Four Wheel Steering: Maneuverability and High Speed Stabilization

A theoretical analysis of four-wheel-steering (4WS) cars is presented. A discussion of low speed maneuvering shows why significant improvements in parallel parking cannot be expected. Using the classical two degree-of-freedom “bicycle model” of the automobile, comparisons of highway maneuverability are made between 4WS and FWS (front-wheel steering) cars. The 4WS lateral response has less phase lag, which permits rapid lane changes with less high frequency motion of the steering wheel. In addition, 4WS vehicles can make more efficient use of tires during transient maneuvers. An extended mathematical model which treats steer angle as a degree of freedom shows that a free control FWS mode is stabilized by either of two 4WS mechanisms considered. This weave oscillation can be excited by a rapid application of steering torque, so moderating the resonance with 4WS probably helps drivers maintain control during emergency maneuvers.