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STATES OTMENT OF 1ERCE

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NBS TECHNICAL NOTE 566

Wet Traction of

Tractionized Tires

U.S.

RTMENT

OF

MMERCE

National

Bureau

of

:> ndards

J.

ttt BUREAU

ltd

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OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 566

ISSUED FEBRUARY 1971

Nat. Bur. Stand. (U.S.), Tech. Note 566, 14 pages (Feb. 1971)

CODEN: NBTJA

Wet Traction of Tractionized Tires

A. H. Neill, Jr.

Office of Vehicle Systems Research

National Bureau of Standards

Washington, D.C. 20234

•<*«„ of *

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402.

(Order by SD Catalog No. C 13.46:566). Price 20 cents.

WET TRACTION OF TRACTIONIZED TIRES*

A. H. Neill, Jr.

A series of dynamic vehicle tests was performed at

NBS to evaluate the performance of tractionized or siped tires. Stopping distance and lateral breakaway data is presented from a two wheel diagonally

braked automobile which clearly shows that siped

tires do not represent any improvement in the

lateral stability or stopping distance characteristics of a typical passenger automobile.

Key words: Siping; stability; stopping distance

tires; traction; tractionizing.

1. Introduction

For many years a process known as siping, or tractionizing, has been

widely publicized as a means of increasing traction at the tire-road interface. The process is said to offer great improvement in stopping distance,

lateral breakaway, and driving traction especially under wet conditions.

The purpose of this paper is to present the results of tests carried out

by NBS for the National Highway Traffic Safety Administration to test

these claims. The reader is also referred to a paper by Domandl [1]

which presents longitudinal friction data from a skid trailer and is com- plementary to the results arrived at in this work.

Several methods of siping tires have been presented to the public. Two

of the most representative methods were chosen for evaluation by NBS. In method A, the tire is taken off the car, placed on a hub and axle, trued,

and then cuts are made in each rib of the tire, eight cuts per inch at a depth of 5/32 to 6/32 at 90 to the angle of rotation. In the second,

method B, the tire is cut with seven si pes per inch at a depth of 7/32

(new tires) to 4/32 (well worn) at an angle of 65 to the direction of

rotation.

*This work was carried out at the National Bureau of Standards under the

sponsorship of the Department of Transportation, National Highway Traffic

Safety Administration (FH- 11 -6090). The opinions, findings, and conclusions expressed in this publication are those of the author and not

necessarily those of the National Highway Traffic Safety Administration.

1
In order to facilitate understanding, the paper is divided into two

sections, one for si ping method A, the other for si ping method B.

2. Method A

Sixteen belted bias tires were chosen for this portion of the tests.

Eight new tires were broken in for 100 miles. The remainder were worn

under actual driving conditions to a depth of 0.298 inch in the inner

groove, 0.326 inch in the outer groove which took 8000 vehicle miles. The

assumption was that if 16 tires all of the same tread design and compound

were used, any deviations in the results must be due to either tire wear,

si ping, or a combination of the two.

The tests were run on four different surfaces with different skid num- bers [2]. Pavements 2, 4, and 5 were used for stopping distance tests,

and 1 and 2 were used from the J-curves. In table 1 a description of the

skid pads is presented and the skid values as defined in ASTM E-274-69 are

listed in a range of speeds from 20 to 50 mph.

The surfaces offered a wide range of coefficients and surfacing materials

used on today's highways. A standard 500 gallon watering truck with a 12-foot watering bar was used to keep the surfaces wet at a depth of

approximately 0.05 inch. One pass was made before each skid by the truck.

The exact depth of the water could not be determined because of differences in the textures of the pavements. The same volume of water was used

on each pad.

The four sets of tires were chosen at random as were the order of pads and

speeds for each set. However, one tire set remained on the car for all

the stopping distance tests before the next set was run. The J-curve

tests were run in the same manner. Thus any drifts in coefficient or changes due to ambient temperature would be minimized. All tires were statically balanced and in addition the siped tires were also trued, i.e.,

high spots cut off to make the tire round.

The stopping distance data was taken under two wheel diagonal braking on

a specially instrumented 1968 Chevrolet loaded to 1270 lb/wheel. This

allowed the driver to retain control of the car and provided a result

which was independent of spinning or lateral acceleration. A digital

readout gave the distance traveled from the point of full lock-up.

Figure 1 is a graphical representation of the data in table 2 for each

skid pad. Although there is a significant difference in stooping distance

between surfaces there is insufficient evidence to support the hypothesis

that siped tires stop in a shorter distance than unsiped tires. Only on pad 2 is there a slight indication that siped tires may perform significantly better than unsiped tires. In the four instances showing improvement, the greatest margin is less than 18 percent. The coefficient of

variation for this portion of the tests, method A, was less than 12 percent,
In order to check for any improvement in lateral breakaway, tests were

run on two surfaces, pad 1 and pad 2. These were in the form of a J-curve

with a radius of curvature of 288 ft. The results are given in table 3

for these tests. Again no significant differences were seen between the

siped and the unsiped tires. It should be noted however that the unsiped

tires appear slightly better than the siped. This is not significant in

terms of the magnitudes involved.

3. Method B

Since no real difference was found between the new and old tires on any

of the pads or at any speeds, eight new bias belted tires identical to the

ones in method A were used in this portion of the tests. As described

above, tires with sipes at an angle of 65 to the direction of rotation

were now evaluated to check their performance.

In table 4, results are presented which clearly show that the siped tires

are again not better or worse than the unsiped tires. The improvement

noted on pad 2 was significant in one case and the greatest significance

was less than 10 percent. The coefficient of variation is less than

17 percent. In figure 2, (a) and (b) represent data taken some 3 weeks

before the data shown in (c), (d), and (e). Each data point is the

average of four runs.

Table 5 gives the results of the lateral acceleration tests as described

in method A. Again no significant improvement was noted; instead a con- sistent average one mph difference is observable showing the unsiped

performance to be better than that of the siped. The lateral acceleration

as measured by accelerometers mounted on the sprung mass of the car give

relative lateral forces induced in the tires just before breakaway. This

data is presented to show comparative lateral forces. It is not the maximum lateral acceleration but bears a linear relationship to it.

The ambient temperature varied within a range of from 73° to 87°F with the mean at 81 F for all the tests.

4. Conclusions

The data above represents a series of tests in which an effort was made to

verify the statementthat siped tires represent a real improvement in stopping distance and lateral control. The results as presented in this paper

do not verify this contention. In two separate experiments on three different occasions the data clearly shows that on a variety of surfaces,

from extremely slick to highly abrasive, the performance of a real vehicle

does not become any more stable or conversely less stable with the use of

tractionized, or siped, tires.

In the lateral breakaway tests where an improvement was expected from the

65 si ping process, method B, a consistent one mph degradation of breakaway speed was noted.
Thus it is clear that the process known as tractionizing, or siping, does

not offer a significant improvement in vehicle stability. Only one type

of tire was used in this evaluation because it was felt that the range

of structures and compounds available in today's tires would not cause enough of a difference in the movement of the contact patch to create a significant difference in the total vehicle performance. The improvements

noted on pad 2 represent a small percentage of the total results and the

number of roads resembling pad 2 is quite small.

5. Acknowledgments

The author wishes to express his gratitude to Messrs. Peter Newfeld and

Perry Rawlins who did all the vehicle testing. Likewise to Messrs. G. Shute

and R. Zimmer for the system of instrumentation and Mr. A. Kondo for his

statistical analysis of the results.

6. References

[1] Domandl , H., Some friction data on Microsiped tires, Report S-27,

Pennsylvania State University, University Park, Penn. (March 1968)

[2] Stocker, A. J., et al , Tractional characteristics of automobile

tires (report on NBS Contract CST-451). Available from the

National Technical Information Service (formerly Clearinghouse),

Springfield, Va. 22151, Accession No. PB 189 272.
Table 1. Skid pad characterization

SN Value

Pavement

No. Description

1 Rounded Silicous Gravel

2 Crushed Silicous Gravel

4 Slag and Limestone Screenings

5 Rounded Silicous Gravel (P.C. Concrete)

Stopping

Distance

Pads

(50-20 mph)

J-Curve

Pads

(40 mph)

42-46 50

43-49 51

7-18 24

42-61 43
Table 2. Stopping distance, method A: each value represents average of

four runs Skidding data (in feet)

Pad 2

mph/ 20 30 40 50

Old siped 38.0 102.8 177.5 268.0

Old unsiped 46.3 98.5 204.5 315.8

New siped 41.8 100.5 187.3 294.5

New unsiped 43.8

Pad

100.8

4

192.0 295.5

mph/ 10 20 30 40

Old siped 19.3 72.5 197.5 387.3

Old unsiped 15.8 76.3 185.0 383.5

New siped 17.8 77.0 205.3 393.5

New unsiped 18.0

Pad

83.0

5

211.8 395.0

mph/ 20 30 40 50

Old siped 46.5 104.8 215.0 330.0

Old unsiped 42.8 107.5 205.5 321.3

New siped 47.5 109.3 208.3 330.5

New unsiped 46.0 110.5 207.3 340.5
Table 3. Lateral breakaway speed, method A

Pad 1 Pad 2

New

New siped

Used

Used siped

41.2 mph 43.7 mph

40.2 43.0

42.3 43.8

42.0 43.9
Table 4. Stopping distance, method B: each value represents average of

four runs

Skiddirig data ( n feet)

Pad 2

October 2, 1970 October 28, 1970

Speed

(mph) Unsiped Siped Unsiped Siped

10 - - 10 10

20 41 40 40 42

30 100 90 94 93

40 190 184 182 176

50 303 296

Pad 4

308 294

10 - - 20 21

20 - - 97 99

30 - - 238 247

40

Pad 5

453 461

10 - - 10 10

20 46 45 40 46

30 no 108 98 118

40 202 218 184 208

50 334 347 316 347
Table 5. Lateral breakaway speed, method B

Max Lateral Max Lateral

Speed at Acceleration Speed Just Acceleration

Spinout Before Spinout Below Spinout Just Before

Tire Pad (mph) (q) (mph) Spinout (g)

2 42 0.625 41 0.65

39 .50 38 .50

4 40 .475

Unsiped

40 .50

42 .70 41 .70

5

42 .725

42 .65 40 .580

2 41

41

.625

.625

39 .55

Siped 38 .525 37 .50

.550

41 .65 40 .62

41 .65 40 .63

40 .65
400

300

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A UNSIPED

a SIPED

O UNSIPEO

OLD

NEW

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a

10 20 30 40 50

SPEED(mph) Figure 1. Distance traveled from the point of full lock-up. (a) Pad 2

10
400

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AUNSIPEDP LU A

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10 20 30 40 50

SPEED(mph)

Figure 1. (b) Pad 4

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10 20 30 40 50

SPEED(mph)

Figure 1 . (c) Pad 5

12
SIPED

UNSIPED

(a ) ^500

r pad 2

LU

°400

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1-

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CO 300 U

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a

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(J) 1 1 1 I 1 10 20 30 40 50

(b)

PAD 5

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- A

o

-

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A

A

1 1 I I 10 20 30 40 50

SPEED(mph)

(c) (d) (e)

-^500 " PAD 2 r PAD 4 " PAD 5

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S^400

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1- Q

CO 300 I

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6 111 _J • i i i i 6 111 I 10 20 30 40 50 10 20 30 40 50 10 20 30 40 50

SPEED(mph)

Figure 2. Stopping distance, method B, siped and unsiped tires (table 4).

13

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