1. Field of the Invention:
This invention relates to a torque distribution control system for a four-wheel drive vehicle which controls the torque distribution to the individual wheels.
2. Description of the Prior Art:
There have been known various systems for controlling distribution of the engine output torque to the front wheels and the rear wheels in a four-wheel drive vehicle. For example, the four-wheel drive vehicle disclosed in Japanese Unexamined Patent Publication No. 60(1985)-248440, a center differential is provided to distribute the engine output to the individual wheels, the braking system is arranged so that the braking forces applied to the individual wheels can be separately controlled, and when a wheel slips, braking force is applied to the wheel. That is, when the slipping wheel is braked, the torque exerted by the wheel becomes different from that exerted by the other wheels, whereby the driving forces of the individual wheels can be made appropriate.
Generally, a vehicle is in contact with the ground by way of four wheels, and accordingly, the critical performance of the vehicle is limited by the frictional force of the tires which solely depends upon the product of the friction coefficient .mu. of the road surface and the load W on the tires. Driving force, braking force and centrifugal force act on the tires, and the vehicle cannot be stabilized in running unless the sum of the vectors of these forces is within the frictional force of the tires.
The load acting on the tires is shifted by the longitudinal acceleration or the lateral acceleration generated during running and causes load differences among the tires, which results in difference in supporting performance between the tires. This is significant especially when the friction coefficient .mu. of the road surface is small.
In the four-wheel drive vehicle, the load on one tire is reduced by distributing the driving force to the four tires in order to stabilize the vehicle on a road having a low friction coefficient .mu.. However, when the front wheels and the rear wheels are connected by way of the center differential, the driving torque is uniformly distributed to the front wheels and the rear wheels, and since slip is more apt to occur in the wheel bearing thereon lighter load, the performance of the vehicle is limited by the tire having the worst performance.
By varying the torque distribution to the front wheels from that to the rear wheels, performance of each tire can be better used. However, in order to directly control the torque distribution to the front wheels and the rear wheels by controlling, for instance, engagement of a clutch disposed in the center differential, the system must be large in size because of large engine output torque to be distributed. This adds to the weight and the cost of the system and to limitation in incorporating the system in the vehicle.
On the other hand, in the system in which the slipping wheel is braked when slip occurs in the front wheels or the rear wheels, the vehicle is decelerated in response to application of the brake to the slipping wheel.