NBSIR 75-972

Skid Resistance Measurement

Tests at the FHWA Eastern

Field Test Center

R. W. Kearns and J. F. Ward

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

August - October 1974

Final

Issued July, 1976

Prepared for Federal Highway Administration

Office of Development

Washington, D. C.

NBSIR 75-972

SKID RESISTANCE MEASUREMENT

TESTS AT THE FHWA EASTERN

FIELD TEST CENTER

R. W. Kearns and J. F. Ward

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

August - October 1 974

Final

Issued July, 1976

Prepared for Federal Highway Administration

Office of Development

Washington, D. C.

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 t
CONTENTS

Page

1 . INTRODUCTION 1

2. PREPARATION, MEASUREMENT AND ADJUSTMENT OF SUBSYSTEMS 2 2.1 Test Preparations 2 2.1.1 Auxiliary Test Equipment 3 2.1.2 Test Tires 3 2.2 Skid Trailer Weight Distribution 5 2.3 Tow Vehicle Displacements 5

2.4 Speed Measuring Subsystem 5

2.5 Pavement Wetting Subsystem 10

2.5.1 Flow Rate Measurement 10

2.5.2 Water Distribution - Static Measurements 10

2.5.3 Water Distribution - Dynamic Measurements 15

2.5.4 Wetted Width Measurement 22

2.6 Force Measuring Subsystem 24

2.6.1 Transducer Excitation Stability 29

2.6.2 Orientation of the Transducer 29

2.6.3 Vertical Load Channel Calibration 29

2.6.4 Traction Channel Calibration 31

2.6.5 On-Board Calibration System Equivalent Values 31

2.7 Suspension Subsystems 31

2.7.1 Tow Vehicle and Trailer Displacements under 33.

Static Traction Load

2.7.1.1 Displacement of the Test Wheel Hub 33

2.7.1.2 Displacement of the Trailer Body 35

through Roll Axis Rotation

2.7.1.3 Displacement of the Trailer Body 35

through Pitch Axis Rotation

2.7.2 Trailer Displacements under Transient Loading 35

2.7.2.1 Effective Coulomb Friction of the 35

Trailer Suspension

2.7.2.2 Effective Spring Rate of the 39

Trailer Suspension

2.7.2.3 Effective Natural Frequencies of the 39

Trailer Suspension

2.7.2.4 Effective Viscous Damping of the 39

Trailer Suspension

i
Page

3. THE DYNAMIC SKID RESISTANCE MEASUREMENT OF SELECTED SURFACES 42

AND EARS- IRS CORRELATION

3.1 Surfaces Selected 42

3.2 Tests Planned 42

3.3 The Test Matrix 45

3.4 Tests Accomplished 54

3.5 Control of Test Variables 54

3.6 Measurement of Uncontrolled Test Variables 56

3.7 The Correlation Test Plan 58

3.8 Test Matrix Results 58

3.9 Skid Resistance Speed Gradient 70

3.10 The Correlation Equation 73

3.11 Use of the Correlation Equation 73

4. RECOMMENDATIONS 76

5. REFERENCES 78

Appendix A. Exploratory Tests of the Dynamic Distribution Gage 79

Appendix B. The Test Matrix 84

Appendix C. General Description of the Eastern Area 87

Reference System

ii
TABLES

1. EARS Flow Rates.

2. DDG Data: IRS, 40 mph (64 km/h)

3. DDG Data: Comparison of Water Collected with Water Pumped.

4. Wetted Width Measurements.

5. Tests Planned and Accomplished.

6. Test Flow Sequence.

7. Weather during Test Plan Time Periods.

8. Ranking of Possible Sources of Dispersion.

9. Correlation Summary.

A-l DDG Data: Comparison of Water Collected with Water

Pumped, Dry Basin.

iii
ILLUSTRATIONS

1. Test tire radial spring rate test setup.

2. Change in tow vehicle hitch height with load.

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

4. Radar test of EARS speed system.

5. Simultaneous speed data recorded by the stationary radar

and the EARS tow vehicle in motion (fifth wheel).

6. EARS pavement wetting based on 7.5 in (19.1 cm) wetted

width compared with ASTM E274-70 water layer tolerance.

7. IRS flow channel, showing water flow into SDG basin.

8. SDG sight tube array.

9. Static water distribution at 20 mph (32 km/h).

10. Static water distribution at 40 mph (64 km/h).

11. Static water distribution at 60 mph (97 km/h).

12. EARS pavement wetting system test using the DDG

13. Dynamic distribution. Depth scale based on slot dimensions.

14. Measurement of water trace width after the trailer

has passed.

15. Wetted width by delayed measurement on pavement.

16. Arrangement for photography of wetted width.

17. Application of a simulated traction force to a locked

test wheel through an air-bearing force plate.

18. EARS wheel transducer linearity - vertical load

channel after trailer modifications.

19. EARS wheel transducer linearity - traction channel.

20. Displacement of the test wheel hub due to static traction.

21. Roll displacement of the trailer body due to static

traction.

22. Pitch displacement of the trailer body due to static

traction.

23. Coulomb damping test response.

24. Axle drop test response.

25. Effective viscous damping envelope.

26. Effect of different damping factors on second-order

linear systems.

iv
27. Location of selected surfaces and pylons at EFTG.

28. Surface temperatures during dynamic testing.

29. Texture of skid resistance surface number 1.

30. Texture of skid resistance surface number 2.

31. Texture of skid resistance surface number 3.

32. Texture of tire wear on surface number 1.

3 3. Texture of tire wear on surface number 2.

34. Texture of tire wear on surface number 3.

35. IRS results from measurements made at EFTC during

single time periods.

36. EARS results from measurements made at EFTC during

single time periods.

37. IRS results from measurements made at EFTC during

multiple time periods.

38. EARS results from measurements made at EFTC during

multiple time periods.

39. Possible tire dispersion calculated from IRS measurements

on surface 3 at the EFTC.

40. Possible tire dispersion calculated from IRS measurements

on surfaces 1 and 2 at the EFTC.

41. Possible lane dispersion calculated from IRS measurements

on surface 3 at the EFTC.

42. Possible lane dispersion calculated from IRS measurements

on surfaces 1 and 2 at the EFTC.

43. Possible time period dispersion calculated from IRS

measurements on surfaces 1, 2, and 3 at the EFTC.

44. EARS-IRS correlation.

A-l Dynamic distribution (dry basin).

A-2 IRS operating on the DDG.

A-3 IRS pavement wetting on solid pavement ahead of the

DDG grate.

A-4 Severe splashing over the DDG.

C-l EARS test cycle sequence of events.

Y

SKID RESISTANCE MEASUREMENT TESTS

AT THE

FHWA EASTERN FIELD TEST CENTER

August and October, 1974

ABSTRACT

The measurement of the skid resistance of highways, under wet weather conditions, is part of the

Federal Highway Administration (FHWA) skid accident

reduction program. To standardize and to improve the

precision of the measurements, the program includes

national and regional reference systems to which the

highway measuring systems may be systematically re- lated. This report describes evaluation tests con- ducted at the FHWA Eastern Field Test Center (EFTC)

which included the use of the Eastern Area Reference

System (EARS) and the Interim Reference System (IRS)

maintained by the National Bureau of Standards. The

performance characteristics of the EARS, its subsystems and a correlation equation between the EARS

and the IRS are presented. A method of quantifying

sources of dispersion is applied to the measurements

made by the IRS on the EFTC reference surfaces. Recommendations for modifications to the EARS, the test

procedures, and EFTC facilities are made.

Key Words: Accident reduction, skidding; correlation,

skid resistance; highway safety; 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 lockedwheel skid measurement systems, which are reported to be in substantial

compliance with the requirements of ASTM Designation E274-70 [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 hierarchial

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

this report.
system for the measurement of pavement skid resistance [2]. At the

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

the regional level, the program provides for area reference systems

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

skid resistance measuring systems are maintained to inventory and rank

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

The plan to assure measurement consistancy between the states includes

the monitoring and development of test procedures and equipment by NBS

to relate the measurements made within the 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 equipment are expected as the program develops with the increased measurement capabilities being transferred to the highway measurement systems at the FTC.

This report describes tests conducted under the direction of NBS

personnel at the FHWA Eastern FTC (EFTC) during periods in August and

October, 1974.

2. PREPARATION, MEASUREMENT AND ADJUSTMENT OF SUBSYSTEMS

Previous results from skid trailer correlation test programs indicated the measurements lacked precision, agreement among trailers using selfwatering 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 peformance of the total EARS

system, adjustments and modifications were made or recommended.

2.1 Test Preparations

Auxiliary test equipment and facilities of the EFTC were used to

prepare the EARS for test. Test tires were prepared for test.

2
2.1.1 Auxiliary Test Equipment

Skid resistance is reported in terms of skid number 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 tirepavement interface are the principal quantities to be measured. To re- late 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 Ibf (1.1 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 transducer of the trailers (Section 2.6).

A turbine flow rate meter was calibrated, to a 0.06 gpm (0.23 liters/

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 liters/min) resolution. The system is part

of the on-board IRS instrumentation, but is adaptable to measuring the

flow rate in the ARS (Section 2.5).

A differential radar speed meter, whose time base was calibrated

against the standard frequency and whose radiated frequency is specified

to be well within its 0.1 mph (0.16 km/h) resolution, is used to record

the speed of the system during a skid resistance measurement (Section 2.4).

Other auxiliary equipment includes a voltmeter, calibrated to one millivolt uncertainty by the Electrical Instruments Section; an inflation

pressure gage, calibrated to 0.05 psi (345 Pa) uncertainty by the Pressure

and Vacuum Section; and an automatic sequential displacement recording

system (Section 2.7).

2.1.2 Test Tires

In preparation for the testing at the EFTC, each of 20 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 spring rate value was 1280 lbf/in (224 kN/m) at

24 psi (165 IPa) 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 shown in Figure 1.

3
Figure 1. Test tire radial spring rate test setup.

4
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 measured weight distribution

for the EARS was:

Driver's side test wheel load = 1099 lbf (4888 N)

Passenger's side wheel load = 1084 lbf (4821 N)

Hitch vertical load = 167 lbf (743 N)

Load adjustments to bring the trailer within the desired range of

1080 to 1090 lbf (4800 to 4850 N) were to be made by EFTC personnel during

the planned time period between the subsystem testing and the dynamic skid

resistance measurement testing.

2.3 Tow Vehicle Displacements

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

measured as a function of operator and passenger 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 ro- tated such that a component of the vertical load may be measured as traction (Section 2.6). Changes in tow vehicle rear wheel radius change the

water pump rate. The results, shown in Figure 2, show that hitch height

displacements are much more sensitive to water load than passenger load

in this case. The IRS is less sensitive to water load changes than the

EARS. Figure 3 indicates the wheel radius changes nearly one percent

over the range of a full water load even though dual wheels are used on

the EARS.

2.4 Speed Measuring Subsystem

The EARS uses separate speed measuring systems for driver aid purposes and for test recording purposes. The driver aid signal was derived

from a fifth wheel pulse transducer system on the tow vehicle while the

recorded signal was generated by a tachometer generator on the right wheel

of the trailer. To avoid confusion and possible error from use of this

dual system, a commercial digital-to-analog (D/A) converter was installed

so that the recorded speed signal was also derived from the fifth wheel.

This recorded signal had a one mile per hour step resolution.

After the above modification, the EARS system (fifth wheel) was adjusted on the measured mile [5] and then checked against the IRS radar

system (Fig. 4). The comparative results of a test at about 40 mph

(64 km/h) are shown in Figure 5. These tests showed the differences

between the radar and EARS to be less than 0.5 mph (0.8 km/h) at speeds

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

5
a ) E A R S TOW VEHICLE:

200

kg

400 600 800

= W ATER LOAD CHANGE*

T 7.6

=150 lb ( 68 kg PASSENGER WEIGHT ADDED TO:

LF= L EFT FRONT SEAT

R F = R I G HT FRONT SEAT

LR = L E FT REAR SEAT

RR=RIGHT REAR SEAT

LF

5.1

2.5

400

b] IRS TOW VEHICLE:

200

800 1200

Ibf

LOAD REMOVED

kg

400 600

1600 2000

800

7.6

. = WATElf LOAD CHANGE

B =150 lb ( 68 kg ) PASSFNGER WEIGHT ADDED TO:

LF= LEFT FRONT SEAT

RF=RIGHT FRONT SEAT

LR= LEFT REAR SEAT

2.5

0

0 400

Ibf LOAD REMOVED

1200 1600 2000

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

0.15

EARS TOW VEHICLE:

€9

0.10 h

c-o

0.05 -

- 0.25

0.00 l&

0

0.0 0 400 800 | b f 1200

LOAD REMOVED

1600 2000

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

7
Figure 4. Radar test of EARS speed system.

8
RADAR RECORD

mph 40 - 64. 4 km/h

TIME

ON-BOARD RECORD

TIME

Figure 5. Simultaneous speed data recorded by the stationary radar

and the EARS tow vehicle in motion (fifth wheel)

9
After these tests, but before the dynamic correlation tests, EFTC

personnel reported a malfunction in the D/A converter and temporarily

reverted to the tachometer generator system. This system has a resolution of 2 mph (3.2 km/h) (0.88 mm/division) and the tachometer accuracy

was not determined. Speed records obtained with the IRS radar system

were used for the data reduction for this report.

2.5 Pavement Wetting Subsystem

The pavement wetting system was tested for water flow rate, uniformity of water distribution, and width of the water trace at the tirepavement interface. The dynamic water distribution measurements obtained

are considered to be unreliable for reasons to be mentioned later.

2.5.1 Flow Rate Measurement

By prearrangement , the EARS water distribution system had been modified to allow replacement of a straight pipe section on the trailer with

the flow rate meter pipe section normally installed in the IRS. EARS was

also modified to provide valves and pressure measuring ports so that the

back pressure of the flow meter could be duplicated when the straight pipe

section was reinstalled.

The water distribution pump of the EARS is driven from the tow vehicle

drive shaft. Therefore, it was necessary to make the stationary tests with

the tow vehicle drive wheels on dynamometer rollers. The average water

flow rate for 75 seconds was determined for simulated vehicle speeds of 20,

40, and 60 mph (32, 64, and 97 km/h) as measured by the vehicle speedometer.

The results, given in Table 1 and Figure 6, show excellent agreement with

flow rates based on the value of 7.5 in (19.1 cm) assumed by EFTC personnel

to be the wetted width.

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 contact the pavement

surface. The SDG includes a basin divided into equal width slots mounted

at floor level (Fig. 7) [6]. The basin is equipped with a sliding cover

that is removed 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. 8).

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

wheels were driven at the desired speed with water flowing from the nozzle.

10
Table 1 - EARS Flow Rates

Simulated speed Measured flow rate Desired flow rate (a)

mph (b) gpm (c) gpm (c)

20 13.5 13.5

40 27.2 27.0

60 40.2 40.5

(a) for assumed 7.5 in (19 cm) wetted width.

(b) 1 mph =1.6 km/h.

(c) 1 gpm =3.8 liters/min.

11
km/h

32 64 97 129

0 20 40 60 80

mph

SPEED

Figure 6. EARS pavement wetting based on 7.5 in (19.1 cm) wetted

width compared with ASTM E274-70 water layer tolerance.

12
Figure 7. IRS flow channel, showing water flow into SDG basin.

13
14
The sliding cover was removed until water rose to a predetermined, approximate level in the sight tubes, and then the cover was quickly replaced.

The gage 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.

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.

The SDG was not available for NBS to conduct these tests on EARS.

The tests were made by EFTC shortly after the evaluation period, and the

results were furnished to NBS. These results are shown in Figures 9, 10,

and 11 for 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 aero- dynamic conditions. The DDG consists of an uncovered container mounted

flush with the pavement having 20 equal 7/8 in (2.2 cm) wide slots

(Fig. 12). Provisions are made for quickly removing the container to a building where the water in each slot is absorbed with paper towels and

weighed. Electrical sensors record the path of the test tire across the

slots. In principle, the DDG measurements could show the wetted width

(but not at the tire-pavement interface as desired) , the alinement of

the test tire with the wetted width, the lateral uniformity of the water

distribution and the depth of the water layer.

Distributions were determined by EFTC personnel and presented in

terms of relative quantity only. However, since the measurements are in

terms of the weight of water collected in each gage slot and the slot

dimensions are known, we are able to calculate the depth of the water

layer. Figure 13 is a typical example of a DDG distribution plotted in

terms of depth. Note that the average depth of water collected under

the tire is 0.038 in (.096 cm), surprisingly high since the flow rate

had been adjusted in accordance with ASTM E274-70 which leads to an ex- pected depth near 0.020 in (0.051 cm).

The total water collected during the run represented in Figure 13 was 168 grams (Table 2) . From the speed and the flow rate for this run,

which were recorded independently, (39.8 mph (64. G km/h), 27.5 gpm

(104.5 liters/min)) we calculate that 69.8 g of water were pumped by the

skid system as it travelled 28.2 in (716 cm), the length of the gage

slots. Therefore, the amount of water collected by the DDG was far in excess of the expected amount.

15
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Figure 12. EARS pavement wetting system test using the DDG.

19
i

IRS 40 mph 1 64 km/h ) RUN 1: i

SLOT

Figure 13. Dynamic distribution. Depth scale

based on slot dimensions.

20
Table 2 - DDG Data: IRS, 40 mph, Run 1

Tire Path: Slots 9 through 15

Slot number Net weight, grams

1 0.0

2 0.5

3 1.0

4 1.5

5 2.0

6 5.0

7 5.5

8 8.5

9 9.0

10 12.5

11 13.5

12 19.5

13 18.5

14 17.0

15 19.0

16 12.5

17 7.5

18 7.0

19 5.0

20 3.0

Total 168.0 grams

21
Indication of excess water was typical for all the runs. The discrepant data are summarized in Table 3.

Exploratory tests to determine the reason for these errors were con- ducted by NBS personnnel. Included was a test for collection efficiency.

A graduated cylinder was filled to the 100 millimeter mark. The net weight

measured on the balance was 100.01 g. The water was distributed among the

slots of the gage in the DDG lane outside. The gage was then brought into

the control center and the slot contents were absorbed and weighed as in a normal test. The elapsed time for weighing was 15 minutes. The amount of

water weighed was 95 g. This test indicates that the excess water collected was associated with the dynamic collection process rather than the

measurement of the water.

Further tests indicated the sources of excess water include:

a) water remaining from prior DDG tests was splashed from beneath the grate and basin.

b) the fog or cloud of mist following in the path

of the test wheel continued to be collected

after the skid trailer had passed the basin.

The results of these additional exploratory tests are discussed in

Appendix A.

In view of the fact that the DDG sample includes a large quantity of

water entering the gage by uncertain paths, the results of this test cannot be considered a valid measurement of the water distribution and are

not given in this report.

2.5.4 Wetted Width Measurement

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

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

tire-pavement interface. 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 trailer, and photographing the pavement from the moving trailer showing the water trace width at the position

of the interface of the 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 and measured after trailer

passage contain an increment of width due to spreading which necessitates

an assumption about the width at the time of passage. Measurement of photographs of the wetted width from the moving skid trailer involves 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.

22
Table 3 - DDG Data: Comparison of Water Collected with Water

Pumped

Speed,

mph (a) Unit

Coll ected,

grams

Pump ed

,

grams

Collected

Pumped

20 EARS, run 1 86.27 68.2 (b) 1.26

20 EARS, run 2 79.59 68.2 (b) 1.17

20 EARS, run 3 76.52 68.2 (b) 1.12

20 IRS, run 1 70.0 65.0 1.08

20 IRS, run 2 71.0 64.5 1.10

20 IRS, run 3 89.0 64.7 1.38

40 EARS, run 1 102.5 68.4 (b) 1.50

40 EARS, run 2 106.0 68.4 (b) 1.55

40 EARS, run 3 115.5 68.4 (b) 1.69

40 IRS, run 1 168.0 69.8 2.41

40 IRS, run 2 168.0 69.0 2.43

40 IRS, run 3 196.0 69.8 2.81

60 EARS, run 1 96.00 68.5 (b) 1.40

60 EARS, run 2 97.11 68.5 (b) 1.42

60 EARS, run 3 107.81 68.5 (b) 1.57

60 IRS, run 1 150.5 71.0 2.12

60 IRS, run 2 180.0 71.3 2.52

60 IRS, run 3 158.5 70.4 2.25

(a) 1 mph = 1.609 km/h.

(b) Computed.

23
In this evaluation, the span of slots collecting water during the

SDG tests at 40 mph (64 km/h) was used as a measure of wetted width. The

resolution of the measurement is limited by the slot width, 5/8 in (1.6 cm).

Measurements from the pavement were performed with the test wheel ex- tended beyond the water trace (Fig. 14). The pavement wetting outlet re- mained in its normal position. This arrangement was chosen to minimize the

change of the aerodynamic disturbance of the trace and the disturbance due

to the tire running in the water trace. Three measurements were made of

each trace with increasing delay times to allow extrapolation back to the

moment of passage (Fig. 15). Uncertainties in the use of this method include measurement between the indistinct boundaries of the spreading trace

and the validity of linear extrapolation.

A high speed movie camera was installed temporarily on the IRS trailer

with a downward view of the water trace opposite the extended test tire

(Fig. 16). Photographs were obtained from which the wetted width could be

estimated in relation to objects of known size. Preliminary tests of this

method show promise. No tests using this method were made with EARS.

The measurements obtained by these methods are presented in Table 4.

None of the methods used to measure wetted width during these tests

is considered satisfactory. The SDG measurements represent a trace width

before the tire-pavement contact line while the measurement on the pavement can occur only after the test vehicle has passed. These values can therefore best be considered as lower and upper bounds on the correct

value. The development of the photographic method might be expected to

provide a better wetted width measurement.

2.6 Force Measuring Subsystem

The transducer of the force measuring system is located in-board

from the wheel, 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 a force acting along vectors in three-dimensional space

with respect to the centroid of the transducer. Displacements occurring

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

displacements are accounted for by using the tow vehicle and trailer system as a fixture while calibrating the transducer "in situ". Vertical

load and traction forces are measured 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, effects, such as the transverse force at the tire-pavement interface and the yawing orientation of the trailer usually present during

skid testing are not accounted for by this test. Displacements of the

interface are larger when the tire is "hot" as in a running condition.

The tests described here were conducted with the tire at room temperature.

24

EARS:

12

11

10

9

8

7

6

1 1

60 mph

- ^^^97 km/h)

~

40 mph

*|64 km/h)

~ ^

20 mph

B"(32 km/h)

i 10 20 30

TIME AFTER PASSAGE, sec

40

30

25

20

15

Figure 15. Wetted width by delayed measurement on pavement

26
Figure 16. Arrangement for photography of wetted width.

Figure 17. Application of a simulated traction force to a locked

test wheel through an air-bearing force plate.

27
Table 4 - Wetted Width Measurements

Speed Flow rate," Wetted width,

mph (a) Unit ggm ( b ) Method in (c

)

20 EARS 13.5 SDG 6.2

20 EARS 13.5 Pavement 7.4

20 EARS 13.5 Photo

20 IRS 14.5 SDG 7.5

20 IRS 14.5 Pavement 8.7

20 IRS 14.5 Photo 7.6

40 EARS 27.1 SDG 7.5

40 EARS 27.1 Pavement 8.9

40 EARS 27.1 Photo ---

40 IRS 29.0 SDG 8.1

40 IRS 29.0 Pavement 8.9

40 IRS 29.0 Photo 8.9

60 EARS 40.2 SDG 7.5

60 EARS 40.2 Pavement 9.3

60 EARS 40.2 Photo

60 IRS 43.5 SDG 8.8

60 IRS 43.5 Pavement 10.4

60 IRS 43.5 Photo 9.9

"Nominal values.

(a) 1 mph =1.6 km/h.

(b) 1 gpm =3.8 liters/min.

(c) 1 in = 2.54 cm.

28
2.6.1 Transducer Excitation Stability

The EARS uses a transducer containing two Wheatstone strain-gage

bridge sensors. The bridge excitation from the on-board power supply

was measured. The load channel excitation voltage was 10.016 dc volts

and the traction channel excitation was 10.025 dc volts. There was no meaningful ac voltage viewable on the 0.05 volt/cm scale of an oscilloscope. There was no change in these values as the engine speed was increased from idle.

2.6.2 Orientation of the Transducer

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

To reduce the effects of any cross-axis misalignment, the hitch height

was changed until the removal and application of the static vertical

load had a minimal effect on the traction channel output. This condition occurred with 11-7/8 inches between a level surface and the bottom

of the welded flange under the trailer hitch assembly. A change of 0.1

in (0.2 cm) in this measurement is discernable with the on-board traction instrumentation. This orientation was maintained for these tests

and should be maintained during trailer use.

2.6.3 Vertical Load Channel Calibration

The EARS trailer (with the left test wheel resting on the force

plate) was alined with the tow vehicle. A weight of 150 lb (68 kg) was placed in each seat of the tow vehicle. (A weight is removed whenever

a person occupies a seat during testing) . The hitch height was set for

the proper transducer orientation. The vertical load at the test wheel

was varied over a range from about 100 lbf (445 N) to about 1450 lbf

(6450 N) . Since the EARS was not compatible with the IRS vertical load

air cylinder, the load at the test wheel was reduced by lifting the

trailer axle with a floor jack. The vertical load at the test wheel was increased by stacking sand bags on top of both sides of the trailer symmetrically. A force plate test was conducted using the on-board amplifiers of the EARS for the transducer load signal and the amplifiers of

the IRS for the force plate load signal. The outputs of the vertical

channels of the force plate and transducer were recorded point-by-point

on the X and Y axes of an X-Y recorder respectively. During the initial

tests, the recorded values were nonlinear at loads above 1100 lbf (4900 N)

A series of tests were conducted which revealed that the problem was in

the EARS transducer or its mount . EFTC personnel found that the trailer

fender, when the new larger E501 test tires were used, interfered with

the transducer-axle-tire assembly when the higher forces were applied

to the trailer body. Modifications were made to provide fender clearance and new springs of the same type were substituted in the trailer

suspension system. After these modifications, the vertical load test was repeated, showing linear results within 15 lbf (67 N) and negligible

hysteresis (Fig. 18).

29
flVOl 3iVld 33H0J

^0
2.6.4 Traction Load Channel Calibration

The trailer was alined with the standardized tow vehicle. A

weight of 150 lb (68 kg) was placed in each seat of the tow vehicle.

The hitch height was set for the proper transducer orientation. A force

plate traction test was conducted using the on-board amplifiers of the

EARS for the transducer traction signal and the amplifiers of the IRS

for the force plate traction signal. The results were plotted point-bypoint on an external X-Y recorder. The transducer was linear within

10 lbf (45 N) under an increasing load as shown in Figure 19. During the

decreasing load portion of the test, the test wheel brake slitmed sufficiently to abort the test. In repeat tests 3 the slinoase occurred again.

Further investigation revealed that the brake hvdraulic fluid was leaking

back to the reservoir such that the reservoir pump had to be energized

every 55 seconds to maintain the brake pressure. However, the pump is

automatically de-energized when the test brakes are applied in the EARS,

and therefore, does not turn on during a force plate test. EFTC personnel repaired the leak at a later date after receipt of new parts. A

hysteresis curve was not completed. However, numerous tests on the same model of transducer have shown negligible hysteresis.

2.6.5 On-Board Calibration System Equivalent Values

To adjust the gain and offset of the transducer instrumentation, two

known values are required for each channel. EFTC procedures use no load

for one of the values and a resistor, shunting the transducer bridge

circuit for the other value to simulate known forces. The equivalent

force values with the bridge circuits shunted were found to be 1081 lbf

(4808 N) for the vertical load channel and 501 lbf (2228 N) for the

traction channel.

2.7 Suspension Subsystems

The measurement of pavement skid resistance at normal traffic speeds

is a dynamic test. While data reduction is to occur during a period of

steady state friction, the fact is that transient oscillations dominate

the entire trace. Trailer motions introduce transient dynamic forces

into the system and vice versa. The effective dynamic characteristics

of the suspension subsystems need to be measured and reported in analytical form in order that dynamic analyses of the system can be performed

for comparison with experimental results and for use in design parameter

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

characteristics that might affect measuring performance and account for

these differences, the effective spring rates, natural frequencies,

Coulomb and viscous friction of the suspension systems were determined.

Measurements of certain static displacements were also made as the traction force at the tire-pavement interface was increased to approximately

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

31
LO

CO

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COCD

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OO CO

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CD CD z ^

CD o <3

«£ Q_ CD Q_

O

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CD

CD

Cvl

IIOI13VHi 3iVld 33N0J
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.

These suspension system tests were conducted in August 1974. The

test program plan utilized the month of September for system modifications

before the dynamic correlation tests occurred in October. As reported in

Section 2.6, to eliminate a nonlinearity in the vertical load channel, the

fenders were raised and the suspension springs were replaced. It was learned later that trailers of this design subsequently manufactured and

used with the larger E501 test tires only required the raising of the

fender to correct the nonlinearity. The suspension springs in the EARS

were replaced with the same type having the same spring rate (Section

2.7.2.2).* The dynamic characteristics reported here were measured prior

to the modification and are proportional to the square root of the spring

rate, but allowance for a four percent difference in spring rate changes

the values by only two percent.

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 EARS for intercomparison purposes.

2.7.1 Tow Vehicle and Trailer Displacements under

Static Traction Load

2.7.1.1 Displacement of the Test Wheel Hub

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

displacement is the resultant of motions throughout the system. Tow

vehicle tire and suspension deflections, trailer suspension deflections,

and trailer yaw contribute.

The component of hub displacement along the direction of the initial

alinement of the tow vehicle and trailer was measured with respect to the

floor. The measured displacements for the EARS and the IRS are plotted

in Figure 20. The displacement sensor at the EARS hub went off scale near

400 lbf (1780 N) traction, however, the slope below 400 lbf (1780 N)

suggests that the displacement at 800 lbf (3560 N) may have been larger

than the corresponding displacement of the IRS hub. At 800 lbf (3560 N)

the IRS hub was displaced 0.72 in (1.8 cm) to the rear.

*Information regarding springs was obtained from K. J. Law, Inc. in a letter dated December 16, 1975.

33
a] EARS:

o 1000

N 2000 3000

1.0

OS

' 1 « 0.8

2 0.6

=c in

LjlJ S 0.4

C-9 «*

Q_

COS 0.2

DQ

=3

3=

0.0

1 1 A = INCREASING TRACTION

V= DLCREASING TRACTION

T

1.0

0.8

0.6

s= in

0.4

0.2

0.0 -

0 200 400

Ibf STATIC TRACTION

b] IRS: N

0 1 000 20 00

i 1

600

T

3 000 —I

A" INCREASING TRACTION

V= DECREASING TRACTION

2.5

- 2.0

- 1.5

S£ ^& A -

400

Ibf STATIC TRACTION

cm

1.0

0.5

0. 0

800

2.5

2.0

1.5 cm

1.0

0.5

0. 0

800

Figure 20. Displacement of the test wheel hub due to static traction.

34
2.7.1.2 Displacement of the Trailer Body Through

Roll Axis Rotation

The body of a two wheeled skid trailer may roll about a longitudinal

axis during skid testing. The suspension and tires of the trailer re- spond to asymmetric forces developed by test wheel traction loading.

Roll displacement due to static traction loading was measured with

respect to the floor by two sensors applied vertically to the back corners

of the trailer body. Lateral separation of the sensors was 46 in (117 cm).

The EARS and IRS responded quite differently (Fig. 21). At 800 lbf

(3560 N) static traction loading, the EARS trailer body fell 0.35 in

(0.89 cm) on the left (test wheel) side and rose 0.62 in (1.6 cm) on the

right side corresponding to a roll axis rotation of 72 minutes of arc. At the same loading the IRS trailer body rose 0.05 in (0.12 cm) on the

left (test wheel) side and rose 0.04 in (0.10 cm) on the right side corre- sponding to a roll axis rotation of 0.7 minutes of arc.

2.7.1.3 Displacement of the Trailer Body Through Pitch

Axis Rotation

The average of the displacements measured by the two trailer body

roll sensors represents the vertical displacement of the center of the

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

the vertical displacement measured by a third sensor at the trailer hitch,

yields trailer body pitch. The pitch displacements for the EARS and the

IRS are plotted in Figure 22. The longitudinal separation of the front

and rear sensors was 137 in (348 cm).

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

rose 0.01 in (0.02 cm) at the hitch and rose 0.14 in (0.35 cm) at the rear corresponding to a pitch axis rotation of three minutes of arc. At

the same loading the IRS trailer body fell 0.07 in (0.18 cm) at the

hitch and rose 0.05 in (0.13 cm) at the rear corresponding to a pitch axis

rotation of three minutes of arc.

2.7.2 Trailer Displacements under Transient Loading

2.7.2.1 Effective Coulomb Friction of the Trailer Suspension

To enhance the measurement of the Coulomb friction in the suspension,

the shock absorbers were temporarily disconnected. Vertical load at the

tire-pavement interface and the displacement between the trailer body and

axle were measured versus time after the trailer was lifted at the rear bumper and dropped abruptly. The measurements are shown in Figure 23

where the linear decay of amplitude is a measure of the Coulomb friction.

The Coulomb friction associated with one wheel was determined to be 9.4 lbf

(41.8 N).

35
a) EARS:

o 1000 20 00

0.8 _ i A INCREASING TRACTION

V=DECR EASING TRACTION

0.4

in

0.0

0.4

3000

! A = LEFT SIDE

V =R IGHT SIDE

b] IRS

:

o 1000

400

Ibf STATIC TRACTION

2000 3000

0.8

0.4

in

0.0

0.4

I I A INCREASING TRACTION

V=DECREASING TRACTION

1 A= LEFT SIDE

V-RIGHT SIDE

" 2.0

1.0

cm

1.9

1.0

800

x

2.0

1.0

cm

0.0

1 .0

200 400

Ibf STATIC TRACTION

600 800

Figure 21. Roll displacement of the trailer body due to static

traction

.

36
a] ears:

1000 2000 000

0.2

0.1

o.i -

I I A INCREASING TRACTION

^s7 = DECREASING TRACTION

A = HITCH

^ = REAR

- 0.51

«ff y

/ v

^ &

0.25

J.O

0.25

b) IRS

0

200 400

Ibf STATIC TRACTION

1 000 2 000

600

3000

800

N

0.2

0.0

A INCREASING TRACTION

V= DECREASING TRACTION

A = HITCH

V = REAR

0.51

0.25

0.0

0.25

200 400

Ibf STATIC TRACTION

Figure 22. Pitch displacement of the trailer body due to static

traction.

37
Figure 23. Coulomb damping test response.

38
2.7.2.2 Effective Spring Rates of the Trailer Suspension

In the above test, the change in force and "displacement from their

equilibrium values is a measure of the spring rate. The body-to-axle

spring rate was determined to be 350 lbf/in (61 000 N/ra).*

With the body-to-axle and tire spring rate known (Section 2.1.2)

the body-to-pavement effective spring rate was determined to be 275 lbf/in

(48 000 N/m).

2.7.2.3 Effective Natural Frequencies of the

Trailer Suspension

There are two significant natural frequencies of the EARS skid trailer

since there are two degrees of freedom:

a) the body can rotate in a vertical plane

about the hitch, and

b) the axle can rotate in a vertical plane

about its trailing arm hinge.

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. There are two dynamic tests which

are easy to conduct which approximate the independent modes such that the

measurement of each natural frequency is enhanced. 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 as shown in Figure 23. The low undamped natural

frequency is measured to be 1.3 hertz. 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

as shown in Figure 24. The high undamped natural frequency is measured to

be 12.7 hertz. After the transient decays to where there is no further

relative movement in the suspension, the oscillation of the trailer on the

tires persists since there is little damping in the tires. The natural

frequency of the trailer on the tires at this low amplitude is 3.1 hertz.

2.7.2.4 Effective Viscous Damping of the Trailer Suspension

To estimate the effective amount of viscous damping in the system,

the shocks were connected and the bumper drop test repeated. The response

is shown in Figure 25. Since the transient is now damped by both the

Coulomb and viscous friction, the damped envelope is shown redrawn to

account for the Coulomb friction occurring in each cycle.

*Subsequent to the replacement of the suspension springs this test was

repeated. When the force amplitude was 44 lbf (196 N) the displacement

was 0.125 in corresponding to a body-to-axle spring rate of 350 lbf/in

(61 000 N/m).

39
t 1 1 r AT START OF TRANSIENT, AXLE FALLS

FASTER THAN BODYJNCREASING

THE RELATIVE DISPLACEMENT

BODY FALLING AND TIRE BOUNCING

REDUCES DISPLACEMENT

DURING AXLE LIFT BODY RISES

0.06in (0.15 cm) WITH RESPECT TO AXLE

FORCE PLATE RESPONSE

100 Ibf (445 N

LOAD ON AXLE IS REDUCED or BY LIFTING OF AXLE |n 0 25 cm

0.5 sec

M 1 >

J I I I I lm J L

Figure 24. Axle drop test response,

40

A nondiraensional plot for estimating damping in systems represented

by a differential equation of second degree is shown in Figure 26 [10].

As drawn, the response envelope (Fig. 25) is approximated by a 250 lbf

(1100 N) drop test, with a first overshoot of 130 lbf (580 N) and a second overshoot of 65 lbf (290 N) . Comparing these results with the

family of curves in Figure 26, the effective damping factor, £, is approximately 0.2. Another way of estimating the amount of damping present

is to measure the rate of decay of oscillation using the logarithmic decrement. This calculation also indicates a damping factor of 0.2.

3. THE DYNAMIC SKID RESISTANCE MEASUREMENT OF SELECTED

SURFACES AND EARS-IRS CORRELATION

The purpose of the dynamic skid resistance measurement test program

was

:

a) to measure selected surfaces at the EFTC,

b) to measure the consistency and precision in the

measurements of both the EARS and the IRS, and

c) to correlate the degree of match in the measurements between the IRS and the EARS.

3.1 Surfaces Selected

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

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

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

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

automatic test sequence for each test speed. The locations of the selected

surfaces and pylons are shown in Figure 27.

3.2 Tests Planned

The test program is designed to yield the following information:

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

test speed,

b) skid resistance - test speed gradient, 8 , as a function of test speed,

c) variability in SN with choice of time period for

the test,

d) variability in SN with choice of test tire, and

e) variability in SN with choice of lane within the

surface

.

42
Figure 26. Effect of different damping factors

on second-order linear systems.

43

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) water flow rate.

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 5.

The test sequence within each time period is given in Table 6. 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 ]

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-Asi 2 3 4 5 6 7 S23456 7 1 S+AS 4 5 6 7 1 2 3

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

the 7 lanes comprising the surface [8]. 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 5 consists of a test matrix where the

variables are time periods, lanes and speed.

45
Table 5 - Tests Planned and Accomplished

Surface

Test Matrix

Nominal Number of No. of No. of Planned Accomplished

Test Speed Time Periods Tires Lanes IRS EARS

40 3x7 3x7

20

40

60

3x7 3x3

3x7 3x7 3x4

3x7 3x4

20

40

60

7 3x7 3x73x5

7 3x7 3x7

7 3x7 3x7

40

40

40

3x7 3x6 3x5

3x7 3x5 3x5

3x7 3x5 3x5

Total Measurements 210 153 93

Accomplished as a Percent

of Planned 73% 44%

46
Table 6 - Test Plan Sequence

Time Period M

Speed

Surface Lane mph (a) Tire

2 1 38 I

2 5 42 I

2 7 40 I

3 1 38 J

3 5 42 J

3 7 40 J

1 1 38 H

1 5 42 H

1 7 40 H

1 1 38 A

1 2 40 A

1 4 42 A

1 5 38 E

1 6 40 E

1 1 42 E

1 2 38 B

1 3 40 B

1 5 42 B

1 6 38 F

1 7 40 F

1 2 42 F

1 3 38 C

1 4 40 C

1 6 42 C

1 7 38 G

1 1 40 G

1 3 42 G

1 4 38 D

1 5 40 D

1 7 42 D

47
Table 6 - Test Plan Sequence (Continued)

c

, Time Period Tu Speed

Surface Lane mph (a) Tire

1 1 40 H

1 2 38 H

1 6 42 H

3 1 40 J

3 2 38 J

3 6 42 J

2 1 40 I

2 2 38 I

2 6 42 I

2 1 38 A

2 2 40 A

2 4 42 A

2 5 38 E

2 6 40 E

2 1 42 E

2 2 38 B

2 3 40 B

2 5 42 B

2 6 38 F

2 7 40 F

2 2 42 F

2 3 38 C

2 4 40 C

2 6 42 C

2 7 38 G

2 1 40 G

2 3 42 G

2 4 38 D

2 5 40 D

2 7 42 D

48
Table 6 - Test Plan Sequence (Continued)

Time Period W

Speed

Surface Lane mph ( aTire

1 2 40 H

1 3 38 H

1 7 42 H

2 2 40 I

2 3 38 I

2 7 42 I

3 2 40 J

3 3 38 J

3 7 42 J

3 1 38 A

3 2 40 A

3 4 42 A

3 5 38 E

3 6 40 E

3 1 42 E

3 2 38 B

3 3 40 B

3 5 42 B

3 6 38 F

3 7 40 F

3 2 42 F

3 3 38 C

3 4 40 C

3 6 42 C

3 7 38 G

3 1 40 G

3 3 42 G

3 4 38 D

3 5 40 D

3 7 42 D

49
Table 6 - Test Plan Sequence (Continued)

. Time Period Th Speed

Surface Lane mph (a) Tire

1 1 42 H

1 3 40 H

1 4 38 H

2 1 42 I

2 3 40 I

2 4 38 I

3 1 42 J

3 3 40 J

3 4 38 J

3 1 18 A

3 2 20 A

3 4 22 A

3 5 18 E

3 6 20 E

3 1 22 E

3 2 18 B

3 3 20 B

3 5 22 B

3 6 18 F

3 7 20 F

3 2 22 F

3 3 18 C

3 4 20 C

3 6 22 C

3 7 18 G

3 1 20 G

3 3 22 G

3 4 18 D

3 5 20 D

3 7 22 D

50
Table 6 - Test Plan Sequence (Continued)

c

. Time Period F Speed

Surface Lane mph (a) Tire

1 2 42 H

1 4 40 H

1 5 38 H

2 2 42 I

2 4 40 I 2 5 38 I

3 2 42 J

3 4 40 J

3 5 38 J

3 1 58 A

3 2 60 A

3 4 62 A

3 5 58 E

3 6 60 E

3 1 62 E

3 2 58 B

3 3 60 B

3 5 62 B

3 6 58 F

3 7 60 F

3 2 62 F

3 3 58 C

3 4 60 C

3 6 62 C

3 7 58 G

3 1 60 G

3 3 62 G

3 4 58 D

3 5 60 D

3 7 62 D

51
Table 6 - Test Plan Sequence (Continued)

j Time Period Sa

Speed

Surface Lane tnph (a) Tire

42

40

38

H

H

H

42

40

38

42

40

38

18

20

22

18

20

22

18

20

22

18

20

22

18

20

22

18

20

22

18

20

22

I

IIA

A

A

E

E

E

B

B

B

F

F

F

C

C

C

G

G

G

D

D

D

52
Table 6 - Test Plan Sequence (Continued)

Speed

Time Period Su

Surface Lane mph (a) Tire

1 4 42 H

1 6 40 H

1 7 38 H

3 4 42 J

3 6 40 J

3 7 38 J

2 4 42 I 2 6 40 I 2 7 38 I

2 1 58 A

2 2 60 A

2 4 62 A

2 5 58 E

2 6 60 E

2 1 62 E

2 2 58 B

2 3 60 B

2 5 62 B

2 6 58 F

2 7 60 F

2 2 62 F

2 3 58 G

2 4 60 C

2 6 62 C

2 7 58 G

2 1 60 G

2 3 62 G

2 4 58 D

2 5 60 D

2 7 62 D

(a) 1 mph =1.6 km/h

53
I 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.

Seven tires were used by each system on the seven lanes of surfaces

1, 2, and 3 for the single time period test matrices (14 tires). One

tire was used by each system on each surface every testing day for the

multiple time period test matrices (6 tires) . Twenty test tires were

used in the program.

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. The weather conditions during the two weeks are given in

Table 7. The testing time periods are related to the date and weather

conditions in the table. Two days were lost due to rain at the beginning

of the week with frost occurring on Saturday, followed by snow that evening. Thursday of the second week was lost due to wet surfaces and a heavy fog. The number of tests completed are shown in Table 5.

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 con- stant of the test wheel vertical load. Variations in hitch height which

also rotate the two-axis force transducer cause a shift in the zero of

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

the vertical load channel, from the expected value. Two axis force transducers are used on both the EARS and the IRS. The static tests (Section

2.3) had demonstrated that the hitch height depends on occupant and water

weight

.

Variations in inflation pressure of the test tire have also been

found to cause a shift in the zero of the traction force instrumentation

and 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 k Pa) 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

(Section 3.9). Variations in the inflation pressure of the tow vehicle

rear tires change the water laying system pump rate with test speed.

54
Coocu

o>

4-> u-4 < M

Q)

CO

(1) &

6Tc

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•Hu

<u p ,

a)C1 E_lw CO miu Pm<c

&41 1 O

(A 4-J QJ H a) £ CO H I—

H

ti

<-t Qh

Q)r1 jj

CO

0)

4-J 03 Q

0)

i—i

cd H

4-J X

60OC

COC

CO 1 T3 <-> 3 c jj 1 O c u

i—

3 CO 1 u CO P-i 1c

•r-l CO os

g3

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OS COXH

CO COXH 3

COI03 CO

60C

•i-l c

(1) >

(-1 cO

PS

cno> >> o N co

w !>• 14-1 o 8 C i-4 o u c 3

fa CQ CO

4-1

u

C lf> 3 ?^ o CO CO 1—4 4-J 4-1 o C 4J W & w c u

4-J § C

M o o o c

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12Q CO CM CO En CO

OsI3

CO

CO

60o>

CO

CD 33w

o>O

CO

14-4 U3

CO

4J

0)

<t- St <t <f <*

r- r~- r~ r— r~ r~ l>- O o o

t—r—i—4 i-4 i—4 —I i—i i-4 i—i i—4 i—4

UO vo r-~ CO CT> o i—4 CM

m <f

i—4 1—4 r-4 —4 1—1 i—l CN CM CM CM CM

S-i 5-4

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4-1 4J 4-J 4-1 4J 4-J 4-J 4-1 4-1 4J 4-1 o CJ o o o o o CJ o o O o o o o o o o O o o O

55
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 con- ditioned 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 k Pa). The hitch height was then readjusted to 12-5/8 in (32.07 cm)

for the IRS and to 11-7/8 in (30.16 cm) for the EARS 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 k Pa) four times

per test day. The shock absorber pressure was checked at least once each

day. 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 the test program. 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.

To control surface dryness, at least one hour was allowed between

tests on any lane. A longer time was allowed if necessary for the surface

to dry.

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

(Section 3.9). A 0.5 mph (0.8 km/h) difference in test speed on surface 3

(Fig. 35) 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 Measurement of Uncontrolled Test Variables

The weather conditions around the surfaces during the dynamic testing

time period are given in Table 7.

The temperature of the surfaces was measured with a sensor located at one end of each surface. The temperature sensitive area of the sensor was placed in contact with the top of the aggregate. A conductive grease was

used to provide thermal contact between the sensor and the sides of the

protruding aggregates and the binder of the surface. The sensors were

read frequently during the testing period. The surface temperatures measured are given versus time of day in Figure 28. The surface temperatures ranged from 40 °F to 92 °F (4 °C to 33 °C) during this period.

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

test and the tread depth at each groove across the width of the tire at

the lock-up position was measured and recorded after each test. The tire

surface temperature was a further guide in locating the lock-up position.

Analysis of these data enable the amount of tire wear and the number of

tests occurring at that tire location to be determined.

56
CM

O DO

Q

O

Q B

OO Q O

OQ_o<j

CD

O o- .Coa

a o Q9

a o

-I '

in

cm

CO

CMco

CM

oo

CM

CO

CM

CM

CM

CD

CO

CM

CM

CM

CM

CM t CT3

CMCO

CMCO

CM

2 ->

00c

u

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COc

60c

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W

01

4-1 CO

l-l 0)a

e

<u

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a;o

co

CO

CN

a)

>-i 3

60 •H

3HniVH3dW31
The type of wear varied with the surface. The textures of surfaces

1, 2, and 3 are shown in Figures 29, 30, and 31 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 32. The surface of tire I, which skidded exclusively on surface 2,

had 0.12 in (0.32 cm) diameter by 1 to 3 in (2.5 to 7.5 cm) long strands

of rubber carved from it as shown in Figure 33. The surface of tire J,

which skidded exclusively on surface 3, was "grated" as shown in Figure 34

3.7 The Correlation Test Plan

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

correlate the degree to which results obtained from measurements using

the IRS and EARS systems match.

The correlation test plan is designed to separate the sources of

dispersion listed in Section 3.2. If their existence can be proven and

quantified, the effect on the results can be taken into account to yield

a more precise correlation. Eventual knowledge of the bounds of these

sources of dispersion will be useful in establishing an uncertainty band

around the correlation result.

Statistical confidence in the results can be improved by increasing

the number of tests. Confidence in the equipment and measurement is en- hanced by finding consistency in the measurement results and the determina

tion of data points within small confidence intervals.

3.8 Test Matrix Results

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

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

about the mean value.

The results from measurements made at the EFTC during single time

periods are shown in Figure 35 for the IRS and in Figure 36 for the EARS.

The results during multiple time periods are shown in Figure 37 for the

IRS and in Figure 38 for the EARS.

The deviation from the mean calculated for the test tires at 20, 40,

and 60 mph (32, 64, and 97 km/h) on surface 3 from measurements made by

the IRS are shown in Figure 39. The deviation from the mean calculated

for the test tires at 40 mph (64 km/h) on surfaces 1 and 2 are shown in

Figure 40.

The deviation from the mean calculated for the lanes of surface 3

at 20, 40, and 60 mph (32, 64, and 97 km/h) from measurements made by the

IRS are shown in Figure 41. The deviation from the mean calculated for

the lanes of surfaces 1 and 2 at 40 mph (64 km/h) are shown in Figure 42.

58
Figure 29. Texture of skid resistance surface number 1
Figure 31. Texture of skid resistance surface numb

Figure 32. Texture of tire wear on surface number

60
Figure 33. Texture of tire wear on surface number 2.
100

km/h

10 20 30 40 50 60 70 90 100 110

90-

SURFACE 3

SN

C =89.3-0.78V + 0.005V

0

C =0.78 + 0.010V

70

60*

50-1

40-

30-

SH20 =75.5

8 20 -1.15

L - +1.8 SN40 =66.3

840 =-0.29

L=-+ 19 SN 6o =60.4

0

60, -0 04

• -B- o

l ^'2

SURFACE 2 SN40 =53.2

8.40 =+ 0.41

L

= ±1.7

SURFACE 1 SN 40 = 37.6

8 40

~- -0.TM

L = ±1.75

20-

10"

10 20 30 40

mph

TEST SPEED

50 60 70

Figure 35. IRS results from measurements made at EFTC during

single time periods.

62
100

90

km/h

10 20 30 40 50 60 70

—i 1 1 I I I l_

90 100 110

_i 1 i

CO

CO

CO

80

70

60-

50-

40H

30-

20-

10-

SURFACE 3

SN 20 -68.1

L =±0.9

SURFACE 2 SN20 ~53.9

?2o-+ 0.27

L =+2.0

SfU =48.9

l~ = + 40

40

SN 60 =48.9

8 60 -0.25

L =+2.2

SN C =62.4 0.54V + 0.0053V

B

c = 0.54 + 0.0 106V

0- -1 1 1 1

10 20 30 40

mph

TEST SPEED

0 60 70

~T"

50

Figure 36. EARS results from measurements made at EFTC during

single time periods.

63
100

km/h

10 20 30 40 50 60 70 90 100 110

90-

CO

CO

CO

co

70-1

60

50

40-

30-

20

10 H

0

J I i I L J l_

a

0

SURFACE 3 SN =62.9

L =+1.4

SURFACE 2 SN 40 = 55.1

L = ±2.5

SURFACE 1 SN 40

=37.3

L =±2.0

nT 20 ~^0~ 40 ~50~ ~60" 70

mph

TEST SPEED

Figure 37. IRS results from measurements made at EFTC during

multiple time periods.

6£
Km/h

100

ay

20 30 40 50 60 70 80 90 100 110

- I I o_ I 1 1 1 1 1

70

60

50-

40-

30-

20-

10

0

SURFACE 3 SH 40 = 58.4

L = +2.7

SURFACE 2 SN 40 48.3

L = ±4.4

SURFACE 1 SN 40 = 34.9

L

~- ±3.5

-T

0 10 20 30 40 50 70

mph

TEST SPEED

60

Figure 38. EARS results from measurements made at EFTC during

multiple time periods.

65
+ 4.0

42. OH

0

- 2.0 H

+6.0-1

+4.0

+2.01

•2.0 J

A B

20mph (32 km/h) TEST MATRIX

Th TIME PERIOD SN 20 = 75.5

D

TIRES

E F G

40mph 164 km/h) TEST MATRIX

W TIME PERIOD SN 4n = 66.3

B C D [ TIRES

+2.0n

0

2.0

60mph (97 km/h) TEST MATRIX

F TIME PERIOD SN 60 = 60.4

tzt

o

A B C D E F G

TIRES

Fieure 39. Possible tire dispersion calculated from IRS measurements

on surface 3 at the EFTC.

b6
SURFACE 1 40 mph (64km/h) TEST MATRIX

M TIME PERIOD SN 40 = 37.6

+2.0-

2.0-

4.0J

+2.0i

n

D E F G TIRES

SURFACE 2

40mph (64km/h) TEST MATRIX

Tu TIME PERIOD SN 40 = 53.2

n i—

"

|_|

lr

D

TIRES

Figure 40. Possible tire dispersion calculated from IRS measurements

on surfaces 1 and 2 at the EFTC.

67
20mph (32 km/h) TEST MATRIX

TH TIME PERIOD SN 20 =75.5

n

l_l U

1 2 3 4 5 6 7 LANES

40mph C64 km/h) TEST MATRIX

W TIME PERIOD SN 40 = 66.3

1 1

- u

2 3 4 5 LANES

6 7

+2.(h

CO

2.0J

60mph (97 km/h) TEST MATRIX

F TIME PERIOD SN 60

= 60.4

1-

1—

1

1 I—» l_l

1 2 3 4 5 LANES

6 7

Figure 41. Possible lane dispersion calculated from IRS measurements

on surface 3 at the EFTC.

6?
+4.0

+2.0

0-

2.0-

4.IH

+4.0i

+2.0-

2.0"

40mph (64km/h) TEST MATRIX

M TIME PERIOD SN 40 = 37.6

3 4 5

LANES OF SURFACE 1

40mph (64km/h) TEST MATRIX

TU TIME PERIOD SN 40 = 53.2

-4.0H

1 2 3 4 5 6 7

LANES OF SURFACE 2

Figure 42. Possible lane dispersion calculated from IRS measurements

on surfaces 1 and 2 at the EFTC.

69
The deviation from the mean calculated for the multiple time

periods on surfaces 1, 2, and 3 are shown in Figure 43.

There was not a sufficient number of tests conducted by the EARS

to calculate surface and tire nonuniformities from the test matrices.

Consequently, there is no verification of the pattern or magnitude of

these sources of dispersion at this time. As additional data becomes

available on these surfaces, more definitive statements may be made.

The ranking of these sources of dispersion calculated from the IRS

test matrices are given in Table 8. The dispersion associated with the

choice of time period for the test was the largest source calculated

while the choice of lane within a surface was the least. Special care had been taken to make each surface uniform during construction.

3.9 Skid Resistance Speed Gradient

It is known from test data generated at largely different test

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

section it is shown that, for one surface, the skid resistance speed

gradients measured using incremental speeds, AS, of 2 mph are consistent

with a continuous polynomial expression for skid resistance as a function

of test speed, while for another surface this consistency was not found.

The skid resistance for surface 3 at the EFTC as measured by the IRS can be expressed as:

SN

c (V) = 89.3 - 0.78V + 0.005V2 where V is the test speed.

Differentiation of this expression results in a skid resistance-test

speed gradient which is also a function of test speed

' 9 (V) = d(SN) =-0.78 + O.OlOV

~dV~

The skid resistance for surface 2 at the EFTC measured by the EARS

can be expressed as:

SN (V) = 62.4 - 0.54V + 0.0053V2 c Differentiation of this expression results in a skid resistance-test

speed gradient which is also a function of test speed

0 (V) = d(SN) =-0.54 + 0.0106V

C dV

70
Table 8 - Ranking of Possible Sources of Dispersion

Possible

Source Sample Sample

of Std. Speed, Std.

Rank Dispersion Dev. Sub -Rank Surface roph (a) Dev.

1 Multiple 2.63 A 1 40 2.8

Time-Period B 3 40 1.8

C 2 40 1.6

7 Tires 2.43 A 3 40 2.9

B 3 20 2.8

C 1 40 2.0

D 2 40 1.3

E 3 60 1.0

7 Lanes 2.0 A 1 40 2.4

B 2 40 2.3

G 3 40 1.5

D 3 20 1.2

E 3 60 0.8

(a) 1 mph = 1.609 km/h.

71
SURFACE 1 TIRE H

1 1

M TU W TH F SA SU

TIME PERIOD

SURFACE 2 TIRE I

1 1

M TU W TH F SA SU

TIME PERIOD —i

( .SURFACE 3 TIRE J

M TU W TH F

TIME PERIOD

SA SU

ieure A3. Possible time period dispersion calculated from IRS

measurements on surfaces 1, 2, and 3 at the EFTC.
The values computed from the above equations can be compared to

values obtained from the slopes of straight lines through average values

taken over small speed increments, AS, about ±2 roph (3.2 km/h) . When

this is done for measurements on surface 3 reasonably consistent values

are found. These are:

V e . cal c

6 IRS8 20 -.58 -1.15 -.79

40 -.38 -0.29

60 -.18 -0.04

when 0 is calculated from the above equation and G and 0 . are calc H IRS U£ARS

the values measured over a small speed range. However, when similar com- parisons are made for surface 2, the results are not consistent and the

measured and calculated values are of opposite sign. The results for

surface 2 are:

V 6 r.alc

9 IRS

9 EARS

20 -.34 -- +.27

40 -.14 +.41 +.60

60 +.06 -- -.25

3.10 The Correlation Equation

The correlation equation between the EARS and the IRS from the un- rounded test data is:

SN

EARS -1.^0+0.886 SN

IRS

The equation has a correlation coefficient of 0.996. A two-sigma band

about the calibration line is about 2.5 skid numbers.

A plot of the data points is shown in Figure 44. The lengths of

the crosses are equal to the 99 percent confidence interval, L.

The results of the dynamic skid resistance measurement test program

conducted on the three surfaces at the EFTC are rounded and summarized

in Table 9.

3.11 Use of the Correlation Equation

Based on the five pairs of values given in Table 9, a straight line

fitted to the data yields:

SN

EARS " U59 + °- 883 SN

IRS

Residual standard deviation 1.37

Standard deviation of intercept 2.86

Standard deviation of slope 0.049

Standard deviation of predicted value 1.14 (SN 40)

(a point on the line) 0.65 (SN 50-60)

1.10 (SN 70)

73
t 1 1 1 1 1 1 r

90%C0NFIDENCE BAND

0 10 20 30 40 50 60 70 80

IRS SKID RESISTANCE MEASUREMENT, SN

Figure 44. EARS-IRS correlation.
3

tn

cu DSCO

OS

<d

3

w

CD OSco

OS

M

4-1 CO

4-1 w

CD H

q * -[BAja^UJ in

• • • o• CM• m• ON• Duapijuoo %66 CO CM CM CM CO o

s^saj.

<N CM O O 0 CM <f IO *OM i—l —

i—l i-4

uoT^eTAaa m r> ON r—

co CM CM

oo 00 in 00 On in

• • • • • • • CO CM l—l -4 CM l—l

o r» m NO

i i nO

• CM

• CM 1 r-• + +

•

1 i

on co o> o> ON i—i kTC UE3U

CO 00 CO 00 00 00 CO <* <f m <t m NO

m

o m ON oo CM ouap-tguoo %66

• CM

•

i—i

• CM•

l—l

• •

l—l

•

l—l

• i —I

00 —vO —m l—l l—l i—l jo »on 1—4 CNl t—

CM i—i CM CM CM

UOT \_J. \_/ l TBTA3n 4 ^ * TM *-* 11 ^J- CO ^J- © CM CM 00 O CM •—I o o • • • • • • • • co CN vt CM CM CO CO CM m

<t i—l ON —I <t 9 ' quaTpean, i Is-

•

1 •

1 CM

•

• —I O•

1 + 1 1 l

CO nO l—l CM ON CO m

• r-

• • m

• co

• CM

• NO

• m• o

CO co m m NO NO r- NO

X co X co co CO X CO co co

CO S CO x S X CO X S a

«ijdiu * pa ads o o O o o o o o o o

<t <* <f CM NO <± CM NO

aoegans 1-) —4 CM CM CM CM CO CO CO co

• I CO NO l—l • 0) Cu

,-4 -r4 60 4J

II C -i

•H 3 X co S

a II II

i—i co 2

75
For 90 percent confidence limits, multiply the standard deviation

of predicted values by 2.353. For 95 percent confidence limits, multiply the standard deviation of the predicted values by 3.182.

The above information may be interpreted as follows for 90 percent

confidence limits: "In repeated calibration of EARS using the same general procedure, the results are not expected to differ from the present calibration by more than 2.7 SN at the two ends of the calibration

line (SN 40 and 70), and by more than 1.5 SN in the middle range

(SN 50-60)."

In using the calibration line, presumably EFTC personnel would like

to predict IRS values from their own measurements, using

SN

IRS

=_1 - 27 + 1 - 123 SN EARS

The uncertainty of a predicted IRS value for a given value of (EARS)

is

:

SN Number Uncertainty

40 3.0

50-60 2.0

70 3.0

computed as before for 90 percent confidence limits. In addition, the

random error component of SN,,,,,. must also be taken into consideration.

EARS

A graphical procedure is perhaps the easiest way to estimate the total

uncertainty. The example shown in Figure 44 is drawn for a 90 percent

confidence band.

The uncertainty given for the calibration line itself is, in a sense, systematic in nature from EARS point of view, i.e., if EARS is

comparing the SN of two different surfaces, this systematic uncertainty

cancels out. However, if EARS compares its result with another ARS, then

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

consideration

.

4 . RECOMMENDAT ION S

In order 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.

76
2. The EARS speed measuring system should be modified so that

the recorded speed and drivers aid are derived from the same fifth wheel source.

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.

The EARS 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 C describes the EARS and

contains a listing of the values of recognized EARS variables in effect

during these tests.

Acknowledgement

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. Appleton and Mr. R. G. Russell in the conduct of the

tests and other members of the NBS staff, of Mr. W. Goff of the Transportation Research Center of Ohio for the operation of the high speed camera,

of personnel of the FHWA Western, Central and Eastern Field Test Centers,

and the Federal Highway Administration for their cooperative efforts.

77
5. REFERENCES

[lj "Standard Method of Test for Skid Resistance of Paved Surfaces Using

a Full Scale Tire" Designation E274-70. Annual Book of ASTM Standards,

Part II. 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 70-80 (December 1972).

[3 J 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., "A Method and Means of Calibrating

an Air-Bearing Force Plate for Use with a Towed Pavement Friction

Test Trailer", National Bureau of Standards (U.S.), NBSIR 75-738

(December 1974).

[5] "Procedure for Speed and Distance Calibration of a Fifth Wheel Equipped

with Either Analog or Digital Instrumentation" Designation F457

,

American Society of Testing and Materials, Philadelphia, Pa. (1975).

[6] Federal Highway Administration Procedures for Evaluating the Skid

Measurement Process at Field Test and Evaluation Centers, Department

of Transportation, Federal Highway Administration, Washington, D.C.

(December 1973 ).

[7] 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).

[8] 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).

[9] Home, W. B. "Traction Performance of New (E524) and Old (E249)

ASTM Smooth Tread Test Tires on Several Wet and Dry Pavements" presented at the ASTM Committee E-17 Meeting in Montreal, Canada (June

24-25, 1975).

[lOj Chestnut, H. and Mayer, R. W. , Solution of Linear Differential

Equations, Chapter 3, Servomechanisms and Regulating System Design,

Vol. 1, p 62, John Wiley & Sons, Inc., N.Y. (1951).

78
APPENDIX A

Exploratory Tests of the Dynamic Distribution Gage

Measurements made by NBS personnel using the DDG (Section 2.5.3)

showed that there is more water collected than is pumped by the EARS

as it travels the length of the collecting basin. Since a large quantity of water enters the gage by uncertain paths, the results of this

test cannot be considered a valid measurement of the water distribution.

The following exploratory tests were conducted to help identify the source of the problem. NBS personnel performed DDG runs removing all

accumulated water from the basin before each run to minimize splash.

The IRS was used at 20, 40, and 60 mph (32, 64, and 97 km/h) . Table A-l

shows the continuing discrepancy between the water distributed and the

water collected. The distributions are shown in Figure A-l. For one

40 mph (64 km/h) run (Fig. A-ld) , a wheel extender was used so that the

test wheel was out of the path of the distributed water. The fender of

the test wheel was also removed for this run.

Photographs were made of IRS runs with and without the wheel ex- tender. High speed cameras were located on the skid trailer and on the

ground near the DDG. Figure A-2 is a typical view showing fog and splash.

It was apparent from these films that more than one cause of splash

existed. Accumulated water splashed during the passage of the test vehicle.

Motion of the grate and gage under the vehicle tires contributed to this

splash. Heavy splashes also occurred when water issuing from the pavement

wetting system encountered the grate and gage structures. Figure A-3

shows the smooth deposit of water over solid pavement ahead of the grate

while Figure A-4, taken later in the same run, shows heavy splashing over

the gage. Splashing of the streams may be caused by such mechanisms as airflow through the grate or collision of the stream with the crossbars

of the grate. The latter mechanism, considered in terms of relative

motion, may occur because the vertical faces of the crossbars act as swatters. A smooth pavement may not exhibit this splashing mechanism.

79
Table A-l - DDG DATA: Comparison of Water

Dry Basin

Collected with Water Pump ed

,

Speed, m ph ( a ) Unit

Collected,

^rams

Pumped

,

grams

Collected

pumped

20 IRS 91.38 68.7 (b) 1.33

40 IRS 182.43 66.4 (b) 2.75

40 IRS 125.44 68.7 (b) 1.83

40 IRS (c ) 173.79 73.2 (b) 2.37

60 IRS 176.65 73.2 (b) 2.41

(a) 1 mph = 1.609 km/h.

(b) Computed.

(c) Wheel extender used.

80
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81
a) Normal

82
Figure A-3. IRS pavement wetting on solid pavement ahead

of the DDG grate.

Figure A-4. Severe splashing over the DDG.

83
APPENDIX B

The Test Matrix

A subdivision of the test plan consists of the following test matrix:

Time Period

Surface

S - AS 1 2 3 4 5 6 7

(LANES)

S+-AS

S

4

2345671

5 6 7 1 2 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

tire A with all the other tires. To

eliminate any measurement deviations

and at different speeds.

effect of making a measurement with

do this effectively, we want to elimidue to skidding in different lanes

Let a measurement result be represented by the notation

(SN + £) LT(S-AS)

where:

SN = a nominal skid number measured value,

+e = an increase in the SN value due to testing at

lower than the nominal test speed by the

increment AS,

L = lane identification of the test measurement, and

T = the tire identification of the test measurement.

84
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 + £) 1A(S - AS) " < SN

>1G(S) " (SN "

£ >1E(S + AS) " A

AGE + 3s <«

2(SN) 2A(S)

- (SN- E) 2B(S + As)- (SN + £ ) 2F(S _ AS) . A

ABF (2)

2(SN "

£) 4A(S + AS) " (SN »4C(S) " <SN + E) 4D(S - AS) = A

ACD " 3e TM

where AA

= twice the difference between using tire A and the average

of using tires G and E in lane 1,

^ABF

= twi ce *~h.e difference between using tire A and the average

of using tires B and F in lane 2,

^ACD

= tw:i- ce t-k e difference between using tire A and the average

of using tires C and D in lane 4.

Note that Equations 1, 2, and 3 are each independent of lane effects

Note also that the sum of the three equations is independent of any speed

effects (assuming AS is small). Consequently, the sum is free of both

lane and speed effects. To simplify the notation, eliminate the lane

and speed variations from the sum, and we get

6(SN) A

- (SN)

B

- (SN)

C

- (SN)

D

- (SN) £- (SN)

p

- (SN>G

(4)

We may add to this equation (8) , 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

- |sNA

+ SN

B

4- SN

C + SN^ + SN

£ + SNp + SHG j ? ^

(6)

85
Now, the measurement performance with tire A can be compared with th< performance of all the other tires by writing

= mean of all

measurements + /yx

7

In a similar manner, the test matrix can be rearranged to yield, for ex- ample, the measurement performance on lane 1 compared with the performance

on all the other lanes. The results are then a measure of surface

uniformity

.

86
APPENDIX C

General Description of the Eastern Area Reference System

(Tow Vehicle and Trailer)

L. System Identification

1.1 System manufactured by: K. J. Law.

1.2 Model: 1270.

1.3 Tow vehicle ID number: DOT 40444.

1.4 Trailer ID number: DOT 40409.

2. Tow Vehicle

2.1 Make: Chevrolet.

2.2 Model year: 1973

2.3 Body style: C30 series "Camper Special" pickup. 10,100 lbGVW

rating.

2.4 Engine: 454-4/SE V8.

2.5 Fuel tank capacity: main 20 gallons, auxiliary 16 gallons.

2.6 Seating: bucket seats, 2 front, 2 rear.

2.7 Normal occupancy: front seats only.

2.8 Air conditioning: yes.

2.9 Wheel base: 164-1/2 inches.

2.10 Track: front 66 inches, rear dual 64 inches inside, 84 inches

outside.

2.11 Tire size: front 8.75-16.5C(6 PR)(TL), rear 8.75-16.5C(6PR(TL).

2.12 Rear wheels: dual.

3. Trailer

3.1 Construction: welded box section chassis with angular sheet

metal body.

3.2 Length: center of hitch to center of axle 120 inches.

3.3 Track 64-5/16 inches (E501 tires).

3.4 Suspension: coil springs, "parallelogram" trailing arms, sway

bar. One air spring shock absorber at each wheel. Air springs

operated at 50 psi.

3.5 Test brakes: hydraulic disk with hydraulic actuator, selectable

for left or right wheel tests.

3.6 Skid measurement transducers: left and right wheels instrumented.

Strain gaged beams between axle and brake backing plate. Outputs

for traction force and normal force.

4. Pavement Wetting

4.1 Water tank capacity: 290 gallons.

4.2 Pumps: gear type, one for left wheel tests, one for right

wheel tests, powered by drive shaft.

4.3 Flow rate control: approximately proportional to speed. Adjusts

by changing drive pulleys.

4.4 Nozzles: Penn State type, mounting adjustable for pitch angle

and height.

87
5. Instrumentation

5.1 Speed: Labeco Tracktest fifth wheel and full scale digital

driver aid readout. Independent tachometer generator on trailer wheel for full scale analog data recording readout.

5.2 Water supply: gage readout in cab.

5.3 Flow rate: no gage.

5.4 Inflation: Common tire gage referenced to precision gage.

5.5 Data recorder: Brush Model 440, strip chart, four

channels side by side, pressure ink pens.

5.6 Signal amplifiers: (2) Brush 13-4215-12 for vertical load

and traction force; (4) Brush 13-4215-32, one for speed, one

for SN, and two for pen position of force signals.

5.7 Channel assignments: Ch 1, traction force; Ch 2, normal force;

Ch 3 , speed; Ch 4 skid number.

Comments: signal recorded on Ch 4 is the instantaneous ratio

of the traction force signal to the normal force signal. One

second time marks are also recorded. Chart rate is 5 cm/sec.

Channels are 4 cm wide with 50 divisions.

5.8 Transducer excitation: 10.02 Vdc.

6. Trailer suspension:

Tire radial spring rate 1280 lbf/in

Effective spring rate: test wheel-to-body 350 lbf/in

body-to-pavement 275 lbf/in

Effective Coulomb friction (without shocks) 9.4 lbf

Natural frequency, high: 12.7 Hz

Natural frequency, low: 1.3 Hz

Natural frequency, tire: 3.1 Hz

Effective viscous damping factor: 0.2

Operating Parameters of the Eastern Area Reference System

The information below represents the values of recognized EARS vari

ables in effect during this comparison with the IRS.

1. Test tires: standard pavement test tires, G 78-15, ASTM

Designation E501.

2. Test tire inflation: 24 psi, maintained during testing.

3. Hitch height: 11-7/8 inches from level pavement to bottom

of trailer hitch flange, maintained during testing.

4. Test side of trailer: left only.

5. Calibration signals: left traction 501 lbf, left normal 1081

lbf; speed, none.

6. Test Sequence: Figure C-l

7. Consumable cargo: pavement wetting tank refilled frequently,

nominally after 10 runs at 60 mph.

88
89
8. Condition of test pavements: dry before each test.

9. Flow rates: 20 mph, 13.5 gpm; 40 mph, 27.2 gpm; 60 mph,

40.2 gpm.

10. Crew

7.1 Driver: Herb Litteral.

7.2 Operator: Bill Edwards.

7.3 For further information, contact:

E. A. Whitehurst

Principal Investigator

Field Test and Evaluation Center

for Eastern States

Post Office Box 249

East Liberty, Ohio 43319

90
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1 PIIH1 If'ATIDM OR RKPDRT \!n

NBSIR 75-972

2. Gov c Accession

No.

3 • Recipient s Accession No.

A TIT I P AVn sI 'RTITI K

SKID RESISTANCE MEASUREMENT TESTS AT THE

FHWA EASTERN FIELD TEST CENTER

5. Publication Date July 1976

nU • lPpCllUIIIllUg r f <~\ r m in o IvlIroan KAII iI£aLlUU 7at inn (^UUt nn f

7. AL'THOR(S) R. W. Kearns and J. F. Ward

8. Performing Organ. Report No.

7. rtiVr UivMllNU VJi\OAlNiiL,AI 1 WIN *\ A Mt AINU AIJ 1J r\ too NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2130440

11. Contract/Grant No.

•*Z • Sririfi^nrinp (ft oan i73rinn N a mp an n I n m n 1 r AHnrpcc frop t i7 t\i S t a t & /I P ^ r £> g uLl l£. at 1 ull -> olllc dll l_l *•— L' 111 L- 1 C Lt '>UU1 Coo 1 *J Li Ct ( , v 1 l y , ^ tu I C , ii /

Federal Highway Administration

Office of Development

Washington, D. C.

13. Type of Report & Period Covered Final

August-October 1974

14. Sponsoring Agency Code

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 measurement of the skid resistance of highways, under wet weather

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

reduction program. To standardize and to improve the precision of the measurements, the program includes national and regional reference systems to which

the highway measuring systems may be systematically related. This report describes evaluation tests conducted at the FHWA Eastern Field Test Center (EFTC)

which included the use of the Eastern Area Reference System (EARS) and the

Interim Reference System (IRS) maintained by the National Bureau of Standards.

The performance characteristics of the EARS, its subsystems and a correlation

equation between the EARS and the IRS are presented. A method of quantifying

sources of dispersion is applied to the measurements made by the IRS on the

EFTC reference surfaces. Recommendations for modifications to the EARS, the

test procedures, and EFTC facilities are made.

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;

pavement, skid resistance; pavement wetting system; tire-pavement interface

forces; wet pavement.

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