Handling characteristics of steer angle dependent four wheel steering system

The rear wheel control technique for the Honda Steer Angle Dependent four wheel steering system (Honda 4WS) is based on a combination of two methods, of which one controls the rear wheel steer angle without delay by using only the steering wheel angle for control input, and the other controls the rear wheel steer angle as a nonlinear function of the steering wheel angle. These methods have been adopted to efficiently reduce a delay in lateral acceleration response during maneuvers in straight road driving by generating lateral force at the rear wheels as quickly as possible, and, in addition, to offer a higher yaw responsiveness when making sharp turns. This report tries to demonstrate the effects of these two control methods from the viewpoint of dynamics and then, based on a quantitative analysis with mathematical models, it compares these and other control methods with respect to their effects on steering responses. The findings indicate that a 4WS system which controls the rear wheel steer angle without delay, by using the steering wheel angle, may better reduce the delay in the higher frequency range of lateral acceleration at the vehicle center of gravity and at the rear axle, than another type of 4WS system which uses only the side force of the front tires as the control input. Under operating conditions close to straight road driving, the former system can achieve the desired steering response characteristics. Compared with a 4WS system using a linear function, a nonlinear system, which reduces the rear to front steer angle ratio as the steering wheel angle increases, loses less yaw responsiveness in a curved course, and therefore, can maintain the desired quick response to steering for large steer input.