In modeling the mechanics of an ATV in acceleration, stopping, hill climbing or descending maneuvers, it is necessary to understand the nature of the frictional forces on the tires of the vehicle. The tire's force characteristics in the longitudinal direction are not as simple as for automotive tires acting on paved surfaces. The interaction of the lugs of the tires with the soil, grass, rocks, roots, and surface anomalies all affect the longitudinal forces transmitted to the vehicle. The typical modeling of the tire's interaction as a force being equal to the normal force times some constant friction coefficient is totally inadequate. Unlike normal tires, the ATV tire has a pulsing effect while at limit conditions. Even on level pavement the pulsing persists which indicates that is not necessarily a surface interaction phenomenon. The frictional spikes are significantly above 1.0 rising as high as 1.89 and will affect one's prediction of the motion of the vehicle.
In this paper the experimental results of skid tests of an ATV on dirt, gravel, dirt with a bump, a hole and on asphalt are presented. Using the deceleration rates of the vehicle CG, comparisons between a typical friction coefficient presented by other experimenters and the actual coefficient are depicted. A simple model using the actual coefficients shows how to calculate a vehicle's performance in limit maneuvers of braking and accleration.