Abstract:  A variable steering system transmission ratio function was proposed based on considering vehicle speeds and road adhesion coefficients to improve the steering portability at low speed and the steering stability at high speed of vehicles. After analyzed the design process of an ideal transmission ratio curve and the drive mechanism of active front steering (AFS) system, proposed some solutions including: (1) Modifying the ideal yaw rate gain according to the error variation law between the actual yaw rate gain and the ideal yaw rate gain of the vehicle positive; (2) Considering comprehensively the influence of vehicle speeds and road surface adhesion coefficients on transmission ratio with the help of deriving the functional relationship between road surface adhesion coefficients and vehicle speeds and transmission ratios. A simulation model was built for vehicle trajectory tracking in a Carsim/Matlab platform to simulating. The results show that the designed transmission ratio function can calculate the reasonable transmission ratio of steering system at different vehicle speeds and different road adhesion coefficients on low adhesion roads. Therefore, it improves the handling stability and steering portability for vehicles. 

Key words: vehicle safety, active front steering (AFS), variable steering ratio, stability and portability, road adhesion coefficient