On the directional stability of motor cars when approaching and exceeding the adhesion limits between tire and ground

This paper is concerned with the investigation of the directional phenomena occurring when the limit of adhesion of tires to the ground is reached or overcome. To this purpose, an original mathematical model assuming the car as a system with six degrees of freedom was first developed. The mathematical model includes a model simulating the tire-ground mechanism through which the tangential forces are developed also when the limit of adhesion is exceeded. In the mean time, an original test car was designed and built in order to compare the actual behavior of the car on the road with that of the model. Much of the research time was devoted to checking and setting up the mathematical model for a correct simulation of the behavior of the car on the road. Thereafter, the mathematical model was used alone to determine time histories of important variable of the motion and parameters as lateral attitude, ground-wheel forces, over-steer-under-steer degree etc., describing the directional behavior of the car during exceptional situation as, for instance, the “tête à queue”.