Golf Mk2 Syncro, Passat B3, B4, Sharan Syncro (1996-2001)(Edit)
viscous coupling automatic all wheel drive system.
A viscous coupling installed instead of a center differential, with freewheel mechanism to disconnect the driven axle when braking. Open rear differential. Open front differential.
In normal conditions, this is a front wheel drive vehicle. In normal driving conditions 95% of torque transferred to front axle. Because viscous coupling is considered to be "slow" (some time is needed for silicone fluid to warm-up and solidify), 5% of torque is transferred to rear axle at all times to "pre-tension" the viscous coupling and reduce activation time. The coupling locks when slipping occurs and up to near 100% of torque is automatically transferred to rear axle. In on-road conditions the car will not move if one front wheel and one rear wheel lose traction.
The freewheel segment, installed inside the rear differential, lets rear wheels rotate faster than front wheels without locking the viscous coupling and preventing ABS from applying brakes to each wheel independently. Because of the freewheel, torque can be transferred to rear axle only when vehicle is moving forward. For all wheel drive to work when reversing, a vacuum-actuated "throttle control element" is installed on the differential case. This device locks the freewheel mechanism when in reverse gear. The freewheel mechanism unlocks when the gear shift lever is pushed to the right pass the 3rd gear. The freewheel is not unlocked immideately after leaving reverse gear on purpose - this is to prevent the freewheel from cycling from locked to unlocked if the car is stuck and driver is trying to rock the car by changing from 1st to reverse and back.
Disadvantages of this all wheel drive system are related to actuation time of the viscous coupling. 1: When cornering under acceleration on a slippery surface, rear axle is engaged with delay causing sudden change in the car's behaviour (from understeer to oversteer). 2: When starting on a sandy surface, front wheels can dig into the sand before all wheel drive is engaged.
Figure: Volkswagen Golf MkII Syncro
Figure: Volkswagen Golf MkII Syncro power transfer to the rear axle
Figure: Volkswagen Golf MkII Syncro rear axle gear unit with visco-clutch, free-wheel unit, and bevel gear differential.
Figure: Volkswagen Golf Mk2 Syncro freewheeling mechanism(12)
Figure: Volkswagen Golf Mk2 Syncro throttle control element(15)
T3 Syncro (1985-1992)(Edit)
Rear wheel drive-based automatic all wheel drive system with torque transfer to the front axle via viscous coupling. Freewheel mechanism(?). Manually locking rear and front differential.
Figure: Volkswagen T3 Syncro viscous coupling
T4 Syncro(Edit)
Automatic all wheel drive system with torque transfer to the rear axle via viscous coupling. Freewheel mechanism. Manually locking rear differential.
Figure: Volkswagen T4 Syncro propeller shaft
Figure: Volkswagen T4 Syncro throttle control element in the rear differential
Figure: Volkswagen T4 Syncro viscous coupling
Touareg(Edit)
Full-time all wheel drive. 50/50 torque split under normal conditions. Multiplate clutch lockable center differential. Optional rear lockable differential. Front open differential. Low gear. Differential locks are engaged by ECU, but also can be engaged manually by the driver. The car is also equipped with Hill Descent Control, Hill Start Assist.
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