A visco-hydraulic clutch represents an alternative to a lockable transfer case as a link between the driven axles. In this concept, the vehicle is driven only by one axle connected directly with the engine. A speed balance is achieved at low values of slip as a result of the second axle being driven freely on the road. The visco-hydraulic clutch produces a torque on the second slower running axle only at differing speeds between front and rear axles, e.g. at increased slip on the driven axle.
A hydraulic multi-disk clutch which can be controlled using the clutch pressure replaces the transfer case as a simple (small mechanical expense) but effective all-wheel drive concept. Control electronics take over the control of the locking effect between both axles. Using various sensors the system can react to a specific driving condition. Based on the amount of effort invested, the locking effect can be controlled step-wise or continuously variable. Special requirements such as the complete release of the lock during braking, can be taken into consideration.