NBSIR 76-1133

Skid Resistance Measurement

Tests of FHWA Reference

Systems at the Western 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

March - April 1 975

Final

Issued October 1 976

Prepared for

Federal Highway Administration

Office of Development

Washington, D. C.

NBSIR 76-1 133

SKID RESISTANCE MEASUREMENT

TESTS OF FHWA REFERENCE

SYSTEMS AT THE WESTERN 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

March - April 1 975

Final

Issued October 1976

Prepared for

Federal Highway Administration

Office of Development

Washington, D. C. 20590

0*

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Acting Director
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 Auxiliary Test Equipment 3

2.1.2 Test Tires

^ 2.2 Skid Trailer Weight Distribution 3

2.3 Tow Vehicle Displacements

3

2.4 Speed Measuring Subsystem

y 2.5 Pavement Wetting Subsystem y 2.5.1 Flow Rate Measurement 7 2.5.2 Water Distribution - Static Measurements 9

2.5.3 Water Distribution - Dynamic Measurements 15

2.5.4 Wetted Width Measurement 30

2.6 Force Measuring Subsystem 27

2.6.1 WARS Transducer 27

2.6.2 WARS Transducer Evaluation 33

2.6.3 On-Board Signal Generator Equivalent Value 33

2.6.4 Transducer Response to Changes in Operating 33

Variables

2.6.5 Transducer Response to Changes in Wheel Radius 31

2.6.6 Transducer Response to Changes in Inflation 35

Pressure

2.6.7 Transducer Response to Changes in Hitch Height 35

2.7 Suspension Subsystems 35

2.7.1 Experimental Determination of the Effective 38

Unloading Constant

2.7.2 Tow Vehicle and Trailer Displacements under 40

Static Traction Load

2. 7. 2.1 Displacement of the Test Wheel Hub 40

2. 7. 2.Displacement of the Test Tire Interface 40

2. 7. 2.

3

Displacement of the Trailer Body through 40

Roll Axis Rotation

2.7.3 Suspension Characteristics under Transient 44

Loading

1
Page

3. THE DYNAMIC SKID RESISTANCE MEASUREMENT OF SELECTED SURFACES 53

AND WARS-IRS CORRELATION

3.1 Surfaces Selected 33

3.2 Tests Planned 33

3.3 The Test Matrix

3 g

3.4 Tests Accomplished 7 q

3.5 Control of Test Variables 7 q

3.6 Test Tire Inflation Pressure Tests 7

3.7 Measurement of Uncontrolled Test Variables 73

3.8 The Correlation Test Plan

g 4

3.9 Test Matrix Results

qg

3.10 Ranking of the Sources of Dispersion 33 Q

3.11 Skid Resistance Speed Gradient 322

3.12 The Correlation Equation 329

3.13 Use of the Correlation Equation 329

4. THE IRS GROUND STATION 132

4.1 Introduction 332

4.2 Features of the Ground Station 332

4.3 Evaluation of the Ground Station ^33

5. RECOMMENDATIONS 1A4

6 . REFERENCES 146

Appendix A. General Description of the Western Area Reference 3 4 7

System (Tow Vehicle and Trailer)

Appendix B. Operating Parameters of the Western Area 33 Q

Reference System.

Appendix C. The Test Matrix. 353

11
TABLES

1. WARS Engine Tachometer Readings (Second Gear).

2. WARS Flow Rates, 8.1 Inch Wetted Width.

3. Comparison of Average Flow Rates Determined by Two Methods.

4. Wetted Width Measurements.

5. Effect of Test Tire Inflation Pressure on WARS

Transducer Output.

6. Tests Planned and Accomplished.

7. Test Plan Sequence.

8. Weather During Test Plan Time Periods.

9. Ranking of Possible Sources of Dispersion in WARS Results.

10. Ranking of Possible Sources of Dispersion in IRS Results.

11. Dispersion Sample Standard Deviations.

12. Correlation Summary.

13. Skid Number Values Computed by the Ground Station.

iii
ILLUSTRATIONS

1. Tc'sL Tire' Radial Spring Rale Test Set-Up.

2. WARS Tow Vehicle Displacements with Load.

3. WARS Piping Modified for Flowmeter and 3 Sets of Adjusting Values.

4. Verification of WARS Flow Rates.

5. SDG Sight Tube Array.

6. Static Water Distribution at 20 mph (32 km/h).

7. Static Water Distribution at 40 mph (64 km/h).

8. Static Water Distribution at 60 mph (97 km/h).

9. WARS Dynamic Pavement Wetting at 60 mph (97 km/h).

10. Measurement of Water Trace Width after the Trailer has Passed.

11. Wetted Width by Delayed Measurement on Pavement, WARS at

20 mph (32 km/h)

12. WARS Pavement Wetting During a Locked Wheel Skid at

20 mph (32 km/h)

13. WARS Pavement Wetting During a Locked Wheel Skid at

40 mph (64 km/h)

14. WARS Pavement Wetting During a Locked Wheel Skid at

60 mph (97 km/h)

15. Effect of Bubble Formation on WARS Pavement Wetting at

20 mph (32 km/h)

16. Side View of WARS Pavement Wetting at 40 mph (64 km/h)

17. A Simulated Traction Force Being Applied to a Locked Test Wheel

Through an Air-Bearing Force Plate.

18. WARS Transducer Performance.

19. Adjustable Radius Test Wheel.

20. Variation in WARS Transducer Output with Test Wheel Radius.

IV
21. Variation in WARS Transducer Output with Hitch Height.

22. Experimental Determination of WARS Effective Unloading Constant.

23. Displacement of the WARS Test Wheel Hub Due to Static Traction.

24. Displacement of the WARS Test Tire Interface Due to Static Traction.

25. Roll Displacement of the WARS Trailer Body Due to Static Traction.

26. Effect of Different Damping Factors on Second-Order Linear Systems.

27. Trailer Transient Drop Test Equipment.

28. IRS Transient Drop Test Set-Up.

29. IRS Displacement Response to Center-of-Gravity Drop Test.

30. WARS Displacement Response to a Center-of-Gravity Type Drop Test - Before Modification.

31. WARS Displacement Response to a Center-of-Gravity Type Drop Test - After Modification.

32. IRS Force Response to a Center-of-Gravity Drop Test.

33. IRS Force Response to an Axle Drop Test.

34. WARS Force Response to a Center-of-Gravity Type Drop Test.

35. WARS Force Response to an Axle Drop Test.

36. Delay in the Drying of a Wet Pavement when Deicing Agents are

Present

37. Histogram of Test Tire Inflation Pressure After a 5 Mile (8 km)

Drive.

38. Histogram of Test Tire Inflation Pressure After Skid Test.

39. Change in Test Tire Carcass Temperature Versus Inflation Pressure.

40. Thermometer Locations.

41. Temperature Versus Time of Day, April 29, 1975.

42. Temperature Versus Time of Day, May 3, 1975.

v
43. Surface 3 North End Temperature Minus Base Temperature Versus Time

of Day, April 23 - May 8, 1975.

44. Surface 3 North End Temperature Minus Base Temperature Versus Wind

Direction, April 23 - May 8, 1975.

45. Surfaces 1, 2, 3 North End Temperature Minus Base Temperature

Versus Time of Day, Wind Calm at Time of Measurement. Various

Days

46. Surface 3 North End Temperature Minus Surface 3 South End Temperature Versus Wind Direction, April 23 - April 26, 1975.

47. Observations of Wind Direction, April 23 - May 8, 1975.

48. Texture of Skid Resistance Surface Number 1.

49. Texture of Skid Resistance Surface Number 2.

50. Texture of Skid Resistance Surface Number 3.

51. Texture of Tire Wear on Surface Number 1.

52. Texture of Tire Wear on Surface Number 2.

53. Texture of Tire Wear on Surface Number 3.

54. Tire Wear on Surface 1.

55. Tire Wear on Surface 2.

56. Tire Wear on Surface 3.

57. Confidence Interval Versus Number of Measurements.

58. WARS Measurement Results at WFTC - Spring, 1975, Single Time Period

Measurements

59. IRS Measurement Results at WFTC - Spring, 1975, Single Time Period

Measurements

60. WARS Measurement Results at WFTC - Spring, 1975, Multiple Time

Period Measurements.

61. IRS Measurement Results at WFTC - Spring, 1975, Multiple Time

Period Measurements.

62. Deviation from the Mean Skid Number Measured by the WARS on

Surface 1 at the WFTC as a Function of Test Tire.

vi

I
63. Deviation from the Mean Skid Number Measured by the IRS on

Surface 1 at the WFTC as a Function of Test Tire.

64. Deviation from the Mean Skid Number Measured by the WARS on

Surface 2 at the WFTC as a Function of Test Tire.

65. Deviation from the Mean Skid Number Measured by the IRS on

Surface 2 at the WFTC as a Function of Test Tire.

66. Deviation from the Mean Skid Number Measured by the WARS on

Surface 3 at the WFTC as a Function of Test Tire.

67. Deviation from the Mean Skid Number Measured by the IRS on

Surface 3 at the WFTC as a Function of Test Tire.

68. Deviation from the Mean Skid Number Measured by the WARS on

Surface 1 at the WFTC as a Function of Lane.

69. Deviation from the Mean Skid Number Measured by the IRS on

Surface 1 at the WFTC as a Function of Lane.

70. Deviation from the Mean Skid Number Measured by the WARS on

Surface 2 at the WFTC as a Function of Lane.

71. Deviation from the Mean Skid Number Measured by the IRS on

Surface 2 at the WFTC as a Function of Lane.

72. Deviation from the Mean Skid Number Measured by the WARS on

Surface 3 at the WFTC as a Function of Lane.

73. Deviation from the Mean Skid Number Measured by the IRS on

Surface 3 at the WFTC as a Function of Lane.

74. Deviation from the Mean Skid Number Measured by the WARS at

the WFTC as a Function of Time Period.

75. Deviation from the Mean Skid Number Measured by the IRS at

the WFTC as a Function of Time Period.

76. Skid Resistance Characteristic, Surface 2 at FHWA, WFTC.

77. Skid Gradient Characteristic, Surface 2 at FHWA, WFTC.

78. Skid Resistance Characteristic, Surface 3 at FHWA, WFTC.

79. Skid Gradient Characteristic, Surface 3 at FHWA WFTC.

80. WARS-IRS Correlation, Single Time Period Results.

vii
81 . IRS Ground Station.

82 .

83 .

84 .

85 .

86 .

87 .

88 .

A-l

Ground Station Plot.

Measured Location of Surface Within the Pad Corresponding to SN

Data.

Variation in SN^ with Starting Point.

Second Degree Polynomial Fitted to Data in Interval.

Differential Radar System Plot of Test Speed.

Plot On-Board Tow Vehicle.

Ground Station Response to Known Inputs.

Sequence of Events.

viii

I
SKID RESISTANCE MEASUREMENT TESTS OF

FHWA REFERENCE SYSTEMS

at the

Western Field Test Center

March and April, 1975

R. W. Kearns and J. F. Ward

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 correlation results

between the IRS and the Western ARS (WARS) are

given

sn

wars

- 4 - 55 + O'" sn irs

where the computed standard deviation of a mid-range predicted value is 1 SN . SN is described

as a function of test speed, V, in mph

(1 mph = 1.609 km/h): for surface 2;

SN (V) = 76.5 - 0.71V + 0.0070V2

and the speed gradient is

d

vj

SN)

= -0.71 + 0.014V.

dV

The test program is explained from test preparations, tuning and calibration of subsystems, thru

dynamic measurement of the surfaces. Controlled and

uncontrolled variables are identified, discussed.
and in some cases, experimentally verified. A

ranking of the sources of dispersion is given.

A discontinuity in SN of surface 1 is identified

due to the unique test plan. A ground station

for improved SN calculation precision and

on-site statistical analysis is found to meet

design objectives.

Key Words: Accident reduction, skidding; correlation

skid resistance; highway safety; measurement, skid

resistance; pavement, skid resistance; pavement wetting

system; tire-pavement interface forces; wet pavement.

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 compliance

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 consistency between the state reports

includes the monitoring and development of test procedures and equipment

by NBS to relate the measurements made within the hierarchial system to

the national measurement system.

The measurement assurance program for the 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 equipments are expected as the program develops with the increased measurement

capabilities being transferred to the highway measurement systems at the

FTC.

*Figures in brackets indicate the literature references at the end of

this report.

2
This report describes tests conducted under the direction of NBS

personnel at the FHWA Western FTC (WFTC) during periods in March and

April 1975.

2. PREPARATION, MEASUREMENT AND ADJUSTMENT OF SUBSYSTEMS

Previous results from skid trailer correlation test programs

indicated the measurements lacked precision, agreement among trailers

using self-watering systems was poor, and the causes for the differences

between trailer measurements was 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 requirements

or conditions were found that would adversely affect the performance of

the total WARS system, adjustments and modifications were made or recommended .

2.1 Test Preparations

Auxiliary test equipment and the facilities of the WFTC were used

to prepare the WARS for test. Test tires were prepared for test.

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 thickness of the water layer, the speed, 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.

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

A turbine flow rate meter was calibrated to a 0.6 gallons per

minute (gpm) (2 liters 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).

|

3
Figure 1. Test tire radial spring rate test set-up.

4
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 mph (0.16 km/h) resolution

was used to record the speed of the system during a skid resistance

measurement (sec. 2.4).

Other auxiliary equipment included a voltmeter, calibrated to one

millivolt uncertainty by the Electrical Instruments Section; and 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 the WFTC, each of the ASTM E501

test tires to be used was mounted on a wheel, balanced, and broken in

with a 200 mile (320 km) drive. In addition, the nominal radial spring

rate was determined at NBS. The value was 1280 lbf/in (224 kN/m) at 24

psi (165 Pa) inflation pressure under 1085 lbf (4800 N) load after break

in. This value was determined from radial load versus deflection data

obtained with the test setup in Figure 1.

2.2

Skid Trailer Weight Distribution

To measure the static vertical wheel loads of the test trailer, a force plate was placed under each trailer tire and a vertical load cell

was placed under the hitch. The air bearings of the force plates were

inflated, water placed in the trailer hose, and the trailer leveled.

Readings were then taken simultaneously. The initial test wheel load

was 1098 lbf (4880 N). After adjustment the distribution was:

Left (test) side wheel load = 1087 lbf (4830 N)

Right side wheel load = 1083 lbf (4820 N)

Hitch vertical load = 178 lbf (792 N)

2.3

Tow Vehicle Displacements

Changes in the tow vehicle hitch height and rear wheel radius were

measured as a function of occupant 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 rear wheel radius change the water pump rate. In Figure 2,

points 1 correspond to the removal of 75 gal (280 1) of water. Points 2

correspond to the removal of 80 gal (300 1) of water and a 150 lb (330

kg) deadweight representing the driver's weight. The driver is the only

occupant of the WARS.

5
REAR

WHEEL

RADIUS

INCREASE

HITCH

RISE

LOAD REMOVED

b) N

Ibf

LOAD REMOVED

Figure 2. WARS tow vehicle displacements with load.

h
2.4 Speed Measuring Subsystem

The WARS speed signal was generated by a fifth wheel mounted

within the wheelbase of the tow vehicle. Two electronic circuits with

separate calibration adjustments were operated from a single tachometer

generator

One circuit provided a full range readout which was recorded on the

tow vehicle strip chart recorder. The second circuit displayed an

expanded scale signal to the driver on a center-zero analog meter. The

null point corresponded to any desired setting preselected by the driver.

WFTC personnel adjusted the speed system while the fifth wheel was

rotated by the surface of rotating drums driven by a synchronous motor.

The system was then checked by driving over a measured mile at near

constant speed while the traverse time was measured with a stop watch.

When checked against the IRS radar, which has a 0.1 mph (0.2 km/h) resolution, the difference was less than 0.5 mph (0.8 km/h) at speeds of

20, 40, and 60 mph (32, 64, and 97 km/h). Corresponding engine tachometer

readings were noted (Table 1) , while driving in second gear as in skid

testing.

During each day of dynamic skid testing, both the WARS and IRS

speed systems were checked against the radar. It was found that the

WARS preselected null settings necessary for agreement with the radar

were not constant. After repairs to the null circuit, setting variations

were reduced from a few miles per hour to a few tenths of a mile per

hour

Records obtained with the IRS radar system during the correlation

testing were used for the reduction of the speed data of both the WARS

and the IRS.

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. The dynamic distribution measurements are considered to be

unreliable for reasons to be mentioned later.

2.5.1 Flow Rate Measurement

To measure flow rate with a resolution of 0.1 gpm (0.4 1/min) the

WARS water system was modified to allow replacement of a straight pipe

section in the tow vehicle with the flow meter assembly normally installed

in the IRS. The modification included valves and pressure measuring

ports so that the back pressure of the flow meter assembly could be

duplicated when the pipe section was reinstalled.

7
Table 1 - WARS Engine Tachometer Readings

(Second Gear)

Speed Tachometer

mph3

rpm

20 1425

AO 2600

60 3800

3

1 mph = 1.609 km/h .

8
The water pump is driven from the tow vehicle drive shaft. During

stationary tests, this shaft is driven with the transmission in second

gear at the engine tachometer readings noted during the speed system

testing (Table 1). Initial tests showed that the flow rate was insufficient for the desired 8.1 inch (21 cm) wetted width. The pump drive

pulley ratio was changed and the piping converted to the design utilized

in the IRS (Fig. 3). The flow rate was then trimmed to the desired

values corresponding to test speeds of 20, 40, and 60 mph (32, 64, and

97 km/h). The pressure readings measured at the ends of the flow meter

assembly under each flow condition are given in Table 2.

Normally, to measure an average flow rate, WFTC personnel operate a water system at nearly constant speed while collecting the water in a 55

gallon (203 1) tank and measuring the time of collection. Results of

tests using this method are compared in Table 3 with the average turbine

output. In one case, the results differ by nearly five percent.

After the turbine assembly was removed from the WARS, the upstream

pressure at the pipe section was adjusted to 8.5 psi (58 kPa) and the

downstream pressure to 4 psi (27 kPa) (downstream valve fully open) for

the 40 mph (64 km/h) condition. To check the system flow rate after

these adjustments the IRS force plate was sealed in a waterproof bag,

placed under the tank and the water discharged was weighed versus time

(Fig. 4). These results confirmed that the system was properly adjusted

to within three percent of the desired values.

2.5.2 Water Distribution - Static Measurements

The objective of static distribution gage (SDG) pavement wetting

system measurements is to determine the distribution of water across the

width of the water trace at the point where it would normally contact

the pavement surface. The SDG includes a basin divided into equal width

slots mounted at floor level. The basin is equipped with a sliding

cover that is used to divert the water flow except during the time that

a sample is being collected. When the cover is removed, the incident

stream is divided by the slots and displayed in sight tubes arranged to

form a bar graph (Fig. 5)

The stationary trailer was positioned with a normal hitch height

and with the test tire centered on the basin such that water issuing

from the nozzle entered the slots. The tow vehicle drive wheels were

placed on dynamometer rollers. To obtain a sample of the water distribution, the drive wheels were turned at the desired speed with water

flowing from the nozzle, and the sliding cover was removed until water

rose to a predetermined approximate level in the sight tubes. Then the

cover was quickly replaced. The basin size is such that the time required

to fill to the predetermined level was comparable to, or longer than,

the duration of a skid test. The relative distribution of water was

then noted from the sight tubes.

9
Figure 3. WARS piping modified for flowmeter and 3 sets

of adjusting valves.

10
Table 2 - WARS Flow Rates , 8.1 Inch (20.6 cm) wetted width

Speed Flow rate

Pump

outlet

pressure

Flowmeter

upstream

pressure

Flowmeter

downstream

pressure

mpha gpmb psi c psi c psi c

20 14.6 19 3 1/4 2

(1425 rpm) d

40 29.2 32 8 1/2 3 1/4

(2600 rpm) d

60 43.9 ^60 M8 6

(3800 rpm) d

a

1 mph = 1.609 km/h.

1 gpm = 3.7 1/min .

C

1 psi = 6.9 kPa .

^ Engine tachometer readings correspond to 2nd gear, the WARS

automatic transmission setting where 3rd gear is locked out.

11
Table 3 Comparison of Average Flow Rates Determined

By Two Methods

Flowmeter Flow Tank Difference

gpma gpma percent

14.4 14.2 -1.4

28.7 27.3 -4.9

44.2 44.1 -0.2

1 gpm = 3.7 1/min .

12
400

o

in

o

o

O) o

If)

If) d

o

o

o

CNJ -Q

o

c

!

LU

H

NOiiviniAinoov hbivm

i i

Figure

4.

Verification

of

WARS

Flow

Rates.
Figure 5. SDG sight tube array.

14
The location of the test tire relative to the slots filled was

noted to check the lateral positioning of the nozzle. The number of

slots collecting water was noted as a measure of wetted width at the

point of surface contact.

Testing of the WARS revealed a partial obstruction of the flow

nozzle. After the obstruction was removed, the distributions shown in

Figures 6, 7, and 8 were obtained for flow rates corresponding to speeds

of 20, 40, and 60 mph (32, 64, and 97 km/h) respectively.

2.5.3 Water Distribution - Dynamic Measurements

The objective of dynamic distribution gage (DDG) pavement wetting

system measurements is to determine the distribution of water across the

width of the water trace at the surface contact line under actual

aerodynamic conditions. A receiving basin equipped with slots is used

but remains uncovered after passage of the trailer. Since the test

speed, flow rate, and length of the basin are known, the expected quantity

to be collected can be calculated. Prior measurements made with the DDG

at the Eastern FTC (EFTC) showed the collections averaged 70 percent

larger than expected. The additional water collected distorted the

distribution, thereby negating the objective of the test. Observed

sources of additional water collected included water splashed from the

gage structure and the spray that follows the trailer.

Since the DDG at the WFTC is of a design similar to the gage at the

EFTC, no dynamic distribution measurements were made. Until proper

sampling of the water trace can be demonstrated consistently, the DDG

should not be used to characterize pavement wetting system performance.

NBS personnel discussed the problems with WFTC representatives and

suggested some steps to minimize splashing over the DDG. They recommended

that the receiving basin be kept dry, that the grate and gage be secured

against wobbling, and that the grate be flush filled to cover the crossbars.

These recommendations were followed in tests involving operation of the

WARS over the DDG conducted by WFTC after the departure of NBS personnel.

Photographs made during the tests by a high frame-rate cine camera

mounted on the WARS trailer were forwarded to NBS. These photographs

recorded the dispersal of water from the pavement wetting nozzle. The

photographs showed that the nozzle position was higher than intended,

approximately 6 inches (15 cm) above the pavement. Films made at 20,

40, and 60 mph (32, 64, and 97 km/h) showed no splashing directly attributable

to the presence of the gage or grate. Dynamic effects on the nozzle

stream were greatest at 60 mph (97 km/h). Typical frames at 60 mph (97

km/h) showing flow onto the solid pavement ahead of the grate and over

the DDG are shown in Figure 9. No gage samples were reported so it is

not known whether excess water was collected.

15
a)

WARS

RUN

1;

b)

WARS

RUN

2:

c)

WARS

RUN

3'.

AlllNVnO 3AU.XTI3H

AlliNVnU 3AllXn3U AlllNVnO 3AI1V33H

16

Static

water

distribution

at

20

raph

(32

km/h).
CM

• • CM

D

0C

(/) oc

a>

CM

O

(/)

AlllNVnD 3AI1V13H

t

17

Figure

7.

Static

water

distribution

at

40

mph

(64

km/h).
Io

CO

18

Figure

8.

Static

water

distribution

at

60

mph

(97

km/h).
(a) ON SOLID PAVEMENT:

p«:; :

; WP

(b) OVER THE DDG:

Figure 9. WARS dynamic pavement wetting at 60 mph (97 km/h).

19
2.5.4 Wetted Width Measurement

In order 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. The various ways of measuring wetted width include using SDG and DDG distribution widths, measuring

the water trace on the pavement after the passage of the skid tester,

and photographing the pavement from the moving skid tester showing the

water trace width at the position of the interface of the test tire.

Each measurement technique has known sources of error. Width

readings of both distribution gages do not include the spreading that

begins when the water contacts the pavement. The SDG reading does not

include dynamic effects. Water traces laid down on pavement contain an

additional increment of width due to spreading and lose resolution,

necessitating assumptions about the width at the desired time. Measurements from photographs of the wetted width from the moving skid tester

involve the use of geometry to account for changes in scale with distance

from the camera. At this point, an optimal camera arrangement has not

been prepared.

In this evaluation, the span, of slots collecting water during the

SDG tests (shown in section 2.5.2) was used as a measure of wetted

width. DDG measurements were not made (see section 2.5.3).

In making direct measurements on the pavement (Fig. 10), the test

wheel was extended beyond the water trace and the test brake was not

applied. Three measurements were made with increasing delay times after

passage of the test vehicle by an observer with a scale. From these

measurements, the width at the time of passage was estimated using

linear extrapolation.

Data was obtained for the WARS at 20 mph (32 km/h) (Fig. 11). At

higher speeds traces were too diffuse to measure. Large amounts of

spray were generated. Coarse pavement and 10 mph (16 km/h) winds contributed to the scatter.

The results obtained from SDG and pavement measurements are given

in Table 4. The extrapolation to obtain the pavement measurement values

is shown in Figure 11.

• A high frame -rate cine camera was installed temporarily on the WARS

skid trailer with a downward view of the water trace. Examples of

photographs obtained in motion from which the wetted width could be

estimated are shown in Figures 12, 13, and 14. These photographs were

made during locked wheel skids.

20
Figure 10. Measurement of water trace width after the trailer

has passed.

21
TRACE

WIDTH

TIME AFTER PASSAGE

i

Figure 11. Wetted width by delayed measurement on pavement,

WARS at 20 mph (32 km/h)

M
Table 4 - Wetted Width Measurements

Speed Unit

Nominal

Flow Rate Method Wetted Width

mpha gpmb in c

20 WARS 14.6 SDG 6.2

20 WARS 14.6 Pavement 10.5

20 WARS 14.6 Photo VL2

20 IRS 14.2 SDG 6.9

40 WARS 29.2 SDG 7.5

40 WARS 29.2 Photo VL4

40 IRS 29.3 SDG 8.1

60 WARS 43.9 SDG 8.1

60 WARS 43.9 Photo M.4

60 IRS 43.3 SDG 8.8

a

1 mph = 1.609 km/h.

1 gpm = 3.7 1/min .

c

1 in = 2 . 54 cm

23
Figure 12. WARS pavement wetting during a locked wheel skid

at 20 mph (32 km/h)

24
I

Figure 13. WARS pavement wetting during a locked wheel skid

at 40 mph (64 km/h)

i!

25
Figure 14. WARS pavement wetting during

a locked wheel skid

at 60 mph (97 km/h) 
A large amount of spray was observed following the WARS nozzle at

all speeds. Spray formed both when the test wheel was normally positioned

and when the wheel extender was used. Much of the pumped water was

still in the air at the position of the test tire. At 20 mph (32 km/h),

photographs showed formation of large bubbles in the water stream which

burst and left dry areas ahead of the test tire (Fig. 15).

Side view photographs (Fig. 16) showed that the stream reached

ground level near the leading edge of the tire-pavement interface with

little time to settle on the pavement before meeting the tire.

Since the WARS photographs show mostly spray, the measured values

of wetted width from the photographs are unrealistically high. However,

the photographs do demonstrate the poor coupling between the stream and

the pavement which left the wetted width undefined.

2.6 Force Measuring Subsystem

The transducer of the force measuring subsystem is located inboard

on the axle, remote from the force it is to measure at the tire-pavement

interface. This 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 acting at the transducer. Many effects

due to these displacements are minimized by using the tow vehicle and

trailer system as a fixture while 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 during such a test in Figure 17. However, some

effects are not accounted for, such as the transverse force at the tirepavement 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 at least 200 miles (322 km) of use

which provides some additional flexibility.

2.6.1 WARS Transducer

The WARS utilizes a torque transducer to measure traction at the

interface. Torque readings are converted to traction by assuming a constant test wheel radius of 12 15/16 inches (32.9 cm). Variations in

readout of up to 20 ft.lbf (27 N.m) occurred under vertical load with no

traction applied. Variations in readout of up to 8 ft.lbf (10.8 N.m)

occurred with the test tire lifted from the pavement when the brake was

energized as compared to de-energized. The results of additional tests

indicated the variations were smaller when the test wheel hub was dithered

by an impact wrench before placing the test tire on the pavement.

27
(a) EXPANDING BUBBLE ABOUT TO BURST

(b) GAP LEFT BY SAME BUBBLE:

1’igure 15. Effect of bubble formation on WARS pavement wetting at 20 mph (32 km/h).

oo
29
Figure 17. A simulated traction force being applied to a locked

test wheel through an air-bearing force plate.

30
2.6.2

WARS Transducer Evaluation

The trailer was alined with the tow vehicle with the test (left)

wheel resting on the force plate. A 150 lb (330 kg) weight was placed

in the seat of the tow vehicle. The hitch height was leveled which

corresponded to 14 3/8 inches (36.5 cm) from a level pavement to the

bottom of a welded flange under the trailer hitch assembly. A force

plate test was conducted using the WARS on-board electronics for the

transducer signal and the IRS electronics for the force plate signal.

The transducer signal was scaled on the X-Y recorder with the test wheel

in the air. Dither was applied and then the wheel was carefully set on

the force plate. The signal of the floating level force plate was then

scaled. The results were plotted point-by-point as shown in Figure 18.

The data points alternated on either side of a straight line under

increasing traction load and alternated, under decreasing load, on

either side of another line which was parallel and 12 lbf (53.4 N) under

the first line.

2.6.3 On-Board Signal Generator Equivalent Value

The value of the resistor used for the calibration signal generator

was determined to be equivalent to a change of 865 lbf (3850 N) under

these test conditions. This corresponds to the 933 ft. lbf torque (1265

N.m) value used by WFTC personnel when the wheel radius is 12 15/16

inches (32.9 cm).

2.6.4 WARS Transducer Response to Changes in Operating Variables

To evaluate the stability of these results, supplementary force

plate tests were conducted which included changes in wheel radius,

changes in test tire inflation pressure, and changes in hitch height.

2.6.5 Response to Changes in Wheel Radius

The test wheel and tire were removed from the trailer and replaced

with a wheel on which an adjustable radius arm, containing a cavity at

its end, had been welded (Fig. 19). A flat plate containing a similar

cavity was placed on the force plate. The brake of the test wheel was

then locked with a ball bearing located between the cavities and directly

under the center line of the axle.

A traction force plate test was conducted at a nominal value of

radius followed by a higher and lower value of radius. The results

shown in Figure 20 confirm that the transducer output, and hence calibra

tion, varies directly with the wheel radius.

31
WARS

ON

BOARD

CALIBRATION

READING

933

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32

Figure

18.

WARS

transducer

performance.
Figure 19. Adjustable radius test wheel.

33
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34

FORCE

PLATE

TRACTION
2.6.6

Transducer Response to Changes in Inflation Pressure

Force plate tests with test tire inflation pressure as the operating

variable were conducted by WFTC personnel. The results are summarized

in Table 5. They indicate the WARS transducer calibration is a function

of test tire inflation pressure varying in excess of one percent over

the range from 22 to 30 psi (152 to 207 kPa)

2.6.7

Transducer Response to Changes in Hitch Height

Force plate tests were conducted with hitch height as the operating

variable. No change in transducer output was measured over the range

from 1 5/8 inches (4.1 cm) below to 1 3/8 inches (3.5 cm) above normal

hitch height (Fig. 21).

Note however that skid number is a function of hitch height, because

the vertical load changes with changing hitch height (see sec. 2.7.1).

2.7

Suspension Subsystems

The measurement of pavement skid resistance at normal traffic

speeds is a dynamic test. While the data of interest 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 in analytical

form so 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, 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 suspension system were

obtained while the trailer was connected to a ball hitch on a stationary

fixture. The displacements of the trailer test wheel under static 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 ball hitch and the tow vehicle tire-pavement interface.

Since optimum design values have not been demonstrated at this time,

results from some tests conducted on the IRS are presented with those of

the WARS for intercomparison purposes.

35
Table 5 - Effect of Test Tire Inflation Pressure on WARS Transducer

Output

Test tire

inflation pressure

Traction force

applied

WARS

transducer

output

psia lbfb volts

30 800 8.65

28 800 8.63

26 800 8.60

24 800 8.58

22 800 8.55

a

1 psi = 6.9 kPa.

b

1 lbf = 4.45 N.

36
ONiavaa aaonasivivai savM

37

FORCE

PLATE

TRACTION
Since the WARS transducer is a torque instrument, an instantaneous

measurement of the test wheel vertical load is not available. As the

test wheel locks, the traction force generates a moment about the hitch

which reduces the vertical load on the test tires. This abrupt unloading

excites a transient response in the suspension subsystem. The unloading

constant is an important parameter in the dynamic analysis of the trailer.

Further, the variation in vertical load must be known to correctly

reduce traction data to skid number.

2.7.1 Experimental Determination of the Effective Unloading Constant

The vertical load 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 load can be described in terms

of an effective unloading constant [4] which can then be experimentally

determined. The reduction in vertical load is given by:

F = F -F \

WL WL WL \ L / Z Z x\ / o

where Frrr = the longitudinal traction force on the left wheel.

WL

x F^ = the total normal force on the left wheel acting at the

z tire-pavement interface.

Frri = The normal force on the left wheel acting at the

z tire-pavement interface when the applied traction

is zero.

_H = the effective normal force unloading constant where

L H is the hitch height and L is the length from the

hitch to the center line of the axle.

There is an uncertainty in the measurement of H and L because of

the shape and size of the ball hitch.

The effective normal force unloading constant for the IRS was

measured at the WFTC to demonstrate the method.

The transducer vertical load signal was amplified and applied to

the Y-axis of the recorder while the amplified traction signal was

applied to the X-axis. The unloading occurs linearly as shown in Figure

22. At a traction force of 800 lbf (3560 N) the unloading is 85 lbf

(378 N). The effective unloading constant is

38
IRS TRANSDUCER VERTICAL LOAD

H- 00cH

3

to

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H

L F,WL

o

X

On skid trailers not instrumented for vertical load, the vertical

load and traction channels of the force plate may be used. WFTC personnel

used the force plate channels to experimentally determine the effective

unloading constant of the WARS. It was reported to be 0.143.

This value was used in the reduction of the WARS data. The effective

unloading constant was not used in the reduction of the IRS data, rather,

the measured vertical loads and traction forces were used.

2.7.2

Tow Vehicle and Trailer Displacements under Static Traction Load

2. 7. 2.1 Displacement of the Test Wheel Hub

Under a static traction load of 742 lbf (3300 N) the test wheel hub

moves backward 1.2 inches (3.0 cm) with respect to the pavement, as shown in Figure 23.

Although the decreasing traction data is incomplete, the existence

of a large hysteresis loop is indicated. This displacement includes the

tow vehicle tire and suspension deflections, trailer yaw and trailer

suspension deflections.

2. 7. 2.Displacement of the Test Tire Interface

The test tire-force plate interface moves rearward with respect to

the hub due to the distortion of the tire. No slippage of the tire on

the force plate was noted. The interface displacement is plotted in

Figure 24. Displacement measurements in the loading cycle beyond 370

lbf (1650 N) of increasing traction are not valid because the sensor

went off scale. The shape of the plot at the lower loads indicates that

a displacement of about 0.5 in (1.2 cm) would have been measured at 800

lbf (3560 N).

2.7. 2.

3

Displacement of the Trailer Body Through Roll Axis Rotation

Angular displacement about the roll axis was measured with respect

to the pavement using two vertical sensors mounted at the rear of the

trailer with an effective lateral separation of 46 in (117 cm).

The displacements are shown in Figure 25. Initially, the sensors

were off scale. The displacement measurements between 263 and 555 lbf

(1170 and 2470 N) have been extrapolated to zero traction to establish

the origin of the displacement scale. The corresponding roll angle was

two minutes of arc.

40
HUB

DISPLACEMENT

TO

THE

REAR

HUB

DISPLACEMENT

TO

THE

REAR

a) WARS: N

STATIC TRACTION

b) IRS:

0 1 000 2000 3000

Figure 23. Displacement of the test wheel hub due to static traction.

41
INTERFACE

DISPLACEMENT

TO

THE

REAR

INTERFACE

DISPLACEMENT

TO

THE

REAR 1000 —I

N —2000

1~ 3000 —l—

(a) WARS

0

0.50

A= INCREASING TRACTION

v = decreasing traction

in

0 200 400 600

Ibf

STATIC TRACTION

- 1 .0

cm

- 0.5

- 0

L_

800

(b) IRS

0 1000 2000 3000

Ibf

STATIC TRACTION

Figure 24. Displacement of the test tire interface due to

static traction. 42
REAR

BODY

DISPLACEMENT

REAR

BOD

Y

D

IS

PLACEMENT

RISE

RISE

a) WARS:

1000

N

2000 —i

3000

i

200 400 600

Ibf

STATIC TRACTION

800

b IRS:

N

O 1000 i2000 3000

Ibf

STATIC TRACTION

Figure 25. Roll displacement of the trailer body due to

static traction.

43
2.7.3. Suspension Characteristics under Transient Loading

The effort i v <

• dynamic characteristics of the suspension subsystem

run he nie.-iiiiii «• < I In leimn <>l nnlui.il I i < •<

| uene I pm nml damping larlni's.

There are three natural 1 requencies ol interest. One occurs under

conditions of small variations in vertical load where there is no relative movement within the suspension subsystem. The axle and body are

free to rotate in a vertical plane about the hitch within a small range

of displacement of the tires (section 2.1.2). The other two natural

frequencies occur under conditions of larger variations in vertical load

where there is relative movement within the suspension system and the

shock absorbers are active. In this case there are two degrees of

f reedom:

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

suspension designs of the IRS type, and

about the leaf springs in suspension designs

of the WARS 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 IRS

suspension design revealed two dynamic tests which are easy to conduct

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 [5]. The

first test is to raise the trailer by the rear bumper and to drop it

abruptly. With this initial condition, an oscillation occurs where the

angular displacement of the body and axle trailing arm are out of phase.

In this mode, the low frequency is enhanced. The second test is to

raise the trailer at the axle and to drop it abruptly. With this initial

condition, an oscillation occurs where the angular displacement of the

body and axle trailing arm are in phase. In this mode, the high frequency

is enhanced. A dynamic analysis of suspension system designs of the

WARS type may reveal a better set of tests to conduct to obtain its

characteristics

A nondimensional family of plots which can be used to estimate the

amount of damping in systems representative of second order systems is

shown in Figure 26 [6]. The curves indicate that a system with a damping

factor, of 0.8 reaches near equilibrium before a system with either a damping factor of 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.

44
Figure 26. Effect of different damping factors

on second-order linear systems

(Reference 6)

45
A special air cylinder assembly was designed so a trailer could be

raised at its center-of-gravity or at its axle and dropped abruptly.

The air cylinder system is shown in Figure 27. The large diameter air

cylinder is used to lift the body. The small air cylinder is used to

raise a trailer by its axle. The IRS test set-up is shown in Figure 28.

The IRS was lifted at its center-of-gravity 0.56 in (1.4 cm) and

dropped abruptly. The body-to-axle displacement, measured at the axle,

versus time is shown in Figure 29. By comparing the form of the response

with the family of curves of Figure 26, the effective damping factor

between the body and axle is estimated to be 0.57. The body- to-ground

displacement response, measured at the rear bumper, is also shown. This

response includes the displacement of the tires which are only lightly

damped. The effective body-to-ground displacement response damping

factor is estimated to be 0.45. The damped low frequenrv F . is p<;t-iTnated

the low natural frequency is approximately 3.6 Hz. The small amplitude

natural frequency on the tires is approximately 4 Hz.

The WARS was subjected to a similar drop test, by lifting it 0.7 in

(1.8 cm) at the rear bumper and dropping it abruptly. The body-to-axle

response is shown in Figure 30. The damping factor was estimated to be

0.45. At the recommendation of NBS personnel, a second set of shock

absorbers was installed on the WARS. The test was repeated with the

result shown in Figure 31. After this modification the body-to-axle

damping factor was increased to 0.65. The damped low frequency, F^ , is

estimated to be 3.7 Hz. The low natural frequency is approximately 4.9

The IRS suspension system consists of parallelogram arms trailing

from the body to the axle assembly with coiled springs mounted vertically

between the arms and upper body.

The response of the IRS force measurements to a center-of-gravity

drop test is shown in Figure 32. The effective transducer and force

plate damping factors are 0.4. The damped low frequency response is 2.9

Hz. There is a natural frequency associated with the trailer body and a second natural frequency associated with the axle. The natural frequency

associated with the axle is a higher frequency. The damped high frequency

response is estimated at 8 Hz.

A response curve consists of both the low and high frequency components. The amount and relative phase of each depends upon the initial

conditions. When the initial conditions are changed by lifting the

to be 3.2 Hz. Since the low natural frequency, F^,

Hz.

46
Figure 27. Trailer transient drop test equipment.

47
Figure 28. IRS transient drop test set-up.

48
DISPLACEMENT

TIME

Figure 29. IRS displacement response to center-of-gravity drop test.

49
DISPLACEMENT

Figure 30. WARS displacement response to a center-of-gravity type

drop test - before modification.

50
DISPLACEMENT

Figure 31. WARS displacement response to a center-of-gravity type

drop test - after modification.

51
FORCE

Figure 32. IRS force response to a center-of-gravity drop test.

52
trailer by the axle (Fig. 33) and dropping the axle abruptly, the

damping factor is estimated at 0.46 with a damped low frequency component of 2.5 Hz and a damped high frequency component of 9 Hz.

The WARS suspension consists of an axle assembly mounted on leaf

springs

The response of the WARS force measurements to a center-of-gravity

type drop test is shown in Figure 34. The transducer responds to torque

inputs. There is a transducer response to the varying vertical force.

The vertical force plate response is also shown. The damping factor

associated with this response is estimated to be 0.21. The damped low

frequency response is nominally 3.3 Hz. Due to the low damping factor,

the undamped low natural frequency is the same value.

The response of the WARS force measurements to an axle-lift drop

test is shown in Figure 35. Again the torque transducer responds to the

varying vertical force with the high frequency component more apparent.

The high frequency component is also more apparent in the force plate

response. The damping factor associated with the axle-lift drop test

force plate response is estimated to be 0.35. The damped high natural

frequency is estimated to be in the range from 6 to 8 Hz. The corresponding undamped high natural frequency would be 6.4 to 8.5 Hz.

The WARS small amplitude natural frequency on the tires is approximately 4 Hz.

3. THE DYNAMIC SKID RESISTANCE MEASUREMENT OF SELECTED

SURFACES AND WARS- IRS CORRELATION

The purpose of the dynamic skid resistance measurement test program

was:

1) to measure selected surfaces at the WFTC,

2) to measure the consistency and precision in the measure

ments of both the WARS and the IRS, and

3) to correlate the degree of match in the measurements be

tween the IRS and the WARS.

3.1 Surfaces Selected

The surfaces selected were a 100 foot (30 m) longitudinal section

of lanes 1 through 7 of surfaces identified as 1, 2, and 3 at the WFTC.

So that the reduced data would all be within the same 100 foot (30 m)

53
FORCE

TIME

Figure 33. IRS force response to an axle drop test.

33

54
FORCE

TIME

Figure 34. WARS force response to a center of gravity type drop test

55
FORCE

TIME

Figure 35. WARS force response to an axle drop test.

2 56
section at all test speeds, pylons were set out where the operator would

start the automatic test sequence for each test speed. The location of

the selected surfaces was the first 100 feet (30 m) at the southern end

of each surface.

3.2 Tests Planned

The test program is designed to yield the following information:

1) Skid resistance, SN, of a surface as a function of test

speed.

2) Skid resistance - test speed gradient, 0 , as a function of

test speed.

3) Variability in SN with choice of time period for the test.

4) Variability in SN with choice of test tire.

5) Variability in SN with choice of lane within the surface.

The test plan included the control of the following operating

variables

a) hitch height,

b) inflation pressure of the test tire,

c) inflation pressure of the tow vehicle rear tires,

d) inflation pressure of the shock absorbers,

e) surface cleanliness,

f) surface degree-of-dryness

,

g) test speed, and

h) wear texture of the test tires.

The test plan included the measurement of the following uncontrolled

variables:

a) weather,

b) surface temperature,

c) test tire lock-up position, and

d) test tire tread depth.

The planned test program is contained in Table 6.

57
Table 6-

Tests

Planned

and

Accomplished

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The test sequence within each time period is given in Table 7. In

general, at least two tests are conducted in other lanes before a test

is repeated in a given lane. At a spacing of 20 minutes between tests,

a minimum of one hour is allowed for the surface to dry before measuring

the lane again [7].

In the NBS tests conducted the previous fall at the EFTC [5] it was

noted that each surface textured the test tire in a different pattern.

Since the water in the interface can fill the texture previously worn in

the tire, as well as fill the texture of the surface, each tire was

assigned to a specific surface. The same test tire was then used on

both the IRS and the WARS. Seven tires were used on each of three

surfaces (21 tires) for the single time period tests. One tire was used

on each of three surfaces (3 tires) for the multiple time period tests.

Twenty-four (24) test tires were used in the program.

3.3 The Test Matrix

A subdivision of the test plan consists of the following test

matrix:

Time Period

Surface

Tires

Speed A B C D E F G

S - S2345671 AS 1 2 3 4 5 6 7

S + AS 4 5 6 7 1 2 3

Letters A thru G designated 7 tires while numbers 1 thru 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. Another

subdivision of the test plan given in Table 6 consists of a test matrix

where the variables are lanes within a surface and time period of the

test. The same test tire and wheel was always used on the same surface

and all tests were conducted at 40 mph (64 km/h)

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. The test matrix is explained further

in Appendix B.

59
'
Table 7 - WFTC Test Plan Sequence

3 Surfaces

24 Tires

210 Skid Measurements - IRS

210 Skid Measurements - ARS

60-69

TIME PERIOD //

IRS ARS SURFACE LANE SPEED TIRE

X 1 2 40 A-l

X 1 3 40 B-l

X 1 1 38 A-l

X 1 2 38 B-l

X 1 4 42 A-l

X 1 5 42 B-l

X 1 3 38 C-l

X 1 4 38 D-l

X 1 6 42 C-l

X 1 7 42 D-l

X 1 4 40 C-l

X 1 5 40 D-l

X 1 1 40 H

X 1 6 40 E-l

X 1 2 40 H

X 1 5 38 E-l

X 1 4 40 H

X 1 1 42 E-l

X 2 1 40 I X 3 1 40 J

X 2 2 40 I X 3 2 40 J

X 2 4 40 I X 3 4 40 J

X 1 7 40 F-l

X 1 1 40 G-l

X 1 6 38 F-l

X 1 3 42 G-l

X 1 2 42 F-l

X 1 7 38 G-l

NEXT HALF REPEAT ABOVE

BUT IRS & ARS CHANGE ASSIGNMENTS

60

TIME PERIOD n

IRS ARS SURFACE LANE SPEED TIRE

X 2 2 40 A

X 2 3 40 B-2

X 2 1 38 A

X 2 2 38 B-2

X 2 4 42 A

X 2 5 42 B-2

X 2 3 38 C-2

X 2 4 38 D-2

X 2 6 42 C-2

X 2 7 42 D-2

X 2 4 40 C-2

X 2 5 40 D-2

X 1 2 40 H

X 2 6 40 E-2

X 1 3 40 H

X 2 5 38 E-2

X 1 5 40 H

X 2 1 42 E-2

X 2 2 40 I X 3 2 40 J

X 2 3 40 I X 3 3 40 J

X 2 5 40 I

X 3 5 40 J

X 2 7 40 F-2

X 2 1 40 G-2

X 2 6 38 F-2

X 2 3 42 G-2

X 2 2 42 F-2

X 2 7 38 G-2

NEXT HALF REPEAT ABOVE

BUT IRS & ARS CHANGE ASSIGNMENTS

61
TIME PERIOD //3

IRS ARS SURFACE LANE SPEED TIRE

X 3 2 40 A3

X 3 3 40 B-3

X 3 1 38 A3

X 3 2 38 B-3

X 3 4 42 A3

X 3 5 42 B-3

X 3 3 38 C-3

X 3 4 38 D-3

X 3 6 42 C-3

X 3 7 42 D-3

X 3 4 40 C-3

X 3 5 40 D-3

X 1 3 40 H

X 3 6 40 E-3

X 1 4 40 H

X 3 5 38 E-3

X 1 6 40 H

X 3 1 42 E-3

X 2 3 40 I X 3 3 40 J

X 2 4 40 I X 3 4 40 J

X 2 6 40 I X 3 6 40 J

X 3 7 40 F-3

X 3 1 40 G-3

X 3 6 38 F-3

X 3 3 42 G-3

X 3 2 42 F-3

X 3 7 38 G-3

NEXT HALF REPEAT ABOVE

BUT IRS & ARS CHANGE ASSIGNMENTS

62
TIME PERIOD #4

IRS ARS SURFACE LANE SPEED TIRE

X 3 2 20 A3

X 3 3 20 B-3

X 3 1 18 A3

X 3 2 18 B-3

X 3 4 22 A3

X 3 5 22 B-3

X 3 3 18 C-3

X 3 4 18 D-3

X 3 6 22 C-3

X 3 7 22 D-3

X 3 4 20 C-3

X 3 5 20 D-3

X 1 4 40 H

X 3 6 20 E-3

X 1 5 40 H

X 3 5 18 E-3

X 1 7 40 H

X 3 1 22 E-3

X 2 4 40 I X 3 4 40 J

X 2 5 40 I X 3 7 40 J

X 2 7 40 I X 3 5 40 J

X 3 7 20 F-3

X 3 1 20 G-3

X 3 6 18 F-3

X 3 3 22 G-3

X 3 2 22 F-3

X 3 7 18 G-3

NEXT HALF REPEAT ABOVE

BUT IRS & ARS CHANGE ASSIGNMENTS

63
TIME PERIOD 75

IRS ARS SURFACE LANE SPEED TIRE

X 3 2 60 A3

X 3 3 60 B-3

X 3 1 58 A3

X 3 2 58 B-3

X 3 4 62 A3

X 3 5 62 B-3

X 3 3 58 C-3

X 3 4 58 D-3

X 3 6 62 C-3

X 3 7 62 D-3

X 3 4 60 C-3

X 3 5 60 D-3

X 1 5 40 H

X 3 6 60 E-3

X 1 6 40 H

X 3 5 58 E-3

X ] 1 40 H

X 3 1 62 E-3

X 2 5 40 I X 3 6 40 J

X 2 6 40 I

X 3 5 40 J

X 2 1 40 I X 3 1 40 J

X 3 7 60 F-3

X 3 3 62 G-3

X 3 6 58 F-3

X 3 1 60 G-3

X 3 2 62 F-3

X 3 7 58 G-3

NEXT HALF REPEAT ABOVE

BUT IRS & ARS CHANGE ASSIGNMENTS

64
TIME PERIOD #6

IRS ARS SURFACE LANE SPEED TIRE

X 2 2 20 A

X 2 3 20 B-2

X 2 1 18 A

X 2 2 18 B-2

X 2 A 22 A

X 2 5 22 B-2

X 2 3 18 C-2

X 2 A 18 D-2

X 2 6 22 C-2

X 2 7 22 D-2

X 2 A 20 C-2

X 2 5 20 D-2

X 1 6 AO H

X 2 6 20 E-2

X 1 7 AO H

X 2 1 22 E-2

X 1 2 AO H

X 2 5 18 E-2

X 2 7 AO I X 3 6 AO J

X 2 6 AO I X 3 7 AO J

X 2 2 AO I X 3 2 AO J

X 2 7 20 F-2

X 2 1 20 G-2

X 2 6 18 F-2

X 2 3 22 G-2

X 2 2 22 F-2

X 2 7 18 G-2

NEXT HALF REPEAT ABOVE

BUT IRS & ARS CHANGE ASSIGNMENTS
TIME PERIOD #7

IRS ARS SURFACE LANE SPEED TIRE

X 2 2 60 A

X 2 3 60 B-2

X 2 1 58 A

X 2 2 58 B-2

X 2 4 62 A

X 2 5 62 B-2

X 2 3 58 C-2

X 2 4 58 D-2

X 2 6 62 C-2

X 2 7 62 D-2

X 2 4 60 C-2

X 2 5 60 D-2

X 1 7 40 H

X 2 6 60 E-2

X 1 1 40 H

X 2 5 58 E-2

X 1 3 40 H

X 2 1 62 E-2

X 2 7 40 I X 3 7 40 J

X 2 3 40 I X 3 1 40 J

X 2 1 40 I

X 3 3 40 J

X 2 7 60 F-2

X 2 1 60 G-2

X 2 6 58 F-2

X 2 3 62 G-2

X 2 2 62 F-2

X 2 7 58 G-2

NEXT HALF REPEAT ABOVE

BUT IRS & ARS CHANGE ASSIGNMENTS

66
PAD 1 TIRE

H

TIME

SPEED 1 2 3 4 5 6 7

40 1 2 3 4 5 6 7 40 2 3 4 5 6 7 1

40

4 5 6 7 1 2 3

TIME

SPEED 1 2 3 4 5

PAD 2 6

TIRE I 7

40 1 2 3 4 5 6 7

40 2 3 4 5 6 7 1

40

4 5 6 7 1 2 3

TIME

SPEED 1 2 3 4 5

PAD 3 6

TIRE

J 7

40 1 2 3 4 5 6 7

40 2 3 4 5 6 7 1

404 5 6 7 1 2 3
PAD 1 40 MPH

TIME PERIOD #1

TIRE

SPEED A-l B-l C-l D-l E-l F-l G-l

38 1 2 3 4 5 6 7

40 2 3 4 5 6 7 1

42 4 5 6 7 1 2 3

TIRE

SPEED AB-2 C-2 D-2 E-2

PAD 2 20 MPH

TIME PERIOD #6

F-2 G-2

18 1 2 3 4 5 6 7

20 2 3 4 5 6 7 1

22 4 5 6 7 1 2 3

TIRE

SPEED AB-2 C-2 D-2 E-2

PAD 2 40 MPH

TIME PERIOD #2

F-2 G-2

38 1 2 3 4 5 6 7

40 2 3 4 5 6 7 1

42 4 5 6 7 1 2 3

SPEED

TIRE

AB-2 C-2 D-2 E-2

PAD

TIME

F-2

2 60 MPH

PERIOD //7

G-2

58 1 2 3 4 5 6 7

60 2 3 4 5 6 7 1

62 4 5 6 7 1 2 3

68
PAD 3 20 MPH

TIME PERIOD //4

TIRE

SPEED A- » it t c- i D- t E » K J 0 i

18 1 2 3 4 5 6 7

20 2 3 4 5 6 7 1

22 4 5 6 7 1 2 3

PAD 3 40 MPH

TIME PERIOD #3

TIRE

SPEED A- 3 B-3 C-3 D-3 E-3 F-3 G-3

38 1 2 3 4 5 6 7 40 2 3 4 5 6 7 1 42 4 5 6 7 1 2 3

TIRE

SPEED A3B-3 C-3 D-3 E-3

PAD 3 60 MPH

TIME PERIOD #5

F-3 G-3

58 1 2 3 4 5 6 7

60 2 3 4 5 6 7 1 62 4 5 6 7 1 2 3

69
3.4 Tests Accomplished

The test plan utilized seven time periods, with each time period

containing two days. The plan required 14 extended working days of

dynamic testing.

Except for a few periods of high wind speeds, during which dynamic

testing was halted, the weather remained good. As shown in Table 6,

both the WARS and NBS crews conducted 98 percent of the planned tests.

3.5 Control of Test 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 two-axis force transducer used on the IRS 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. The static tests had demonstrated (Fig. 21) that the WARS traction

transducer output is rather insensitive to changes in hitch height. The

static tests (sec. 2.3) had demonstrated that the hitch height depends

on occupant and water weight.

Variations in inflation pressure of the WARS test tire change the

transducer output signal as demonstrated in the subsystem tests (Table

5). Variations in inflation pressure of the IRS test tire shift the

zero of the traction force instrumentation. Variations in inflation

pressure of the test tires on both systems change the area of contact

between the test tire and the surface. On dry pavements the variation

in contact area changes the 2 mph (3.2 km/h) skid resistance of smooth

tires approximately one skid number per 2.5 psi (17 kPa) in inflation

pressure [9]. The 2 mph (3.2 km/h) skid resistance is a measure of the

first term of the skid resistance functional equation (sec. 3.9).

Variations in the inflation pressure of the tow vehicle rear tires

change the water laying system pump rate with test speed.

Each of ten tires were used for three tests during a test plan time

period. Prior to the set of three tests, the test tire was conditioned

with a five-mile drive. Immediately upon return to the test site, the

inflation pressure of the test tire was readjusted to 24 psi (165 kPa)

The hitch height was then readjusted to 10.5 in (26.7 cm) for the IRS

and to 14.30 in (36.5 cm) for the WARS for the dimension from the surface

to under the hitch flange at the same level location on the test site.

The inflation pressure of the tow vehicle rear tires of both systems was

readjusted to a nominal 45 psi (310 kPa) four times per test day. The

shock absorber pressure was checked at least once each day on the IRS.

I
The shock absorber inflation pressure of the WARS was maintained at

zero. Variations in shock absorber inflation pressure result in a shift

in trailer body orientation which changes the static vertical test wheel

load

The surfaces were swept at the beginning of each test day. There

is a considerable amount of rubber deposited on the surfaces as evidenced

by the tire wear and "black residue" accumulated on the trailer fenders.

There is also evidence that this sweeping was not sufficient to maintain

a constant skid resistance value of the surface (sec. 3.10). Recommendations

concerning surface cleanliness are made in another section of this

report

The degree-of-dryness of the surface was controlled above a minimum

level. The test plan is designed for a 20 minute time interval between

skid tests on a given surface. Also, the succeeding tests are not run

on adjacent lanes but are spaced laterally in position and sequentially

in time so sufficient time was available for surface drying before

another test was conducted in a given lane. Figure 36 shows the delay

in recovery from initial wetness with time for water [8]. Tests were

delayed further, as required, until the test program director ruled a

particular lane was dry.

The skid resistance of a surface is recognized to vary with test

speed (sec. 3.9). A 1.0 mph (1.6 km/h) difference in test speed on

surface 3 at 20 mph (32 km/h) changes the measurement result by approximately one percent. Test speed was recorded with a stationary differential

radar system. The resolution of the system is 0.1 mph (0.2 km/h).

3.6 Test Tire Inflation Pressure Tests

Test tire inflation pressures were recorded throughout the testing

day as described in section 3.5. The inflation pressures of two other

test tires were also recorded. One was located in the shade and the

other was located in the sun. The measurements are shown in Figures 37,

38, and 39.

The largest variation occurs with ambient temperature. At the WFTC

the mornings were cool 67°F (19°C) but the carcass temperature of the

tire in the sun would reach 122°F (50°C) in the early afternoon. Pressure

variations from 24 psi (165 kPa) to 27.8 psi (192 kPa) were recorded

over this temperature range as shown in Figure 39. The test tires were

stored in the sun during the day. After a tire was mounted on a trailer,

the inflation pressure was adjusted to 24 psi (165 kPa) before starting

the 5 mile (8 km) run. The range of variation measured upon the trailers

return to the test site is shown in Figure 37. The average inflation

pressure was increased on the IRS by 1.7 psi (12 kPa) and on the WARS by

1.4 psi (10 kPa) during the warm-up run. The inflation pressure was

readjusted to 24 psi (165 kPa) before the skid test. The inflation

pressure was then measured immediately after the skid test. The average

inflation pressure was decreased on the IRS by 0.1 psi (689 Pa) and

decreased on the WARS by 0.2 psi (1380 Pa) during a skid test.

71
SKID

NUMBER

DRY

WATER

ROCK SALT

CALCIUM CHLORIDE

DRYING TIME, min.

Figure 36. Delay in the drying of a wet pavement when deicing

agents are present (Reference 8) .

72
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73

INFLATION

PRESSURE
NUMBER

OF

OBSERVATIONS

40

36

32

28

24

20

16

12

8

4

0

INFLATION PRESSURE

BEFORE SKID TEST

IRS

WARS

..J

r

T_.

J

a

ri

'

i

L.

X S

IRS 23.9 0.27

WARS 23.8 0.30

L.J

XIXJ

.2 .4 .6 .8 2 .4 .6

23.0 24.0

INFLATION PRESSURE, psi

.8

25.0

Figure 38. Histogram of test tire inflation pressure after skid test,

74
CARCASS

TEMPERATURE

kPa

165 170 175 180 185 190

Inflation pressure

Figure 39. Change j.n test tire carcass temperature versus

inflation pressure.

75
Since variations in inflation pressure change the transducer outputs

of both systems, and the weather conditions at the EFTC are quite different

than those at the WFTC during the calibration period for state measuring

systems, an estimate of the extreme differences which may occur when

inflation pressure is not controlled follows.

Consider that the inflation pressure of the test tires to be used

each day are set in the morning to 24 psi (165 kPa) before leaving the

garage. During the night and early morning the garage temperatures at

the Eastern and Western FTC are maintained at 67°F (19°C). During the

year, correlation tests are conducted in ambient temperatures of 60°F

(16°C) at the EFTC while others are conducted at 110°F (43°C) at the

WFTC. Using Figure 39, the inflation pressure of a tire at the EFTC

test surface is estimated to be 23.5 psi (162 kPa) while a tire at the

WFTC test surface is estimated to be 27 psi (186 kPa) . Because of the

higher ambient temperature, the WFTC tire pressure may further increase

by the larger values given in Figure 37 by the end of the five mile

warm-up drive. The EFTC tire pressure may increase only by the smaller

values. Under these assumptions, correlation tests at the WFTC may be

conducted with test tires which are at 6 psi (41 kPa) greater inflation

pressure than those used at the EFTC.

The effect of this pressure difference would depend on the design

of the measuring systems being correlated. One effect would be that the

WARS transducer output at a traction force of 800 lbf (3559N) would

increase 0.8% (Table 6). Ideally, there should be no difference in the

calibration result of a state measuring system conducted at either

center. Recommendations concerning inflation pressure are made in

another section of this report.

3.7 Measurement of Uncontrolled Test Variables

The weather conditions around the surfaces during the dynamic

testing time period are given in Table 8.

The temperatures of the skid pad surfaces vary with environmental

conditions. The surfaces used to provide a range of skid resistances

vary in composition, coloration, roughness, etc. Their response to

environmental changes depends on their characteristics of radiative,

conductive, and aerodynamic heat transfer. In addition, pavement wetting

may affect the surface temperature for some time following the pass of a skid tester.

The area of the skid surface is so large and the wheel tracks of

the vehicles so varied that the temperature of the surfaces corresponding

to the data reduction has to be measured indirectly. Surface thermometers

were used to measure the temperatures of the dry skid pads, the base

material on which the surfaces were inset, and the air in the shade at

ground level.

76
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78
The temperature sensitive area of the sensor was placed in contact

with the top of the aggregates. A conductive grease was used to provide

thermal contact between the sensor and the sides of the protruding

aggregates and the binder of the surface. The 9ensors were read throughout

the testing period. The locations of the sensors are shown in Figure

40. The surface temperatures measured are given versus time of day in

Figure 41. The surface temperatures ranged from 78°F (25°C) to 130°F

(54°C) during skid testing. Figures 41 and 42 show examples of the

diurnal variations measured. The peak temperatures roughly correspond

to solar heating, but note that the skid surfaces may appear warmer or

cooler than the base pavement. When the temperature difference between

a skid surface and the base is plotted versus time of day for 14 days

data, the erratic tracking is evident (Fig. 43). Since the radiative

and conductive properties of the pavements are unlikely to change, the

aerodynamic component is suspected.

In Figure 44, temperature differences are grouped according to the

wind direction at the time of each measurement. The line through the

group averages shows correspondence to wind direction, the greatest

positive temperature difference occurring with south winds.

In periods of calm, the three skid surfaces all tend to be hotter

than the base pavement (Fig. 45).

If wind from the south travels the length of the skid surface and

becomes warmer, its cooling effect will diminish, causing the temperature of the skid surface to increase from the south end to the north.

The reverse gradient will result from north wind and, due to the location

of the base thermometer during these tests, heated air from the skid

surface will exaggerate the measured difference. Measured temperature

differences between the ends of the surfaces are shown in Figure 46.

During these tests, north or northwest winds prevailed in the

morning, often shifting toward the south by afternoon (Fig. 47). The

wind direction thus tended to reinforce the sun angle in producing the

daily peak.

Another uncontrolled variable monitored during the tests was tire

wear. During each test an observer noted the test wheel lock-up position.

The IRS crew would then measure and record the tread depth at each

groove across the width of the tire at the lock-up position using the

tire surface temperature as a further guide to position. Analysis of

these data enable the amount of tire wear and the number of tests occurring

at that tire location to be determined. The texture of surfaces 1, 2,

and 3 is shown in Figures 48, 49, and 50 respectively. A distinctive

type of wear is associated with each surface. The surface of tire H,

which skidded exclusively on surface 1 was "blistered" as shown in

Figure 51. The surface of tire I, which skidded exclusively on surface

2, "carved" 0.1 inch (0.2 cm) diameter by 1 to 3 inch (2 to 7 cm) long

strands of rubber from the tire surface. The surface of tire I is shown

in Figure 52. The surface of tire I, which skidded exclusively on

surface 3, was "grated" along the outer ribs and "lightly grated" on the

inner ribs (Fig. 53).

79
SKID SURFACES

Figure AO. Thermometer locations.

NORTH

80
TEMPERATURE,

°C

80r T T T

Figure 41. Temperature versus time of day, April 29, 1975.

(Temperatures of surfaces 1, 2, 3 measured at

north end.)

81
TEMPERATURE

80 T

10

± _L

8 10 12 h 14

STANDARD TIME

16 18

Figure 42. Temperature versus time of day, May 3, 1975.

(Temperatures of surfaces 1, 2, 3 measured at

north end .

)

82
TEMPERATURE

DIFFERENCE

Figure 43. Surface 3 north end temperature minus base temperature

versus time of day, April 23 - May 8, 1975.

I 83
TEMPERATURE

DIFFERENCE

+ 8

+ 6 —= AVERAGE

+ 4

+ 2

0

°G

- 2

• « « •••

/

/

.\

\

V

• \ / ...

• « 9

/

- 4

- 6

0 •# •

9

- 8

NE

N

SE SW

W

NW

N

WIND DIRECTION

Figure 44. Surface 3 north end temperature minus base temperature

versus wind direction, April 23 - May 8, 1975.

Q /.
TEMPERATURE

DIFFERENCE

+8

+6

+4

+2

°C

0

A

OCA

O

O

A o

A

O

-2

A

-6

-8

SURFACE 1

SURFACE 2 A

SURFACE 3 O

8 10 12 h 14 16

STANDARD TIME

18

Figure 45. Surfaces 1, 2, 3 north end temperature minus base

temperature versus time of day, wind calm at time

of measurement. Various days.

85
TEMPERATURE

DIFFERENCE

WIND DIRECTION

Figure 46. Surface 3 north end temperature minus surface 3 south

end temperature versus wind direction, April 23 - April 26, 1975.

86

1
(a)

STANDARD

TIME

7

to

10

h:

(b)

STANDARD

TIME

10

to

IS

SN0UVAU3S80 SN0llVAU3Sa0

87

Figure

47.

Observations

of

wind

direction,

April

23

-

May

8,

1975.
Figure 48. Texture of skid resistance surface number 1.

88
Figure 49. Texture of skid resistance surface number 2

89
Figure 50. Texture of skid resistance surface number 3

90
Figure 51. Texture of tire wear on surface number 1.

91
92
Figure 53. Texture of tire wear on surface number 3.

93
Examples of measurements from which the tire wear on surfaces 1, 2,

and 3 can be estimated are shown in Figures 54, 55, and 56 respectively.

A narrow white strip was painted radially on the test tire side wall at

the beginning of the tire serial number. The locked position of the

tire during a skid test was then related to "time" where the strip

corresponds to the hour hand of a clock. The data illustrated corresponded

to the tire position of 6:30, 12:00, and 8:30 o’clock on surfaces 1, 2,

and 3 respectively. Tread depth measurements were made only when the

test wheel was mounted on the IRS. The locked wheel position was noted

as it skidded on both trailers. Consequently, the reduction in tread

depth between the measurements made on lane 5 and lane 3 of surface 1 occurred in two skids, one made by the IRS and one made by the WARS.

The skids occurred in the top-to-bottom order shown in the legend of the

figures

The length of the contact patch at the tire-pavement interface is

nominally six inches (15 cm) while the measurements are made in a 0.25

in (0.6 cm) transverse band. There was no assurance that the tire

position was in integer multiples of one-half hour increments, that the

measurement band was in the center of the contact patch or that migration

of the rubber did not occur as it was textured. With these qualifications

the figures indicate that tire wear was least on surface 1, the most on

surface 2 and intermediate on surface 3.

3.8 The Correlation Test Plan

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

correlate the degree to which the IRS and WARS measuring system results

match. The results include three sources of dispersion which degrade

the correlation:

1)

different time periods in which the measurements are made,

2) any nonuniformity between test tires, and

3) 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.

Statistical confidence in the results can be improved by increasing

the 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

is enhanced when the data points are found to be within small intervals

of variation.

94
TREAD

DEPTH

READING

.380

in

.320

.300

TIRE H ON SURFACE 1 40 mph

LOCK POSITION = 6:30 o'clock

O © T.P. - 1 LANE 4

q T.P. - 2 LANE 5

WARS T.P. - 3

A A T.P. - 7 LANE 3

.280

1 2 — OUTBOARD INBOARD < TIRE GROOVE

Figure 54. Tire wear on surface 1,

$

9-5

--^ 9.0

- 8.5

mm

- 8.0

- 7.5

-17.0

6

95
TREAD

DEPTH

READING

.360

.340

.320

.300

in

.280

.260

.240

OA

TIRE I ON SURFACE 2 40 mph

LOCK POSITION - 1 2 o'clock

WARS T.P. - 1

O O T.P. - 3 LANE 6

T.P - 6 LANE 2

WARS T.P. - 6

A , T.P. - 7 LANE 7 - 9.0

LAST SKID

-OT.P. - 7 LANE

> \

/

/

/

A

/ \

\‘

w

\

\

\ \

\

\s \

\]

\

N-

- -A

_L

8.5

8.0

mm

7.5

7.0

6.5

6.0

1 2 3 4 5 - OUTBOARD INBOARD

TIRE GROOVE

Figure 55. Tire wear on surface 2.

96
TREAD

DEPTH

READING

OUTBOARD INBOARD

TIRE GROOVE

Figure 56. Tire wear on surface 3.

97
Testing of the IRS with the Central FTC (CFTC) ARS was conducted in

May, 1972. A statistical analysis of the results of these tests provides

a basis for computing the number of tests needed to determine the skid

number within a given confidence level. Statistical inference techniques

are used in the calculations [9].

The results of the calculations for one surface at the CFTC are

given in Figure 57. For example, to report a mean skid number within +1

skid numbers in May, 1972 would have required 185 tests on the surface

by the IRS. This number of tests on the same pavement at the same speed

was not practical. Consequently, the IRS has been modified, instrumentation subsystems added, and the degree of test control improved with the

development of more complete test procedures. In anticipation of improved

precision due to these changes, the test program was designed using the

expected IRS results shown. A larger number of tests were planned for

40 mph (64 km/h) than for 20 and 60 mph (32 and 97 km/h) since this is

the more usual test speed. The test plan contained 210 skid measurement

tests to yield an expected skid number interval of less than four (+2)

skid numbers.

3.9 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.

The results from measurements made at the WFTC during single time

periods are shown in Figure 58 for the WARS and in Figure 59 for the

IRS. The results for multiple time periods are shown in Figure 60 for

the WARS and in Figure 61 for the IRS.

The deviation about the mean calculated for the set of test tires

used on surface 1 from measurements made by the WARS are shown in Figure

62 and from measurements made by the IRS are shown in Figure 63. The

pattern of the deviations about the mean is similar but of opposite

sign. That is, while the measurement made by the WARS using test tire

A-l was below the mean of the measurements made by the WARS, the measurements made by the IRS using test tire A-l was above the mean of the

measurements made by the IRS.

The deviation from the mean calculated for the set of tires used on

surface 2 from measurements made by the WARS are shown in Figure 64 and

from measurements made by the IRS are shown in Figure 65.

The deviation from the mean calculated for the set of tires used on

surface 3 from measurements made by the WARS are shown in Figure 66 and

from measurements made by the IRS are shown in Figure 67.

98
NUMBER

OF

MEASUREMENTS

Figure 57. Confidence interval versus number of measurements.

99
SKID

RESISTANCE.

SN

km/h

V.mph >0

TEST SPEED

Figure 58. WARS measurement results at WFTC - spring, 1975,

single time period measurements.

100
SKID

RESITANCE.

SN

km/h

Figure 59. IRS measurement results at WFTC - spring, 1975,

single time period measurements.

101
100

90

80

70

60

50

40

30

20

10

0

km/h

20 40 60 80 100 —i 1 —i 1 1—

SURFACE 3

SN = 66.8

Mean

L = ± 1.3

SURFACE 2

SN =62.6

Mean

L =t 1.1

SURFACE 1

SN = 48.3

Mean

L = t 3.7

10 20 30 40 50 60 70

V,mph

TEST SPEED

igure 60. WARS measurement results at WFTC - spring, 1975,

multiple time period measurements.

102
SKID

RESISTANCE,

SN

km/h

0 20 40 60 80 100

1 00

90

80

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70

V, mph

TEST SPEED

SURFACE 2

SN -60.1

MEAN

L =t 1 .7

SURFACE 3

SN =60.2

MEAN

L = - 2.4

SURFACE 1

SN =47.3

MEAN

L = ± 3.3

Figure 61. IRS measurement results at WFTC - spring, 1975,

multiple time period measurements.

103
DEVIATION

FROM

THE

MEAN,

SN

SURFACE 1

TEST TIRE

Figure 62. Deviation from the mean skid number measured by the WARS

on surface 1 at the WFTC as a function of test tire.

L04
DEVIATION

FROM

THE

MEAN,

SN

SURFACE 1

TEST SPEED 40 mph (64 km/h)

-4 . 0-1

A-1 B-1 C-1 D-1 E-1 F-1 G-1

TEST TIRE

Figure 63. Deviation from the mean skid number measured by the IRS

on surface 1 at the WFTC as a function of test tire.

105
DEVIATION

FROM

THE

MEAN,

SN

TEST SPEED 20 mph (32 km/h)

SN 20 = 67.9

TEST SPEED 40 mph (64 km/h)

SN 40 = 62.0

TEST SPEED 60 mph (97 km/h)

A-2 B-2 C-2 D-2 F-2 E-2 G-2

TEST TIRE

Figure 64. Deviation from the mean skid number measured by the WARS

on surface 2 at the WFTC as a function of test tire.

106
DEVIATION

FROM

THE

MEAN.

SN

+ 2.0

TEST SPEED 20 mph (32 km/h)

SN .= 65.2

20

0

- 2.0

A-2 B-2

TEST SPEED 40 mph (64 km/h)

SN - 59.4

40

TEST SPEED 60 mph (97 km/h)

SN = 59.2

60

C-2 D-2 E-2 F-2

TEST TIRE

G-2

Figure 65. Deviation from the mean skid number measured by the IRS

§

on surface 2 at the WFTC as a function of test tire.

107
DEVIATION

FROM

THE

MEAN,

SN

TEST SPEED 20 mph (32 km/h)

SN ,0

=73.5

TEST SPEED 40 mph (64 km/h)

TEST SPEED 60 mph ( 97 km/h)

SN = 60.9

60

-4.0 J

A-3 B-3 C-3 D-3 E-3 F-3 G-3

Figure 66. Deviation from the mean skid number measured by the WARS

on surface 3 at the WFTC as a function of test tire.

108
DEVIATION

FROM

THE

MEAN.

SN

+2.0*i

TEST SPEED 20 mph (32 km/h)

SN 20 = 70.1

2.0J

+4.0-

+2.0i

0 -

- 2 . 0

-

-4.0-

-6.6J

+4.01

+2. 0 -

0

-2.0 J

A-3

TEST SPEED 40 mph (64 km/h)

SN = 60.2

40

rEST SPEED 60 mph ( 97 km/h)

SN = 54.4

60

B-3 C-3 D-3 E-3 F-3 G-3

TEST TIRE

Figure 67. Deviation from the mean skid number measured by the IRS

on surface 3 at the WFTC as a function of test tire.

109
The deviation from the me^n calculated for the lanes used on

surface 1 from measurements made by the WARS are shown in Figure 68 and

from measurements made by the IRS are shown in Figure 69. The relative

pattern of deviation is similar for the two systems.

The deviations may be large enough to be outside the random error

band of the test so that the similar patterns are evident. To reinforce

the validity of the pattern of deviations it should be noted that lanes

1 thru 4 were constructed on one pass of the pavement laying equipment

while lanes 5 thru 7 were constructed on a second pass. Thus there may

be a discontinuity in skid resistance between lanes 4 and 5.

The deviation from the mean calculated for the lanes used on surface

2 from measurements made by the WARS are shown in Figure 70 and from

measurements made by the IRS are shown in Figure 71.

The deviation from the mean calculated for the lanes used on surface

3 from measurements made by the WARS are shown in Figure 72 and from

measurements made by the IRS are shown in Figure 73.

The deviation from the mean calculated for the space of time that

tests were conducted on surfaces 1, 2, and 3 from measurements made by

the WARS are shown in Figure 74 and from measurements made by the IRS

are shown in Figure 75. The deviation pattern is similar on surface 3

for both systems. The measurements tended to be higher at the beginning

of the tests and decreased toward the end of the test period.

3.10 Ranking of the Sources of Dispersion

The ranking of these sources of dispersion calculated from the WARS

test matrix results are given in Table 9 while the ranking calculated

from the IRS results are given in Table 10.

The sample standard deviations of skid number, with the calculated

tire and lane dispersions, are listed according to test matrix in Table

11. With a few exceptions, the standard deviation of the calculated

tire and lane dispersion is small compared to the standard deviation of

the skid number measurement result. One exception, which is confirmed

by the measurement of both systems, is the lane dispersion on surface 1.

The pattern of lane dispersion is similar for both systems on surface 1.

The results indicate that the measurements in lanes 1, 5, 6, and 7 gave

higher than average values while the measurements in lanes 2, 3, and 4

yielded lower than average values.

As discussed in section 3 the test program is divided into two sets

of ten test matrices. One set is for the IRS, the other for the WARS.

Generally, two days of testing were planned for a test matrix which

110
DEVIATION

FROM

THE

MEAN,

SN

SURFACE 1

TEST SPEED 40 mph (64 km/h)

1 2 3 4 5 6 7

LANE

Figure 68. Deviation from the mean skid number measured by the WARS

on surface 1 at the WFTC as a function of lane.

Ill
DEVIATION

FROM

THE

MEAN,

SN

I

+ 8 . 0 -»

+ 6 . 0 -

+4. 0 -

SURFACE 1

TEST SPEED 40 mph (64 km/h)

SN =44.9

3 4 5 6 7

LANE

Figure 69. Deviation from the mean skid number measured by the IRS

on surface 1 at the WFTC as a function of lane.

112
DEVIATION

FROM

THE

MEAN,

SN

TEST SPEED 20 mph (32 km/h)

SN 20 = 67.9

TEST SPEED 40 mph (64 km/h)

TEST SPEED 60 mph ( 97 km/h)

SN =62.3

60

Figure 7Q. Deviation from the mean skid number measured by the WARS

on surface 2 at the WFTC as a function of lane.
DEVIATION

FROM

THE

MEAN,

SN

TEST SPEED 20 mph (32 km/h)

SN =59.4

TEST SPEED 40 mph (64km/h)

TEST SPEED 60 mph (97 km/h)

1 2 3 4 5 6 7

LANE

Figure 71. Deviation from the mean skid number measured by the IRS

on surface 2 at the WFTC as a function of lane.

114

DEVIATION

FROM

THE

MEAN,

SN

116
DEVIATION

FROM

THE

MEAN,

SN

+ 4j0i

+ 2. 0 -

- 2. 0 -

4.0-1

+ 4.On

+2. 0

*

-2. 0

"

-4.0-1

25 28 30

APRIL

APRIL

SURFACE 1

TIRE H

TEST SPEED

40 mph (64km/h)

6 8

MAY

SURFACE 2

TIRE I

MAY

SURFACE 3

TIRE J

TEST SPEED 40 mph (64km/h)

RVvm

24 26 29

APRIL

1 2

DATE

5 7

MAY

Figure 74. Deviation from the mean skid number measured by the WARS

at the WFTC as a function of time period.

117
DEVIATION

FROM

THE

MEAN,

SN

+ 2.0

- 2.0

24 26

APRIL

+ 2.0

- 2.0

24 26

APRIL

+4.0

+2.0

- 2.0

4.0

29

29

SURFACE 1

TIRE H

TEST SPEED 40 mph (64km/h)

1

li

2 5 7

MAY SURFACE 2

TIRE I TEST SPEED

40 mph (64 km/h)

5 7

MAY

surface; 3

TIRE J

TEST SPEED 40 mph (64 km/h)

<

it

25 28 30

APRIL DATE

6 8

MAY

Figure 75, Deviation from the mean skid number measured by the IRS

at the WFTC as a function of time period.

lid
Table

9

-

Ranking

of

Possible

Sources

of

Dispersion

in

WARS

Results

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119
Table

10

-

Ranking

of

Possible

Sources

of

Dispersion

in

IRS

Results

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120

1

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=

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km/h.
X*

CD coo 00 00 <r cn

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121
includes a share of one of the three between time period test matrices.

In operation, the tests conducted on the first day were all repeated on

the second day with each equipment changing test assignments on the

second day. Thus, while we may expect differences in measurement between the two systems, we would not expect a difference between the

average measurement made the second day from that of the first. It is

therefore significant that in each of the seven two-day pairs, the

average skid number measured on the second day is lower than the average

skid number measured on the first day. In those instances when a portion

of the second day’s test program was conducted during the first day

also, the second value measured on a given lane at the same speed,

corrected for speed gradient, was lower than the first. When the skid

measurement results are grouped by tire and date there are 37 test

comparisons where the first day's result exceeds the second and 27 test

comparisons where the reverse is true. When the skid measurement results

are grouped by lane and date the number of test comparisons where the

first day's results exceeds the second rises to 66 and there are 24 test

comparisons where the reverse is true. This implies that the reduction

in skid resistance value on the second day of test is associated more with

the lane rather than the tire used. The average skid resistance value

measured on the second day was 2.2 percent below that measured on the

first day. With few exceptions, three skid tests were made per lane on

the first day and three per lane on the second day.

A black film did build up on the fenders of the trailer as the skid

testing progressed. This fact, in combination with the discussion

above, implies the surfaces may have to be "washed" rather than "swept"

to restore the skid resistance of the surfaces.

3.11 Skid Resistance Speed Gradient

From test data generated at largely different test speeds, we know

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

we have shown, analytically and experimentally, 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 consistent with a continuous polynomial expression of

skid resistance as a function of test speed.

The synthesis of a skid resistance expression for surface 2 at WFTC

utilized the results from three test matrices measured by the IRS. The

expression is:

SN (V) = 76.5 - 0.71 V + 0.0070V2 c where V is the test speed in mph (1 mph = 1.609 km/h).

122
Differentiation of this expression results in a skid resistancetest speed gradient which is also a function of test speed.

9 (V) = = -0.71 + 0.014V

c d (.V;

A similar expression for the skid resistance of surface 2 at the

WFTC can be synthesized from the results of three test matrices measured

by the WARS. That expression is:

SN (V) = 79.9 - 0.76V + 0.0077V 2 c Differentiation of this expression results in a skid resistancetest speed gradient which is also a function of test speed.

0 (V) = -0.76 + 0.0154V

c

The skid resistance characteristic of surface 2 as synthesized from

the measurements of the IRS and the WARS are shown in Figure 76. The

results show quite good agreement over the test speed range from 0 to 80

mph (0 to 129 km/h). The curves are practically parallel differing by

3.4 SN at 0 mph (0 km/h) and 4.2 SN at 80 mph (129 km/h).

The skid gradient characteristic of surface 2 as synthesized from

the measurements of the IRS and WARS are shown in Figure 77. The results

show quite good agreement over the test speed range from 0 to 80 mph (0

to 129 km/h)

The synthesis of a skid resistance expression for surface 3 at the

WFTC utilized the results from three test matrices measured by the IRS.

The expression is:

SN (V) = 85.2 -0.87V + 0.0060V 2 c

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

Differentiation of this expression results in a skid resistancetest speed gradient which is also a function of test speed.

0 (V) = TTlPr = -0.87 + 0.012V

c d (V)

An expression for the skid resistance characteristic of surface 3

at the WFTC was synthesized using the results from three test matrices

measured by the WARS. The expression is:

SN (V) = 80.4 -0.36V + 0.0006V 2 c

Differentiation of this expression results in a skid resistancetest speed gradient which is also a function of test speed.

123
km/h

TEST SPEED

Figure 76. Skid resistance characteristic, surface 2 at FHWA, WFTC

124
SKID

GRADIENT,

km/h

20 40 60 80 100 120 140

Figure 77. Skid gradient characteristic, surface 2 at FHWA, WFTC.

125
0 (V) = -0.36 = 0.0012 V

c

The skid resistance characteristic of surface 3 as synthesized from

the measurements of the IRS and the WARS are shown in Figure 78. The

WARS results are more linear than the IRS results. Since the surface is

constructed of the same aggregate as used on surface 3 at the EFTC, the

results of the IRS measurements at the EFTC made in the fall of 1974 [5]

are shown also. The EFTC results tend to confirm the nonlinear characteristic of the IRS results since the curves are practically parallel and

also tend to confirm the higher values measured by the WARS.

The skid gradient characteristic of surface 3 as synthesized from

the measurements of the IRS and the WARS are shown in Figure 79. The

skid gradient results of the WARS differ from those of the IRS. The

skid gradient characteristic of surface 3 at both the WFTC and the EFTC

show good agreement as measured by the IRS.

Consistency of the measurements can be tested by determining the

slope measured with each test matrix and comparing it with the value

calculated from the formula.

For surface 2:

IRS WARS

mph

6 c0

m

0 c0

m

20 -.43 -.53 -.40 -.26

40 -.11 -.03 -.14 -.14

60 + .13 + .15 + .17 +.35

1 mph = 1.609 km/h

Both measuring systems show

gradient, 0 , and the calculated

For surface 3:

consistency between the measured skid

skid gradient,

IRS WARS

mph 0 c

0

m

0 0 c m

20 -.63 -.71 -.34 -.38

40 -.38 -.49 -.31 -.38

60 -.15 + .56 -.29 -.45

1 mph = 1.609 km/h

126
SKID

NUMBER

km/h

0 20 40 60 80 100 120

TEST SPEED

Figure 78. Skid resistance characteristic, surface 3 at FHWA, WFTC

127
km/h

TEST SPEED

Figure 79. Skid gradient characteristic, surface 3 at FHWA, WFTC

128
The measured skid gradient 0 , made by the IRS at 60 mph (97 km/h)

is not consistent with the corresponding calculated skid gradient, 0 . The WARS measured values are all consistent with the calculated values.

3.12 The Correlation Equation

The results of the dynamic skid resistance measurement test program

conducted on the three surfaces at the WFTC are summarized in Table 12.

The linear correlation equation for the seven sets of single time

period results is:

SN

WARS = 4.55+0.99 SN TDC 1Kb

Correlation coefficient

Residual standard deviation

Standard deviation of intercept

Standard deviation of slope

Standard deviation of predicted

value (a point on the line)

0.974

2.01

6.12

0.103

1.6 (SN 45 ) 0.88 (SN 50-60 )

1.4 (SN

yo

)

A plot of the equation and data points is shown in Figure 80. The

length of the crosses correspond to the 99 percent confidence interval,

L.

3.13 Use of the Correlation Equation

To predict a value with 95 percent confidence limits, multiply the

standard deviation of predicted values by 2.571. To predict a value

with 90 percent confidence limits, multiply the standard deviation of

predicted values by 2.015.

The correlation equation data may be interpreted as follows for 90

percent confidence limits:

"In repeated calibration of WARS using the same general

procedure, the results are not expected to differ from the

present calibration by more than 3.2 SN at the low end of the

calibration line (SN^,.) , by more than 1.8 SN in the middle

range ( SN c _ , A ) and by more than 2.8 SN at the high end

(sn

70

)." 5 °-60

In using the calibration line, presumably WFTC personnel would like

to predict IRS values from their own measurements, using

129
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130
WARS

SURFACE 1

SPEED 40 mph (64 km/h)

SURFACE 2

SURFACE S

SPEED 20 mph

(32 km/h)

SPEED 60 mph (97 km/h)

80 *

70 •

60

SURFACE 3

SPEED 60 mph (97 ) km/h)

50

SIM

40

30

20

10

WARS

MEASURED VALUE

xiizizr

SURfACEJ

SPEED 40 mph

(64 km/h)

WARS MEASUREMENT

UNCERTAINTY

UNCERTAINTY

OF PREDICTION^

RANGE OF UNCERTAINTY

OF PREDICTED VALUE

i

i i

SURFACE 2

SPEED 20 mph

(32 km/h)

SURFACE 2

SPEED 40 mph

(64 km/h)

10 20 To

SN

IRS

50 60 70 80

Figure 80. WARS-IRS correlation, single time period results.

131
SN

IRS

-1.34 + 0.969 SN

WARS

The uncertainty for 90 percent

value for a given value of WARS is

SN Number

45

60

70

confidence limits of a predicted IRS

Uncertainty

3.4

1.7

2.6

computed as before. In addition, the random error component of WARS

must also be taken into consideration. A graphical procedure is perhaps

the easiest way to estimate the total uncertainty. The example shown in

Figure 80 is drawn for a 90 percent confidence band.

The uncertainty given for the calibration line itself is in a sense

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

comparing the SN of two different surfaces, this systematic uncertainty

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

then the uncertainty of the two calibration lines will have to be taken

into consideration.

4. THE IRS GROUND STATION

4.1 Introduction

The skid resistance measurement process includes the conversion of

the measurement instrumentation outputs to skid number. Measurement

precision may be degraded due to differences in judgement between data

analysts. Further, the precision of repeated measurements depends upon

the identification of the specific section of surface that corresponds

to the skid number result.

To further improve the skid resistance measurement process in this

regard the IRS was modified to include a ground station (Fig. 81). The

ground station was partially evaluated during the dynamic skid testing

at the WFTC. In this section, the features of the ground station and

the results of the evaluation tests are given.

4.2 Features of the Ground Station

There are three outputs from the ground station:

1) a magnetic tape record of all the test data

scaled in engineering units, preceded by

a test description.

132
START SWITCL

/

J

GROUND STATION

a) BLOCK DIAGRAM

b) SET-UP WITH A VAN AT THE WFTC

Figure 81. IRS ground station.

133
2 ) an automatic plot of the te.st variables as

a function of longitudinaL location on the

surface, and

3) a statistical analysis of Lhe SN results

between whichever longitudinal distance

limits are selected.

When test data are recalled from the magnetic tape, the ground

station automatic printout includes:

a) file number.

b) date of test.

c) time of day.

d) surface number. lane number, nominal test speed

e) skid trajectory start lane and finish lane, and

f) static test wheel load.

The on-board analog strip chart recorder plots the test variables

versus time. Traction, vertical load and water flow rate are simultaneously recorded in the digital on-board memory. The memory is unloaded

through the umbilical cable to the ground station. The automatic plot

from the ground station records the test variables versus distance 1 rom

a start switch (Fig. 82). The start switch is segmented such that the

starting and finishing transverse location is known in one foot (0.3 m)

increments

As soon as the start tape switch is energized, the ground station

integrates the radar speed data with respect to time to obtain the

longitudinal distance coordinate of the test variable outputs.

Test speed is plotted to a resolution of 0.1 mph (0.2 km/h). Water

flow rate is plotted to a resolution of 0.1 gpm (0.4 I /min). Traction

and vertical load are plotted to a resolution of 0.5 lbf (2.2 N)

The combination of lateral lane position and test parameter variations plotted as a function of longitudinal position from a known start

point locates the section of surface that corresponds to the skid number

result

The data analyst is required to judge the mean value of both the

traction recording and the vertical load recording over a one second

interval. Previous experiments by NBS personnel, using data which had

also been reduced at the CFTC, indicated that differences between data

analysts can result in five percent differences in skid numbers measured.

134
Figure

82.

Ground

station

plot.
Skid number, as calculated by an analyst, is then taken as the quotient

of the two mean values. The skid number values plotted by the ground

station are the instantaneous quotients of the traction and vertical

load. The mean value calculated by the ground station is the mean of

the quotients.

Further, to comply with the ASTM method, the data analyst must use

judgement to reference the one second interval to the start of steady

state friction. From Figure 83, it can be shown that steady state

friction does not generally start after a fixed delay in time from the

start of brake application, but rather, requires more time delay at the

higher test speeds.

There are three results printed automatically when the ground

station statistical analysis feature is used. These are

1) the mean value of SN between

two longitudinal limits,

2) the standard deviation of SN with

respect to this mean value, and

3) the best, in a least squares sense,

second degree polynomial fitted to

the data between the limits.

By using this feature repeatedly, while moving the same interval length

through the data. Table 13 and Figure 84 can be constructed. The location where the standard deviation about the mean is a minimum can be

found. Using this as a mathematical definition of steady-state, the one

second data interval can be located without the use of data analyst

judgement. The corresponding second degree polynomial is given as a function of longitudinal position along the surface in Figure 85.

4.3 Evaluation of the Ground Station

The evaluation of the ground station was based upon a comparison of

ground station results with other IRS measurements.

The data analyst established a skid number value of 59 for this

test. The mean value determined by the ground station using the minimum

standard deviation technique is 58.6.

The radar was tracking the vehicle during the test. The resulting

differential radar record is shown in Figure 86. The test wheel "lock"

(L) and "unlock" (U) events were marked manually after being seen

visually. The ground station plot mimics the radar speed information

perfectly starting at the 400 foot position. The "lock" and "unlock"

event marks are also properly synchronized with the traction force

trace. The radar indicates that the test speed was constant at 40.2 mph

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DISTANCE

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SURFACE

2
TEST

SPEED

Figure 86. Differential radar system plot of test speed.

140
Table 13 Skid Number Values Computed by the Ground Station

DATE* 5 / 5 / 73

TIME* 1405

SURFACE: 2740

INTEGRATED DISTANCE = 643

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(64.7 km/h) for approximately one second prior to wheel lock. At wheel

lock the test speed was 40.0 mph (64.4 km/h), declined to 39.7 mph (63.9

km/h), recovered to 39.8 mph (64.0 km/h), and declined again to

39.4 mph (63.4 km/h) at wheel unlock after which the speed increased

rapidly through 40 mph (64 km/h). The on-board fifth wheel record is

shown in Figure 87. It indicates that the speed was 40.4 mph (65.0

km/h) just prior to lock-up of the test wheel. The speed declined to

39.7 mph (63.9 km/h) recovered to 39.9 mph (64.2 km/h), and declined

again to 39.5 mph (63.6 km/h) just after wheel unlock after which the

speed increased rapidly through 40 mph (64 km/h) . (The rapid increase

is not reproduced here.) Both the radar and fifth wheel instrumentation

have a digital uncertainty of 0.1 mph (0.2 km/h). A comparison of the

records indicates the fifth wheel instrumentation may be higher than the

radar by approximately 0.1 mph (0.2 km/h).

Both the ground station plot and the on-board instrumentation indicate that the flow rate was 29.1 gpm (107.7 1/min) when the speed was

39.8 mph (64.0 km/h). The desired flow rate at 39.8 mph (64.0 km/h) is

27.9 gpm (103.4 1/min). The flow rate may have been high by 3 percent.

Since the traces are reasonably parallel, the flow rate was proportional

to test speed.

There is also good agreement between time as measured on the onboard recorder, test speed, and distance as computed by the ground

station. Since flow rate is recorded versus time and distance it is

used for the comparison. A 1.2 second interval on the time scale

corresponds to 70 feet (21 m) on the distance scale. This is nominally

correct at 39.7 mph (63.9 km/h), approximately the average test speed

during the interval.

The test wheel brake application occurred at 410 feet (125 m)

beyond the start tape, which is 5 feet (1.5 m) after the beginning of

the test surface (see Fig. 83). This is closer to the surface beginning

than desired but the average speed during the approach was low. The

test tire "shudders" during the free rolling to lock-up transition. The

traction force at the tire-pavement interface generates a moment with

respect to the hitch which unloads the vertical force causing the

trailer to bounce vertically after lock-up.

To further evaluate the ground station, a test was simulated using

known inputs. The ground station radar signal input was set to 40.0

mph(64.4 km/h). The flow rate differential was set to +0.2 gpm (0.7

1/min) above the set point of 28.8 gpm (106 1/min). The dynamic calibrator was then cycled twice during the automatic test time interval. Each

cycle generated a sequence of signals on the traction and vertical load

channels which was equivalent to known values of force. The test results

were then stored on magnetic tape by the ground station and plotted at a later date. The results are plotted in Figure 88. The calculated skid

number segments appear correct within the resolution of the scales.

Second degree polynomials were fitted to four intervals of skid number.

The fitted results are shown superimposed on the data intervals.

142
LOAD

TRACTION

FLOW

RATE

SPEED

Ibf

Sbf

gpm

mph

+ 100 -n 0 -

- 200 -

T]T d

j

|

' I.-J.4 . «-'l : i ll : . Uj

!

1 sec

Figure 87. Plot on-board tow vehicle.

143
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144

Figure

88.

Ground

station

response

to

known

inputs.
In each test sequence the test tires are changed after three skid

tests. There is a time delay between each set of three while the new

tire is mounted, subjected to a five-mile (8 km) warm-up, etc. The plan

is to record two of the test results on tape and to plot the three

results during the time delay. In this manner, a test can be conducted

every ten minutes which is compatible with the drying time of the surfaces.

The ground station performance has met the design objectives.

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:

1. A measured distance for adjusting the fifth wheel system

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.

2. The WARS speed measuring subsystem should be modified so that

both the driver's aid readout and the recorded speed signal

are conditioned through a single set of calibration adjustments.

3. The DDG and its method of use should be thoroughly studied and

modified as necessary to improve its reliability.

4. A photographic method should be developed for determining the

width of the water trace at the point that the tire enters it.

5. The change in transducer readout which occurs when the brake

system is energized should be eliminated.

6. The height of the WARS flow nozzle should be reduced in compliance with ASTM E274.

7. The test tire inflation pressure should be adjusted to 24 psi

(165 kPa) after the 5 mile (8 km) conditioning drive and

immediately before the skid resistance measurement test.

8. The weighing on the force plate of the flow tank versus time

should be considered by WFTC personnel as a means of measuring

the average flow rate.

9. The IRS skid resistance measurement results obtained by use of

the ground station should be compared to the results obtained

from a data analyst. The comparison should be made for a

large number of tests during the time restraints of a correlation program.

145
The WARS evaluations and correlation results reported here are

valid only if no modifications are made, if no changes occur in mechanical and electrical components, and if the system is operated in the

same manner as it was at the time of these tests. Appendix A describes

the WARS and contains a listirg of the values of recognized WARS variables in effect during these tests.

It is a pleasure to acknowledge the helpful discussions and assistance of Dr. Hsien H. Ku concerning the statistical analysis of the test

results, of Mr. W. H. Appletor and Mr. R. G. Russell in the conduct of

the tests, of other members of the NBS staff, and of personnel of the

FHWA Western, Central and Eastern Field Test Centers and the Federal

Highway Administration for their cooperative efforts

146
REFERENCES

[1]

"Standard Method of Test for Skid Resistance of Paved Surfaces

Using a Full Scale Tire," Designation E274-70, 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, Vol. 37, No. 3, pp 77-80

(December 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 1969).

[4] Kearns, R. W. and Ward, J. F.,

an Air-Bearing Force Plate for

Test Trailer," National Bureau

(December 1974)

[5] Kearns, R. W. and Ward, J. F.

,

at the FHWA Eastern Field Test

ards (U.S.), NBSIR 75-972.

"A Method and Means of Calibrating

Use with a Towed Pavement Friction

of Standards (U.S.), NBSIR 75-738

"Skid Resistance Measurement Tests

Center," National Bureau of Stand-

[6] Chestnut, H. and Mayer, R. W. , Solution of Linear Differential

Equations, Chapter 3, Servomechanisms and Regulation System Design,

Vol. 1, p 62 (John Wiley & Sons, Inc., N.Y. 1951).

[7] Garfinkel, Mann and Youden, "Design and Statistical Procedures for

The Evaluation of an Automatic Gamma-Ray Point-Source Calibrator,"

National Bureau of Standards (U.S.), Spec. Publ. 300, Vol. 1, 436

pages (February 1969)

[8] Mortimer, T. P. and Ludema, K. C. (University of Michigan), "The

Effects of Salts on Road Drying Rates, Tire Friction and Invisible

Wetness," Pavement Friction Characteristics and Water Depth, Paper

396, Highway Research Record, (1972).

[9] "Locked-Wheel Pavement Skid Trailer Correlation and Calibration

Techniques," Final Report NCHRP project 1-12(2). February, 1973.

(Pennsylvania State University Transportation and Traffic Safety

Center)

147
APPENDIX A

General Description of the Western Area Reference System

(Tow Vehicle and Trailer)

1. System Identification

1.1 System manufactured by: Ford Motor Company

1.2 Model: Arizona Proving Ground Torque Trailer System

1.3 Tow Vehicle ID Number: 000T024

1.4 Trailer ID Number: 13

2. Tow Vehicle

2.1 Make: Ford

2.2 Model year: 1974

2.3 Body style: F350

2.4 Engine: 460 with dual exhaust

2.5 Fuel tank capacity: main tank removed, auxilary 22 gallons

2.6 Seating: driver's seat only

2.7 Normal occupancy: driver only

2.8 Air conditioning: yes

2.9 Wheel base: 140.6 inches

2.10 Track: front 66.5 inches, rear 63.5 inches

2.11 Tire size: front 9.50-16.5, rear 9.50-16.5 load range E

2.12 Rear wheels: single

3. Trailer

3.1 Construction: welded steel chassis and box with streamlined

fiberglass body

3.2 Length: center of hitch to center of axle, 105.2 inches

3.3 Track: 66 inches (E 501 tires)

3.4 Suspension: leaf springs with two air spring shock absorbers

at each wheel. Air springs operated at 0 psi.

3.5 Test brakes: non-retracting disk type, air-actuated hydraulic

3.6 Skid measurement transducer: left wheel, strain gaged

torque tube between axle and brake backing plate

4. Pavement Wetting

4.1 Water tank capacity: 235 gallons

4.2 Pump: one gear pump, powered by drive shaft

4.3 Flow rate control: roughly proportional to speed. Coarse

control by drive pulley. Fine control by means of trim

valves adjusted for 20, 40, 60 mph

4.4 Nozzle: Pennsylvania State Type, mounting adjustable for

pitch angle and height.

148
5. Instrumentation

5.1 Speed: analog fifth wheel with nullmeter driver aid readout,

full scale data recording readout.

5.2 Inflation: common tire gage referenced to user calibrated

gage.

5.3 Data recorder: Brush, 11 2111 28 galvanometers, filtered

inputs, strip chart, two channels, light pens.

5.4 Signal amplifier, torque transducer: Daytronic model 878A.

5.5 Channel assignments: torque, speed. Comments: chart paper

is 6 in wide with 100 percent channel overlap. Grid

lines are recorded while paper is in motion. Timing

marks are recorded at 1/10 second intervals.

5.6 Transducer excitation: 3 kHz carrier, modulated and

demodulated internally in the signal amplifier.

6. Test Sequence

6.1 Sequence of events: See figure A1

7. Crew

7.1 Dr iver/Operator : Eugene L. Kaylor

7.2 For further information, contact:

John L. King

Principal Investigator

Field Test and Evaluation Center for Western States

Post Office Box 1031

Kingman, Arizona 86401

149
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APPENDIX B

Operating Parameters of the Western Area Reference System

The information below represents the values of recognized WARS

variables in effect during this comparison with the IRS.

1. Test tires: standard pavement test tires, G 78-15, ASTM designation E 501.

2. Test tire inflation: 24 psi, maintained during testing.

3. Hitch height: 14 3/8 inches from level pavement to bottom of

trailer hitch flange, maintained during testing.

4. Test side of trailer: left only.

5. Calibration signal: left wheel, 865 lbf (933 ft-lbf at 12 15/16

inches wheel radius)

6. Static vertical loads: left trailer wheel, 1087 lbf; right trailer

wheel, 1083 lbf; hitch, 178 lbf.

7. Trailer suspension:

Tire radial spring rate = 1280 lbf per inch.

Effective damping factor (after modification):

C body-to-axle = 0.65,

C body-to-ground = 0.42,

C force plate = 0.35.

Suspension system:

damped low natural frequency =3.7 Hz,

damped high natural frequency = 6-8 Hz.

Vertical frequency response on tires without suspension relative

movement = 4 Hz.

8. Consumable cargo: pavement wetting tank refilled frequently,

nominally after three runs.

9. Condition of test pavements: dry before each test.

10. Flow rates: 20 mph, 14.6 gpm; 40 mph, 29.2 gpm; 60 mph, 43.9 gpm.

151
APPENDIX C

The Test Matrix

A subdivision of the test plan consists of the following test matrix

D

3 4

(LANES)

4 5

Time Period

Surface

E

5

6

1

6

7

2

G

7

1

3

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

where:

(SN + e) LT(S-AS)

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.

152
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)

2 ( SN

)

2A(S)

2 (SN - e)

1A(S - AS)

- (SN + e)

4A(S + AS)

(SN >1G(S)

- (SN - e) lE(S + AS)

aage

+ 3e (1)

2B(S - AS)

(SN “ £ ^2F(S+ AS)

aabf (2)

(SN) 4C(S)

- (SN + £) 4D(S - AS)

aacd

" 3e (3)

where A^^ = twice the difference between using tire A and the average

of using tires G and E in lane 1,

^ABF

= tw* ce t *ie difference between using tire A and the average

of using tires B and F in lane 2,

twice the difference between usii

of using tires C and D in lane 4,

^ACD

= tw:*- ce t ^ie difference between using tire A and the average

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)

p

- (SN) G = ^

We may add to this equation [7]

,

the equation

(SN) A

- (SN) A

= 0

( 4 )

(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

)

a

- <SNA + SN

B + SN

C + SNd + SNg + SNy + SNg

i E ( 6 )

153
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 example, the measurement performance on lane 1 compared with the performance

on all the other lanes. The results are then a measure of surface

uniformity.

U3C0MM-NBS-DC 154
From the test matrix, tire A was used in lanes 1, 2, and A 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 A 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)

2 ( SN )

2A(S)

2 (SN - e)

1A(S - AS)

- (SN + e)

AA(S + AS)

(SN >1G(S)

- (SN - e) lE(S + AS)

aage

+ 3e (1)

2B(S - AS) (SN " e) 2F(S+ AS)

aabf (2)

(SN) AC(S)(SN + e) AD(S - AS)

aacd

" 3e (3)

where A

AGE

A

ABF

ACD

twice the difference between using tire A and the average

of using tires G and E in lane 1,

twice the difference between using tire A and the average

of using tires B and F in lane 2,

twice the difference between using tire A and the average

of using tires C and D in lane A.

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)

fi - (SN)

C

- (SN) d

- (SN)

E - (SN)

p

- (SN) G = ^7 (A)

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)

a

- SN. + SN,, + SN + SN + SN„ + SN^ + SN, A B C D E F C t E ( 6 )

153
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 example, the measurement performance on lane 1 compared with the performance

on all the other lanes. The results are then a measure of surface

uniformity.

U3C0MM-NBS-DC 154
N BS-1 1 4 A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR RKPORT NO.

NBSIR 76-1133

2. Gov’t Accession

No.

4. TITLE AND SUBTITLE

SKID RESISTANCE MEASUREMENT TESTS OF FHWA REFERENCE SYSTEMS AT

THE WESTERN FIELD TEST CENTER

7. AUTHOR(S)

R. W. Kearns and J. F. Ward

3. Recipient’s Accession No.

5. Publication Date

August 1976

6 . Performing Organization Code

NBSIR 76-1133

8 . Performing Organ. Repoit No.

I

9. PERFORMING ORGAN I/.ATION 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. 1-1-1261

12. Sponsoring Organization Name and < omplete Address (Street, City, State, ZIP)

Federal Highway Administration

Office of Development

Washington, D. C. 20590

13. Type of Report & Period

Covered Final

March-April, 1975

14. -Sponsoring Agency C ode

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual 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. Inter

comparisons between these reference systems and the highway measuring systems at the

state level, provide measurement assurance.

In this report, the first correlation results between the IRS and the Western ARS

(WARS) are given: SN = 4.55 + 0.99 SN where the computed standard deviation of

WAKo IRb

a mid-range predicted value is 1 SN. SN is described as a function of test speed, V,

in mph (1 mph = 1.609 km/h): for surface 2; SN(V) = 76.5 - 0.71V + 0.0070V2 and the

speed gradient is d(SN) = -0.71 + 0.014 V.

The test program^is explained from test preparations, tuning and calibration of

subsystems, thru dynamic measurement of the surfaces. Controlled and uncontrolled

variables are identified, discussed, and in some cases, experimentally verified. A

ranking of the sources of dispersion is given. A discontinuity in SN of surface 1 is

identified due to the unique test plan. A ground station for improved SN calculation

precision and on-site statistical analysis is found to meet design objectives.

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; tire-pavement

interface forces; wet pavement.

18. AVAILABILITY [XU Unlimited

[3Z] F° r Official Distribution. Do Not Release to NTIS

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

165

Order From Sup. of Doc., U.S. Government Printing Office

Washington. D.C. 20402. SD Cat. No. 03

20. SECURITY CLASS

(THIS PAGE)

22. Price

D3 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151 UNCLASSIFIED $ 6.75

USCOMM-DC 29042-P74