Response, stability, and driver control of a rear wheel steering vehicle

The lateral dynamics of rear wheel steering (RWS) vehicles are explained in detail, with comparisons to the front wheel steering (FWS) case. The predictions of low order mathematical models are confirmed by data recorded from an instrumented, full scale experimental RWS vehicle. A two degree of freedom mathematical model describes the response to steer angle imputs. An analysis of steady state cornering shows that RWS vehicles might understeer or oversteer depending on weight distribution, just as FWS vehicles do, although RWS vehicles have a greater sideslip angle in the steady state. The transient response to steer angle imputs is characterized by a yawing response which is the same as in the FWS case. The RWS lateral acceleration response is characterized by reverse action, so the rear of the vehicle initially accelerates in the opposite direction to any steer angle change. The major difference between FWS and RWS vehicles is in the response to steering torque, which is modeled with only three degrees of freedom for the first time. Whereas FWS vehicles have a damped free control weave mode at approximately 1 Hz, the corresponding mode in the RWS case is unstable. Weave, which has not previously been explained, can occur in any wheeled vehicle at high speeds because tire slip angles at the steered wheels are significantly greater than the steer angle. When the steering wheel is released, the steer angle goes to zero and overshoots, and in the RWS case there is a transient increase in the tire slip angle which drives the weave mode. Whereas the FWS-RWS response difference to steer angle inputs diminishes with increasing vehicle speed, the RWS weave instability becomes worse with increasing speed, so it is the most likely reason why RWS vehicles are normally operated at low speed only. The implications for FWS is that the weave mode, although stable, may be detrimental to high speed safety during rapid lateral maneuvers, a potentially important problem which has received scant attention in the driver steering control literature.