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An idealization of the elastic compression of the car on the basis of representing the car-body as a cylindrical shell with elastic wrinkles shows that three main factors govern the rebound velocity and hence the coefficient of restitution. These are the 3/2 power of acceleration at maximum dynamic crush, the square of the ratio of car-body mass to overall car mass and the half power of the position of the plastic/elastic crush interface. The model is applied to predict mean rebound characteristics for the car population. A comparison with published experimental data of the coefficient of restitution of the car population in frontal impacts with rigid barriers at impact speeds up to 100 km/h was made. The model predicted values of e similar in magnitude to those of the car population and also predicted a similar reduction in e with increasing impact speed.

This paper examines on a fundamental basis the elastic deformation of car structures during frontal impact and proposes models for the elastic stiffness of cars. The paper shows that the rebound energy and Vr, rebound velocity is a function of the impact force at the instant of maximum dynamic crush and of the elastic stiffness. Theoretical predictions of the level of Vr, rebound velocity and it’s manner of variation with approach velocity, Va, for full width barrier impact are similar to the pattern of Vr, rebound velocity variation found from staged tests. The implications of the model for Vr, rebound velocity in offset collisions are reviewed and an explanation is provided for both tangential and normal restitution coefficients at angled impact surfaces.

In temperate climatic conditions the water depths on wet roads are generally low, typically less than 1 mm. In this paper we examine the various types of road surface and the manner in which they can be classified in terms of macro and micro-texture. We propose a simplified representation of the tyre road interface in which the tyre footprint is divided into two zones, a dry zone in which dry road friction levels are obtained and an initial wet zone in which there is a water layer between the tyre and road and which gives no retardation. A generalised relation for the variation in the size of the wet zone with speed is proposed. The model is applied to published data for road surfaces of differing characteristics with fully treaded and smooth tyres. The model is shown to give a good representation of the variation in locked wheel retardation with speed and highlights the sensitivity of stopping distance to variations in road surface and tyre tread depth.