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A steering stabilizing algorithm for automobiles commands transient steering control torques so that the sum of natural steering restoring torque and the control torque is more nearly in phase with steer angle than the natural restoring torque alone. The resulting reduction in the phase lag from steer angle to restoring torque mitigates the steering weave mode. Natural restoring torque determined from a measured lateral acceleration signal can be compared to expected steady-state restoring torque calculated from steer angle and vehicle speed. Commands to a steering torque actuator depend on the difference signal, which is nonzero during rapid transients only. The character of control torques required is different from passive steering damping, so that an active control torque gives the best, response. Simulations show that a rapidly-controlled variable steering damper could apply the desired weave control torque a fraction of the time, resulting in significant weave mode mitigation.

The steering weave mode is explained with the aid of slip angle diagrams and a third order mathematical model. Nonlinear simulations show why the weave mode is not noticed during normal driving, and that it is worse with large amplitude maneuvers beyond the linear range of lateral tire force curves. A steering stabilizing method is described, in which transient torques are applied to the steering system using feedback from the instantaneous dynamic state of the vehicle. The steering stabilizer operates by reducing the phase difference between steer angle and steering torque, thereby damping the weave mode and reducing overshoot. The driver has a better torque feel, and the vehicle is more stable if the driver releases the steering wheel, both of which are expected to be beneficial. Suggestions for implementing the steering stabilizer are described.

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.

A three-wheel car concept is described, in which steering of the front wheels is limited to small angles for packaging reasons, so rear-wheel-steering is needed. Arguments for minimizing weight and aerodynamic drag suggest that this particular configuration is the optimum two-passenger vehicle for maximizing fuel economy. The limitation on steering of the front wheels requires that rear-wheel-steering is dominant in tight turns at low speed. The feasibility of this proposed steering scheme is supported by experimental evidence, as well as by the literature on driver behavior and all-wheel-steering cars. Preliminary tests with a prototype vehicle indicate that rear-wheel-steering alone is sufficient below 10 meters per second (22 mph), which would permit a front steer angle limit of .1 radians (6 degrees).