LSD rear end ID

[no694terry]

seems i spoke too soon. I finally found a very useful link, very suprised how similar they are. according to this, i do have a LSD

LSD vs Open guts

[jp1203]

Wow, subtle difference! So when the LSDs go, it's because the clutch plates have worn out?

I wonder how much more expensive the LSD is to produce...doesn't look like it could be all that much worse. Makes you wonder why Subaru didn't put it in everything that didn't have VDC.

[no694terry]

No clutches. Its a fluid based coupling sort of like the torque converter in an automatic transmission, but the fluid in a vlsd is silicone based and thickens up as its heated. The coupling I beleive is attached to the smaller gear that the axle slides into inside the ring gear. The coupling tries to keep that smaller gear and the ring gear spinning at the same speed, when slippage occurs the fluid heats up and start the "effect". Once the smaller gear and ring gear are spinning the same speed again, the spider gears can then tranfer the power to both wheels evenly. This all happens in split seconds

[plain OM]

Great pics, and link! Thanks.

Originally Posted by jp1203

Wow, subtle difference! So when the LSDs go, it's because the clutch plates have worn out? . . .

The plates themselves are not in contact with each other (at least they're not supposed to be), so normally they don't wear out. However, the plates are very close to each other and are perforated. There's a special viscous fluid between the plates. Alternating plates are attached to one side gear and to the differential case. When they turn at different speeds, which happens when one wheel spins relative to the other, the fluid between the plates heats up due to the "shearing" effect between the plates. As a result, the fluid "gels". Because the gelled fluid is not only between the plates but through the perforations, when it becomes stiff it resists the relative movement of the alternating plates.

I believe the most common failure in the V-LSD is the seals, which separate the plate area from the rest of the differential. (It's that extended "housing" to the left of the crown gear in the pics.) Once the seals go, and the viscous fluid either leaks out, or becomes mixed with the gear oil in the differential, the gelling effect is lost. Loss or mixing of fluid could happen very slowly over a long time. The seals might still cause the drag that leads to both wheels turning in the same direction when manually rotated, but the LSD won't be effective.

See also the attached description from my 2007 FSM, and http://en.wikipedia.org/wiki/Viscous_coupling_unit, which has photos of the plates (not Subaru LSD but essentially the same idea).

(p.s., woops, see I doubled with no64terry).

[no694terry]

Other than the bushing bolts, the swap was painless. Axles clicked right in but wont be able to test it until I put the motor back in.