Dynamic Yaw Rate Regulation for Moderate Understeer in Four-Wheel Steering Vehicles with Zero Sideslip Angle

The pursuit of maintaining a zero-sideslip angle has long driven the development of four-wheel-steering (4WS) technology, enhancing vehicle directional performance, as supported by extensive studies. However, strict adherence to this principle often leads to excessive understeer characteristics before tire saturation limits are reached, resulting in counter-intuitive and uncomfortable steering maneuvers during turns with variable speeds. This research delves into the phenomenon encountered when a 4WS-equipped vehicle enters a curved path while simultaneously decelerating, necessitating a reduction in steering input to adapt to the increasing road curvature. To address this challenge, this paper presents a novel method for dynamically regulating the steady-state yaw rate of 4WS vehicles. This regulation aims to decrease the vehicle's sideslip angle and provide controlled understeer within predetermined limits. As a result, the vehicle can maintain a zero-sideslip angle during turns with constant speed and exhibit a neutral or slightly understeer behavior during turns with varying speeds. The relationship between vehicle speed, yaw rate, and the understeer gradient is rigorously analyzed, following the definition of the understeer gradient in the Guiggiani’s formulation. Yaw rate is influenced by vehicle speed and steering wheel angle during turns, resulting in a moderate understeer gradient and a slight deviation from the baseline—i.e., the zero-sideslip angle condition. To address this, a regression algorithm is developed to facilitate the realignment of the steady-state yaw rate with the baseline as speed and steering wheel inputs change, thereby maintaining minimal sideslip angles. Simulations validate the proposed method, demonstrating its effectiveness in achieving a moderate understeer gradient and eliminating counter-intuitive and discomforting steering actions. Ultimately, this dynamic regulation of steady-state yaw rate promises to enhance the handling performance