GPL Salisbury diff preload

Hi all,

Not sure if RAS is stilled filled with car sim creators as back in the
day. ;-)

I'm currently revising my differential code (in Racer) and trying to get
Salisbury diffs to work as a variation on LSD's (limited slip diffs).

For an LSD, I define:
- power_ratio; torque bias ratio for power-on
- coast_ratio; same for coasting (letting go off the throttle)
- preload; base spring-loaded torque

GPL defines diffs as 60/30/2 for example (power/coast/clutches).
From back in the day Niels Heusinkveld explained that:
- ratio=cos(angle)*(1+clutches).

So for example, cos(60)*(1+2) = 0.5*(1+2) = 1.5. So for the above diff
you get 1.5 torque bias ratio. Anything inbetween that keeps the diff
locked.

But for preload; how does GPL do that? Without some preload, the diff
would become a bit unstable, esp when one wheel is on the grass. Surely
all the clutches result in some preload torque. But how much friction
torque exactly?

Thanks for any ideas,
Ruud