Analysis of the dynamic response of four wheel steering vehicles at high speed

In the past few years some motor car manufacturers have displayed a great deal of interest in developing four wheel steering systems for passenger vehicles as a means of enhancing their dynamic characteristics. Papers published to date present theoretical and experimental research into various control strategies used to determine optimal rear wheel, and sometimes front wheel, steering control methods. Most published theoretical analyses utilise a simple two degree of freedom, linear, bicycle type model to analyse steering control strategies. This paper presents the development of a vehicle model used to simulate the dynamic behaviour of a typical passenger car in which the kinematic effects of suspension systems, as well as the effects of lateral weight transfer on tyre properties have been included. Each suspension system is represented as a three dimensional mechanism and spatial kinematics are utilised to simulate suspension motion. A tyre model based on an extended friction ellipse concept and utilising Calspan tyre measurement has been employed to generate tyre forces as functions of normal loads. The vehicle simulation model presented in this paper can predict the dynamic behaviour of various vehicle designs and differing steering control methods in combined steering and braking manoeuvres. Vehicle response is investigated using a ramp steering input and compared for four different steering control strategies.