In Japanese Patent Application Publication No. JP-A-H6-107156, a conventional control is disclosed which operates when a vehicle is running on a μ-split surface, in other words, when a friction coefficient (hereinafter referred to as “surface μ” or just simply as “μ”) of a road surface differs between a left side wheel and a right side wheel of the vehicle. In the conventional control, the front wheel on an area of the μ-split surface with higher surface μ is alternately under an independent limiting control and a select-low control. The area of the μ-split surface with higher surface μ is hereinafter referred to as a high-μ surface. This control is aimed at gaining as much braking force as possible while suppressing the spin of the vehicle.
The independent limiting control is a control for limiting, based on a control for the front wheel with a larger slip, upward tendency of braking force at the other front wheel.
The select-low control includes a control in the case that the ABS control is started for the wheel on an area of the μ-split surface with lower surface μ. The wheel on an area of the μ-split surface with lower surface μ is referred to as a low-μ surface. In this case, the select-low control starts a decompression control in the ABS control for the wheel on the high-μ surface as well as for the wheel on the surface-μ irrespective of whether the wheel on the high-μ surface satisfies a condition for starting the ABS control.
The independent limiting control can gain as much braking force as possible at the wheel on the high-μ surface by generating on the μ-split surface a higher braking force at the wheel on the high-μ surface than that at the wheel on the low-μ surface. However, when the difference between the surface μs at the right side wheel and the left side wheel is large, the difference in the slip ratio between the left front wheel and the right front wheel becomes large. This makes it difficult for the vehicle to get its yaw torque under control, and the vehicle may accordingly start spinning.
Therefore, in the case that the difference in the slip ratio between the left front wheel and the right front wheel is large, the select-low control operates. Although the select-low control cannot gain as much braking force as the independent limiting control, it can prevent the vehicle from spinning because it suppresses the yaw torque applied to the vehicle by generating the same the braking force at the left and right front wheels.