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superfans 10-11-2005 11:32 PM

Differential (LSD)
Stupid Question - Can anyone explain what is the purpose for having a limited slip differential (or LSD)? Does the new 997 with PASM has it? What happens to a car if it does not have any differential (or LSD)? Thanks in advance.

Jack Olsen 10-12-2005 12:09 AM

A differential does a few key things. It transfers the power from the output end of the transmission at a 90-degree angle, so that it can power the wheels. Simultaneously, it reduces the gearing down a final time before the power reaches the wheels. But it also allows each wheel to spin independently of the other, since during a turn they need to rotate at different speeds. Normally, it applies equal torque to each wheel. But in instances where one wheel has less traction, like if one wheel was raised off the ground, an open differential will allow that wheel to continue to spin, without applying power to the other wheel.

With a limited slip differential, you limit this phenomenon -- an LSD reduces the difference in torque distribution between the two wheels, so that the wheel getting less traction and the wheel getting good traction both continue to turn, with the 100%/0% distribution changing to something more equitable (although only in a locked differential would both wheels always receive the same amount of torque).

If a car doesn't have an LSD, it's more prone to wheelspin in some situations where the difference in traction between the two wheels is great. In some situations (like tight turns on wet pavement) the LSD can make things worse, not better. But as a rule, it provides better, more stable, power distribution to the two wheels.

At least, that's how I understand it.

911teo 10-12-2005 05:38 AM

A side effect of an LSD is the fact that it increases a car's tendency to understeer as the rear wheels try to go on a straight line (same torque to both wheels).

There are several ways to counter that... (sway bars adj etc) but that's another story...

DW SD 10-12-2005 07:10 AM

I've heard that it is also useful under braking where you want both wheels to apply their braking force. It may keep a rear wheel from locking up if it has been unloaded due to cornering forces during trail braking. I can't say I know this phenomenon first hand.


Yellowbird RS 10-12-2005 07:28 AM

Introduction to Limited Slip Differentials (LSD)

rick-l 10-12-2005 07:29 AM

I am not all that familiar with PASM but I believe one aspect that it controls is wheel spin by applying a brake to the wheel that is spinning. It also does a lot of other stuff that the software programmer thinks you should do to stabilize a vehicle approaching the limits of adhesion going around a turn based on yaw rate sensors and accelerometers.

cashflyer 10-12-2005 07:39 AM

Another effect to be aware of when using an LSD diff is that if you break traction with one wheel, you have broken traction with both wheels.

This is something that I had to become aware of when I was using a sure-grip rear diff in an old Plymouth. In hard accelleration (drag) the benefit is that both wheels are pulling to get you down the strip - as long as you have good grip. However, under hard accelleration in a sharp turn the posi/sure-grip/lsd (whatever you want to call it) can allow both rears to loose traction and "come around".

Of course, as Porsche drivers we all know what it's like when the backend tries to outrun the car in a turn.

Makes for one heck of a donut burner, tho....

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