1. Field of the Invention
The present invention relates to a method of controlling the steer angle of a road wheel of a motor vehicle, and more particularly to a method of controlling the steer angle of rear road wheels of a four-wheel steering motor vehicle.
2. Description of the Relevant Art
Steer response characteristics of a motor vehicle having only two front road wheels which are steerable (hereinafter referred to as a "2WS motor vehicle") will be described below.
A 2WS motor vehicle has a different yaw response to movement of the steering wheel while the motor vehicle is running straight ahead from a yaw response during turns. More specifically, the yaw rate gain of a 2WS motor vehicle tends to be nonlinear so that it is lower as the lateral acceleration yo of the motor vehicle is higher when the motor vehicle is making a turn. As is generally known, the reason for this is that the cornering power Cf of front road tires is lowered as the lateral slip angle cf of the front road wheels is increased. As described above, the yaw response of a 2WS motor vehicle during a turn is governed by the characteristics with which the lateral force F1 of the front road tires is generated with respect to the lateral slip angle .alpha.f.
In order to change the yaw response during turns without varying various parameters such as the weight m of the motor vehicle, the yawing moment Iz of inertia, the wheelbase l, the tread T, and the aspect ratio and material of the tires, it is general practice to adjust the roll stiffness and changes in alignment of the front and rear suspensions of the motor vehicle. However, such an adjusting process is based on the principle that the characteristics with which the lateral forces F1, F2 of the front and rear wheel tires are varied depend on the turning condition of the motor vehicle while passively utilizing the response of the motor vehicle body during the turn. Therefore, it is difficult to largely vary the yaw response of the motor vehicle. It is still more difficult to finely vary the yaw response depending on various values of motion of the motor vehicle such as the vehicle speed V and the lateral acceleration yo.
In the 2WS motor vehicle, during a turn in which the lateral acceleration yo applied to the motor vehicle is large, the cornering power Cf of the front wheel tires is almost zero, i.e., the lateral force F1 generated by the front wheel tires to turn the vehicle body in a yawing direction is saturated. Accordingly, even if the suspensions are finely adjusted to vary the yaw responsiveness, the yaw rate gain is approximately zero and any suspension adjustment is ineffective during turns in which the motor vehicle is subjected to large lateral accelerations yo.
Japanese Laid-Open Patent Publication No. 60(1985)-166561 published Aug. 29, 1985, for example, discloses a steering control process for a motor vehicle in which two front road wheels and two rear road wheels are steerable (hereinafter referred to as a "4WS motor vehicle"). According to the disclosed steering control process, the steer angle ratio k of the rear wheel steer angle .delta.r to the front wheel steer angle .delta.f is varied depending on the lateral acceleration yo on the motor vehicle. More specifically, during a turn in which the front and and rear road wheels are turned in the same direction, when the lateral acceleration yo exceeds a certain level, the steer angle ratio k is switched to a small value. With this steering control process, however, it is impossible to obtain smooth yaw responsiveness in a wide lateral acceleration from a small lateral acceleration yo to a large lateral acceleration yo.
The present invention has been made in an effort to eliminate the aforesaid drawbacks of the conventional methods of controlling the road wheel steer angle of a motor vehicle.