviations from the mean skid number observed for the lanes

of surfaces 2 and 3.

80
Table

10.

Ranking

of

possible

sources

of

dispersion

in

ARS

1

results

3

X) o

0) 3 •H rH n3 -P co no CO CN On rH ID 00 CN r—

1

CX X) c0 MON CTn V£> rH CTN vd" 1—1 00 e c •H

cfl CO > CM CN i—

1

1

1 i—1 r—1 1“

i

rH rH i—1 rH o m 3 cu 3 T3

X) 3

3 3= o o o o o o O o o o o o 0) CX CN V£) vO CN c£> -d- CN vD CN

ex B

3

o

CO

4-1 co co co CN CN CN CO m m CN CN CN 3

3

in

3>53

CO3 < W CJ c B C < B C A B CJ -Q3

m

3

XJ o

3 3 •H rH Cd U CO co LO

(X X) CO Cl m CO B 3 •H • • • • cO co > 1 1 «—

1

i—

1

1—

1

m 4_i ai CO X)

CD

CJ3 CN 1—

1

3 3 o O 0-> cx 3 •H 3 CO CN co /-“N 3 /-n

CO O <D 3 3 O 3

3 4-1 3 3 3 CD 3 3 3 3 3

'—

1

o CD CxO °H CD 3 CD 3 M) 3

JD a 3 3 3 3 3 •H

•H CO CD 4-1 11 4-1 1

1 3 3

3 •H 3 3 3 3

CO XI •H w 3 3 •H o H m m H

Ph

3d3

5

rH CN CO

CX> O

,3

CXB

3

81
Table

11.

Ranking

of

possible

sources

of

dispersion

in

ARS

2

results

XI o

QJ V-1 *i—

H tO 4-1 CX XI 03 E C -H

cd CO >

CO 4-1 0)

CO X)

X

0)

CDCX| CO

lO N O'

r-( O 00

O' H CO

cn oo oj

NON O rH vD 00

<d <r CO CN rH

cdX

cxE o o o cd <r cn o o o cd cni <r o o o CN VD <r o o o CN vD

CN CN CN r~i m cn

emu <mu emu e pq i

X) o

CD X •H

r—

n3 4-1 00 rH rH

Q* T) X CN] kO LO E C •H • • • • cd cd > CN] rH rH

CO 4-1 0)

co X)

3 c CX /-s CX ^ O o 3 co ro N CN 3 co

CO •H O CD CA CO O CD • CO X U CD CD CD CD X )-i -3

0) 4H 00 -H O C Cj 3 00 -H

rH O a) 4-1 cd a) cd cd 4-1 "e

40 a CD U-l rH <4-4 i—0) 4*i •H CO X r" X 3 Cj i-—

CO °rl •H ^ 3 r^ 3 t" •H O'

co X) H CO CO w H o

o cD

(X, •

cxE

82
Table

12.

Ranking

of

possible

sources

of

dispersion

in

ARS

3

results

c

x> o

03 X -H

1—I CO 4-* CX X> CO B C -H

co ra >

C/3 U OJ

C/3 TO

o cm o o <r

CM CM i—

I

on o- r-- CXO nD CO

co <}- o <30 CM CM

i—[ i—I r—

{

in o co on -o- <r

i—( T—( o

X3

0303a

03 XexB

o o o no cm <r o o o cm <r no o o o o o o

NO CM NO CNI <1-

03O

CO nwX

o

03

cm cni cni rococo cococo cmcmcm

3

cO

5-i J33

03

< « o < « o cmo

e

X) o

03 X -H

r-H CO 4J CX XJ <0 B 3 -H

CO CO >

03 4J 0)

03 X)

ON NO

130 C30 CO

ONco

03

CJ

5-i 3o c

C/3 o CM *-

N

•H 03

03 <4-4 X 03 CD rH O 0) CJ C

X> & CO CC

•rH 03 U-4 r-H

C/) •H X

0) XI 3

O 03

(X

oCX /—

*

3 c/3 O 03

5-i 5-J 60 -H

4-1 03

>-i r—

•HH

co

CO

CX 4—

S

3 03 0) O 03

03 <D X X o C 60 -H

CO XI

4-3 1—

1

03 X X O' 3 r-. •H n_4 03 H

-3 'b

ONoNO

83
Table

13.

Ranking

of

possible

sources

of

dispersion

in

IRS

results

c X o

<U S-i -H

i—i cc -u CX X 0) B C -H mum as ctf >

US X

in m h

I—I o\ o

CN i—I i—

tfl (M O

H m st

CN i—I i—

O CO N O t-H O

m oo o O 00 O

'-IOO

X

<U0)CX

c/s

jC

(X£

o o o CN O O’ O O O

O O’ CN o o o o o o

O’ O CN CN O’ O

<D

CJ

03

4-1

S-4 3

co room cnjcncn room cncmcn

XC3

n3

S-i X>3

CO emu emu emu emu

c X o

as S-I •H

1—

03 U

cx no 03 e e •H

<3 <T5 >

CO 4-J as

us x

CNl 00

CN vD CNJ 00

r—

i—

O

0/

CJ

S-i O’ r-H 3 c o o CX ,-n. CX r-v

c/s *H 3 us 3 us

US o as o as

as 4-4 S-i S-i S-i S-i S-i

i—

O as 00 -H 00 -H m cx 4-1 4-1 •H US as CL) co S-i S'- S-4 1''-

C/3 X •H N—•H s ' o H H

cx

CO CN /—N

C/3 C/3 . as a) as 0) -C u c a C

cti 03 0) 03 £

4-1 rH 4-1 rH

S-4 S-4 3 3 o CO co o

\D

II

e

03

cx&

CNJ m <r

i—

03

84
Table

14.

Ranking

of

possible

sources

of

dispersion

in

the

overall

results

e

T3 O

03 Cl "H

i—i o3 4-i «

S C -H

03 CO >

CO 4-1 CD

Cfl TO

CO

UT f'. cO

cn <r

<f -<f <r

i—i i—i i—

i

03U

Cl co CN r—

1

3 e

O o Cl a /—*N Cl ^ Cl /-v

co •H 3 cn 3 CO 3 CO 3 Cfl CO /-s CN /—V

CO O QJ O QJ O 03 O c1) c/3 0) a> 4-i C Ci Cl C Ci Ci *4 03 03 0) a> rH o 03 00 •H oo •H 00 -H oc •H O c u c

X3 Cl 4_> 4-1 4-1 4-1 03 0) cd cd •H CO 0) O 03 03 4-1 T—

1

4-4 rH

Cfl •H Ci Cl r^ C r-. Ci r\ C *4

cn T3 •H •H •H N—

'

•H 3 D

o H H H H CO w C/3 'w' cc

c

03 OS

85
The sample standard deviations of skid number, with the calculated

tire and lane dispersions, are listed according to system, surface,

and speed in table 15. The standard deviation of the calculated tire

and lane dispersion is comparable to or less than the standard deviation

of the skid number measurement result in each case.

3.10. Deviation from Desired Test Speeds

The frequent use of nominal speeds in skid number data re- duction is justified when actual speeds fall within appropriate limits.

Limits are needed because SN is speed-dependent. E 274-70 sets bounds

on the allowable deviation: +0.5 mph (0.8 km/h) for test speeds at or below 40 mph (64.4 km/h) and + 1 mph (1.6 km/h) at higher speeds. Data

reduction can compensate for off-target speeds only when measured speeds

and speed gradients are available.

In general, each skid represents a driver's attempt to perform a test as near to a specified speed as possible. For the tests under discussion, speeds of all four vehicles were accurately recorded during all

skids. Later, the observed speed for each skid was averaged over the

portion of the skid used for skid number calculation. Here we present

the dispersion of the averaged speeds about the target values.

Bar graphs illustrate the speed dispersions found for each system

(figs. 30, 31, and 32). Specified speeds were 18, 20, and 22 mph (29.0,

32.2, and 35.4 km/h); 38, 40, and 42 mph (61.1, 64.4, and 67.6 km/h);

and 58, 60, and 62 mph (93.3, 96.5, and 99.8 km/h). Data is graphed by

speed range since the effects of operational conditions such as time

available to reach test speed and test wheel traction were clearly

associated with slow, moderate, or fast ranges. ARS 1, 2, and 3 were mechanically similar. Crew technique may be considered as a cause of

consistent differences seen in ARS speed performance (e.g., wider scatter

in speeds of the ARS 2). The IRS differed mechanically from the ARS

units

.

Test conditions were good: speed subsystems were well calibrated,

tests were repetitive, and in comparison to roadway testing, test pavements were good and traffic distractions were minimal. Even so, observed speeds exceeding limits for the three speed ranges totalled 3 (2%

of the skids) for ARS 1, 26 (20%) for ARS 2, 16 (12%) for ARS 3, and 4

(3%) for the IRS. It can be seen from the graphs that distributions

were not balanced around the desired speed. Slow runs were more numerous

in ten of the twelve distributions. The mean values of the distributions are listed in table 16. These fall within the previously quoted

limits, however each is derived from approximately 45 tests.

The results underscore the importance of speed measurement. The

data analyst must have assurance that the speed system operated suitably

close to the desired speed.

86
Sample

standard

deviation

ARS 1

results

ARS 2

results

ARS 3

results

IRS

results

cu

m 00

O o oo c O 00 NO O NO rH

cfl • • • • • • -1 ?—1 o O rH rH rH

0)

O CM oo in rH in

<r

m 1—1 i—i

vO c^

•H • • • • • • H iH rH CM CM r—H

rH

CM CM CO o CO

m

S3 CM oo Mj o VD V£> CO • • • • • • CM T—1

CM CO CN CM co

•H

Cfi

0)aw

•HQ

JOH

X)

OJcua,

cn oom

-om

LO

CN

<r

ooo

no

nto

UOO'

<r

oo

XGE oCN

O

<r o

vO

<r

CN

cn

oo

00

OC oCM

r-~ NO

OCN

cn

00

cn

no

CM

CN r^ CO rH CO o rH 00 CM CO

CN rH i—i

rH rH CM

CN CM

m vO OC

00 o rH vD <r 1 rH CM CO rH rH 1

CM vO fH 00 r^ 00 CO O CM vD co • • • • • • CM CM CO CM rH CM

<1) ON CO CM 00 vD rH C rH rH

U5 • • • • • • J rH rH rH rH rH rH

CD -d~ tH

CM <r vO 00 <r 00 CM o

'H • • • • • • H rH O rH CM rH CM

GA o v£> o Z CO CO CM 00 00

cn • • • • • • CM CM CN CM rH CM O o

nC E

ONo

nOaE

87
i

i

coa

CO

U-o

cc

UJ

co

Z>Z

km/h

0.0

m ph

DEVIATION FROM SPEED

(20 mph RANGE)

Figure 30. Average deviations from nominal speed observed during skids

at 18, 20, and 22 mph (29.0, 32.2, and 35-^ km/h).

88
NUMBER

OF

SKIDS

km/h

mph

DEVIATION FROM SPEED

(40 mph RANGE)

Figure 31. Average deviations from nominal speed observed during skids

at 38, 40, and h2 mph (6l.l, 6U.4, and 67.6 km/h).

89
NUMBER

OF

SKIDS

km/h

-4.8 -3.2 -1.6 00 + 1.6 r r

, T r

ARS 1

j 1 L 00

00

mph

DEVIATION FROM SPEED

(60 mph RANGE]

Figure 32. Average deviations from nominal speed observed during skids

at 58, 60, and 62 mph (93.3, 96.5, and 99-8 km/h).

90
Table 16. Mean deviation from nominal speed

Mean deviation

Unit 20 mph range 40 mph range 60 mph range

ARS 1

b

(46)

mph

3

+0.1 (43)

mph

-0.1 (46)

mph

-0.3

ARS 2 (46) 0.0 (46) -0.4 (36) -0.7

ARS 3 (46) -0.1 (45) 0.0 (46) -0.8

IRS (46) -0.1 (56) -0.1 (48) -0.3

a

1 mph = 1.609 km/h

The number of tests considered is shown in parentheses.

91
3.11. Skid Resistance Speed Gradients

It is known from test data generated at largely different test

speeds that pavement skid resistance varies with speed. In this section

we show that the skid resistance - test speed gradients measured using

the 2 mph (3.2 km/h) differential test speeds, AS, of the test matrix,

in conjunction with the differential radar system, are generally con- sistent with the slope of a continuous polynomial expression of skid

resistance as a function of test speed.

The synthesis of the polynomial skid resistance expression for a skid measuring system-surface combination at EFTC utilized the results

of the respective test matrices of time periods 1 through 6 over the

full speed range. Least squares techniques were used to fit the data

with an expression of the form:

SN = A + BV + CV 2 c

where V is the test speed in mph (1 mph = 1.609 km/h).

Differentiation of SN produces an expression for calculation of

the skid resistance - test speed gradient which is also a function of

test speed:

d (SN ) _ c_

c “ d(V)

B + 2CV.

The SN curves, shown plotted on separate ordinates in section 3.8, are re-plot£ed with common ordinates in figure 33 to illustrate the

agreement obtained between measuring systems. The systems tend to

produce similar curves on a surface; that is, the speed gradients characteristic of the system-surface combinations in this experiment appear

to be more dependent on the surface than on the choice of measuring

system. It can be seen that the SN differences between systems on a surface could be largely corrected Ey an adjustment of the constant

term.

For all four systems, the SN curve on surface 2 is concave upwards

with zero slope (speed gradient) at a point above 40 mph (64 km/h).

This agress with measurements previously reported on surface 2 at EFTC

[6, sec. 3.8] and on the similar surface 2 at WFTC [7, sec. 3.9].

On surface 3, the SN results for the four systems are more linear,

and are divided between sSightly concave and convex curves. Again, the

earlier measruements [6, sec. 3.8] [7, sec. 3.9] agree, in that the SN

results for surface 3 were more linear than for surface 2. Of the

curves developed on surface 3 by the IRS, the present measurement is

convex while the earlier two were concave.

92

•ITrfarT—

'

-
SKID

RESISTANCE,

SN

0

70

CN

LUu

<

U_

Ctl 3

CO

60

km/h

30 60 90 120

-T | 1

1

1

co

LU

Q£Z>CO

0 20 40 60 80

mph

SPEED

Figure 33*. Skid resistance as a function of speed, calculated for

surfaces 2 and 3 at EFTC.

93
The 0 results are shown in figure 34 for surface 2 and in figure

35 for surface 3. If the results are taken as system-independent and

averaged, the dashed lines in the figures represent the average 0 for

the respective surfaces. The expressions for the averages are:

(surface 2) 0^

= - 0.557 + 0.0111V

and

(surface 3) 0

£ = - 0. 332 - 0. 0010V.

Experimental values of the speed gradient, © , measured using

variations about the nominal test speeds, are shown in table 17. Each

value of 0 is the slope of a least squares line through the group of

data points, for the particular system-surface combination, representing

skid numbers obtained at radar-measured differential speeds about the

nominal speed. The 0 values are paired with the 0 values for com- parison. They can be seen to be generally consistent in sign, and also

consistent in magnitude in about half of the comparisons. The averaged

magnitudes are more consistent.

3.12. Supplementary Comparisons between Systems

The test plan consists of direct comparisons between ARS 1 and IRS,

and between ARS 2 and ARS 3.

Steps were taken in planning to ensure that the pairs of skid

resistance measuring systems would indeed measure the same property.

Common elements of the measurements made by each pair of systems during

a time period included surface, speed range, and tires, with the time

span kept as short as possible consistent with surface drying. Drying

time and the number of tests required per system per tire precluded a direct four-way comparison.

The test plan provides for measurement of the range of surface

variability over the term of the tests and for the measurement of tire

variability. If these effects are known, the test results can be ex- tended to interrelate all four systems, using surface and speed as the common elements.

The loss of several test days (sec. 3.4) prevented completion of

the final two planned time periods; consequently, surface variability

over the testing time was not measured. An estimate is available from

previous experiments [7, sec. 3.9] but it was felt worthwhile to use the

last remaining test day to make direct comparisons between all pairings

of the four systems, even though the number of data points obtainable

for each match would be small.

94
SPFED

GRADIENT,

krrj/h

0 30 60 90 1 20

SPEED

Figure 34. Skid resistance speed gradients calculated from measurements

on surface 2 at EFTC

.

95
SPEED

GRADIENT

+ 0.4

30

k m/h

60

T

90

T

120

T

+0 2 -

0 -

4>

Q_

E

- 0.2 -

- 0-4 -

- 0-6 -

- 0.8 -

-10 X

20 40

mph

SPEED

60 80

Figure 35. Skid resistance speed gradients calculated from measurements

on surface 3 at EFTC

.

96
Table

17.

Measured

and

calculated

skid

resistance

speed

gradients

<r 1—1

rH

m Ok

CJ CO 1—1 i—1

CJ

m cn cn

CD . . • CD • • QJ o o o 0) o o o GO 1 1 + 00 i i i cti Cti Cj Sj

0) 00 1—1 CM 0) rH kO > £ CM o r—1 > E m co rH

<C CD * • • < © • • • o o o o o o 1 + + i 1 1

03 vO CO rH CO

m o CM l“H O o CM co •<r CD • • • © • • • o1 o1 o1 Oi o o © 03 l 1 i Pi Pi M 00 <r 03 M 00 m £ LO CO m E 00 rH CM

CD • • • © • • • o o o O o o 1 + + 1 1 1

<r CO 00 C\| rH

CJ <3" 1—1

rH o CO cn

CD • • • © • • • o o o o o o • • CO 1 1 + • * co 1 i 1 CM c^

CO CO

<D pi o CN 0) Pi kO kO o

CJ < £ CN CM o CJ < £ cn cn

cti CD • • . 03© • • • M-J o o o <H o o o U 1 1 + u 1 i i 3 D

V) 03 u U o O

P-i

I—1

03 CO o o o

CJ CO c rH CJ <r

CD . • • © • • • o o o o o o

CM 1 1 + CM 1 1 i CO 03

Pi co <r 'Cl- pi o> cn 00 < E o o in < £ m CM

CD • . • © • • • o o o o o o 1 + i i i + o oo <]- kO 00 Ok

CJ CO o i—i

CJ cn CM

CD . • . © • • • o o o o o O t—1 1 1 + iH i i 1 CO CO

Pi o UP 03 Pi CM »—i £ CO o CO < £ i—1 m CM

CD • . . © • • • o o o O o o 1 1 + 1 i 1

03 cti -cti 0) cu <u ex o o o cu ex o o o

ex E CN <0 vO ex E CM kO

CO CO

97 1

mph=

1.609

km/h.
The supplementary test plan sequence shown previously in table 6 was followed. Each system made two measurements at each speed on each

surface measured. Tires from tire group 2, which showed the least wear,

were used.

The resulting SN values did not compare well with the average SN

values from the first six time periods. When the two measurements were corrected for speed deviation and averaged, sixteen of the twenty-four

results agreed with the previous averages within limits of two standard

deviations, whereas eight were quite far apart. The new results were predominantly lower (table 18).

This large percentage of excessive discrepancies suggests that the

SN measurements are still not under statistical control, perhaps due to

factors that have not been taken into account.

One procedural change in particular may have contributed to the

discrepancy. The tires used in time periods 1 through 6 were each used

on only one surface, and so each skid occurred on either virgin tread or tread with the wear pattern characteristic of that surface [6, sec. 3.6].

In the supplementary comparisons, tires which had previously been used

only on surface 3 were now used alternately on surfaces 2 and 3. The

variations in tread wear texture may have affected the measured SN. It

may be desirable in the future to include skids on virgin tread as a control to determine this effect.

Confidence

discrepancies

.

in the supplementary comparisons is clouded by the

Least squares fits to the comparison data points are:

SN

ARS 1

SN

ARS 2

SN

ARS 3

SN

ARS 2

SN

ARS 3

SN

ARS 3

= 14.9 + 0.703 SN TT3C 1Kb

=-15.8 + 1.102 SN tdc IRb

= 125.6 - 1.101 SN toc IR b

- 327.3 - 4.682 SN

ARS 1

- 6.3 + 0.985 SNars 1

" 170.0-2.140 SNars 2

These lines and the data points are plotted in figure 36.

98
Table 18. Difference between two sets of average SN measurements

System Surface Speed SN discrepancy

mph^

ARS 2 3 40 -9.5 \ ARS 2 3 20 -8.1 1 ARS 1 2 20 . / Exceed

- / . 4 / IRS 2 20 -j f 2 std. dev.

IRS 2 40 -4.9 ) fT

ARS 3 3 40 -4.7 \

iF 1 b ARS 2 2 60 - , \ values -3.4 1 ARS 1 2 40 i CN 00

ARS 1 3 40 -1.9

IRS 3 40 -1.7

ARS 2 2 40

i

—

i

i

IRS 3 60 -1.5

ARS 1 2 60 -1.4

ARS 1 3 20 -1.2

ARS 3 2 20 o

i

ARS 3 3 20 1o ARS 1 3 60 -0.1

ARS 3 2 40 +0.5

ARS 3 3 60 +0.6

IRS 2 60 +0.6

ARS 3 2 60 +1.1

ARS 2 3 60 +1.9

ARS 2 2 20 +2.0

IRS 3 20 +2.6

For each system-surface-speed combination, the discrepancy is the

difference between the average SN measured on the final test day

and the average SN measured during time periods 1 through 6.

b

1 mph = 1.609 km/h.
o

CN CO

Z

to

2 u u

< < <

U_ LL. cm cm 3 3

to CO

I

I •

m

fl£ < z

to

100

Figure

3

6.

Direct

comparisons

between

systems

on

the

last

day

of

testing.
3.13. The Correlation Equations

A summary of the results of the dynamic skid resistance measurement

test program conducted on the two chosen surfaces at the EFTC is given

in table 19.

As pointed out in section 3.12, the results include direct com- parisons between ARS 1 and the IRS, and between ARS 2 and ARS 3, because

these system pairs measured the same surfaces contemporaneously. Comparisons drawn between other combinations of the systems are inferred on

the premise that the effects of a few days interval between like measurements can be neglected.

The comparisons are expressed as linear correlation equations.

Coefficients for the equations have been calculated by least squares,

using the mean SN values, and are listed in table 20.

3.14. Use of the Correlation Equations

Each of the correlation equations results from six measurement

pairs. Therefore, to predict a value with 95 percent confidence limits,

the standard deviation of predicted values is to be multiplied by 2.78.

To predict a value with 90 percent confidence limits, the standard

deviation of predicted values is to be multiplied by 2.13.

At present, the national FHWA standard for skid number measurement

is the IRS. Field Test Center personnel would presumably like to predict, for ongoing ARS SN measurements, the values that would be obtained

by the IRS, using the appropriate correlation equation.

A plot of a representative equation (for ARS 1 and IRS) is shown in

figure 37. The lengths of the crosses correspond to the 99 percent con- fidence intervals, L, of the measured mean skid numbers from which

the correlation line was derived, while the curves symmetric to the correlation line represent the 90 percent confidence limits for the predicted

values

.

Of course, the random error component of the new ARS SN measurements must also be taken into consideration. A graphical procedure is

perhaps the easiest way to estimate the total range of uncertainty.

With reference to figure 37, the projection of the ARS measurement

uncertainty from the 90 percent confidence band to the IRS axis re- presents the range of uncertainty of the predicted value.

The uncertainty for the correlation line itself is, in a sense,

systematic in nature from the ARS point of view, i.e., if an ARS is

comparing the SN of two different surfaces, this systematic uncertainty

cancels out. However, if an ARS compares its results with another ARS

via the correlations with IRS, then the uncertainty of the two correlation lines will have to be taken into consideration.

101
Table

19.

Correlation

summary

<Du

c

<D 0\ *H On COo

0) >0) XJ

cla • 03 XJ

CO i-» «

c

0) •H £ T3 O

CT3 Uo

c Z

0) CO s

O CO AJ

• CO O Q> Z 4J

TJ

0) O

B *H

*H Vj H (1) CL

CL U 3

•H O H U*GO

T3

(U

OJCL

CO

B

(!) U

CO>N

CO

in v£i vj oo n n <4 on rH un o o vO o oo cm on <4 <j h \£) o vO o

rH r—| «—( rH rH rH H H (N rH CM »-H rH rH »—< r-H f-H rH »—< i-H rH rH rH

41 +| +, +| +| +| +| +1 41 41 +1 41 41 41 4J +1 +| 41 41 41 41 41 41

O' O r» n n

v£> O

0O ON 00

CM v£> rH CO o CM 00 r' 00

sT VO n

m o <r

in vo oo

CO

00

vr> CM CM (N CO

CM 00 vO o co tr> O VO vo

CM CM CM CM rH CM CM CM CO (M H CM CM rH CM rH CM CM CM rH CM CO CM CM

o m ^ non CM rH i** h m CM

cnO o m ^ n oo

lO <4 CM O 0> CM

CM CM O

lO lO o <4 on on

00 <f ON m n in00 <4 lO

00 H CM O O O

i i + o o o

I I I O O p I I + oop i i 4 o o o

I I I ??? o o o

I I I

vD rH <4 n io o

o <r

CM 00 ON

VO rH CO m cm vo

oo o m o co

on on <4

vo oo m

rH m on

oo o\ o

cm cm on 00 N Sf ^ vO

tH 00

rH P^* 00 vo m mvO O" O vo m m

cn in m

n n m

m n oo

so m <r on o- oo vo m in n o onvo vo m

rs on h

vO vO vO

on -d- m

vo vo m

H 00 H rH O »H rH »H O'* H H CO rH r—I H H H H rH rH rH rH rH rH

CM rH CM CM CM CM N CS| H CM CM H CM CM CM CM CM CM CM CM CM CM CM CM

on rH m <tcmvo >j cm vo n h in cm vo n h in on h m >4 cm vo

i—i h h cm cm cm n on n cm cm cm on on on ^ ^3- rH ^ <4 <r

-3000 CL CM <4 O

a 000 cm <r vP OOO cm <r vo 000 CM sj v£) OOO CM <4 vP OOO CM >J v£) OOO CM <4 vO OOO CM <4 vp

cm cm cm on on m cm cm cm on o on cm cm cm n on on cm cm cm on on m

CM

in3

Uio

s

102

1

mph

*

1.609

km/h.
U

•H <U

4-> •H rH o <r ON CN r—< o CO CJ uO uo UO oo CN UO UO UO

r—1•H o> on

O ON ON ON CN CN

0) 4-4 • • • • • .

5-i 4-4 o o o o o o o o o 5-i <y O o

CJ

u >

<u "O -3 i—

H

CN c r>. o v£> CO CO CN

OC • • • * • • « • • • •H r—1 i—I 1—1 o rH o rH rH rH

T3 -3

4-J Z

CO co

<u 3 3 3 00 uO C^

CO rH 0) * • • • • • • • • e CO E o o c o o o o o o 5-i > Z

<u CO

4_» "3

CU

S-i CO 4-J o <u CJ o uo vD o UO CO CO CN

5-1 3 •H r—1 • • • • • • • • •

S-l I-H T3 z rH 1—1

rH o 1—1 o i—1

rH rH

0i TO cu CO > 5-i + Cu z X CO

PP

C 1—1 • + co 03 >

<U 3 cu CO rH CN o CN o o < E 03) 33 ON CS-N CN VD ^3

•H

II <4-1 CO • rH rH i—i o rH o rH 1—1 rH o <y 03) X pp 4-) CO CO

S-i • • *H t

ion 6

pa

00 CO \D U0 1—1 rH CO

CO CO CM CO CN rH cr\ CTi 3 O o CO CN CM o CN 00

cr 4-j PC o o c o C7\ 00 00

cu • T3 1—1 1—1 1—1 rH

O o o o o 3 <U O w

•i-l 4-1

4-J -H

CO >4-1 CO r—1 ' 3 cu o 5-i •H 5-i uo CN c UO o 00 00 \£>

4-J

O CN CN 00 ON o CO CO

O

CO CJ < 3 u3 CN UO CN o O

cr 1 i—l 1 1 rH rH 1—1 <U i CO

•H

4J

ctf T—1

/—N rH rH CN rH CM co

CD

X Z CO CO CO

M CO DC

X Pi CO CO CO CO CO cn

Su 'w' I—i M

M PP pp pp pp PP pp

O < <d < < C <c oo

CN

0) rH CM CO CN CO CO

r—1cn CO CO x> >1 Z CO CO CO cn CO CO pp pp PP

03 cn

X

X pp pp pp pp

MMM

H

w c C c < < C

10 3
‘g

e

37.

Use

of

the

correlation

equations

to

predict

IRS

values.
Indeed, it is a consequence of the choice of an artifact standard

such as a reference measurement system, rather than a purely procedural

standard, that the uncertainty of the relation increases with each com- parison step separating a user- level system from the ultimate reference.

In the skid resistance case, two comparisons apply, since the user-level

skid measurement systems have access to the national reference system

only through an area reference system. The uncertainty increase in a two-step correlation is illustrated by figure 38. In order to obtain an acceptable level for the overall uncertainty, great care in measurement

is needed at each step to achieve narrow confidence bands on the relations

between systems.

4. The IRS Ground Station

The features of the ground station have been described in detail

previously [7, sec. 4]. Briefly, the ground station provides a magnetic

tape record of the data obtained by the skid test system, automatically

calculates SN from the data, plots the data and SN versus location on the test surface, and provides a printed statistical analysis of the SN

results. The calculation, plotting, and analysis can be done on site

while skid tests proceed or can be done at a later time, using the

magnetic tapes. The measurement process is improved in two ways by use

of the ground station — first, the portion of the surface measured is

identified, and second, automatic data processing eliminates the subjective judgements of the data analyst as a source of error.

The ground station was developed partly in the expectation that, once its usefulness had been demonstrated, the calculator could be used

to process inputs from all skid test vehicles engaged in a correlation.

At present, only the IRS has been provided with a digital on-board

memory to transfer data to the ground station. The original analog

strip-chart recorder remains on the IRS as the primary means of re- cording data.

During this program of skid resistance measurements, the ground

station was operated daily to evaluate its reliability. Examination of

ground station plots, as mentioned in section 3.5, first detected the

unexpected drift of the IRS vertical load transducer signal.

The ground station functioned well throughout the testing period.

The calculator and associated electronics performed reliably. Mechanical

difficulties with the start and finish tapeswitch assemblies across the

test surfaces caused some loss of ground station data. Data-taking

resumed when the assemblies were temporarily bypassed with manually

operated switches.

105
.

USER-ARS

CORRELATION >

LU

QC U_

LUO Uz3 z

CO

oc

Q_

106

Figure

38.

Uncertainty

increase

ina

two-step

correlation.
A number of the magnetic data tapes recorded at EFTC have been

played back at NBS. The SN results are compatible with the data ana- lyst's interpretation of the corresponding analog strip chart records.

Ground station plots made during skid tests at EFTC have been duplicated

from the tapes. Because traction, vertical force, speed, etc. are recorded by the ground station as separate data channels, it is possible

to vary the calculator program to obtain, for instance, SN in terms of

either the quotient of instantaneous traction and vertical load values

or the quotient of instantaneous traction and the vertical load values

obtained using the H/L unloading constant.

In overall performance, the ground station has been found to be

versatile and reliable.

5 . RECOMMENDATIONS

To improve the reliability of subsystem evaluation, skid measurement correlation results, and the convenience of conducting the tests,

the following recommendations are made:

a) A measured distance for adjusting the fifth wheel subsystem

should be established at a place more convenient to the skid

test surfaces, where it would be possible to make test runs

in both directions, and where extended driving distance would

not be required between runs.

b) Drivers assigned to reference skid measuring systems should

check the speed record frequently to develop driving techniques

for maintaining the specified speed during skids.

c) Means should be provided to increase the sensitivity of ARS

recording ranges for speed, flow, vertical force, and traction

by a factor of ten or more. A series of reference signals

should be provided for each channel to allow linearity checks

and to facilitate zero suppression.

d) ARS flow readouts should be adjusted regularly, using the flow

sensor calibration results provided by NBS. An additional

connector should be installed in each ARS to make this possible.

e) The damping factor of the ARS trailer suspension should be in

creased. This might be accomplished by the installation of

additional shock absorbers or the use of double-acting shock

absorbers

.

f) Reference surfaces should be clean when skid tests are con ducted. It is recommended that the surfaces be hosed down with

water and allowed to dry prior to each day of testing.
g) The temperature drift of the IRS wheel transducer vertical load

channel should be eliminated. Until this is done, an operator

should adjust the vertical load channel immediately before each

skid test.

During the program conducted by NBS involving correlations at the

Field Test Centers, it has become apparent that problems exist in the

skid resistance measurement process that require study beyond the scope

of the correlation tests. The following recommendations deal with some

of those problems:

a) The effects on SN due to changes in pavement wetting variables

should be determined. One variable is water layer depth. The

nozzles that have been used in these correlations do not produce a uniform water layer. This is largely because the air

stream and the speed difference between the water and the pave

ment violently affect the water as it falls from the nozzle onto

the pavement. The wetted width on the pavement and the water

distribution across that width are not well formed. As a result, there is presently no satisfactory method for measuring

the effective depth of the water layer encountered by the test

tire or for determining that the water quantity specification

of E 274-70 is met. Another variable is soak time. Pavement

wetting serves as a lubricant for the skidding tire and, to some degree, simulates wet weather conditions. Water applied

by the nozzle hits the pavement only milliseconds before it is

encountered by the tire. In this time span, the pavement may

become superficially wet or the water may remain beaded on the

dry surface. Actual wet weather persists for an extended time,

and the pavement becomes soaked. When the effects of changes in

water layer depth and soak time are known, it may be possible to

choose optimum values objectively.

b) The effect on SN due to changes in test tire inflation pressure

should be determined. Unless the effect of changes such as those that may occur during skid testing is found to be negligible,

test tire inflation pressure should be controlled at the spec

ified value. It should be determined whether the inflation

pressure specified for the standard tire in E 274-70 results in

the best skid resistance consistent with other requirements

for an automotive tire such as ride quality and tread life.

c) It should be determined whether the calibration procedures that

have been developed for force-sensing skid measurement systems

are adaptable to torque-sensing systems and, if they are not,

new procedures should be developed for torque.

108
d) The effect of tread wear texture on SN should be determined.

Each skid wears a patch of tread with a texture characteristic

of the pavement. At present, a test tire may lock and skid at

random on worn or virgin tread. If the effect of wear texture on SN is not shown to be negligible, lock position should be

controlled such that each skid measurement is made with virgin

tread

.

e) It should be determined whether correlations obtained on the

reference surfaces are valid on common highway pavements. The

present reference surfaces are constructed with epoxy as a binder and, in some cases, as an overcoat. This is not typical

of highway construction. Skids on the reference surfaces pro

duce unusual wear textures. This suggests that skid resistance

is generated differently on these surfaces than on common pave

ments

.

f) Means should be provided to improve trailer stability. For

example, traction force on the locked test wheel of a twowheeled skid trailer acts at a distance from the trailer center

line, causing the trailer to rotate in roll, pitch, and yaw.

This results in a deflection of the test wheel that is contrary

to a condition of E 274-70 that "its major plane is parallel

to its direction of motion and perpendicular to the pavement"

while the skid resistance measurement is made. Balanced traction

developed by an anti-skid brake on the opposite wheel might be

used as a means of preventing trailer roll and yaw, and would

also have the desirable effect of eliminating lateral force on the test wheel.

The reference skid measuring system evaluations and correlation

results reported here are valid only if no modifications are made and if no changes are made in mechanical and electrical components that affect

the results, and if the systems are operated in the same manner as they were at the time of these tests.

It is a pleasure to acknowledge the cooperative participation of

personnel of the Federal Highway Administration and the FHWA Field Test

Centers in these tests. Special thanks are also due to the following

NBS staff members; Dr. Hsien H. Ku, for his contributions and guidance

in the statistical treatment of the experiment; Ray Russell, driver of

the IRS, for his excellent control of the system despite the inconvenience of a foot injury, and for doubling in operation of the on-board

instrumentation; Bill Appleton, IRS instrumentation operator on previous

trips, for his contributions to the construction of the system electronic

equipment and for preparing it for use in the field; John Hazzard, for

aid in mechanical design, drafting, and illustration; John Heine, for

additional drafting aid; and Ruth Davenport, for patient and careful

reading of the strip chart skid records. Their support has been invaluable to the completion of this complex task.

109
6. REFERENCES

[1] Standard method of test for skid resistance of paved surfaces

using a full-scale tire, ASTM Designation E 274-70, Annual Book

of ASTM Standards, Part 11 (American Society for Testing and

Materials, Philadelphia, Pa., 1973).

[2] Watson, John R. , and Cook, Larry M. , A national program to standardize skid resistance measurements. Public Roads, A Journal of

Highway Research, 37 , No. 3, pp. 7780 (Dec. 1972).

[3] Smith, L. L. , and Fuller, S. L. , Florida skid correlation study

of 1967 - skid testing with trailers. Highway Skid Resistance

STP 456 (American Society for Testing and Materials, Philadelphia,

Pa., 1969).

[4] Kearns, R. W. , and Ward, J. F. , A method and means of calibrating

an air-bearing force plate for use with a towed pavement friction

test trailer, Nat. Bur. Stand. (U.S.), NBSIR 75-738 (Dec. 1974).

[5] Standard specification for standard tire for pavement skid resistance tests, ASTM Designation E 501-73, Annual Book of ASTM

Standards, Part 15 (American Society for Testing and Materials,

Philadelphia, Pa., 1976).

[6] Kearns, R. W. , and Ward, J. F. , Skid resistance measurement tests

at the FHWA Eastern Field Test Center, Nat. Bur. Stand. (U.S.),

NBSIR 75-972 (July 1976).

[7] Kearns, R. W. , and Ward, J. F. , Skid resistance measurement tests

of FHWA reference systems at the Western Field Test Center, Nat.

Bur. Stand. (U.S.), NBSIR 76-1133 (Oct. 1976).

[8] Chestnut, H. , and Mayer, R. W. , Solution of linear differential

equations, Chapter 3, Servomechanisms and Regulation System

Design, _1 p. 62 (John Wiley and Sons, Inc., New York, N. Y. , 1951).

110
Appendix A. Flow Turbine Calibration Reports

rooM NBS-443

(REV. 11-69) KRBrplb U.S. DEPARTMENT OF COMMERCE

national bureau of standards

WASHINGTON, D.C. 20234 213 . 06/2130440

July 2, 1976

REPORT OF CALIBRATION

OF A TURBINE TYPE FLOW TRANSDUCER

Maker: ITT Barton

Monterey Park, California

Serial: 7101-2567

submitted by

Engineering Mechanics

Mechanics Division

National Bureau of Standards

Washington, D.C. 20234*

A calibration has been performed on the meter described above by

counting the total pulses generated during discharge of a weighed

quantity of water converted to gallons.

As tested, this assembly consisted of upstream tubing 1 inch inside **• diameter and 28 inches in length preceded by an elbow, the meter,

and exit tubing 20 inches in length. The assembly mounted horizontally

with the meter pickoff coil in a vertical position was connected by

a straight AN nipple to AN - 16 hose. Test temperature and downstream

pressure level were 22 ± 4.5°C and 38 psig, respectively, and the

pulse frequency was controlled to within one-half percent of the

values listed.

The results given in the attached table are the arithmetic means of

ten separate observations taken in groups of five successive runs on each of two different days. The reported values of pulses per gallon

have an estimated overall uncertainty of ± 0.19 percent based on a standard error of 0.03 percent and an allowance of ± 0.1 percent for

possible systematic error.

For the Director,

L. K. Irwin

Acting Chief

Fluid Meters Section

Mechanics Division, IBS

111
Appendix A. Flow Turbine Calibration Reports - Continued

Results of Water Calibration

ITT Barton Turbine Meter, S/N 7101-2567

Pulses/Sec Pulses/Gal

235 1371.6

460 1367.8

681 1370.9

920 1371.5

1160

213.06/2130440

July 2, 1976

1372.4

112
Appendix A. Flow Turbine Calibration Reports - Continued

FOAM NBS-443

IMEV. 11-43)

KRBrplb U.S. DEPARTMENT OF COMMERCE

national bureau of standards

WASHINGTON, D.C. 20234 213.06/2130440

July 2, 1976

REPORT OF CALIBRATION

OF A TURBINE TYPE FLOW TRANSDUCER

Maker: ITT Barton

Monterey Park, California

Serial: 7101-2568

submitted by

Engineering Mechanics

Mechanics Division

National Bureau of Standards

Washington, D.C. 20234

A calibration has been performed on the meter described above by

counting the total pulses generated during discharge of a weighed

quantity of water converted to gallons.

As tested, this assembly consisted of upstream tubing 1 inch inside

diameter and 28 inches in length preceded by an elbow, the meter,

and exit tubing 20 inch'ds in length. The assembly mounted horizontally

with the meter pickoff coil in a vertical position was connected by

a straight AN nipple to AN - 16 hose. Test temperature and downstream

pressure level were 24.3 ± 0.8°C and 38 psig, respectively, and the

pulse frequency was controlled to within one-half percent of the

values listed.

The results given in the attached table are the arithmetic means of

ten separate observations taken in groups of five successive runs on each of two different days. The reported values of pulses per gallon

have an estimated overall uncertainty of ± 0.19 percent based on a standard error of 0.03 percent and an allowance of ± 0.1 percent for

possible systematic error.

For the Director,

L. K. Irwin

Acting Chief

Fluid Meters Section

Mechanics Division, IBS

113
Appendix A. Flow Turbine Calibration Reports - Continued

Results of Water Calibration

ITT Barton 1-in Turbine Meter, S/N 7101-2568

Pulses/Sec Pulses/Gal

236 1365.4

462 1363.9

684 1366.4

920 1368.2

1151

213.06/2130440

July 2, 1976

. 1368.6

114
Appendix A. Flow Turbine Calibration Reports - Continued

FORM NBS-443

VflEV. 11-43)

KRBrplb U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D.C. 20234 213.06/2130440

July 2, 1976

REPORT OF CALIBRATION

OF A TURBINE TYPE FLOW TRANSDUCER

Maker: ITT Barton

Monterey Park, California

Serial: 7101-2569

submitted by

Engineering Mechanics

Mechanics Division

National Bureau of Standards

Washington, D.C. 20234

A calibration has been performed on the meter described above by

counting the total pulses generated during discharge of a weighed

quantity of water converted to gallons.

As tested, this assembly consisted of upstream tubing 1 inch inside

diameter and 28 inches in length preceded by an elbow, the meter,

and exit tubing 20 inches in length. The assembly mounted horizontally

with the meter pickoff coil in a vertical position was connected by

a straight AN nipple to AN - 16 hose. Test temperature and downstream

pressure level were 22.5 ± 3°C and 38 psig, respectively, and the

pulse frequency was controlled to within one-half percent of the

values listed.

The results given in the attached table are the arithmetic means of

ten separate observations taken in groups of five successive runs on each of two different days. The reported values of pulses per gallon

have an estimated overall uncertainty of ± 0.19 percent based on a standard error of 0.03 percent and an allowance of ± 0.1 percent for

possible systematic error.

For the Director

L. K. Irwin

Acting Chief

Fluid Meters Section

Mechanics Division, IBS

115
Appendix A. Flow Turbine Calibration Reports - Continued

Results of Water Calibration

ITT Barton Turbine Meter, S/N 7101-2569

Pulses/ Sec Pulses/Gal

236 1348.3

461 1347.8

682 1352.5

922 1352.5

1160 1353.0

213.06/2130440

July 2, 1976

116
Appendix A. Flow Turbine Calibration Renorts - Continued

FORM NBS-443

(REV. 12-43)

KRB:plb U.S. DEPARTMENT OF COMMERCE

national bureau of standards

WASHINGTON, D.C. 20234 213.06/2130440

July 2, 1976

REPORT OF CALIBRATION

OF A TURBINE TYPE FLOW TRANSDUCER

Maker: ITT Barton

Monterey Park, California

Serial: 7101-2570

submitted by

Engineering Mechanics

Mechanics Division

National Bureau of Standards

Washington, D.C. 20234

A calibration has been performed on the meter described above by

counting the total pulses generated during discharge of a weighed

quantity of water converted to gallons.

As tested, this assembly consisted of upstream tubing 1 inch inside

diameter and 28 inches in length preceded by an elbow, the meter,

and exit tubing 20 inches in length. The assembly mounted horizontally

with the meter pickoff coil in a vertical position was connected by

a straight AN nipple to AN - 16 hose. Test temper.ature and downstream

pressure level were 24.3 ± 0.8°C and 38 psig, respectively, and the

pulse frequency was controlled to within one-half percent of the

values listed.

The results given in the attached table are the arithmetic means of

ten separate observations taken in groups of five successive runs on each of two different days. The reported values of pulses per gallon

have an estimated overall uncertainty of ± 0.19 percent based on a standard error of 0.03 percent and an allowance of ± 0.1 percent for

possible systematic error.

For the Director

L. K. Irwin

Acting Chief

Fluid Meters Section

Mechanics Division, IBS

117
Appendix . Flow Turbine Calibration Reports - Continued

Results of Water Calibration

ITT Barton Turbine Meter, S/N 7101-2570

Pulses/Sec Pulses/Gal

235 1368.4

463 1367.8

685 1369.8

921 1372.3

1151 . 1372.7

213.06/2130440

July 2, 1976

118
Appendix A. Flow Turbine Calibration Reports - Continued

FORM NBS-443

(REV. IHS)

HHA: plb U.S. DEPARTMENT OF COMMERCE

national bureau of standards

WASHINGTON, D.C. 20234 213 . 06/2130440

July 2, 1976

REPORT OF CALIBRATION

OF TWO TURBINE TYPE FLOW TRANSDUCERS

Maker : ITT Barton

Monterey Park, California

Serial: 7101-2571

7101-2566

submitted by

Engineering Mechanics

Mechanics Division

National Bureau of Standards

Washington, D.C. 20234

A calibration has been performed on each of the meters described

above by counting the total pulses generated during discharge of

a weighed quantity of water converted to gallons.

For each meter tested, the assembly consisted of upstream tubing

1 inch inside diameter and 28 inches in length preceded by an elbow,

the meter, and exit tubing 20 inches in length. Each assembly,

mounted horizontally with the meter pickoff coil in a vertical

position, was connected to AN - 16 hose 10 inches long. Test

temperature and downstream pressure level were 24.2 ± 0.6°C and

35 psig, respectively, and the pulse frequency was controlled

to within one percent of the values listed.

The results given in the attached table are the arithmetic means of

four separate observations taken in groups of two successive runs

on each of two different days. The reported values of pulses per

gallon have an estimated overall uncertainty of ± 0.25 percent based

on a standard error of 0.05 percent and an allowance of ± 0.1 percent

for possible systematic error.

For the Director

L. K. Irwin

Acting Chief

Fluid Meters Section

Mechanics Division, IBS

119
Appendix A. Flow Turbine Calibration Reports - Continued

Results of Calibration

ITT Barton, Serial 7101-2571

Pulses/Sec Pulses/Gal

233 1324.3

458 1319.4

682 1318.8

913 1318.7

1208 1319.6

ITT Barton, Serial 7101-2566

Pulses/Sec Pulses/Gal

231 1371.5

464 1370.2

676 1369.6

911 1369.6

1205 1371.5

213.06/2130440

July 2, 1976

120
Appendix B. The Test Matrix

A subdivision of the test plan consists of the following test matrix

Time Period

Surface

Letters A thru G designate 7 tires while numbers 1 thru 7 identify

the 7 lanes comprising the surface. The nominal test speed is S, while

AS is an incremental change in the speed.

The mean of the 21 skid number measurements represents the skid

number of the surface measured during the time period using the average

tire at the nominal test speed. The novelty of the test matrix is that

it yields the skid gradient of the surface, as well as a measure of the

surface and tire nonuniformities.

Suppose we want to compare the effect of making a measurement with

tire A with all the other tires. To do this effectively, we want to

eliminate any measurement deviations due to skidding in different lanes

and at different speeds.

Let a measurement result be represented by the notation

(SN + e) LT(S-AS)

where:

SN = a nominal skid number measured value,

+e = an increase in the SN value due to testing at

lower than the nominal test speed by the

increment AS,

L = lane identification of the test measurement, and

T = the tire identification of the test measurement.

121
Appendix B, The Test Matrix - Continued

From the test matrix, tire A was used in lanes 1, 2, and 4 at three

different speeds. Lane 1 was used with tires A, G, and E at three different speeds, while lane 2 was used with tires B, A, and F at the three

different speeds, while lane 4 was used with tires D, C, and A at three

different speeds.

Let us write an equation for the measurement results made in each

lane

2(SN + e) 1A(s _ AS) - (SN) ig(S) " (SN ~ e) lE(S + AS)= AAGE + 3e (1)

2(SN) 2A(S)

” (SN+ £) 2B(S - AS) " (SN ” e) 2F(S+ AS) ~ A

ABF (2)

2(SN - e) 4A(S + AS)- (SN) 4C(S)- (SN + £) 4D(S - AS)

= A

ACD

’ 3e < 3 >

where A^g = twice the difference between using tire A and the average

of using tires G and E in lane 1,

A

ABF

= twice the difference between using tire A and the average

of using tires B and F in lane 2,

AACD

= tw ^ ce t ^ie difference between using tire A and the average

of using tires C and D in lane 4.

Note that equations 1, 2, and 3 are each independent of lane effects.

Note also that the sum of the three equations is independent of any speed

effects (assuming AS is small). Consequently, the sum is free of both

lane and speed effects. To simplify the notation, eliminate the lane

and speed variations from the sum, and we get

6 ( SN

) A

- ( sn > b

" < SN ) C - ( sn > d

“ ( sn > e “ ( sn > f- < sn > g

= (4)

We may add to this equation [7]

,

the equation

(SN)

A

- (SN) a

= 0 (5)

which simply says that the skid number without lane and speed dispersion

is equal to the skid number measurement using tire A without lane and

speed dispersion with no measurement error. Then

7 (SN) AJA- {SN. + SNb + SN + SN + SN„ + SN^ + SN_

D L L) lj r O

122
Appendix B. The Test Matrix - Continued

Now, the measurement performance with tire A can be compared with the

performance of all the other tires by writing

mean of all

measurements ( 7 )

In a similar manner, the test matrix can be rearranged to yield, for ex- ample, the measurement performance on lane 1 compared with the performance

on all the other lanes. The results are then a measure of surface

uniformity.

123
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PURER A TTON OR REPORT NO.

NBSIR 77-1213

2. Giov’t Accession

No,

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Skid Resistance Measurement Tests of New FHWA

Reference Systems at the Eastern Field Test Center

5. Publication Date February 1977

6. Performing Organization Code

7. AUTHOR(S) , T „ T7 , R. W. Kearns and J. F. Ward

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME. AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2130440

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, 6lP)

Federal Highway Administration

Office of Development

Washington, D. C. 20590

13. Type of Report & Period Covered

Final

Feb-April 1976

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRAC T (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The Federal Highway Administration (FHWA) is developing a program to improve the

method of measuring wet weather pavement skid resistance (SN) and to reduce the variation in results. At the national level, an interim reference system (IRS) is

maintained and operated by the National Bureau of Standards. At the regional level,

an area reference system (ARS) is maintained and operated at each FHWA Field Test

Center. Intercomparisons between these reference systems and the highway measuring

systems at the state level provide measurement assurance.

In this report, the first correlations between three identical, newly manufactured

systems (ARS 1, 2, 3) and the IRS are given. Computed standard deviations of midrange predicted SN values are typically less than 0.1 SN. SN is given as a function

of test speed for each system, on two test surfaces. Speed gradients of SN are found

to be characteristic of the surface.

The test program is explained, from preparations and calibrations of subsystems

through dynamic measurement on the surfaces. Controlled and uncontrolled variables

are identified, discussed, and in some cases, investigated experimentally. A ranking

of the sources of dispersion is given.

A ground station for improved SN calculation precision and on-site statistical

analysis, operated in parallel with traditional SN data acquisition methods, is found

to perform reliably.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Accident reduction, skidding; correlation, skid resistance; highway safety; measurement, skid resistance; pavement, skid resistance; pavement wetting system; skid

resistance, measurement; tire-pavement interface forces; wet pavement skid resistance.

18. AVAILABILITY Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

] For Official Distribution. Do Not Release to NTIS

UNCLASSIFIED 131

Order From Sup. of Doc., U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. Cl 3

20. SECURITY CLASS

(THIS PAGE)

22. Price

fTtj Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151 UNCLASSIFIED $6.00

USCOMM.DC 29042- P 74