Patent ID: 7905312

Claim:
A driving force transmission device for a four-wheel-drive vehicle, the driving force transmission device comprising: a switch mechanism for switching between a four-wheel-drive mode in which a fastening power of a multi-disc clutch mechanism is changed continually and a driving force distribution transmitted to front and rear wheels is automatically controlled depending on running conditions, and a two-wheel-drive mode in which the multi-disc clutch mechanism is released and the driving force distribution is transmitted only to the front wheels; a front wheel differential that receives a driving force from a power source and outputs the driving force to left and right front wheel drive shafts and the multi-disc clutch mechanism; a propeller shaft that transmits the driving force distributed by the multi-disc clutch mechanism; a rear wheel differential that receives the driving force from the propeller shaft and outputs the driving force to left and right rear wheel drive shafts; and a disconnection/connection mechanism that disconnects and connects the rear wheel differential and one or both of the left and right rear wheel drive shafts, wherein a dragging torque of the multi-disc clutch mechanism is made smaller than a rotation resistance of a rear wheel driving force transmission section from the multi-disc clutch mechanism to the disconnection/connection mechanism in the two-wheel-drive mode, and the rear wheel differential and one or both of the left and right rear wheel drive shafts are disconnected by the disconnection/connection mechanism and thereby a rotation of the rear wheel driving force transmission section is stopped.