Some aspects of friction and wear of tyres arising from deformations, slip and stresses at the ground contact

Abstract

Beginning with the subject of friction of rubber on a dry surface which has been theoretically and experimentally studied at the Vehicle Research Laboratory, the important role of tread patterns in adhesion on wet roads is emphasized. The tread patterns cause the stress distribution in the area of contact to be wavy and variable both in time and space. The deformation of an inflated tyre against a hard, flat surface induces a symmetrical and inwardly-directed shear stress distribution at the ground contact. The shear stresses have been calculated from frictional consideration under certain simplifying assumptions. Apart from the importance of cornering and stability of automobiles, the study of a tyre set at a slip angle (the angle between the direction of travel, and the plane of the tyre) is also important from the point of view of wear, especially because of the high temperatures prevailing at the tread surface under these conditions. The forces developed on wet road surfaces are considered. The results obtained show that cornering forces on well-lubricated surfaces are very low. This leads to the relatively new concept of employing a high hysteresis tread rubber to give additional friction on such surfaces. The tests carried out elsewhere at high speeds on roads sprayed with water have been of limited value in evaluating the hysteresis component of friction resulting from deformations since the change in tread properties resulting from unknown contact temperatures due to surface heating was not considered. Measurements on the glass plate apparatus with speed of only 4 1 2 cm/sec on dry and lubricated surfaces indicate that the contribution due to hysteresis can be appreciable. The cornering forces developed by two tyres with different resilience and hardness were measured on dry and well-lubricated surfaces with different roughness. Finally, a short note is added describing some of the theories explaining the mechanism of wear occurring at the contact between rubber and a hard surface.