DYNAMIC TRACTION / ROLLING RESISTANCE

Prepared for:

U.S. Environmental Protection Agency

Office of Research and Development

By:

SMITHERS RAPRA NORTH AMERICA, INC.

1150 North Freedom Street

Ravenna, Ohio 44266

Report Date: 

June 6, 2011

Every precaution was taken to ensure the accuracy of the test report. However, the information is 

provided subject to the condition that Smithers Scientific Services, Inc., will not be liable for any loss or 

damage resulting from use of these data. Should the results of this testing be considered for use in any 

advertising or promotional material, it should be noted that Smithers Scientific Services, does not allow 

the use of its name to be contained in advertising and/or promotion material.
i

TABLE OF CONTENTS

Background .................................................................................................. 1

Summary of Results..................................................................................... 2

Description of Snow Traction Test Method.......................................... 3 – 7

Description of Rolling Resistance Test Method ......................................... 8

Description of Footprint Analysis Method ......................................... 9 – 13

Appendices

A1 Traction – Overall Performance Ratings.......................... A1-1 – A1-28

A2 Traction – Control Rating Analyses...................................A2-1 – A2-57

A3 Traction – Raw Data .......................................................... A3-1 – A3-95

A4 Rolling Resistance Data........................................................A4-1 – A4-2

A5 Footprint Data .......................................................................A5-1 – A5-2

A6 Tread Depth Data..................................................................A6-1 – A6-2
1

Background

In Fall 2010, the U.S. EPA contracted with Smithers Rapra North America, Inc. to conduct a variety of 

tests on a group of 27 medium-duty truck tires. The testing included:

 Driving traction on snow

 ISO 20580 rolling resistance

 Physical measurements

 Measurements of tire contact (footprint) geometry

This report presents those measurements. 

One goal of this program was to determine whether there was any significant correlation between rolling 

resistance performance of typical-use truck tires and their winter driving traction performance. 

The tires were chosen to include a mixture of traction and long haul tires, with both SmartWay and nonSmartWay tires represented. Seven tire manufacturers participated in this program. The tires were provided 

by the manufacturers, with each manufacturer providing two to five tire specifications for testing. 

All tires in this program were either size 295/75R22.5 or 275/80R22.5 and all were load range G. Different 

physical tires were used for the traction testing than for the rest of the testing. 

To prevent any concerns over use of these data for commercial purposes, the data in this report have been 

blinded. Each tire spec. in the program has been assigned a random code number. The same code number 

will be used to identify that specification for all of the tests. In that way, data from the various tests can be 

cross-plotted without the need to identify any specific tire brand or model.

The table on the next page summarizes the results of the testing.
2

EPA Traction / Rolling Resistance Summary

Code

# Category Shoulder

Snow 

Driving 

Traction 

Rating 

(%)

ISO 28580 

Rolling 

Resistance 

Coefficient 

(N / 1000N)

Durometer 

(A scale)

Average 

Tread 

Depth 

(mm)

Tire 

Weight 

(kg)

Footprint 

Length 

(mm)

Footprint 

Width 

(mm)

ASTM 

F870 

Void 

Ratio 

(%)

Net 

Contact 

Area 

(mm2)

Gross 

Contact 

Area 

(mm2)

1 Long Haul Closed 105.4 8.30 65 24 57.6 241 228 15.9 27420 37570

2 SmartWay Closed 78.0 7.00 68 25 58.1 247 221 17.3 25870 36970

3 Long Haul Closed 108.7 8.94 64 25 59.9 231 236 14.2 28300 37880

4 Traction Open 132.9 7.97 65 21 59.0 255 212 16.8 26210 37200

5 SmartWay Closed 104.5 6.95 66 20 55.3 233 211 15.4 26210 35010

6 Long Haul Closed 103.3 8.51 69 24 58.1 233 229 14.6 27530 35990

7 Long Haul Closed 115.6 8.01 66 23 59.4 234 215 12.3 28080 34870

8 SmartWay Closed 106.3 6.39 65 19 54.4 235 208 7.6 29930 34480

9 Traction Open 141.5 8.19 65 23 57.2 235 231 16.0 27810 38470

10 SmartWay Closed 89.0 6.32 62 21 53.5 234 210 13.6 26650 34340

11 Traction Open 145.3 9.41 65 23 56.3 247 218 16.4 26480 38520

12 SmartWay Closed 113.6 6.78 67 20 54.0 233 216 19.5 24290 35930

13 SmartWayOpen 147.7 7.74 70 21 56.7 231 238 14.3 27590 37270

14 Traction Open 119.5 7.44 65 19 51.7 244 205 10.4 29080 35030

15 Long Haul Closed 83.2 7.55 67 25 59.4 243 221 17.7 26170 36880

16 SmartWay Closed 83.4 5.90 65 22 57.6 231 227 13.6 27500 35820

17 SmartWay Closed 106.4 6.27 64 22 56.3 244 219 15.0 27440 36690

18 SmartWayOpen 146.6 6.44 64 21 53.1 255 213 16.2 27270 37570

19 SmartWay Closed 104.7 7.97 67 24 54.4 246 216 17.5 25610 36070

20 Long Haul Closed 102.9 7.50 67 24 54.9 247 218 16.7 26080 36510

21 SmartWayOpen 105.2 8.55 68 20 51.3 238 208 18.6 23820 35310

22 SmartWay Closed 118.3 6.45 65 25 58.5 235 237 14.8 29290 38720

23 Long Haul Closed 103.3 7.51 67 22 53.1 242 218 15.9 26060 35240

24 SmartWay Closed 95.1 5.70 65 20 55.8 235 216 19.9 24310 36350

25 Long Haul Closed 80.6 6.85 65 22 58.5 229 227 14.4 27580 36540

26 SmartWayOpen 110.6 6.67 64 20 54.9 234 196 11.5 26860 33190

27 SmartWay Closed 108.4 7.53 65 24 55.3 234 231 16.0 27790 37400
3

Description of Snow Traction Test Method

Dynamic traction testing is performed using Smithers’ truck tire drive traction test vehicle (shown in 

Figures 1 and 2). The basic operation of the Smithers’ truck tire drive traction test vehicle is similar to that 

of Smithers’ passenger and light truck tire drive traction test vehicle. The right rear axle of the vehicle is 

instrumented to measure the driving torque applied to the test tire. Brakes are applied to the three 

remaining rear wheel positions to prevent the vehicle from accelerating beyond test speed due to the 

application of driving torque to the test tire. Measurement of the drive axle torque, in combination with a 

known tire rolling radius and a predetermined load, allows calculation of the driving forces and the traction 

coefficient developed by the tire. Computerized instrumentation in the vehicle digitally records the data 

while the operator controls throttle application.

During a test, the operator manually applies brakes to the non-test positions to maintain the ground speed of 

the vehicle at 3 mph. The throttle is also applied to accelerate the test wheel to 5 mph DIV (this 

corresponds to 170% slip.) The computer program requires the ground speed to be maintained within + 0.5 

mph, and the test wheel to remain at least 150% slip throughout the test. After the test tire speed has 

stabilized, the average torque developed in the axle over a period of four seconds is recorded by the 

computer. The torque developed by the tire is then converted to an equivalent force using the known tire 

radius. No correlation between the previously used drawbar method and the driving torque method has 

been performed. However, it should be noted that using the same brand tire, the force indicated by the 

drawbar method is 10% lower than the force indicated by the torque method. This is to be expected, since 

the drawbar method cannot measure all of the force developed by the tire. It can only measure the tire 

traction force minus the force necessary to move the vehicle. 

Actual testing proceeds until ten good test runs have been made on each test tire. At the completion of the 

10 test runs, data analysis proceeds in the same fashion as for passenger tire drive traction. The method is 

as follows: An initial mean and standard deviation are computed based on the original 10 data samples. 

The data are then examined for any values that fall outside the range of +1.5 standard deviations from the 

original mean. These runs are removed and a new mean and standard deviation are computed. In addition, 

the coefficient of variation express as a percentage (CV%), is calculated and displayed. In ideal testing 

conditions, a CV% of less than 7% is readily attainable.

Figure 3 shows a sample printout of a raw data sheet. Section 1 contains information showing the test 

number and date. Other identifying information including tire size, identification numbers, load, pressure, 

and surface and ambient temperatures are also provided here. Section 2 shows the raw data obtained for 

the 10 runs. These data are in pounds, which are obtained by dividing the torque in the axle by the rolling 

radius of the tire. Section 3 shows the calculation of the initial mean and standard deviation and identifies 

runs that were cast out due to being more than +1.5 standard deviations from the initial mean. A second 

mean and standard deviation and coefficient of variation expressed as a percentage are also shown.

The test sequence always begins with a control tire, followed by a maximum of two test tires, and ends with 

another control tire. This sequence continues until all test tires in a particular program have been run. Each 

tire is rated against the average of the control tire before and after it. Figure 4 shows a sample of a rating 

analysis printout. Again, Section 1 shows the test number and other information about the tire being 

analyzed. In Section 2, the average force developed by the test tire is divided by the average of the two 

bounding controls and expressed as a percentage. Section 3, which is similar to the raw data sheet, contains 

mean 1 and mean 2 calculated as above. In addition to the computation of an overall percentage rating, a 

least significant difference, based upon a student “t” Test at 90% confidence, is calculated and reported 

with final data. 

Tests on each tire are run a minimum of two times. The results from the first and second tests are then 

compared. Depending upon test conditions, workload priority, and client instructions, a third test is made if 

the difference between the first two ratings is more than 15% of the average of both rating. Results from all 

runs of each tire are pooled to create a final rating for the tire. Figure 5 shows a sample of the overall 

rating sheet.
4

Figure 1: Smithers' Truck Traction Vehicle

Figure 2: Truck and Passenger Traction Vehicles and Tire Changing Area
5

TEST NUMBER:

DATE:

MANUFACTURER:

CLIENT NAME:

PROGRAM NUMBER:

TEST SURFACE:

TEST VEHICLE:

TEST OPERATOR:

CTI INDEX:

TIRE NAME:

TIRE SIZE:

TIRE IDENTIFICATION:

AMBIENT TEMPERATURE:

SURFACE TEMPERATURE:

TIRE PRESSURE:

CONTROL TIRE:

TEST LOAD:

RUN #

1 690

2 786

3 773

4 754

5 789

6 777

7 804

8 781

9 782

10 797

Average Force 773 Lbs

Standard Deviation 32

CV% 4.2

Runs Cast Out

1

Average Force 783 Lbs

Standard Deviation 15

CV% 1.9

Figure 1

Smithers Scientific Services

Traction Test

Raw Data

SAMPLE

4250 lbs

Section 1 Section 2 Section 3

AVERAGE FORCE (LBS)

16 F

20 F

105 psi

TRUE

74

ABC

275/80R22.5

DUMMY

TRAC R1

MEDIUM PACK

VOLVO

MWS

1/25/2002

987

Demo

Sample

Figure 3
6

TEST NUMBER:

DATE:

MANUFACTURER:

CLIENT NAME:

PROGRAM NUMBER:

TEST SURFACE:

TEST VEHICLE:

TEST OPERATOR:

CTI INDEX:

TIRE NAME:

TIRE SIZE:

TIRE IDENTIFICATION:

AMBIENT TEMPERATURE:

SURFACE TEMPERATURE:

TIRE PRESSURE:

CONTROL TIRE:

TEST LOAD:

RUN # Rating

1 70.6

2 80.5

3 79.1

4 77.2

5 80.8

6 79.5

7 82.3

8 79.9

9 80.0

10 81.6

MEAN 1 = 79.1

STDDEV1 = 3.3

MEAN 2 = 80.1

STDDEV2 = 1.49

FINAL RATING IS 80.1

LST-SIG-DIFF IS 2.88

Figure 2

SAMPLE

DATA CAST OUT

1 Section 1 Section 2 Section 3

4250 lbs

Smithers Scientific Services

Traction Test

Rating Analysis

Control Test Number

986 and 989

By Average Method

16 F

20 F

105 psi

TRUE

74

ABC

275/80R22.5

DUMMY

TRAC R1

MEDIUM PACK

VOLVO

MWS

987

1/25/2002

Demo

Sample

Figure 4
7

CLIENT NAME: Sample

MANUFACTURER: Demo

PROGRAM NUMBER: TRAC R1

TEST TYPE: Traction

TEST SURFACE: MEDIUM PACK

TEST VEHICLE: VOLVO

TIRE NAME: ABC

TIRE SIZE: 275/80R22.5

TIRE IDENTIFICATION: DUMMY

TIRE PRESSURE: 105 psi

TEST LOAD: 4250 lbs

OVERALL RATING 80.7

OVERALL LST-SIG-DIF 2.78

Figure 3

Overall Performance Rating

Test Numbers

987

995

SAMPLE

Smithers Scientific Services

Figure 5
8

Description of Rolling Resistance Test Method

Rolling Resistance was measured using the ISO 28580 force method. The tires were rolled against a 1.707 m 

diameter drum, covered with a friction material simulating pavement microtexture. Reaction forces were measured 

at the spindle and converted to rolling resistance. Load / spindle force interactions (cross-talk) were characterized 

and minimized software, and other misalignment effects were mitigated by running the entire test in both clockwise 

and counterclockwise directions and averaging the two results. Room temperature is also controlled.

To ensure the long-term repeatability of the machine, a program of daily and weekly control checks is used. To 

further improve the repeatability of results, the system uses an equilibrium algorithm. This algorithm waits to take 

measurements, until the time derivative of the measured rolling resistance is less than ±0.13 N/min over a period of 

at least 90 seconds.

Measurement and control software provides automatic control of speed, load, and inflation pressure. The control 

system provides digital data acquisition of time, normal force, rolling resistance force, inflation pressure, speed, 

loaded radius, and ambient temperature.

The measurement accuracies are:

Normal Force (Tire Load): ±10 N or ±0.1%, whichever is greater

Rolling Resistance Force: ±0.5 N or ±0.5%, whichever is greater

Loaded Radius: ±1 mm

Test Surface Speed: ±0.32 km/h

Inflation Pressure: ±0.75 kPa 

Ambient Temperature: ±0.2 C 

The control or dynamic accuracies are:

Tire Load: ±35 N 

Test Surface Speed: ±2.0 km/h

Inflation Pressure: ±3.0 kPa

Ambient Temperature: ±2.0 C

The system meets the requirements of SAE J1269 and ISO 28580.
9

Description of Footprint Analysis Method

The footprint measurements for this program were conducted using the method of ASTM F870, but the analysis was 

done using modern digital image processing techniques. 

Each tire was mounted on a 22.5 x 8.25” rim, and inflated to 100 psi. Footprints were measured at 6175 lbs (the 

tires’ rated load). Per F870, mold vents and flash were trimmed and sanded smooth.

Per ASTM F870, each tire was marked in three locations, approximately 120 degrees apart. Footprints were made at 

each location using an ink block-printing method. To make each footprint, the tire was pressed onto a plate coated 

with black ink at approximately 110% of the desired load; it was then unloaded and pressed onto a white poster 

board sheet up to the desired load.

When the ink had dried, the footprints were scanned in color at 300 DPI (dots per inch). An example scan is shown 

in Figure 6. 

Figure 6: Scanned Footprint
10

Figure 7 is an enlargement of the upper left corner of Figure 6 showing the scanned detail.

Figure 7: Enlargement of Figure 6
11

To facilitate analysis, the color image was converted to a black and white image. The image was then manually 

edited as follows: Any gray areas in the middle of tread elements that had been rendered as white were manually 

edited to black and any black areas in the middle of voids were edited to white. Figure 8 shows the edited black and 

white image created from Figure 6. This black and white image was used for the analysis.

Figure 8: Black and White Image Created from Figure 6
12

Per the method of F870, the edges of the footprint were manually identified, using a custom-designed image 

processing program. Figure 9 shows the parts of the footprint similar to the way they were identified with the 

program. 

Figure 9: Areas Selected for Analysis

The left and right extents of the footprint are shown as blue lines at the top and bottom of Figure 9. The distance 

between these lines (scaled to the number of dots per inch) is the footprint width in the summary data table. The 

front and back extents of the footprint are shown in green. The distance between these lines is the footprint length in 

the data table. 

Once the front and back of the footprint are identified, F870 specifies that the void area calculation be conducted on 

a region from 20 to 80% of the distance between the front and back (the yellow lines in Figure 9 show the 20% and 

80% locations) and extending from the right to the left line. This rectangle of interest (shown in pink) was identified 

automatically by the program. 

The program then determined the total number of pixels in the rectangle of interest and counted the number of these 

pixels that were white to determine the void ratio. Void ratio is defined as:

 ( ) 

 

 
13

Two other values were also calculated. The Net Contact Area is the total area of the black in Figure 9. It is 

calculated as the number of black pixels in the area bounded by the green and blue lines in Figure 9, divided by the 

area of each pixel (which is 1 / 3002 in2).

To calculate the Gross Contact Area, the image of Figure 8 was again processed to remove all grooves and voids 

within the footprint area. The processed image is shown in Figure 10. Gross Contact area was the total number of 

black pixels in Figure 10 divided by the area of each pixel. 

Figure 10: Image Modified for Gross Contact Area Measurement

Since three footprints were made for each tire, the values reported in the summary table are the averages of the 

measurements of the three areas.