Conventionally, there are known a so-called antilock brake system and a traction control system which adjust, when a drive wheel slips due to a brake pedal depression operation (braking operation) and an accelerator pedal depression operation (acceleration operation) performed by a driver, a braking force of the drive wheel in a slip state and an engine output, and applies an optimum driving force to the drive wheel.
For example, in the conventional antilock brake system, a control is performed so that a brake is applied in a region in which a brake friction coefficient becomes a predetermined value or more based on a relationship between a slip ratio and a brake friction coefficient of a wheel. In the characteristics of a slip ratio and a brake friction coefficient of a wheel, in general, when the slip ratio increases from 0%, the brake friction coefficient also increases and becomes a maximum value at a certain slip ratio, and thereafter the slip ratio decreases up to 100%. When the slip ratio decreases from 100%, the brake friction coefficient increases and becomes a maximum value at a certain slip ratio, and thereafter the slip ratio decreases up to 0%. In this case, it is known that when the slip ratio increases and when the slip ratio decreases, the brake friction coefficient increases and decreases through an approximately same path.
Accordingly, in a conventional brake control, an upper limit value and a lower limit value of the slip ratio is set in a stable region in which the brake friction coefficient becomes a predetermined value or more, the slip ratio is monitored in a travel state of a vehicle, and a brake fluid pressure of a brake device is controlled so that the slip ratio does not exceed the upper limit value and the lower limit value of the slip ratio. Accordingly, when the driver applies brake suddenly, the slip ratio can be kept in the stable region, that is, between the upper limit value and the lower limit value, and thus a shortest stop distance can be secured.
Note that the technology described above is described in the following patent documents 1 and 2.    Patent Document 1: Japanese Patent Application Laid-open No. 2007-245767    Patent Document 2: Japanese Patent Application Laid-open No. 2007-145075
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