Disengageable four-wheel-drive transmission system for motor vehicles

Claim 1

A disengageable four-wheel-drive transmission system for a motor vehicle having front and rear wheel drive shafts and a transverse gearbox with a toothed output member, including a toothed input member meshed with the toothed output member of the gear box, a front differential which drives the drive shafts of the front wheels of the vehicle, a rear differential which drives the drive shafts of the rear wheels of the vehicle, and means for distributing the torque between the front and rear differentials constituted by a hydraulic clutch which can be engaged and disengaged gradually and which is operatively associated with an electronic control unit arranged to pilot the gradual engagement of the hydraulic clutch automatically in dependence on operation parameters of the transmission system, starting from a disengaged condition in which the drive is transmitted from the gearbox to only one of the said front and rear differentials in order to control the distribution of the torque between the front and rear axles, wherein the front differential includes a box and the drive shafts of the wheels project from opposite sides therein and wherein the one side of the box of the front differential is coupled permanently to the toothed input member of the transmission system, wherein the hydraulic clutch includes first and second coaxial entrainment members and friction discs coupled alternately to the two entrainment members, and wherein the first entrainment member is coupled permanently for rotation with the other side of the box of the front differential and a toothed output member for transmitting the drive to the rear differential is coupled permanently for rotation with the second entrainment member so that, when the hydraulic clutch is disengaged, the drive is transmitted from the gearbox to only the front differential.