Second Paper: Studies on the Skidding Resistance of Passenger-Car Tyres on Wet Surfaces

Abstract

This paper reports investigations into the effect of tread pattern, pattern modifications, tread material and tyre casing construction, on the skidding resistance of passenger-car tyres at speeds between 25 and 80 mile/h on a range of wet road surfaces. The technique used was to brake the front wheels of a car until these wheels locked, and to record the resulting deceleration of the vehicle. From the deceleration record, both the peak and locked-wheel values of the braking force coefficients were obtained. The method is described and an assessment of the errors involved is given. The experiments showed the importance of adequate drainage of the region of contact between tyre and road, particularly at the higher speeds, in order to remove the lubricating film of water. Such drainage can be provided either by a tread pattern or a coarse road surface, and the present results indicate the importance of each and their inter-relation. The results of various modifications to a standard tread pattern are given; these show that on smooth surfaces worth-while increases in wet road adhesion at high speeds can be obtained by suitable design of tyre tread patterns. The magnitude of the increases in friction due to changes in tread material are given for both peak and locked-wheel coefficients. Ratios of peak to locked-wheel coefficients have been calculated over a wide range of surface and tyre combinations, and they show the potential value of devices enabling a vehicle to make use of these high coefficients.