A theory of viscous hydroplaning

The traction developed in the contact area of a pneumatic tyre rolling and accelerating on a wet road surface is simulated by a block of rubber slipping on a twodimensional sinusoidal surface covered with a thin water film. The resulting pressure-wedge formation on individual asperities of the surface provides a net uplift which tends to force tyre-tread and surface apart, thus promoting the onset of skidding or viscous hydroplaning. The important factor in m_i~imlzing skidding hazards is the provision of an adequate micro-roughness at asperity tips which contributes an adhesiona] mechanism to the overall friction coefficient. It is shown that in comparison with the adhesional contribution to friction, the contribution of hysteresis and viscous shear of the film are insignificant at low slip speeds.