Vehicle steering system

1. A steering system for a vehicle, comprising: a pair of wheel steering devices that respectively steer right and left wheels independently of each other; and a controller configured to control the pair of wheel steering devices,

wherein the controller is configured to:

determine a standard steering amount of each of the right and left wheels in accordance with a steering request;

execute opposite-phase shift steering in which steering amounts of the respective right and left wheels are shifted, by respective shift amounts, in mutually opposite directions with respect to the standard steering amounts determined respectively for the right and left wheels; and

estimate a friction coefficient of a road surface on which the vehicle is running based on steering forces respectively applied to the right and left wheels in the opposite-phase shift steering.

2. The steering system according to claim 1 ,

wherein a vehicle-body slip angle at a time when the right and left wheels are steered by the respective standard steering amounts is defined as a standard vehicle-body slip angle, and

wherein the controller is configured to execute the opposite-phase shift steering so as to maintain the vehicle-body slip angle at the standard vehicle-body slip angle.

3. The steering system according to claim 1 , wherein the controller is capable of executing, in straight running of the vehicle, the opposite-phase shift steering such that the shift amount of the right wheel is equal to the shift amount of the left wheel.

4. The steering system according to claim 1 , wherein the controller is capable of executing, in turning of the vehicle, the opposite-phase shift steering such that the shift amount of one of the right and left wheels located on an outer side in turning of the vehicle is smaller than the shift amount of the other of the right and left wheels located on an inner side in turning of the vehicle.

5. The steering system according to claim 1 , wherein the controller is capable of executing the opposite-phase shift steering such that the shift amount of each of the right and left wheels periodically varies.

6. The steering system according to claim 5 , wherein, when the shift amount of each of the right and left wheels is at its peak, the controller estimates the friction coefficient of the road surface on which the vehicle is running based on the steering forces respectively applied to the right and left wheels.

7. The steering system according to claim 1 , wherein the controller is configured to estimate a degree of the friction coefficient of the road surface based on a degree of the steering force applied to each of the right and left wheels in the opposite-phase shift steering.

8. The steering system according to claim 1 , wherein the controller is configured to identity that the vehicle is running on a low friction coefficient road as the estimation of the friction coefficient of the road surface on which the vehicle is running.

9. The steering system according to claim 8 , wherein the controller identifies that the vehicle is running on the low friction coefficient road when the steering force of at least one of the right and left wheels in the opposite-phase shift steering is smaller than a set steering force.

10. The steering system according to claim 8 , wherein the controller is configured to execute a notifying process of notifying a driver of the vehicle that the vehicle is running on the low friction coefficient road.

11. The steering system according to claim 10 , further comprising: a steering operation member operated by the driver; and a reaction-force applying device configured to be controlled by the controller to apply an operation reaction force to the steering operation member,

wherein the controller is configured to execute the notifying process such that the operation reaction force is made smaller when the vehicle is running on the low friction coefficient road than when the vehicle is not running on the low friction coefficient road.