NBSIR 76-1174

Skid Resistance Measurement

Test Procedures for

Intercomparing FHWA National

and Regional Reference

Systems

Robert W. Kearns and John F. Ward

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1976

Prepared for Federal Highway Administration

Office of Development

Washington, D. C. 20590

NBSIR 76-1174

SKID RESISTANCE MEASUREMENT

TEST PROCEDURES FOR

INTERCOMPARING FHWA NATIONAL

AND REGIONAL REFERENCE

SYSTEMS

Robert W. Kearns and John F. Ward

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1976

Prepared for Federal Highway Administration

Office of Development

Washington, D. C. 20590

v " y

o' *

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

Edward O. Vetter. Under Secretary

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

Federal Highway Administration Policy Statement

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 1

2. PREPARATION, MEASUREMENT AND ADJUSTMENT OF SUBSYSTEMS 5 2.1 Test Preparations 5 2.1.1 Orientation of Personnel 5 2.1.2 Documentation and Description of the ARS 6

2.1.3 Test Tires 7 2.1.4 Tow Vehicles 8

2.1.5 Force Measuring Instrumentation 9 2.1.6 Skid Trailers 10

2.1.7 Force Plate Calibration 11

2.2 Speed Measuring Systems 12

2.2.1 Fifth Wheel 12

2.2.2 Drive Train Tachometer 13

2.2.3 Radar Correlation and Determination of 14

Fifth Wheel Sensitivity to Operating

Parameters

2.3 Water Laying System 15

2.4 Wheel Force Calibration 16

2.4.1 Transducer Orientation 16

2.4.2 Vertical Load Channel Calibration 17

2.4.3 Traction Channel Calibration 18

2.4.4 Determination of the Effective Vertical 18

Unloading Constant

2.4.5 Sensitivity of Wheel Force Calibration to 19

Variations in Operating Parameters

2.5 Trailer Displacements 21

2.5.1 Displacements under Static Traction Force 21

2.6 System - Crew Performance Tests 22

2.6.1 System-Driver Rating 22

2.6.2 System-Operator Rating 23

3. SKID RESISTANCE MEASUREMENTS OF PAVEMENTS 24

3.1 The Test Matrix 24

3.2 Variation of Measurement Results with 27

Operating Parameters

4. INTERRELATION AND CORRELATION OF REFERENCE SYSTEM 28

MEASUREMENTS

5. REFERENCES 30
'
Skid Resistance" Measurement Test Procedures for

Intercomparing FHWA National and Regional Reference Systems

Robert W. Kearns and John F. Ward

ABSTRACT

The measurement of the skid resistance of

highways, under wet weather conditions, is part

of the Federal Highway Administration (FHWA) skidaccident reduction program. A national reference

system and regional reference systems are operated

to improve the method of measurement, to reduce

differences in results between systems and to include measurement assurance in the program. State

highway measuring systems are intercompared with

reference systems at FHWA regional field test

centers. This document describes the objectives,

the scope and the general procedures of the tests

used to intercompare the regional reference systems

with the national reference system operated by the

National Bureau of Standards. Listings of the

general equipment required are included.

Key Words: Friction, pavement; highway safety;

measurement, skid resistance; pavement, skid

resistance; test procedures, skid resistance

measuring systems; tire-pavement interface forces.

1. INTRODUCTION

Most measurements of the skid resistance of highways are carried

out using locked-wheel skid measurement systems. The measurement is

based upon the steady state, friction force developed by a locked test

wheel as it is dragged over a wetted pavement under a constant load

and at a constant speed. The test apparatus consists of an automotive

vehicle with one or more test wheels incorporated into it or forming

part of a suitable trailer towed by a vehicle. It includes force and

velocity transducers and associated instrumentation, a water supply

and dispensing system, and actuation controls for the brake of the

test wheel. The test wheel is equipped with a standard ASTM tire for

pavement tests.

1
In the measurement process, the test apparatus is brought to the

desired test speed, water is delivered ahead of the test wheel and the

braking system is actuated to lock the wheel. The friction force

generated between the test tire and the pavement surface (or some other

quantity that is directly related to this force) and the speed of the

test vehicle are recorded with the aid of suitable instrumentation.

The result is expressed as skid number, SN, which is determined from

the force required to slide the locked test tire at a stated speed,

divided by the effective wheel load and multiplied by 100.

Requirements for this type of locked-wheel skid measuring system

have been set forth in ASTM Designation E274 [1]*, and most of the

systems operated by state highway departments are reported to be in

substantial compliance with these requirements. However, differences

between various systems that comply with the requirements have been

found to cause variabilities between measurements made with different

systems. In order to reduce these measurement variabilities, and also

to improve the precision of the measurements made with a given system,

the Federal Highway Administration (FHWA) is developing a hierarchy for

the measurement of pavement skid resistance [2]. This activity is being

pursued as part of the FHWA's skid accident reduction program.

At the national level the hierarchy provides for an interim re- ference system (IRS) maintained by the National Bureau of Standards (NBS)

.

At the regional level the hierarchy provides for area reference systems

(ARS) maintained at FHWA Field Test Centers (FTC) . The skid resistance

measuring systems maintained by the states are used to inventory and

rank the skid resistances of highways. The FHWA plan to assure consistency between state measurements includes the development and monitoring

of test procedures and apparatus by NBS in order to relate the measurements made within the hierarchy to the national standards of measurement.

The IRS is a skid measuring system developed for FHWA by NBS. It was developed by analyzing and modifying a commercial, two-wheel skid

trailer and a 3/4-ton (680-kg) crew-cab truck. The system, as modified,

is shown in figure 1. The trailer incorporates a strain-gage-based,

beam-type transducer mounted on the left wheel axle. This transducer

is instrumented to measure vertical and horizontal wheel forces and the

torque applied to the wheel. A layer of water is applied to the pavement just ahead of the left wheel by a flow channel water chute specially

designed to have a non-divergent flow pattern and to minimize the effects

of minor perturbations in the water flow. The water is delivered from a tank on the truck by a pump that is belt driven from the drive shaft.

The flow rate is controlled by metering valves and measured with a turbinetype flow meter. The driver is assisted in controlling the vehicle speed

during tests by visual and audio signals that indicate the deviation from

*Numerals in brackets denote the literature references cited at the

end of this report.

2
u

O OC

Z>

3

Figure

1.

The

interim

reference

skid

resistance

measurement

system

(IRS).
the specified test speed. The speed signal is derived from a fifth

wheel attached to the truck. Speed is also measured and recorded by a stationary radar system. During a test the horizontal (traction) force,

vertical (wheel load) force, water flow rate, and speed (as measured by

the fifth wheel) are recorded in analog form in the tow vehicle. These

same quantities, except speed, are also recorded in digital form by a transient recorder also located in the tow vehicle, and speed (as measured

by the radar) is recorded at the ground station. At the conclusion of a test, the digital information is transferred to the ground station for

processing. The ground station is a recording-calculating-plotting

system designed to process the data to obtain the instantaneous measured

skid number as a function of location on the test pavement.

This report outlines the test procedures which are used to elicit

and to compare the performances of the IRS and the several ARS systems.

The objective and the scope of each test procedure is given as well as a check list of the general equipment required. The report is addressed,

primarily, to the FTC engineers who are responsible for comparing the

state systems with the ARSs; to provide them with the opportunity to

adopt, for their work, some or all of the outlined procedures. These

engineers are already familiar with the IRS and its performance, and

with the general nature of these test procedures. Therefore, the procedure outlines are intended to serve only as general guides and reminders,

not as test method standards.

The preparations which should be carried out before testing begins

are summarized in section 2.1 of this report. The calibrations of the

speed measuring and force measuring equipments are outlined in sections

2.2 and 2.4, respectively. The needs for an improved water laying

system, and for an improved test of its performance, are discussed in

section 2.3.

In section 2.5, displacement measurements are summarized which

identify the static motions due to the static traction forces applied at

the tire-pavement interface. However, motions of the trailer during

skid testing involve dynamic forces. Thus, while only a prelude to a dynamic force analysis, the static displacement results can be used to

compare different trailer designs.

Skillful operation of the national and regional reference systems

is required to achieve reliable and repeatable measurements. The test

method is based upon skidding within designated lanes of the surface at

designated longitudinal positions. The accomplishment of a constant

speed during skidding mandates a close relationship between the driver

and the power train. The instrumentation operator needs to know the

timing of his system and be able to compensate for the time lags as- sociated with the system response. The tests outlined in section 2.6

acknowledge the contribution of a skillful crew to the measurement

process and suggest a means of quantifying the crew-system performance.

4
Non-uniformities in tires, pavement characteristics, and weather

conditions contribute to data scatter. Test matrices can be designed to

identify such conditions when the scatter exceeds the normal dispersion

level of the measurements. A test matrix, or test program building

block, is described in section 3.1.

Other parameters which can be controlled by the conditions of

operation of the system have also been shown to affect the skid resistance measurement results. The tests summarized in section 3.2 can establish the various ranges of operational control needed to achieve

the desired degree of measurement precision. Since different pavements

are expected to exhibit different speed and water depth sensitivies,

different degrees of operational control may be required on the different

test surfaces to achieve the same precision of measurement.

The results of the tests conducted in sections 2 and 3 should be

obtained prior to system correlation. If the sensitivity of SN to a particular operating variable is unknown, a constant value of the

variable should be maintained during the correlation tests. To interrelate the measurements of the reference systems, a test program is

designed using the previously mentioned procedures as building blocks.

The desired elements of such a program are given in section 4.

The measurement assurance program for an FTC and its 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 the

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 increased

measurement capabilities being transferred to highway skid resistance

measuring systems at the FTC.

These procedures were developed through a cooperative effort by

personnel of NBS, FHWA and the Western, Central, and Eastern FTCs.

2. PREPARATION, MEASUREMENT, AND ADJUSTMENT

OF SUBSYSTEMS

2.1 Test Preparations

2.1.1 Orientation of Personnel

a. Objective

The test personnel are to be prepared for the test program to be

conducted.

5
b. Scope

(1) An introduction to the test program will be provided.

(2) The operational and safety procedures in effect at the test

site and the proper channels through which requests for

assistance should be processed will be determined.

c. General Procedures

(1) An inspection of the test equipment will be conducted.

(2) An orientation session will be held, consisting of discussions

among the assembled test personnel. The test director will

outline the test program, the preferred test sequence and

schedule, and duties which may require supplementary test

personnel

(3) The notations required on the test records, the amount of preliminary data reduction, the contents of any preliminary letter

reports, and the final report will be discussed.

(4) During the session, questions will be encouraged.

(5) Details of FTC operational and safety procedures will be

solicited by the test director.

d. General Equipment Required

(1) Test plan and schedule.

(2) Lists of personnel duties and requirements.

2.1.2 Documentation and Description of the ARS

a. Objective

General information about the ARS and the testing facilities of the

FTC is to be obtained.

b. Scope

The information can consist of photographs, general technical data

and descriptions including mechanical dimensions, schematics and system

block diagrams.

6
c. General Procedures

(1) Photograph each system tested.

(2) Photograph each test set up.

(3) Record pertinent dimensions of each system and test set up.

(4) Obtain or develop pertinent schematics or block diagram

information

d. General Equipment Required

(1) Camera with film.

(2) System manuals.

(3) Measuring equipment.

2.1.3 Test Tires

a. Objective

A set of tires sufficient for the test program is to be prepared.

b. Scope

(1) The tires will be conditioned for use.

(2) Test data will be obtained for the evaluation of

engineering characteristics of the tires.

(3) The degree of uniformity will be investigated within

and between tires, considering wheel radius, groove

dimensions, rubber hardness, and spring rate.

c. General Procedures

(1) A sufficient number of standard test tires will be

obtained, preferably from one production run.

(2) The tires will be mounted on appropriate wheels and

balanced dynamically.

(3) The tires will be individually numbered on the outboard

sidewall as a means of identification and a radial paint

stripe will be applied as an index mark.

(4) A go/no go gage will be used to determine that groove

width and depth are within tolerance.

(5) Each tire, inflated to 24 psi (165 kPa) will be run under load for 200 miles (320 km) near 40 mph (64 km/h).

7
(5) Each tire, inflated to 24 psi (165 kPa) will be run under load for 200 miles (320 km) near 40 mph (64 km/h)

.

The following procedures may be applied to a representative

sample of the lot.

(6) Each tire will be stored for 24 hours or more at

approximately 72 °F (22 °C) . At 24 psi (165 kPa)

inflation pressure, the tread hardness will be

measured using a Type A-2 durometer with one kilogram

total load.

(7) With a radial load of 1085 lbf (4825 N) applied to

each tire in turn, the inflation pressure will be

re-adjusted to 24 psi (165 kPa) and the tire radius

will be measured at the point of load application.

(8) The radial load will be varied between 700 lbf (3100 N)

and 1500 lbf (6700 N) , and the corresponding changes in

tire radius will be measured. The results will be used

to calculate the radial spring rate. Tangential spring

rate can be determined from traction displacement data

(see section 2.5).

d. General Equipment Required

(1) Tire mounting and balancing equipment.

(2) Go/no go groove profile gage.

(3) Thermometer.

(4) Tire inflation pressure gage.

(5) Type A-2 durometer.

(6) Paint.

(7) Means of applying load and measuring displacement.

2.1.4 Tow Vehicles

a. Objective

The tow vehicles, in turn, will be prepared for the test program.

b. Scope

(1) The normal operating configurations of the tow vehicles

will be determined.

(2) Test data will be obtained concerning the displacement of

the hitch and the changes in radius of the tow vehicle

tires as a function of occupant weight, tire inflation,

and water dispensed.

8
c. General Procedures

(1) The crew complement and seating arrangement of the

tow vehicle will be determined. Normal on-board

storage of auxiliary equipment will be determined

and implemented. Fuel and pavement-wetting tanks

will be filled.

(2) Crew weight and trailer hitch loads will be simulated

with weights.

(3) Hitch displacement and tow vehicle tire radius will be

measured as occupant weights are modified, tire-inflation

pressure is changed, and water is removed from the tow

vehicle.

d. General Equipment Required

(1) Weights.

(2) Displacement gages.

(3) Inflation pressure gage.

(4) Means of removing water from tow vehicle tanks.

(5) Means of measuring volume or weight of water removed.

2.1.5 Force Measuring Instrumentation

a. Objectives

The skid- test systems, in turn, will be checked to determine the

operational readiness of their on-board force measuring instrumentation.

b. Scope

(1) All on-board force measuring instrumentation necessary

for the skid-test program will be checked to ensure

that it is functional.

(2) Tests will be conducted to evaluate resolution, linearity,

frequency response, stability, and other characteristics,

as appropriate, to ensure the reliability of the

instrumentation.

c. General Procedures

(1) Signal generators will be used to simulate force

inputs within normal ranges of amplitude and frequency.

(2) Outputs will be observed for the normal range of instrumentation settings under operator control.

9
(3) Engine speed will be varied and the stability of excitation

voltages, amplifier outputs, etc., will be observed.

(4) An oscilloscope will be used in a high-gain, a-c measuring

mode to identify characteristics of noise.

(5) An auxiliary X-Y recorder will be used to expand the output

signals for the determination of linearity and hysteresis.

(6) The cab heater/air conditioner or other means will be used

to vary instrument compartment temperature. Temperature

stability of each instrumentation channel will be observed.

(7) The wheel force transducer temperature will be varied by

conductive heat transfer from the brake assembly. Several

skid tests (brake applications) will be performed in

succession as a means of heating the assembly. Transducer

outputs will be monitored for temperature stability.

d. General Equipment Required

(1) Signal generators.

(2) Oscilloscope.

(3) Voltmeters.

(4) X-Y recorder.

2.1.6 Skid Trailers

a. Objective

The skid trailers, in turn, will be prepared for the test program.

b. Scope

(1) The desired trailer static weight distribution will be

attained.

(2) Test data will be collected to enable the determination

of engineering characteristics of the trailer suspension

system, such as Coulomb friction, damping factors,

suspension spring rates, and natural frequencies of the

trailer

c. General Procedures

(1) Normal values will be set for trailer operating variables,

e.g., tire and shock absorber inflation pressure, hitch

height, etc.

10
(2) The trailer will be weighed simultaneously at its three

points of support, i.e., the hitch and the two tires.

(3) The total weight and its distribution will be trimmed

such that each tire supports 1085 ± 5 lbf (4825 ± 20 N)

and the hitch supports 150 ± 25 lbf (670 ± 100 N)

.

(4) The trailer body will be lifted slightly through its

center of gravity and released abruptly. The transient

time variations of the following quantities are to be

recorded: body-to-axle displacement; body-to-pavement

displacement; axle- to-pavement displacement; vertical

load on the test wheel as measured by a force plate under

the wheel; vertical load on the test wheel as measured by

the wheel transducer.

(5) Step (4) will be repeated for a variety of lift forces,

both with and without the trailer shock absorbers

connected

.

(6) The trailer will be lifted slightly through the axle

and released abruptly. Again, the transient variations

of the quantities listed in (4) are to be recorded.

(7) Step (6) will be repeated for a variety of lift forces

both with and without the trailer shock absorbers

connected.

d. General Equipment Required

(1) Force plate instrumented for vertical load.

(2) Vertical load transducers.

(3) Inflation pressure gage.

(4) Electrical displacement gages.

(5) X-Y recorder with time-base feature.

(6) Means of lifting and releasing the body and axle.

2.1.7 Force Plate Calibration

a. Objectives

A force plate with multiple sensitive axes is to be calibrated in

a manner traceable to the national standards of force by a procedure

that minimizes sources of error.

11
b. Scope

(1) The force plate is to be calibrated in a fixture that

provides simultaneous combined loading of its sensitive

axes. Loads applied are to be determined by load cells

that have been directly compared with the national

standards of force. Loading conditions will be designed

to simulate conditions of actual use.

(2) Means are provided with the force plate to generate

reference signals under specified conditions. These

signals will be adjusted to provide force plate outputs

equivalent to known loads. The signals can be used to

adjust the gain and offset of readout instrumentation

used with the force plate.

c. General Procedures [3]

(1) The orientation of the traction axis of the force plate

with respect to its surface is to be determined.

(2) Combined loads are to be applied in accordance with the

vertical force unloading-due-to-traction characteristic

of the trailer design with which the force plate is to

be used.

(3) Traction response of the force plate will be recorded

under combined loading. Traction reference signals will

be adjusted under 1085 lbf (4825 N) vertical loading.

(4) Vertical response of the force plate will be recorded

under vertical loading. Vertical reference signals will

be adjusted under no-load conditions.

d. General Equipment Required

(1) Multiaxial force loading fixture.

(2) Load cells traceable to the national standards of force.

(3) Readout electronics.

(4) Means for measuring angular displacement.

2.2 Speed Measuring Systems

2.2.1 Fifth Wheel

a. Objective

A fifth wheel speed measuring system is to be calibrated.

12
b. Scope

The fifth wheel readout is to be compared with known speed and

distance quantities determined by timed operation over a measured

course.

c. General Procedures

Use the procedure given in ASTM Designation F457 [4].

d. General Equipment Required

(1) Measured mile.

(2) Stop watch.

2.2.2 Drive Train Tachometer

a. Objective

The relation between speed and drive train tachometer readings is

to be established as an aid to water flow rate adjustments.

b. Scope

Tachometer readings will be obtained at the speeds designated for

skid testing.

c. General Procedure

(1) The tow vehicle will be put in its normal operating

configuration.

(2) Water and fuel tanks will be filled.

(3) Tires will be warmed up by a five-mile (8-km) drive at

approximately 40 mph (64 km/h)

.

(4) Tow vehicle tires will be set at the desired inflation

pressure for skid testing.

(5) The tow vehicle will be driven at a speed designated for

skid testing and in the gear ratio to be used during skid

testing.

(6) The corresponding drive train tachometer reading will be

recorded.

(7) Steps (5) and (6) will be repeated at each designated speed.

13
d. General Equipment Required

(1)

Inflation-pressure gage.

2.2.3 Radar Correlation and Determination of Fifth Wheel

Sensitivity to Operating Parameters

a. Objectives

(1) Speed measurements made with the calibrated fifth wheel are

to be correlated with radar speed measurements.

(2) Test data are to be obtained to determine the sensitivity

of the fifth wheel to its operating parameters.

b. Scope

(1) Tow vehicle speed will be measured simultaneously by fifth

wheel and radar techniques at each speed designated for skid

testing.

(2) The tests will be repeated with variations of fifth wheel

inflation pressure and vertical force.

c. General Procedures

(1) The fifth wheel is to be calibrated in accordance with

section 2.2.1.

(2) The fifth wheel will be warmed up by a five-mile (3-km)

drive at approximately 40 mph (64 km/h).

(3) The tow vehicle is to be driven directly toward the radar

deviating only near the end of the run in order to avoid

colliding with the radar equipment.

(4) The tow vehicle is to be driven with a speed variation of

approximately ±2 mph (±3 km/h) about the speed specified

by the radar operator.

(5) Simultaneous records are to be made of the fifth wheel

and radar readouts, and synchronized by means of the

speed peaks.

(6) The test will be repeated at each speed designated for

skid testing.

(7) The test will be repeated with fifth wheel inflation

pressure varied from normal by ±10 percent and ±20

percent

.

14
(8) The test will be repeated with the fifth wheel vertical

force varied from normal by ±10 percent and ±20 percent.

d. General Equipment Required

(1) Speed measuring radar system with 0.1 mph (0.2 km/h)

resolution and a recorder output.

(3) Means for determining and adjusting the vertical force

exerted against the pavement by the fifth wheel.

2.3 Water Laying System

The standard method of test for skid resistance, ASTM E274 requires

that the pavement wetting system be suitably designed to ensure that the

water layer encountered by the test tire has a uniform cross section at

all test speeds, with a minimum of splash and over-spray. Thus, the cross section of water to be measured is located at the forward edge of

the tire-pavement interface. While in transit from the nozzle to the

interface, the water is subject to. many disturbances.

At this time, a completely satisfactory pavement wetting system

does not appear to be available, nor suitable equipment and procedures

for measuring water laying system performance.

The static distribution gage (SDG) measures the distribution of

water across the width of the stream, but not at the leading edge of

the tire-pavement interface nor after the water trajectories are modified by impact with the pavement surface under dynamic conditions

(i.e. with the trailer in motion). The dynamic distribution gage (DDG)

is used under dynamic test conditions to meet the latter objection but

is not a test of conditions at the leading edge of the interface. The

DDG, as it is presently designed, collects extraneous water such as the

spray which follows the trailer as well as the water ahead of the test

tire.

The various ways of evaluating wetted width include using the 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 to show the water trace width at the leading

edge of the tire interface. These methods produce widely varying

results

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. Measurements of photographs of the wetted width from the

moving skid trailer require geometric calculations to account for changes

in scale with distance from the camera. At this point, an optimal

camera arrangement with proper lighting has not been established.

15
Consequently, no procedures for measuring water laying system

performance are given here. Additional test descriptions and a discussion of measurement errors are given in reference 5.

2.4 Wheel Force Calibration

2.4.1 Transducer Orientation

a. Objective

The wheel transducer is to be oriented on the trailer such that large

changes in vertical load produce only minimum changes in the traction

force readings.

i b. Scope

i The adjustment is applicable to force transducers instrumented for

traction force.

c. General Procedure

(1) The trailer is to be put in its normal operating position.

(2) The test wheel is to be supported on an air bearing.

(3) The transducer traction instrumentation is to be set on a sensitive range and the reading is to be zeroed.

(4) The test wheel is to be lifted into the air and any change

in traction reading noted.

(5) Steps (2) to (4) are to be repeated while varying hitch

height until there is a minimum change in the traction

reading between loading conditions.

(6) On single test wheel trailers, operation at this hitch

height is an alternative to rotating the transducer.

On dual test wheel trailers at least one transducer will

have to be rotated if the objective is not met initially.

d. General Equipment Required

(1) Air-bearing support for test wheel.

(2) Adjustable hitch on tow vehicle.

16
2.4.2 Vertical Load Channel Calibration

a,. Objectives

(1) The vertical load measuring system is to be calibrated.

(2) The linearity and hysteresis of the system are to be

determined

b. Scope

The performance of the instrumentation is to be recorded externally

using an expanded scale. The variation in vertical load is to be at

least ± 400 lbf (± 1800 N) about the static load value.

c. General Procedure

(1) The trailer frame is to be pinned to a vertical air

cylinder which can unload or load the frame. Alternatively, weights may be placed symmetrically on the

trailer frame to increase the load. An hydraulic jack

may be used to lift the trailer by the body to reduce

the load.

(2) A force plate instrumented for vertical load is to be

placed under the test wheel.

(3) Using an external X-Y recorder as readout, plot the

on-board vertical load instrumentation channel output

versus the force plate vertical load output. The plot

is to cover an increasing and decreasing range of at

least ± 400 lbf (± 1800 N) . At the same time, a voltmeter is to be used to record numerical values.

(4) Under static vertical load, the output(s) of the

on-board vertical load calibration signal generator

is to be recorded.

(5) The data are to be examined to determine the equivalent

force value of the on-board calibration signal (s) and

to evaluate the linearity and hysteresis of the

instrumentation channel.

d. General Equipment Required

(1) Force plate system.

(2) X-Y recorder with high resolution.

(3) Stable voltmeter with high resolution.

17
2.4.3 Traction Channel Calibration

a. Objectives

(1) The traction force measuring system is to be calibrated.

(2) The linearity and hysteresis of the system are to be

determined.

b. Scope

The performance of the instrumentation under combined vertical and

traction loading conditions is to be recorded externally using an ex- panded scale. The variation in traction load is to be from zero to at

least 800 lbf (3600 N) and return.

c. General Procedure

(1) A force plate instrumented for traction is to be placed

under the test wheel.

(2) Using an external X-Y recorder as readout, plot the

on-board traction instrumentation channel output versus

the force plate traction output. The plot is to cover increasing and decreasing traction forces from zero to

at least 800 lbf (3600 N) . At the same time, a voltmeter

is to be used to record numerical values.

(3) Under zero traction force, the output(s) of the on- board traction calibration signal generator is to be

recorded.

(4) The data are to examined to determine the equivalent

force value of the on-board calibration signal (s) and

to evaluate the linearity and hysteresis of the

instrumentation channel.

d. General Equipment Required

(1) Force plate system.

(2) X-Y recorder with high resolution.

(3) Stable voltmeter with high resolution.

2.4.4 Determination of the Effective Vertical Unloading Constant

a. Objective

The effective value of the unloading constant H/L (geometrically,

the ratio of hitch height to horizontal distance from hitch to trailer

axle) is to be determined.

18
b. Scope

The constant is to be determined for the hitch height obtained in

section 2.4.1 and also for this hitch height increased by one inch

(2.5 cm)

.

c. General Procedure

(1) The wheel transducer traction signal is to be applied

to the X-axis of an X-Y recorder.

(2) The wheel transducer vertical load signal is to be

applied to the Y-axis of the X-Y recorder.

(3) The trailer test wheel is to be supported on the

force plate.

(4) The hitch height is to be set to the value determined

in section 2.4.1.

(5) Traction force is to be applied to the plate, increasing

and decreasing the force over a range from zero to at

least 800 lbf (3600 N)

.

(6) The data are to be recorded and a straight line is to be

fitted to the plot. The slope of this line, measured

from the X-axis, is the effective value of H/L for the

trailer

.

(7) Increase the hitch height by one inch (2.5 cm).

Repeat steps (5) and (6). The slope of the plotted line

is the effective value of (H + 1)/L for the trailer

when H and L are expressed in inches.

d. General Equipment Required

(1) Force plate system.

(2) X-Y recorder with high resolution.

2.4.5 Sensitivity of Wheel Force Calibration to Variations

in Operating Parameters

a. Objective

The change in offset and/or sensitivity of the wheel force transducer

readings, due to variations in operating parameters, is to be measured.

19
b. Scope

The operating parameters to be varied may include:

(1) hitch height,

(2) test wheel radius,

(3) test tire inflation pressure,

(4) shock absorber inflation pressure,

(5) trailer static weight,

(6) lateral force at the tire-pavement interface, and

(7) transducer temperature.

c. General Procedure

(1) The transducer instrumentation is to be set up in the

normal manner with the skid testing system in its normal

operating configuration. The test wheel of the trailer

is to be supported on a force plate.

(2) The value of one operating parameter is to be increased

to the upper bound of the range expected during skid

testing.

(3) Traction force is to be applied to the test wheel. The

wheel transducer instrumentation output is to be plotted

versus the force indicated by the force plate, using an X-Y recorder. At the same time, a voltmeter is to be

used to record numerical values. Any difference from

the results with the normal value of the operating

parameter is to be noted.

(4) The value of the operating parameter is to be decreased to

the lower bound of the range expected during skid testing.

(5) Step (3) is to be repeated for the decreased value.

(6) Steps (2) through (5) are to be repeated for each

operating parameter to be investigated.

d. General Equipment Required

(1) Force plate system.

(2) X-Y recorder with high resolution.

(3) Stable voltmeter with high resolution.

(4) Means of varying the operating parameters.

20
2.5 Trailer Displacements

2.5.1 Displacements Under Static Traction Force

a. Objective

Translation and rotation of the skid trailer, in response to static

traction force, are to be characterized.

b. Scope

(1) The trailer, hitched to the tow vehicle, is to be subjected

to static traction forces comparable to the steady-state

forces experienced in skid testing.

(2) Displacement measurements are to obtained over this range

of forces to aid development of analytical representations

of the skid testing system, and to permit comparisons with

other trailer designs.

c. General Procedure

(1) The trailer and tow vehicle are to be hitched together,

in line and on level pavement, and in the normal operating configuration. The test wheel of the trailer is to

be supported on a force plate.

(2) Displacement measuring devices are to be positioned

about the trailer to measure the following responses

to traction force:

(a) trailer axle rotation in roll and yaw,

(b) trailer body rotation in roll, pitch, and yaw,

(c) trailer body translation along longitudinal,

lateral, and vertical axes,

(d) hitch point translation along longitudinal,

lateral, and vertical axes,

(e) change in radius of test tire, and

(f) motions of the trailer suspension.

(g) Tow vehicle tire longitudinal deflection may also

be measured at this time by means of a displacement

measurement at the tow vehicle rear axle.

21
(3) Traction force is to be applied through the force plate

over a range from zero to 800 lbf (3600 N) and return.

Displacements are to be measured over the full range of

increasing and decreasing traction at increments of

approximately 50 lbf (220 N)

(4) The results are to be plotted to show the motion

characteristics of the trailer versus increasing and

decreasing traction force.

i

d. General Equipment Required

(1) Force plate system.

(2) Displacement transducers such as dial gages or electrical

linear potentiometers.

2.6 System - Crew Performance Tests

2.6.1 System-Driver Rating

a. Objective

A performance rating for the system-driver combination of a skid

resistance measuring system is to be developed.

b. Scope

The rating is to include a measure of the driver's ability to

stay within a specified lane of a surface and to maintain a specified

speed throughout a skid test in a particular system.

c. General Procedure

(1) Instrumentation is to be placed at the entrance and exit

of each lane of a surface to indicate passage of the

test wheel.

(2) The driver is to be informed of the specified test speeds

and lanes.

(3) System speed is to be continuously recorded during test

wheel lock for three skid tests at each of three speeds;

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

and exit lanes of each test are to be noted.

(4) A system-driver performance rating (DPR) is to be

calculated for each skid test. Such a rating may take

the form

22
L

DPR

1

T

D

2 dt

where L = 1 if the test wheel entered and exited

on the specified lane,

L = 1/2 if the test wheel entered on the specified

lane but exited on an adjacent lane,

L = 0 if the test wheel entered on the wrong

lane, or if it entered on the specified

lane .but exited two or more lanes away

from the specified lane.

D speed deviation, expressed in 0.1-mph units

(0.1 mph = 0.16 km/h),

t time, expressed in seconds, and

T time duration of the skid, expressed in

seconds

d. General Equipment Required

(1) Lane instrumentation.

(2) Speed recording instrumentation.

(3) Means of indicating the duration of the skid on the speed

record.

2.6.2 System-Operator Rating

a. Objective

A performance rating for the system-operator combination of a skid resistance measuring system is to be developed.

b. Scope

The rating is to include a measure of the operator's ability to

start a locked wheel skid at a predetermined longitudinal position on

the surface while operating a particular system.

c. General Procedure

(1) The driver and operator are to be informed of the

desired starting position of the locked wheel skid.

23
(2) The operator may place reference pylons on the approach

to the surface but shall not make a practice skid on

the surface.

(3) The difference between the desired position and the

actual position of the start of the locked wheel skid

is to be measured in three skid tests conducted at

each of three speeds; 20, 40, and 60 mph (32, 64, and

97 km/h)

(4) A system-operator performance rating (OPR) is to be

calculated for each skid test. Such a rating may take

the form

OPR = -ja

where d = distance in feet from the desired location

(1 foot = 0.3 m)

d. General Equipment Required

(1) Highway pylons.

(2) Length scale.

3. SKID RESISTANCE MEASUREMENTS OF PAVEMENTS

3.1 The Test Matrix

The general method of test is described in ASTM Designation E 274.

The measurement represents the steady-state friction force on a locked

test wheel as it is dragged over a wetted pavement surface under known

vertical load and at constant speed. The test is used to measure

either the difference in skid resistance between various designs of

tires or the difference in skid resistance between various pavements.

In the latter case, a standardized, but special-purpose, full-scale

automotive tire is mounted on the test wheel to direct the sensitivity

of the measurement to differences in surface characteristics. However,

variations in the test results occur which are not due to the measuring

system since the tire is subject to wear, and the surface over which

it is dragged has non-uniform skid resistance characteristics. To

measure the effects of the different operating variables and to account

for disagreement between measuring systems, the non-uniformities of

standard tires and test surfaces need to be identified. Accordingly,

the test program described here employs, as a building block, a test

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

between the lanes comprising the test surface to be determined, while

the significance of a given variable is being tested. A sample test

matrix is shown in figure 2. Twenty-one skid resistance measurements

are made in each test matrix with the tire, lane, and operating

variable values indicated. The mean of the 21 skid resistance measurements is taken as the skid number of the surface measured during the

24
indicated time period using the "average" tire at the indicated test

speed. In addition, the matrix yields a skid number-test variable

gradient for the surface, as'well as an indication of the skid number

dispersion due to lane and tire variabilities.

For example, if it is desired to compare the effect of making

a measurement using tire A with measurements made using all of the

other tires, it is desirable to eliminate any measurement deviations

due to skidding in different lanes and with different values of the

test variable.

Time Period

Surface

Test Speed Tires K

Test

Variable ABC D E F G

(1) (2) (3) (4) (5) (6) (7)

(2) (3) (4) (Lane) (5) (6) (7) (1)

(4) (5) (6) (7) (1) (2) (3)

V + AV

Figure 2. Test matrix.

Letters A through G designate seven tires while numbers 1 through

7 identify the seven lanes comprising the surface. The value of the

test variable is V, and AV is an incremental change in the test variable.

Let a measurement result be represented by the notation

(SN + e)

LT (V - AV)

25
where SN = a nominal skid number measurement value

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

a lower value of test variable

L = lane identification of the test

T = tire used in the test

V = nominal value of test variable

AV = incremental change in the value of the test

variable.

From the test matrix, figure 2, tire A was used in lanes 1, 2, and

4 at three values of the test variable. Lane 1 was used with tires A,

G, and E at three values of the test variable while lane 2 was used with

tires B, A, and F at the three values of the test variable, and lane 4 was used with tires D, C, and A at the three values of the test variable.

An equation can be written for the measurement results from each

lane.

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 test variable effects (assuming AV is sufficiently small that the

gradient is linear). Consequently, the sum is free of both lane and test

variable effects. To simplify the notation then, eliminate the lane and

test variable differences from the sum, and get [6]

2 (SN + e) 1A(V - AV)

(SN) 1G (V)

" (SN e) lE(V + AV)

" A

AGE + 3e (1)

( 2 )

2 (SN - e) 4A(V + AV)

(SN) 4C(V)

(SN + e) 4D(V - AV)

" A

ACD

“ 3e (3)

where A^_,g = twice the difference between using tire A and

the average of using tires G and E in lane 1

AABF

= tw* ce the difference between using tire A and

the average of using tires B and F in lane 2

A

ACD

= tw^ ce t ^ie difference between using tire A and

the average of using tires C and D in lane 4.

6(SN)a

- (SN) e

- (SN) C

- <SH)d

- (SN) e

- (SN) f

<SN)

g

-

26
To this equation add the equation

(SN) A

- (SN) A= 0 (5)

Then

7 -E ( SN

) A

- SNa + SN

b + SN

C + SN

D + SN

£ + SN

p + SNg (6)

Now, the measurement performance with tire A can be compared with

the performance of all of the tires by writing -A .y f!i + ^

1-A 7 7

*= mean of all

measurements + (7)

7

In a similar manner, the data from the test matrix can be re- arranged to yield, for example, the measurement performance on lane 1 compared with the performance on all of the other lanes. These results

are a measure of surface uniformity.

A least-squares linear fit of the test matrix data versus the

measured operating variables yields the skid number-operating variable

gradient.

3.2 Variation of Measurement Results with

Operating Parameters

a. Objectives

(1) The sensitivity of the measurement results on selected

surfaces to changes in operating conditions is to be

determined.

(2) The degrees-of-control required on certain operating

parameters to achieve desired levels of precision are

to be determined.

b. Scope

(1) Data are to be obtained for test matrices where the

operating variable is a parameter such as:

27
(a) test speed,

(b) water depth,

(c) hitch height, i (d) test tire inflation pressure,

(e) test tire static load, or

(f) tow vehicle tire inflation pressure.

(2)

Data are to be examined to determine the effect that

the change in the variable has on the measurement

results.

c. General Procedures

(1) A surface for test is to be selected.

(2) Nominal test conditions are to be specified.

(3) Data are to be gathered for the planned test matrix

with incremental values of the operating variable

covering the expected normal range of variation.

(4) Sufficient time is to be allowed between tests for

the surface to dry.

(5) The data are to be examined as outlined in section 3.1.

c. General Eqipment Required

(1) Means to vary and measure operating variables.

4. INTERRELATION AND CORRELATION OF

REFERENCE SYSTEM MEASUREMENTS

a. Objectives

(1) The difference in skid resistance measurements, obtained

by the reference systems while operating under similar

conditions on the same surface, is to be determined.

(2) The sensitivity of the skid resistance measurements

to incremental changes in operating variables is to

be measured.

(3) Dispersion in the measurements due to differences between

test tires, between the lanes comprising the surface, and

between time periods of test and climatic conditions is

to be identified.

28
b. Scope

(1) The measurements are to be made on at least two surfaces

providing a suitable range of skid resistance values.

(2) The reference systems are to make measurements over the same range of a particular operating variable while

other variables are held constant.

c. General Procedures

(1) Each subsystem is to be prepared to confirm or adjust

its performance within a desired range prior to

starting the interrelation and correlation test program.

(2) Surfaces are to be selected for measurement.

(3) Test matrices are to be planned.

(4) Known operating variables are to be held under control

where possible, such as:

(a) test speed,

(b) water depth,

(c) hitch height,

(d) test tire inflation pressure,

(e) test tire static load,

(f) tow vehicle tire inflation pressure,

(g) surface cleanliness, and

(h) surface degree-of-dryness prior to measurement.

(5) Known operating variables not subject to control are to

be measured, such as:

(a) temperature of ambient air,

(b) temperature of the test surfaces,

(c) wind speed and direction near the test surfaces, and

(d) wear condition of the test tires.

(6) The test program is to be conducted.

29
5. REFERENCES

[1] Standard method of test for skid resistance of paved surfaces

using a full-scale tire, ASTM Designation E274-70, Annual

Book of ASTM Standards, Part 15 (American Society for Testing

and Materials, Philadelphia, Pa., 1976).

[2] Watson, John R. and Cook, Larry M. , A national program to

standardize skid resistance measurements, Public Roads, A

Journal of Highway Research 37 , No. 3, pp 77-30 (Dec. 1972).

[3] Kearns, R. W. and Ward, J. F. , A method and means of calibrating an air-bearing force plate for use with a towed pavement

friction test trailer, Nat. Bur. Stand. (U.S.) NBSIR 75-738

(Dec. 1974). 1

[4] Proposed procedure for speed and distance calibration of a fifth wheel equipped with either analog or digital instrumentation, ASTM Designation F 457, American Society for Testing

and Materials, Philadelphia, Pa.

[5] Kearns, R. W. and Ward, J. F., Skid resistance measurement tests

at the FHWA Eastern Field Test Center, Nat. Bur. Stand. (U.S.),

NBSIR 75-972 (July 1976).

[6] Garfinkel, S., Mann, W. B., and Youden, W. J., Design and

statistical procedures for the evaluation of an automatic

gamma-ray point-source calibrator, Nat. Bur. Stand. (U.S.),

Spec. Publ . 300, Vol. 1, pp 92-102 (Feb. 1969).

30
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBI.K A I ION OR 111. POR I' NO. NBSIR 76-11742. Gov’t Accession

No.

3. Rec ipient’s Accession No.

4. TITLE AND SUBTITLE

SKID RESISTANCE MEASUREMENT TEST PROCEDURES

FOR INTERCOMPARING FHWA NATIONAL AND REGIONAL

REFERENCE SYSTEMS

5. Publication Date December 1976

6. Performing Organization Code

7. author(S)

jjQkgj.j. yj. Kearns and John F. Ward 8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2130440

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Federal Highway Administration

Office of Development

Washington, D. C. 20590

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. It 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. A national reference system and regional reference systems

are operated to improve the method of measurement, to reduce differences in results between systems and to include measurement assurance in the program. State

highway measuring systems are intercompared with reference systems at FHWA re- gional field test centers. This document describes the objectives, the scope and

the general procedures of the tests used to intercompare the regional reference

systems with the national reference system operated by the National Bureau of

Standards. Listings of the general equipment required are included.

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

)

Friction, pavement; highway safety; measurement, skid resistance; pavement, skid

resistance; test procedures, skid resistance measuring systems; tire-pavement

interface forces.

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(THIS REPORT)

21. NO. OF PAGES

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