1. Field of the Invention
The present invention relates to a four-wheel drive device capable of optionally setting the torque distribution ratio to the front wheels and the rear wheels.
2. Related Art Statement
Hitherto, it has been possible to drive a front-wheel drive vehicle straight ahead on a road more safely than a rear-wheel drive vehicle. However, a front-wheel drive vehicle cannot be made to easily turn a corner since a driver must continue to maintain the tire in a position which enables the vehicle to completely turn the corner against the urge of the tire to return to the original straight ahead position. On the contrary, it is possible to make the rear-wheel drive vehicle easily turn a corner. However, it has a problem in that the vehicle of this type easily slips on a slippery road.
Accordingly, there has been disclosed a four-wheel drive device capable of distributing the driving force to the front wheels and the rear wheels so that the vehicle is operated by the driving force thus distributed. However, the four-wheel drive device encounters a problem in that the vehicle is brought to a braking state due to the difference in rotational speed between the front wheels and the rear wheels when the vehicle is rapidly turned at low speed on a road having a large friction coefficient, that is, a tight-corner braking phenomenon is undesirably generated.
In order to overcome the above-described problem, a central differential gear device is provided so as to absorb the difference in the rotational between the front wheels and the rear wheels. That is, in the above-described four-wheel drive device, the rotation of the engine is first devided by the central differential gear device so as to be transmitted to a differential gear device for the front wheels and that for the rear wheels via the propeller shaft.
However, in the conventional four-wheel drive device, torque transmitted from the transmission portion thereof is enlarged by a reduction device before it is transmitted to the transfer portion.
That is, the output shaft of the transmission portion is connected to the input shaft of the transfer portion via the reduction gear so that the torque transmitted from the transmission portion is supplied to the central differential gear device after its level has been changed by the reduction gear.
In this case, the strength of the above-described differential gear device connected to the reduction gear is determined depending upon the level of the torque obtainable from the reduction gear. Therefore, the size of the central differential gear device must be enlarged so as to correspond to a large-capacity engine. As a result, a problem arises in that the size and the weight of the transfer portion cannot be reduced.
Accordingly, an object of the present invention is to provide a four-wheel drive device capable of enlarging the torque transmitted from a transmission and reducing the size and the weight of the transfer portion.