LSD question.

I have always been under the impression that a clutch pack type LSD has a "built-in" design feature that provides more lockup when it's called upon. What I mean is that while the clutch plates allow a certain amount of slip based upon friction surface, stack pressure, number of plates, state of wear, etc......when there's a lot of slip the clutch pack heats up and the expansion of the plates creates less slip. In other words, sort of a built-in thermal based counter measure to continued excess slip.

I was told this by a very experienced and very successful racer - LeMans podium finisher, and more than one SCCA championship. So, I always assumed it was fact and true. Is it?

Randy V

Staff member
Lifetime Supporter
My experience with clutch type LSD’s is not consistent with what others have told you.
Considering all components to be in new or good operating condition - The amount of slippage between the disks and plates is controlled by two things (optimally).
1) Preload of the stack
2) Dynamic load of the stack as provided by the differential side gears interacting with the 2 to 4 pinion gears within the carrier.
Anything else would be pretty inconsistent and would ultimately lead to the failure of the differential system.
Yup, I'm following you here Randy, and that does make total sense.

The last time I had an LSD apart I replaced the (worn out) plates in my LSD-equipped porsche 915. I noticed there was some "cupping" to the plates, which makes sense as this provides the pre-load to the stack. It looked to be a matter of maybe 1-2 thousandth's per plate of cupping across the span. I guess it's possible some thermal expansion could increase the stack pressure, but likely not so much as to take up all the slack (ie. lockup). Interesting......
What you are describing when excessive heat causes the plates to bind is what happens in a Viscous Differential. Depending on application they are built to produce that "locking" effect. Goes the same for a Viscous Coupling. Basic difference between the two is the coupling has the same viscous clutch just not differential gear set. Binding or Humping is seldom seen in production units but a tuneable feature for motorsports applications.
If they build that much heat the oil would break down. They have preload and the plates have a certain coefficient of friction and they will slip a certain amount they dont lock up. Some types you can adjust from the out side by set screws that tighten the preload but the more they slip the more they wear and will eventually not hold and will have to be rebuilt.