Four wheel drive vehicles

Claim 1

A four-wheel-drive vehicle including a prime mover, front and rear wheels drivable by the prime mover, a drive connection between the front and rear wheels, a torque transmitting device having two relatively rotatable parts arranged so that the ratio of input torque to output torque of the device is not at all times equal to the ratio of output speed to input speed of the device, one of said parts of the device being connected with the front wheels and the other of said parts of the device being connected with the rear wheels so that when a speed differential arises between the front and rear wheels said parts of the device rotate relative to each other to transmit torque between the parts thus diverting part of the prime mover torque from the faster rotating wheels to the slower rotating wheels, a normally disengaged clutch connected kinematically in parallel with the torque transmitting device so as to provide an alternative torque transmitting path between the front and rear wheels, and a temperature sensitive element arranged, when the temperature of the torque transmitting device reaches a predetermined value resulting from relative rotation of said parts thereof in response to a speed differential between the front and rear wheels, to cause the clutch to engage, thus relieving the device of its torque transmitting duty and reducing the amount of energy converted into heat in the torque transmitting device.