How does a SUBARU viscous limited slip differential (VLSD) work?

June 10, 2011. Author:

A limited slip differential permits slip (i.e. a difference in rotating speeds) between its two outputs (for example, wheels) before it starts to try to force both wheels into rotation.

In a Subaru viscous limited slip differential (VLSD), a silicone fluid is used in a mechanism between the differential's two outputs.  In normal operation, this fluid is fairly thin and allows for a difference in rotation speed (slip) of the two outputs.  A difference in speed between the two outputs, however, then causes the silicone fluid to heat up and become quote viscous (thick).  As the fluid becomes thicker (more viscous), it in a sense binds the two outputs together, thus limiting the amount of slip between the outputs. 

Now in practice, there is a limit to how much torque the limited slip differential can force onto the output that isn't turning.  If the fluid gets too hot for too long, it tends to stay viscous rather than returning to its normal thinner viscosity when cool.  Then the differential can exhibit torque bind.

Subaru uses VLSD's in the rear differentials on some Subaru's.  Some of the Subaru manual transmissions use a VLSD center differential to transfer power to the rear differential.

The silicone fluid is isolated from the gear oil in the differential and cannot be changed or serviced.

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Do I need limited slip additive in the gear oil for my Subaru limited slip rear differential? Thanks.