Patent ID: 8047322

Claim:
An optionally connectable four-wheel drive vehicle comprising: an engine having a drive shaft; two main drive wheels; a main power train permanently connecting the drive shaft to the main drive wheels, and in turn comprising a transmission presenting a plurality of different velocity ratios, and a main differential; two normally driven secondary drive wheels; and an optionally connectable secondary power train for also connecting the drive shaft to the secondary drive wheels, and in turn comprising a gear drive presenting a limited number of velocity ratios which is lower than the number of velocity ratios of the transmission, and at least one secondary clutch, which is connected on one side to the drive shaft upstream from the transmission, and on the other side to the secondary drive wheels; and a control unit which: keeps the secondary clutch completely open, irrespective of the request to engage the four-wheel drive, when the total velocity ratio of the main power train is longer than the total velocity ratio of the secondary power train; closes completely the secondary clutch, in case the engagement of the four-wheel drive is requested, when the total velocity ratio of the main power train is equal to the total velocity ratio of the secondary power train; and closes partially the secondary clutch, in case the engagement of the four-wheel drive is requested, so that the secondary clutch is made to operate with a certain slip in order to perform the speed adjustment, when the total velocity ratio of the main power train is shorter than the total velocity ratio of the secondary power train; wherein the optionally connectable secondary power train comprises a unit for the reversal of the motion, which is controlled to reverse or not to reverse the direction of the motion.