Method for determining a wheel grip coefficient by simultaneous clamping

1. A method of determining a characteristic quantity representative of a grip condition of a wheel of a vehicle running on a ground surface, the method comprising:

utilizing an output of a control processor to simultaneously impose on two wheels of a same axle of the vehicle a given steering angle variation, the steering angle variation being of a same amplitude for the two wheels but in opposite directions; and

measuring the characteristic quantity for at least one of the two wheels.

2. The method according to claim 1 , wherein

the two wheels of the same axle of the vehicle are two wheels of a rear axle of the vehicle, and

a toe-in angle setting is imposed simultaneously on the two wheels of the rear axle of the vehicle.

3. The method according to claim 1 , wherein

the two wheels of the same axle of the vehicle are two wheels of a front axle of the vehicle, and

a toe-out angle setting is imposed simultaneously on the two wheels of the front axle of the vehicle.

4. The method according to claim 1 , wherein

plural steering angle variations are imposed in succession on the two wheels of the same axle simultaneously, the steering angle variations being of a same amplitude for the two wheels but in opposite directions.

5. The method according to claim 4 , wherein the plural steering angle variations imposed in succession increase monotonically, with a rate of variation being less than or equal to 0.1 degrees/meter traveled by the vehicle.

6. Method according to claim 5 , wherein the measuring of the characteristic quantity includes:

measuring a self-alignment torque for at least one of the two wheels; and

determining, as the characteristic quantity, a steering angle (β c ) for which the self-alignment torque is a maximum.

7. Method according to claim 5 , wherein the measuring of the characteristic quantity includes:

measuring a self-alignment torque for at least one of the two wheels;

determining a steering angle (β c ) for which the self-alignment torque is a maximum; and

determining, as the characteristic quantity, a lateral force (F yc ) corresponding to the steering angle (β c ).

8. The method according to claim 5 , wherein the measuring of the characteristic quantity includes:

measuring a self-alignment torque for at least one of the two wheels;

determining a steering angle (β c ) for which the self-alignment torque is a maximum;

determining a lateral force (F yc ) corresponding to the steering angle (β c ); and

estimating a maximum lateral force (F ymax ) of the at least one of the two wheels as the characteristic quantity.

9. The method according to claim 8 , wherein the maximum lateral force (F ymax ) of the at least one of the two wheels is estimated by:

F ymax =F yc ×K,

in which K is a characteristic coefficient of the at least one of the two wheels.

10. The method according to claim 9 , wherein, when a load (F Z ) on the at least one of the two wheels is known, the maximum lateral force (F ymax ) is used to estimate a maximum grip coefficient by:

μ max =F ymax /F Z .

11. The method according to claim 8 , further comprising determining a maximum steering angle corresponding to a lateral force that is below 85% of the maximum lateral force (F ymax ).

12. The method according to claim 1 , wherein, after the imposing of the given steering angle variation on the two wheels, the characteristic quantity is measured after a running distance of the vehicle greater than half a tire perimeter of a tyre of one of the two wheels.

13. The method according to claim 1 , wherein the characteristic quantity is a self-alignment torque (M Z ) on a tyre of one of the two wheels.

14. The method according to claim 1 , wherein the characteristic quantity is a lateral force (F Y ) on a tyre of one of the two wheels.

15. The method according to claim 1 , wherein the characteristic quantity includes a self-alignment torque (M Z ) and a lateral force (F Y ).

16. The method according to claim 1 , wherein the steering angle variation is between 0.5 and 2 degrees.

17. The method according to claim 1 , wherein the steering angle variation is between 1 and 1.5 degrees.

18. The method according to claim 1 , further comprising, after the characteristic quantity is measured, determining a grip coefficient of a tyre of one of the two wheels using a chart that associates the characteristic quantity with the grip coefficient.

19. The method according to claim 1 , wherein the vehicle is running in a straight line.

20. The method according to claim 1 , wherein, when the axle is a nonsteerable and non-powered axle, a check is made that the axle is not subjected to a braking torque before the steering angle variation is imposed on the two wheels.

21. The method according to claim 1 , wherein, when the axle is a powered axle, a check is made that the vehicle is not in an accelerating phase or a braking phase before the steering angle variation is imposed on the two wheels.