LSD Questions

[timhum2002]

Would one of the technically mind contributors be able to explain the advantages of having an LSD on a road going Porsche. My understanding is that on a track car it is principally to improve traction which certainly doesn't seem to be a problem that afflicts my C2S.

thanks

Tim

 

[Geoff_Fields]

LSD works to increase traction, which by now we all know means an increase in handling. There are different mechanical styles of LSD's and then different settings on them.

Open Differential: A differential allows the tires to run at different speeds. This allows the car to turn, because the inside wheel will always be travelling a shorter line. So the power goes to the outside wheel, which has a harder job to perform. To make it easy, with an open diff, the wheel that is spinning more gets the power. That means that during a high power launch, only one tire will get the power and keep spinning. When an open differential gets one or both wheels in a limited traction situation, like one wheel on ice, it sends 100% of the driving torque to the least laden wheel, which results in you going nowhere fast.

Clutch LSD: (997 type) These transfer power through a clutch in the differential. As one wheel starts to spin, the clutch applies pressure and transfers power to the other wheel. The more pressure, the more power transferred. In limited traction situations, like where both wheels are on ice, the two wheels remain locked to the carrier and will pull together and help pull you out of a problem. In situations where you have one wheel on a limited traction situation, the other wheel will slip, just like an open diff except that the clutch pack assures that it carries some proportion of the available torque. LSD's also provide some major handling advantages, by guaranteeing torque will be transmitted to the outside drive wheel in a turn, even if you have lifted the inside wheel off the ground. Under acceleration the LSD allows you to put more power to the road and helps keep the car straight. Similar advantages are noticed under braking also.

 

[jpcgt]

Would it be fair to say that going for the sports suspension with LSD option on the C2S would be a better choice than going with the standard PASM suspension offering with no LSD?

Interested in opinions on this 997 no cost option.

Thanks Jon

 

[melsomp]

ORIGINAL: Jon987

Would it be fair to say that going for the sports suspension with LSD option on the C2S would be a better choice than going with the standard PASM suspension offering with no LSD?

Interested in opinions on this 997 no cost option.

Thanks Jon

This is an interesting question, and one that I'm wrestling with as I decide on the spec for a 997S. Perhaps for a track car LSD is the way to go, but I'm not so sure LSD is right for a road car. If LSD is so good, then why did Porsche go to all the trouble of developing PASM?

 

[Lancerlot]

Unless you have been experiencing traction problems with your current model, then an LSD will not provide any real gains for you. It's also doubtful whether you will ever need it on the track. Better to spend the money on something else!

 

[Guest]

[/quote]

This is an interesting question, and one that I'm wrestling with as I decide on the spec for a 997S. Perhaps for a track car LSD is the way to go, but I'm not so sure LSD is right for a road car. If LSD is so good, then why did Porsche go to all the trouble of developing PASM?
[/quote]

PASM is to do with suspension, and has the advantage of soft and hard settings, not to do with traction directly. LSD are expensive and ideally you would have PASM and LSD, but I guess Porsche felt that only those pushing their car to the extreme would notice an LSD, and these guys would probably not mind the lower harder sports suspension, so they saved the cost of the PASM and gave them an LSD instead for the same price...

 

[melsomp]

Thanks camcayenne, it's all beginning to make sense now, I think I'll stick with the standard PASM, I like the fact that it gives you options, I don't think I could live with permanent harsh suspension, seems to me that PASM gives you the best of both worlds.

 

[Alex Postan]

ORIGINAL: Geoff997

LSD works to increase traction, which by now we all know means an increase in handling. There are different mechanical styles of LSD's and then different settings on them.

Open Differential: A differential allows the tires to run at different speeds. This allows the car to turn, because the inside wheel will always be travelling a shorter line. So the power goes to the outside wheel, which has a harder job to perform. To make it easy, with an open diff, the wheel that is spinning more gets the power. That means that during a high power launch, only one tire will get the power and keep spinning. When an open differential gets one or both wheels in a limited traction situation, like one wheel on ice, it sends 100% of the driving torque to the least laden wheel, which results in you going nowhere fast.

Clutch LSD: (997 type) These transfer power through a clutch in the differential. As one wheel starts to spin, the clutch applies pressure and transfers power to the other wheel. The more pressure, the more power transferred. In limited traction situations, like where both wheels are on ice, the two wheels remain locked to the carrier and will pull together and help pull you out of a problem. In situations where you have one wheel on a limited traction situation, the other wheel will slip, just like an open diff except that the clutch pack assures that it carries some proportion of the available torque. LSD's also provide some major handling advantages, by guaranteeing torque will be transmitted to the outside drive wheel in a turn, even if you have lifted the inside wheel off the ground. Under acceleration the LSD allows you to put more power to the road and helps keep the car straight. Similar advantages are noticed under braking also.

Ahem, not quite right!

A LSD will increase traction in marginal conditions but it will not improve the handling. It could be said that it might improve the roadholding.

Handling = how the car behaves
Roadholding = the amount of lateral grip generated

A LSD will usually impair handling by increasing understeer unless it is a microprocessor controlled hydraulic system such as fitted to current WRC cars.

For a road car, particularly a modern Porsche, the grip levels generated are such that you are very unlikely to ever find yourself driving on the road and actually need an LSD. The car will be far nicer without and it will handle better. On a track day, and only if you really can drive, with a LSD you might take a half second off your lap times. Big deal!

 

[Alex L]

I was just told by my OPC that LSD is good for Carrera 2 cars which track a lot. Apparent Carrera 4 (and Turbo) models really don't need any extra traction on the road which LSD provides.

 

[Lancerlot]

I agree whole heartedly with Alex P - also with what Alex L has been told, but remember the original question did in fact relate to a C2S.

 

[sawood12]

'Dumb' mechanical LSD's are not the holy grail they are made out to be unless you've got one of the new computer controlled diffs fitted to things like the GT3 and Turbo's. Ever since JC's ignorant comments about the lack of an LSD 'crippling' the performance of the Caymen on their track has fed alot of hype about LSD's. On the track in the hands of someone like the Stig then maybe, but in the hands of a 'normal' person who drives the car on the road for all its life an LSD only helps you in a handful of scenarios where one wheel is likely to loose traction before the other. For example when you are pulling out of a junction and you have to floor it to get through a gap in the traffic - with an open diff there is a risk that the inside wheel will spin with 100% of the drive going to the spinning wheel (the wheel that has lost grip), therefore you are left high and dry in the middle of the lane with no drive and an articulated lorry bearing down on you. An LSD will still provide say at least 40% of the drive to the wheel with grip which should still give you a decent level of accellaration to get you out of trouble.

99% of the time on the road an LSD is probably hindering you. In actual fact an LSD can induce understeer as they are more reluctant to turn in than an open diff - especially on tighter bends where the drive wheels speed difference approaches the % slip limit of the diff. and in some other cases they can induce oversteer. Of course on the track the LSD helps you accellarate out of tight low speed corners like hairpins, where you are flooring it on the exit from a low speed and there is a chance the inside drive wheel might loose traction - especially in the wet.

Modern mecahnical viscous diffs are alot better than the old friction plate diffs so they might be more benign to the average driver and the zero cost option on the 997 might be a good option, especially if you are going to track the car. But do you want to compromise the comfort and adaptability of PASM for it for the odd occasion where you might get the benefit on the track? In any case i'm sure i've read that the PASM 997 Carrera S lapped the ring quicker than the LSD and passive sports-suspension equipped Carrera S in the hands of Walter Rhol - but I don't think there was much in it.

In anycase it's a nice decision to be pondering over!