Examination of ATV Tire Forces Generated on Clay, Grass and Sand Surfaces

891106

Examination of ATV Tire Forces Generated On Clay, Grass, and

Sand Surfaces D. C. Holloway, W. H. Wilson, and T. J. Drach

Dept. of Mech. Engrg. University of Maryland

College Park, MD

Government/Industry Meeting and Exposition

Washington, DC May 2-4,1989

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ISSN 0148-7191 copyright 1989 Society of Automotive Englneers,inc.

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891106

Examination of ATV Tire Forces Generated on Clay, Grass, and

Sand Surfaces D. C. Holloway, W. H. Wilson, and T. J. Drach

Dept. of Mech. Engrg. University of Maryland

College Park, MD

ABSTRACT

A towed tire testing fixture suitable for use in testing all terrain vehicle tires is designed and built. Vertical loads on the tire under test can be varied from 100 to 400 1bf (445-1780N) and the camber and slip angles can be varied from 0 to 20 degrees. In addition, longitudinal slip measurements in braking are possible through the use of a disc brake assembly. Six strain gage loads cells are used to determine the force and moment resultants at the tire contact patch. Data acquisition and processing are done through a Daytronic 10KUD data pack and lap top PC.

This system is used to test seven representative ATV tires operating on surfaces of hard packed clay. Two of these tires are also tested on short and tall field grass, and on beach sand. Information on the lateral force coefficient, and the rolling resistance coefficient, as functions of vertical load on the tire, and slip angle of the tire, are generated. For the two tires tested on clay, grass and sand, one quadrant of the friction ellipse is determined showing the lateral force coefficient vs the braking force coefficient, with slip angle and longitudinal slip ratio as parameters.

THIS PAPER PRESENTS INFORMATION on the forces generated by seven all terrain vehicle (ATV), tires, when operating on surfaces of hard packed clay, grass and sand, as functions of slip angle, longitudinal slip, vertical load, and

vehicle speed. In a few tests, the aligning torque as a function of slip angle was also determined. All of this information was generated in support of the ATV study being conducted by the US Consumer Product Safety Commission, and are essential elements used in the development of an accurate handling model for an ATV.

Any dynamic handling model for an ATV requires information about the forces and moments generated between the ATV tires and the operating surface. A free body diagram of an ATV showing the forces and other important parameters is given in Figure 1. This figure shows a three wheeled ATV negotiating a right-hand turn. For a steady state turn, ie. constant forward speed, the force and moment equations of equilibrium can be solved to determine the conditions under which a turn is possible, if the constitutive relations between the normal loads, slip angles, tire forces, and moments are known. Twenty equations result after including the kinematic constraint equations, the constitutive relations and the six equilibrium equations. A four-wheeled ATV requires the solution of twenty-seven equations. As can be seen, what initially seems to be a simple problem becomes extremely complex in short order. It should be evident that the solution to this problem is only as good as the input data: the forces and moments between the tires and ground.

There is a wealth of information in the literature on the mechanical properties and the forces generated by automobile and truck tires.

0148-7191/89/0502-1106$02.50 Copyright 1989 Society of Automotive Engineers, Inc.


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The book Mechanics of Pneumatic Tires (1)' is an excellent example.

However, tire construction, tread design, operating pressure and surface conditions are significantly different for passenger car and ATV tires; and, while the trends are similar, little can be extracted from this information and applied to an ATV analysis. There has been research conducted with off road agricultural tires and test fixtures; work by Crolla et al. (2), Schwanghart (3,4), Stephens (5), Mcallister (6), Sommer et al. (7), Horton et al. (8), Wismer et al. (9) is representative. Once again however, the characteristics of the tires and the operating loads are sufficiently different from those of ATV tires to necessitate a study specific to ATV tires. There was virtually no information in the literature pertaining to the low pressure type of tire found on ATVs. These tires typically operate in the 2-5 psi (14-34 kPa) range.

A tire is a complex composite structure. It is made from a variety of ply materials oriented in various directions with multiple layers embedded in a rubber matrix. When the tire

* Numbers in parentheses designate references at the end of paper

deforms as it does during acceleration, braking or a turning maneuver, forces and moments are generated in the plane of contact. These forces are highly non-linear functions of the deformation, and are specific to the tire construction, inflation pressure, tread pattern and normal loads.

The tires on first generation ATVs also served as the suspension system of the vehicle. This meant that they had to have a soft spring constant in the radial direction, which in turn led to the use of carcassless tires. In addition, the contact patch was large, thus lowering the contact normal stress. This allowed the vehicle to operate on very soft terrains. The enveloping properties of these soft tires also allowed the vehicle to travel over small obstacles such as logs and rocks. A disadvantage to these early tires was that they would deform extensively with time.

Today, most current ATVs have a mechanical suspension consisting of a spring and shock absorber assembly or assemblies at each end of the vehicle. All future ATVs will be required to have some mechanical suspension. As a result, tire construction has changed as the designers have attempted to optimize the combined effects of tire spring and damping with those of the mechanical suspension. The current generation of ATV tires have carcasses, and come in radial or bias-ply construction. In an examination of nineteen tires studied as part of this project by Holloway (10), the initial radial spring constant of these tires went from 171 to 345 lbf/in (30-60 N/mm). Older tires were not available for testing, but we would expect that their radial spring constants would have been lower. The new tires still maintain the "balloon" appearance, low operating pressures, and the enveloping properties of the earlier ones.

Because there was no information in the literature on the forces generated in the contact plane by these tires as they deform, it was necessary to build a testing fixture to examine representative ATV tires under normal usage conditions.

TIRE TESTING FIXTURE

The tire testing fixture design consisted of a triangular steel frame that mounted to the rear bumper of a towing vehicle. A top view sketch of this apparatus is shown in Figure 2. The tire is located at the rear apex of the fixture, and adjustments allow the slip and camber angles to be varied. Normal


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loads are changed by stacking 50 lbf (222 N) weights on the fixture just slightly forward of the tire location.

Following SAE convention, the tire forces are referenced to the x'y' axis, and in subsequent discussion of the forces, the prime superscript notation will be dropped. Fy is the lateral side force, and Fx for a free rolling tire is the rolling resistance force. Five full-bridge strain-gage ring-type loads cells were constructed and fastened between the bumper and fixture using spherical rod ends at both connections. The cells have a resolution of 1 lb (4.5 N) and were designed to safely withstand an overload of up to 1000 lbf (4440 N). These cells measured the Ax, Ay, Az, Bx, and Bz reactions. Using this information Fx, Fy and Fz can be determined. A sixth cell (not shown in the figure) was mounted to the steering arm, and it measured a component of the aligning torque, Mz. The nominal weight of the fixture with a mounted ATV tire was 115 lbf (511 N). A photograph of the apparatus mounted to the tow vehicle is shown in Figure 3; also seen is a fifth wheel which measured the vehicle speed. Rotational speed of the tire being tested was determined by a tachometer-generator mounted on the wheel-axle shaft.

There have been many improvements and additions to the test fixture and electronics during the course of this project. The first design did not have

braking capability, and so some tires were only tested for their lateral force coefficients as functions of slip angle and vertical load. For these first tests, the force data from the load cells was acquired on strip chart recorders. A plotted data point was then taken to be the average of an eighty second-test, run at fixed conditions of slip angle and weight at a nominal towing speed of 10 mph (16 km/hr). In order to generate a range of data for a particular tire, the vehicle would be stopped; the weight or slip angle would be changed, and a new test run begun. This procedure obviously took some time, and the data reduction was quite laborious.

When a disc brake assembly was added to enable the determination of the forces generated during braking, a DC driven linear actuator was also added to the steering arm so that the slip angle could be swept during a run. Camber information still required mechanically adjusting a mounting plate. In turned out, however, that for the purposes for which this data would be used, the variation in results with camber change was not necessary. As a result, all of the data presented in this paper was taken at 0 degrees of camber.

Improvements were also made in the data acquisition and processing. In its present form, the signal conditioning and data acquisition takes place through a Daytronic 10KUD data pack which is interfaced to a Zenith 183 lap top PC. Software was written to calibrate the load cells, and to control the data acquisition process. Typical data sample rates are about four times a second for eight channels. Once the vertical load on the tire is set, and the tow vehicle is up to speed, the operator may either


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sweep through the full braking range or through the full range of slip angles. A test may still last eighty seconds, but greatly increased amounts of data are collected and are immediately available for further processing or viewing. In general, the first fifteen seconds of the test are used to establish the free rolling conditions since the rolling radius of the test tire decreases with increasing vertical load.

A test record of braking and lateral force coefficients vs time during a swept brake test is presented in Figure 4.

As can be seen there is considerable variation in the braking coefficient during the test. This is due to slight unevenness of the test surface, and to the design of the test fixture. The former is easily understood but the later requires some explanation. The test fixture was mounted to the bumper of the tow vehicle twenty inches above the ground. If one visualizes a side view of the fixture, and considers the summation of moments about the mounting axis,then it is clear that as the braking force increases, the vertical load on the tire must decrease, since there is only a static downward load consisting of the test fixture and any added weights. This changing vertical force creates a slight vertical oscillation of the tire assembly, and near tire lock-up this causes the tire to alternately skid and roll. For the test of Figure 4, the maximum F2 value was 306 lbf (1360 N), the minimum was 221 lbf (983 N), and the average was 265 lbf (1180 N). As a way of removing this artifact of the test

fixture, the data has been digitally filtered by using a simple, equally weighted moving average method. At each increment of time, the value of the force coefficient at that instant plus five values before and after that time are averaged to create a new value. These filtered or averaged values have also been plotted in Figure 4, and in all subsequent plots and discussion such filtered data is presented. The small values of lateral force coefficient, when the tire is supposedly at 0 degrees of slip angle, are a result of the difficulty in exactly determining this angle prior to beginning a test. We appear to be off by about 0.5 degrees in this example.

The force coefficients for the same test are shown as a function of longitudinal slip ratio in Figure 5.

The percent longitudinal slip is determined by first finding an average value of the voltage ratios generated by the tachometers on the fifth wheel and test tire during free rolling conditions, and then comparing this value to similar ratios generated when the brake is being applied. The range of negative values of longitudinal slip is a good indicator of the errors associated with these measurements, since only braking forces were applied. An inspection of this figure indicates an uncertainty of about 3%. This error and the error in slip angles were deemed acceptable when considering the final use of the data.

Figure 6 shows filtered force coefficients as functions of longitudinal slip for a different ATV tire operating on the same surface.


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For this data, the initial slip angle was 9 degrees, and the vertical load during the test ranged from 179 to 287 lbf (796-1280 N) with a test average of 231 lbf (1030 N). This figure clearly shows the loss in lateral force capability as more is demanded from the tire in the longitudinal direction.

DESCRIPTION OF THE TEST SURFACES

Towing tests were conducted on three different surfaces. One was a hard packed surface described as clay, located at the engineering proving grounds of Ft. Belvoir in Springfield, VA. This was a level surface several hundred yards long, that was periodically graded by the Army. The surface was a hard packed clay randomly intermixed with gravel ranging up to 3/4 inches (2 cm) in size. All tests were conducted in similar, dry, conditions. Runs were conducted up and down this surface in the same positions, so that after a few trials, the upper layer of this surface consisted of a fine powder about 1/4 in (6 mm) thick. Towing speed was 10 mph (16 km/hr) , although a few tests were also conducted at 20 mph (32 km/hr).

The grass surface was located at a University of Maryland farm near Upper Marlboro, MD. It can best be described as level pasture with a mixture of K-31 (a tall tough fescue), wild grasses, and weeds with some soil occasionally showing through the turf. Tests were conducted on mowed grass about 7 cm tall, and a nearby region that was unmowed at about

17 cm tall. Attempts were made to conduct each run in a region of undisturbed grass, but this was not always possible. These tests were conducted during a very dry summer, and the grass was still green, but not lush. Because of limited space, the towing vehicle speed was 5 mph (8 km/hr).

The sand tests were conducted on dry beach sand near Cape Hatteras, NC. The test fixture was mounted to a 4x4 light truck, and test run lengths were unlimited. A perforated steel mesh screen was mounted at the rear of the truck frame, and served to level and grade the sand just ahead of the test tire. The towing speed was about 5 mph (8 km/hr).

TEST RESULTS

Table 1 is a summary of the tires and the conditions under which they were tested.

Tire 1: KT465 22x11x8 - clay, strip chart data

This tire was one of the first tested, and was run at 0,5,10, and 15 degrees of slip angle with six normal loads ranging from 81 to 290 lbf (360-1290 N). It was tested at 10 and 20 mph (16 and 32 km/hr). Figures 7 and 8 show the lateral force coefficient when the tire was tested at these speeds. The differences in the data for these two towing speeds are within the variations in data for tests conducted at either fixed speed. From this it was concluded that there is little or no dependence on speed within the range tested. From this point on, the tests on the clay surface were all conducted at 10 mph (16 km/hr). In both figures, there is a slight improvement in performance at the lighter normal loads. This is a trend that is similar to passenger car tires.

Figure 9 shows the rolling resistance force coefficient at 10 mph. As can be seen, the coefficient is about constant at a nominal value of 9% of the normal load. The results at 20 mph are essentially the same. These values are substantially higher than passenger car tires, where the rolling resistance force is typically 1 to 3 percent of the normal load.

Tire 3: Pro-Am H-Trak, 22x11x8 - clay, strip chart data

From the results of the testing described above, it was determined that


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in this and subsequent tests, there should be a finer division in slip angles, and that angles greater than 15 degrees should be examined.

Results for this tire are presented in Figures 10 and 11 in a bar graph format. In these figures, data from two repeated runs at a given slip angle are plotted, and increasing vertical loads on the tire are shown from left to right in each cluster. Results from the repeated tests show the variability in the data for the field conditions on clay.

Figure 10 shows the lateral force coefficient as a function of slip angle. The repeatability of the tests is quite good, with variations that are acceptable when considering the end use of the data. It is also noted that at low slip angles, the tire performance at lower vertical loads is better. However, at larger slip angles, better performance was achieved at the greater vertical loads. At intermediate slip angles, there is quite a variation in lateral force coefficient as vertical load changes. For example, at 4 degrees of slip, Fy/Fz goes from 0.31 at the lightest load to 0.21 at the heaviest.

Figure 11 shows results for the rolling resistance force coefficient. The differences shown are of the same magnitude as the experimental variations. On the average, this tire has a rolling resistance coefficient of about 0.05. It also appears to be independent of both slip angle and normal load.

Tire 4: KT-686 - clay, strip chart data

This tire was the stiffest (in the radial direction) of any of the tires tested. Its lateral performance on clay is shown in Figure 12. Absolute performance limits for this tire are similar to those of the KT-465 (tire 1), and both are slightly higher than the Pro-Am H-Trak. In addition, both of the KT tires (from the same manufacturer) exhibit less sensitivity to vertical load than does the Pro-Am (different manufacturer).

Tires 5a and 5b: Pro-Am H-Trak2 -clay, strip chart and computer data

These tires are similar to tire 3; two with different serial numbers were examined. One (5a) was tested using strip chart recording of the data, and the other was tested using the computer data acquisition system. The lateral force coefficient for tire 5a is shown in Figure 13. The performance limit is nearly identical to that of tire 3, and


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large sensitivity to vertical load is also found for this tire. The average value of rolling resistance force coefficient was 0.07, for the range of vertical loads and slip angles tested as shown in Figure 14.

A plot of aligning torque, Mz, vs slip angle as a function of vertical load is presented in Figure 15. The determination of Mz requires simultaneous force data from all six load cells; consequently there is considerable scatter in this data. It is interesting to note that the aligning torque appears not to return to zero at the larger slip

angles as is the case for passenger car tires. However, when this information was used in the ATV modeling studies of companion papers (11,12) we did force the aligning torque to zero at 18 degrees of slip because of the experimental uncertainties.

A second tire of this type was also tested using the computer based data acquisition system. With this system and the braking capabilities added to the tire fixture, it is possible to generate a quadrant of the friction ellipse showing the cornering and braking forces generated as functions of slip angle an longitudinal slip ratio. Figure 16 is such a plot for tire 5b. Contours of fixed slip angles are shown along the vertical axis, and contours of fixed


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longitudinal slip ratio are shown along the horizontal axis. This plot shows data only to 12 degrees of slip angle; however, saturation occurs at about 15 degrees. Braking data for slip ratios beyond 30% are essentially the same, and show saturation in this direction. There is a slight "nose" in the data where the best braking performance occurs in combination with a slip angle of 6 to 12 degrees. This is a characteristic that also occurs with some passenger car tires.

Tire 6: KT-846 21x7x10 - clay, grass, sand, computer data

Friction ellipse plots of tire 6 on clay, short grass, tall grass and sand are presented in Figures 17-20. This tire has about equal capabilites in braking and cornering on the clay surface. The information on the grass surfaces tends to be scattered. This is the result of variability in the grass test tracks used during the many test runs required to generate these plots. The figures only show the results up to 50% longitudinal slip without a saturation occurring. However, examination of the raw data shows complete saturation at about 60% longitudinal slip. The performance limits are higher on the grass surfaces than they were on the clay surface, because of the interaction between the tire cleats and the turf.

The sand data in Figure 20 is especially interesting. The lateral and braking force coefficients are completely uncoupled beyond the 10% slip point, and the reason for this is that the shear characteristics of the sand dominate over any of the tire properties. As the braking torques were being applied to the tire, it would dig itself into the sand, and near lock up, it simply became a blunt plow submerged several inches into the sand. The raw data shows the braking force coefficient to be a linear function of longitudinal slip ratio with a saturation value of 0.9 at full lock up. In addition, any increase in slip angle


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beyond 12 degrees contributes only to the braking coefficient because of increased plowing action.

Tire 7: KT-847 22x11x10 - clay, grass, sand, computer data

Friction ellipses for this tire on the four different surfaces are given in Figures 21-24. The data for the clay surface is well characterized with about equal performance limits in both directions. The data on the grass surfaces is slightly scattered for the

same reason as given above. However, there does appear to be a slight gain in braking in the taller grass.

Like the last tire, the data in sand shows very little coupling between the lateral and braking force coefficients. However, there is a slight drop in the lateral coefficient with increased braking at the greater slip angles. This tire is wider in cross section and foot print than the last tire, and because of this, the larger slip angles do produce a


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greater lateral force coefficient. The braking force coefficient is a linear function with longitudinal slip ratio, and reaches a maximum value of 0.9 at full lock up.

TIRE COMPARISON AND SUMMARY FOR THE CLAY SURFACE - In order to quantify and summarize the lateral force coefficient as a function of slip angle for these six tires at a common vertical load, an equation of the form Fy/Fz = (l-exp(C*alpha)} was fit to the experimental data using a least squares approach. Correlation coefficients were 0.97 for the KT-686, 0.98 for the Pro-Am H Trak

and 0.99 for the remaining tires. A plot of these functions is shown in Figure 25. In this figure it is seen that five of the tires have similar performance up to about five degrees of slip, and that there is quite a range in saturation values for the lateral force coefficient. The two Pro-AM tires cluster together, as do the three from the KT-465,686,846 series. The KT-847 is unique in that its slope is noticeably greater.


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CONCLUSIONS

A towed tire test fixture was built that measures the forces and moments generated between ATV tires and their contact surfaces as functions of slip angle, vertical load, and longitudinal slip ratio. Seven tires representative of those in use on ATVs were tested on a hard packed clay/gravel surface. Two of these seven were also tested on a tall grass surface, a short grass surface, and on dry beach sand.

Maximum lateral force coefficients ranged from about 0.5 to 0.7 on the clay surface with saturation slip angles varying from 12 to 20 degrees. For the three tires tested in longitudinal slip, the maximum braking force coefficient ranged from 0.6 to 0.7 at a corresponding slip ratio of 50%. Performance limits on the grass surfaces were slightly higher. The data obtained on beach sand was unique in that the shear response of the sand dominated the tests.

For the tires tested in side slip and longitudinal slip, quadrants of friction ellipses were generated for the clay, grass and sand surfaces. For the clay and grass surfaces, the general appearance of these figures parallels that for passenger car tires. However, on the beach sand, the ellipse became a rectangle, and there was very little coupling between lateral and longitudinal force properties of the tires.

ACKNOWLEDGEMENTS

The talents and hard work of several undergraduates who helped in the design and construction of the test fixture, and in the tire testing are greatfully appreciated. Thank you Scott Schmidt, Dan Schartman, Greg Thomas, and John Hamilton.

This project has been funded in part with Federal Funds from the United states Consumer Product Safety Commission under contract number CPSC-C-87-1221. The content of this publication does not necessarily reflect the views of the Commission, nor does mention of trade names, commercial products, or organizations imply endorsement by the Commission.

REFERENCES

1. Mechanics of Pneumatic Tires, Ed. Samuel K. Clark, US Government Printing Office, DOT HS 805 952, 1981.

2. Crolla, D.A., El-Razaz, A.S.A., Alstead, C.J., Hockley, C., "A Model to Predict the Combined Lateral and Longitudinal Forces on an Off-Road Tire," Proceedings of 1987 ISTVS, pp. 362-372, 1987.

3. Schwanghart, H., "Influence of Tyre Tread on Steering Forces with Non Driven Tyres on Hard Surface," Proceedings of 1987 ISTVS, pp.627-635, 1987.

4. Schwanghart, H., Rott, K., "The Influence of the Tyre Tread on the Rolling Resistance and Steering Forces on Undriven Wheels," Proceedings of 1984 ISTVS, pp. 872-888, 1984.

5. Stephens, L.E., "Lateral Force Characteristics of Powered, Steered Tires," SAE paper 851091, 1985.

6. McAllister, M., "A Rig for Measuring the Forces on a Towed Wheel," J. Agric. Engng. Res., 24, pp. 259-265, 1979.

7 . Sommer, M.S., Gee-Glough, D., "Steering forces on Undriven, Angled Wheels," Journal of Terramechanics, 18, pp. 25-49, 1981.

8. Horton, D.N.L., Crolla, D.A., "The Handling Behavior of Off-Road Vehicles, Int. J. of Vehicle Design, vol 5, pp. 197-217, 1984.


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9. Wismer, R.D., Luth, H.J., "Off-Road Traction Prediction for Wheeled Vehicles," ASAE paper 72-619, 1972.

10. Holloway, D.C., "An Examination of Tire and Suspension Properties of All Terrain Vehicles," Report to CPSC on contract CPSC8612500, 1986.

11. Chen, S.Y., Tsai, L.W. , Chen, J., Holloway, D.C, "The Steady-State Handling of Three-Wheeled All Terrain Vehicles (ATVs)", SAE 891109, 1989.

12. Chen, S.Y., Tsai, L.W., Chen, J., Holloway, D.C, "Steady State Handling of Four-wheeled All Terrain Vehicles (ATVs)", SAE 891110, 1989.


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Examination of ATV Tire Forces Generated on Clay, Grass and Sand Surfaces