Volkswagen all wheel drive explained

All Wheel Drive
...Explained
...Safety Concerns
...Quattro Evolutions
...xDrive
...Torsen
...Haldex
...Super Select
...Torque-Vectoring AWD
...Viscous Coupling
...Differential
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Golf Mk2 Syncro, Passat B2 Syncro (Quantum in the US), Sharan Syncro (1996-2001) - 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)

Passat B5 Syncro (1997-1999) / Passat B5.5 4motion (1999-2005) - Torsen-based full-time all wheel drive. See Quattro generation IV.

Passat B6 4motion (2005-...) - Haldex Generation II automatic all wheel drive. Haldex Generation IV automatic all wheel drive since 2007.

Golf IV 4motion, Bora/Jetta 4motion, Beetle Rsi, Sharan 4motion (1999-...) - Haldex Generation I automatic all wheel drive. Haldex Generation II automatic all wheel drive since 2002. Haldex Generation IV automatic all wheel drive since 2007.

Tiguan 4motion (2007-...) - Haldex Generation IV automatic all wheel drive.

T2 Syncro - Automatic all wheel drive. Normally rear wheel drive vehicle with torque transfer to the front axle via viscous coupling. Manually locking rear(?) differential.

Figure: Volkswagen T2 Syncro viscous coupling

T4 Syncro - Automatic all wheel drive. Normally front wheel drive vehicle with torque transfer to the rear axle via viscous coupling. 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

T4 4motion - Haldex automatic all wheel drive. Normally front wheel drive vehicle. Manually locking rear differential.

Iltis -

Last updated: 2010-07-29 17:50

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