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viscous coupling

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How does viscous coupling work?Next

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 fluid becomes very viscous due to it's viscoelasticity as soon as the plates rotate at differentiating speed. The silicone fluid resists the shear generated in it by the plates with differentiating speed, causing a torque transfer from the faster spinning axle to the slower spinning axle. Therefore, slight speed difference is required for torque transfer.

If the rear wheels and driveshaft are slipping and turning faster than the front, friction between the plates increases due to the generated shear in the fluid, 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 differentialNext

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]

viscous coupling integrated into the center differentialNext

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)




  1. http://www.volvoxc.com/

There are 7 comments
Erasmus Gudza
August 07, 2015 - 11:34

what is likely to cause damage to a viscous coupling on a car?

June 09, 2015 - 17:51

How does leaking seals in the rear differential affect the viscous coupling?

March 03, 2015 - 13:29

My Toyota Rav 2008 make noises at the speed about 40mph. It was finally diagnosed that it is coming from the viscous coupler in LSD differential. I was told the only way to solve the noise was to replace the rear differential as a unit. I was advised it has nothing to do with the safety of driving the car. You dont need to replace it. What is your advice to me.


H A Gordon
December 31, 2012 - 11:19

How important is viscous coupling to the 2013 Subaru Forrester?

Reply to H A Gordon
September 09, 2014 - 14:32

pretty important if you like having AWD.
its essentially the clutch that drives power from your front axel to the rear.

Reply to H A Gordon
November 14, 2015 - 17:32

The viscous coupling in a manual Forester controls the allocation of torque between front and rear axles, by loading the centre mechanical differential - there is no clutch between front and rear axles.

March 27, 2009 - 09:58

Thanks for a quick basic lesson. Interesting! I'll definitely be looking into these couplings further, and also be looking at this forum for more info.