## 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 R olling Resistance Test Method

................................ ......... 8

Description of F

ootprint Analysis Method

................................ ......... 9

– 13

Appendices

A1 Traction

– Overall Performance Rating s

.......................... A1

- 1

– A1

- 28

A2 Traction

– Control Rating A nalys es

................................ ... A2

- 1

– A2

- 57

A 3 Traction

– Raw Data

................................ .......................... A3

- 1

– A3

- 95

A 4 Rolling Resistance Data

................................ ........................ A4

- 1

– A4

- 2

A 5 Footprint Data

................................ ................................ ....... A5

- 1

– A5

- 2

A6 Tread Depth Data

................................ ................................ .. A6

- 1

– A6

- 21



## 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 non

SmartWay tires represented. Seven tire manufacturers participated in this progr am. T he t ires were provided

by the manufacturers, wi th 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 resu lts 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 SmartWay Open 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 SmartWay Open 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 SmartWay Open 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 SmartWay Open 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 374003



## 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 t est 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

kno wn 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 appli cation.

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 compu ter 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 s hould 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. I t 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 fashi on 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 deviatio ns 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.

F igure 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 am

bient 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 i nitial 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.

T he 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 a nd 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 divid ed 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 signi ficant 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 condi tions, 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 fina l rating for the tire. Fi gure 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 Area5 

## Figure 36 

## Figure 47 

## Figure 58



## Description of Rolling Resistance Test Method

Rolling Resistance wa s measured using the ISO 28580 force method. The tire s were rolled against a 1.7 07 m

diameter drum, covered with a friction material simulating pavem ent microtexture. Reaction forces we re measured

at the spindle and converted to rolling resistance. Load / spindle force interactions (cross

- talk) were characterized

and minimized software , and other misalignment effects we re mitigated by running the enti re 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 t he 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 c ontrol 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.

T he measurement accuracies are:

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

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

Loaded Radius: ±1 mm

Test Surface Speed: ±0.32 km/h

Inflation P ressure: ±0.75 kPa

Ambient Temperature: ±0.2  C

The c ontrol or dynamic accuracies are :

Tire Load: ± 35 N

Test Surface Speed: ± 2.0 km/h

Inflation P ressure: ± 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 wer e 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 th e desired load.

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

in Figure 6

.

Figure 6 : Scanned Footprint10

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

Figure 7: Enlargement of Figure 611

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

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

Figure 8 : Black and White Image C reated from Figure 612

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

- designed im age

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 a rea calculation be conducted on

a region from 20 to 80% of the distance between the front and ba ck (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 i dentified

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 tot al 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 i n Figure 9 , divided by the

area of each pixel (which is 1 / 300 2 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 proc essed image is shown in Figure 10 . Gro ss 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.