LSD breakaway torques Options

[pcar916]

I have enough mileage on the 40/60 LSD to measure the breakaway torque. Now that the Paul Guard is no longer at GT and having talked to Matt for internal rebuild specs, I have a question for those of you who track that specification.

Excellent customer support by Paul before him, and now Matt, on my diff by the way, but Matt has no way of knowing what Paul dialed into my diff when it was assembled, and I can't find where I wrote it down. He questioned me at length about my driving style and how the car was to be used before building the unit. Now, before anyone brings it up I know there are more things involved than simply overcoming the pre-load created by the Belleville washers, like having more or fewer actual clutch plates, so feel free to include those assumptions in your descriptions if needed...

what breakaway torques (accel/decel) do you clutch-types build into your diff's in our mid-engined machines?

For those who never heard of this the "breakaway torque" is the amount of torque you have to apply to one rear wheel (off the ground) to make it rotate when the other wheel is locked in place and can't turn. The differential has a different number for acceleration than deceleration unless it's setup to the same angle for both directions.

[pcar916]

Richard, outstanding writeup. You are correct that I stated ramp angles when I meant percent lockup on load/overrun. In addition my clutch setup is O l O l on each side. I didn't know to specify that one way or another when I ordered the diff.

So if I want to modify my slip (therefore breakaway torque) I'd rearrange the disc configuration or change the thickness of the entire clutch package to increase/decrease the spring tension.

I'd have to contact GT to find out how much the lockup percentage changes when two friction disks (internal spines in my case) are put next to each other, or when one of them is replaced by a smooth disk so only one is present on each side. I know the smooth disks (externally splined) can be ordered in various thicknesses.

One disk per side, or reducing the spring tension "some amount" might solve my terminal understeer problem when I attempt to run the occasional AX. I know the AX guys must run a higher slip percentage than the track crowd just to be able to get the cars to turn in. Sounds like multiple gearboxes is the best answer.

Of course this isn't a trivial amount of time to spend on tuning. Especially since our (914) gear clusters have to come out before the diff does. Kinda makes me want to invert a 915 so the R&P is beside the side cover. 911 gearboxes are so easy that way. But enough of that ramble.

I'm already running synthetic gear oil so I won't get much more slip with fluid changes... and no, I don't go through sychronizers or dog teeth.

Thanks for the replies guys.