Recently, a great deal of attention has been focused on the development of four wheel steering systems (4WS) for passenger vehicles in order to enhance their handling properties. At high speeds vehicle response can be enhanced by steering the rear wheels in the same direction as those in the front, while at low speeds vehicle maneuverability can be enhanced by turning front and rear wheels in opposite directions. Much of the current theoretical literature explores various methods which can be employed to control the steer angle of the rear wheels. The results of these papers are usually based on analysis of the classic two degree of freedom linear bicycle model.
This paper presents the development of a vehicle model which is capable of simulating the dynamic behavior of a passenger car and includes many important effects ignored by the two degree freedom bicycle model. This nonlinear three degree of freedom vehicle model represents each suspension as a three dimensional mechanism and uses spatial kinematics to simulate suspension motion, giving the simulation the ability to determine the change in wheel steer and camber angles caused by chassis roll. The model also includes the effects of lateral and longitudinal weight transfer on tire frictional and stiffness properties. The tire model employed in the simulation uses Calspan tire data and computes tire forces as nonlinear functions of normal load, slip angle, camber angle, and braking force. Tire limits of adhesion are determined using an extended friction ellipse concept.
The vehicle model presented in this paper is capable of predicting the response of different vehicle designs and steering control methods in combined steering and braking maneuvers. Vehicle response is investigated using a ramp steering input and compared using four different steering control strategies. The paper also investigates the stability properties of the four steering control methods in pure cornering, as well as in combined cornering and braking maneuvers. Vehicle stability analysis is based on the comparison of generated tire forces with the tire limits of adhesion. The results presented in this paper were obtained treating the vehicle as a open loop system in that the effects of driver behavior were not included.
It was determined that the addition of four wheel steering systems improved the responsiveness of vehicles by reducing transient response time, and also reduced undesirable vehicle motions such as fishtailing, making a vehicle easier to control during a potential accident situation. However, it was determined that the addition of a four wheel steering system did not appreciably extend the overall stability margins of a vehicle.