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Viscous coupling is filled with silicone and is not computer controlled. A series of plates with holes and slots turn in the silicone fluid. Some plates are attached to the front axle driveshaft and some are attached to the rear axle driveshaft. Normally the plates turn at the same rate without relative motion. The silicone becomes very viscous as soon as it is heated by friction and shear caused by differences between the motion of the plates. This tends to lock the driveshafts. If the rear wheels and driveshaft are slipping and turning faster than the front, friction between the plates increases, slippage is reduced, the rear wheel spin is reduced and the torque from the input shaft is transferred to the front.
A viscous coupling can be installed in two ways:
viscous coupling acting instead of a center differential
In this case, in normal conditions, all power is transferred to just one axle. One part of the viscous coupling is connected to the driving axle, another part is connected to the driven axle. When driving wheels slip, viscous coupling locks and torque is transferred to the other axle. This is an automatic all wheel drive system.
Figure: Viscous coupling
(25KB)
Figure: Viscous coupling (2) location near the rear differential on VW Golf Mk3 (5KB)
Figure: Volkswagen's viscous coupling parts (61KB)
The disadvantage of a viscous coupling is that it engages too slowly and allows for excessive wheelspin before transferring torque to another wheels. This is especially critical in automatic all wheel drive systems - when cornering under acceleration, the rear end is engaged with a slight delay, causing sudden change in the car's behaviour fron understeer to oversteer. Also, when taking-off in sand, front wheels can become bogged down before all wheel drive is engaged.
In an attempt to reduce the coupling's activation time, VW Golf MkII Syncro always transfers 5% of torque to rear wheels (this is achieved by rear driveshaft rotating slower than front driveshaft in normal conditions, causing viscous fluid warm-up and slight solidification).
At the same time, pre-tensioning the coupling too much leads to undesireable transmission wind-up and makes the system too sensitive to uneven tread wear on front and rear tires. This is why Volvo first reduced the pre-tensioning in 2000 and then replaced the viscous coupling with Haldex clutch on their all wheel drive vehicles in model year 2003 (Volvo s60 has Haldex since 2002). [1]
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viscous coupling integrated into the center differential
In this case, all wheels are powered at all times. Viscous coupling is integrated into the center differential. Central differential distributes power to all wheels and lets them turn at different speeds while cornering. When excessive wheelspin occurs on one of the axles, viscous coupling locks the differential and equalizes the speeds of both axles. Torque is transferred to wheels that have traction. This is a full-time all wheel drive system.
Viscous coupling can also be integrated into the rear differential.
Figure: Viscous coupling locking differential (20KB)
Figure: Viscous coupling (left) and its installation in the rear (top right) and center planetary gear differentials (lower right) (93KB)
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