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Join Date: May 2005
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ABD vs LSD

It seems that the regular 997S with PASM use the "Active Brake Differential" and only the sport suspension with -20mm ride height use the Limited Slip Differential. What is the difference between the Active Brake Differential and Limited Slip Differential? Also, which one is better? Thanks in advance.

Old 10-13-2005, 12:16 AM
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ABD is nothing but ABS applying brake on spinning wheel and thus transfering power to other wheel.

Problem is that it chews lot's of power that could be applied to wheels and loads the brakes in progress.

LSD is better.
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Old 10-13-2005, 03:17 AM
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Yeah, but when LSD clutch disks rub on the inside of diff housing they transmit the torque to other side directly. When ABD tightens brake caliper it will split the torque and heat brake pads with one half and then transfer torque trough intricate mesh of differential gear to other side, loosing even more power.
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Old 10-13-2005, 04:37 AM
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lsd also works on trailing throttle, this stabilizes the car
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Old 10-13-2005, 04:48 AM
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I don't know about ABD ..but LSD changed me forever.
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Old 10-13-2005, 06:33 AM
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I wonder how it handles left foot braking? Wouldn't that mess with the ABD? My Audi (with their version of ABD) doesn't like it one bit--cuts the throttle whenever the brakes are applied.
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Old 10-13-2005, 07:20 AM
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beepbeep,
Thats only half right. A cluch style LSD has to transfer torque though the spyder gears too. The "grip" side clutch pack will transfer torque directly, while the "slip" side clutch pack will transfer the torque back though the spyders to the "grip" side. Since both packs have the same normal force from the ramps, the effort is always split 50/50 between the packs.

Technically if you can get the diff to lock up 100%, then there is no frictional loss. But, thats not practical. The LSD is also eating up power everytime you corner where ABD has no parasitic loss until slippage occurs (which by definition means you have more power than you need, so a little loss in the brakes isn't so bad).

The best of both worlds (for accelerating) would be a quaife ATB and some form of ABD to kick in when your on ice or in the air (curb hopping). In a Hummer H1, you get to do this manually by left foot braking while rock climbing (it has quaifes). Thats easy to do since they are all automatics

ABD works pretty well for being esentially free. It doesn't have the advantage on decel like a real LSD has, but it also doesn't have the low speed chatter or rain/ice decel pecularities because of this either.

For racing, use an LSD. All stock cars w/o an LSD should be equiped w/ ABD - unless they royally goof up the programming like RallyJon says w/ his audi. IMHO, *ALL* cars should have some sort of LSD option from the factory - particularly all FWD cars that have crappy traction to begin with

SMD
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Last edited by smdubovsky; 10-13-2005 at 08:19 AM..
Old 10-13-2005, 08:17 AM
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Quote:
Originally posted by smdubovsky
beepbeep,
Thats only half right. A cluch style LSD has to transfer torque though the spyder gears too. The "grip" side clutch pack will transfer torque directly, while the "slip" side clutch pack will transfer the torque back though the spyders to the "grip" side. Since both packs have the same normal force from the ramps, the effort is always split 50/50 between the packs.

Technically if you can get the diff to lock up 100%, then there is no frictional loss. But, thats not practical. The LSD is also eating up power everytime you corner where ABD has no parasitic loss until slippage occurs (which by definition means you have more power than you need, so a little loss in the brakes isn't so bad).

The best of both worlds (for accelerating) would be a quaife ATB and some form of ABD to kick in when your on ice or in the air (curb hopping). In a Hummer H1, you get to do this manually by left foot braking while rock climbing (it has quaifes). Thats easy to do since they are all automatics

ABD works pretty well for being esentially free. It doesn't have the advantage on decel like a real LSD has, but it also doesn't have the low speed chatter or rain/ice decel pecularities because of this either.

For racing, use an LSD. All stock cars w/o an LSD should be equiped w/ ABD - unless they royally goof up the programming like RallyJon says w/ his audi. IMHO, *ALL* cars should have some sort of LSD option from the factory - particularly all FWD cars that have crappy traction to begin with

SMD
Hmm...when I think about it more, you're right!

I personally don't like ABD but I only drove FWD cars equipped with it. One of things I remember well was smell of overheated brake pads after driving SAAB 9-5 Aero agressivly trough snow. I didn't brake much, just accellerated hard trough snow which triggered lot's of ABD work.
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Old 10-13-2005, 01:03 PM
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ABS is for girls...
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Old 10-13-2005, 01:07 PM
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Quote:
Originally posted by targa911S
I don't know about ABD ..but LSD changed me forever.

Me too. I now know what the color green sounds like. It sounds the same as chicken tastes.

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Old 10-13-2005, 01:29 PM
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