[antzcrashing]

Originally Posted by Remonster

On a normal car with an open diff, if one tire slips all of the power is wasted spinning that tire while the other one with grip essentially gets no power. A limited slip differential limits the speed difference between the wheels to ensure some of the power always goes to the wheel with grip.

When you have a "25% LSD", the maximum difference in power received by the rear wheels is 25%. With a 25% diff, one side can receive up to 62.5% of the total torque while the other gets 37.5%. Higher % can be beneficial in certain situations like accelerating hard out of a corner, but higher % also increases understeer in slow corners and increases oversteer in the rain (sounds counter-intuitive but basically you are making it harder for the two rear wheels to spin at different speeds in a corner).

For a street car, I would say 20-40% is ideal. Older BMWs with LSDs like the E36 M3 had 25% lockup so this new one is actually more aggressive. The fact that it only locks 9% on deceleration means that it actually won't cause understeer on corner entry and the 30% acceleration lock is not enough to make it dangerous in slippery conditions.

The current M cars use an electronically-variable lock which allows them to vary lockup as needed. These diffs are incredible but they are much more difficult to retrofit onto a car since they need to tap into wheel speed sensors and probably the car's computer systems to function so obviously BMW didn't want to offer such a complex unit as an upgrade for the standard cars. I personally think the price and ease of installation are perfect for this LSD when you consider that it won't be noisy like some aftermarket diffs, doesn't require service as often as some aftermarket diffs (I don't believe it's truly lifetime, I'd probably replace the oil every 50-100,000 miles), and comes with a warranty.

I avoided using the term "lockup" because some people describe LSDs the opposite way where 100% lockup means there is NO difference in the speed of the two rear wheels or the power applied to each. This is, as someone above said, like having a welded diff and is what drifters use but it would be terrible to drive around with a fully locked diff all the time, the car would be very difficult to turn.

Thanks for the nice explanation