In contrast to a normal vehicle, a 4-wheel steer (4WS) and 4-wheel independent drive (4WID) vehicle provides more flexibilities in vehicle dynamic control and better handling performance, since both the steer angle and drive torque of each wheel can be controlled. However, for motorsports, how much lap time can be improved with such a vehicle is a problem few discussed. So, this paper focuses on the racing line optimization and lap time improvement for a 4WS &4WID vehicle. First, we optimize the racing line and lap time of three given circuits with the genetic algorithm (GA) and interior-point method, and several objective functions are compared. Next, to evaluate the lap time improvement of 4WS & 4WID, a detailed vehicle dynamic model of our 4WS & 4WID platform vehicle is built in Carsim. To follow the racing line, a path following controller which contains a PID speed controller and a model predictive control (MPC) yaw rate controller is built. Moreover, a sliding mode (SMC) 4WS controller is designed to adjust the steer angle of wheels, meanwhile another SMC direct yaw moment (DYC) controller is proposed to track the target yaw rate and allocate the wheel torque considering the appropriate usage of tire friction. Results have demonstrated that the optimized racing line and lap time have tolerable error (<3%), and the proposed 4WS & 4WID method can reduce 3% of the lap time by enlarging the G-G diagram.
