Physical Factors in Tyre Traction

The ability of an automobile tyre to ‘grip’ the road and avoid skidding is ultimately dependent upon the events in the tyre-road interface. This seems obvious, and yet some 70 years after pneumatic rubber tyres were first put to use we are not able to explain fully the frictional processes between the tyre and the road. Over 650 papers have been published prior to 1970 on some aspect of tyre behaviour (Ludema and Gujrati 1973). The majority of these have been concerned with the development of tyres by empirical methods, and with the tyre as a structural element that influences vehicle dynamics. It will be several years before equations are developed from first principles to aid in tyre tread design. road texture specification and materials selection. This paper will describe some of the variables to be aware of in tyre performance, and it provides a ‘state of the art’ review of the friction processes involved. Considerable uncertainty exists concerning the importance of tyre-road traction (or friction). Intuitively most users of vehicles would endorse high friction on the simple ground that it would increase safety. However, there has not yet been a complete study of driver-vehicleeenvironment system effectiveness to determine how high friction should be. Nor has there been an effort to determine alternatives to high friction. The end result of an incompletely effective system is collision. In industrialized nations one person in 5000 is killed each year in automobile ‘accidents’ and many others are maimed. It is known that in many accidents a high tyre-road friction would not have reduced the severity of the consequences. In many other instances high friction might have been useful, but only if the driver had been trained to handle his vehicle during extreme manoeuvring. Few operators are experienced in vehicle manoeuvres involving accelerations in excess of 0.3 g. A major activity in the United States at this time is the specification of product quality. The tyre industry has been under some pressure to specify