Investigations have shown that a high skidding resistance on wet roads is associated with the presence of sharp edges in the road surface: on these sharp edges high pressures are set up which assist in breaking through the lubricating water film between tyre and road. This paper gives some details of laboratory tests now being made to investigate further the dependence of skidding resistance on such localized pressures.

Pressure distributions beneath rigid spheres and cones pressed into rubber have been calculated, on the basis of the elastic theory, from measurements of their penetration into tyre-tread rubber under load. The experimental evidence confirms that under the conditions of test employed the rubber is behaving as an elastic material.

The effect of the pressures on friction under wet conditions has been investigated by sliding different shapes over wet rubber, using a laboratory friction machine designed to simulate conditions between tyre and road when skidding takes place. These tests indicate that the coefficient of friction recorded under wet conditions is closely related to the pressure over the contact area between slider and rubber, and that, to ensure a satisfactory skidding resistance in wet weather, the shape of individual projections in the road must be such that average pressures of the order of 1000 lb in^-2 are set up on them. Ideally, what is required is that the individual projections in the surface of the road should have angles at their tips of 90° or less: the necessary pressures are unlikely to be obtained with rounded or polished projections, whatever their size or the load applied to them.