First Paper: Tyre to Wet Road Friction at High Speeds

Abstract

The general problem of loss of tyre grip on wet road surfaces is discussed. The initial section of the paper covers a description of the testing techniques employed for the investigations reported. These include both road and indoor rig techniques developed to study limiting tyre braking and cornering properties under a wide range of operating conditions. The level of friction between a tyre and a wet road is primarily related to the ability to remove a water film from the ground contact area. The contact area is considered to be separated into three effective zones. Initially the ‘bulk’ of the film is displaced leaving a thin residual film to be penetrated at, or absorbed from, the interface before substantially dry contact can be established. The size of this dry area, at the rear of the contact zone, has an overriding control on the level of available friction and is dependent upon the time occupied in displacing the water film in the frontal zones. An increase in the peripheral speed of the tyre reduces the time available for water displacement and effectively shortens the area of actual ground contact. In the limiting condition the vertical load on the tyre becomes entirely supported on the water film and the condition of aquaplaning exists. The requirements for this condition to occur are briefly discussed. Tread pattern and road surface design are shown to exert a considerable influence upon the rate of water displacement from the contact area and hence upon the relative sizes of the water-supported zones. The forces developed in the dry zone at the rear of the ground contact are dependent upon the frictional characteristics of the tread material. A section of the paper considers the wide scale effects of tread pattern, tread material and road surface characteristics and discusses the nature of the interactions between these three variables. Some results of recent studies related to different types of tread pattern, specific pattern design features and the effect of tyre casing construction are included. Future potential development of the tyre contribution is reviewed.