Sorry for being OT, I just want to answer the "what does preload do" questions to my best knowledge.
A clutch pack differential tries to equalize the torque acting on both wheels to a certain percentage. For example, if the left wheel is on the ground and the right one in the air, then an open diff would just spin the one in the air, thus providing 100% torque to the freely spinning wheel (
E: Actually this is wrong, as soon as a wheel spins it doesn't apply any torque). A locked diff would always provide a 50/50 split and a clutch pack is inbetween.
Lets say we have an input torque of 400Nm - left wheel on ground, right in the air. Torque applied to wheels as follows:
Locked diff - L: 200Nm R: 200Nm
Open diff - L: 0Nm R: 400Nm ([I]E: Actually not 400, but almost 0Nm[/I])
Clutch pack diff @ 50% - L: 100Nm R: 300Nm
Clutch pack diff @ 75% - L: 150Nm R: 250Nm
Now, in order for the clutch pack LSD to provide a
percentage of locking, the amount of pressure on the clutch plates in the differential must be proportional to the input torque, which means the plates have to be pressed together harder the higher the input torque is. In reality this is defined/archived by the ramp angles used in the differential - the steeper the angle the more the clutch plates are pressed together when torque is applied. You also have two ramp angles to play with, one for each direction of torque (power/coast lock).
If we understand this, we can also see that when no input torque is applied, there is also no locking of the clutch pack diff, because 50% locking of 0Nm torque is still 0Nm. This is where
preload comes into play. It provides a minimum amount of pressure on the clutch plates, so that even with no input torque, there's always a minimum amount of locking applied.
Another short example with a 50% locking clutch pack diff with 150Nm preload, gradually decreasing the input torque:
Input tq, resistance tq by clutch, left wheel, right wheel, real % of locking
-----------------------------------------------------------------------------
Input: 400Nm, Res: 200Nm, L: 100Nm, R: 300Nm, Lock: 50%
Input: 350Nm, Res: 175Nm, L: 88Nm, R: 262Nm, Lock: 50%
Input: 300Nm, Res: 150Nm, L: 75Nm, R: 225Nm, Lock: 50%
Input: 250Nm, Res: 150Nm, L: 75Nm, R: 175Nm, Lock: 60%
Input: 200Nm, Res: 150Nm, L: 75Nm, R: 125Nm, Lock: 75%
Input: 150Nm, Res: 150Nm, L: 75Nm, R: 75Nm, Lock: 100%
Input: 100Nm, Res: 100Nm, L: 50Nm, R: 50Nm, Lock: 100%
I hope I didn't make any errors, and that it's more clear for you guys now
E: Well, I did make errors, but apparently the general idea is still correct (at least no one pointed out any errors yet).
E2: Seems like the tables are wrong too. Better wait for someone who really understands all this stuff