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LSD and MOT

harry

harry

PCGB Member
Member
Hi guys,

The RS with the LSD I understand should not have the brakes tested on the equipment that rotates the wheels individually. However, if the gear is in neutral presumably the LSD is not "engaged" (correct term?) so would it be OK? Otherwise it will have to be done by another method - can't remember what it was called. I'd be interested in your thoughts.

Harry
 
I mention to the mot tester the car has an Lsd and may be damaged by their test equipment....I have known them put a brake test potentiometer (?) on the passenger floor on the car and let me take it round the block,,,other times I mention it braked true and straight the week before at Bedford, getting down from 140 to 100 for the chicane (slow I know [;)]) , this has been enough to pass the car on brakes before without them actually testing them.
 
I have had my diff tested in nuetral....not as conclusive as I had hoped.......what is mileage on your car,,,above 30k miles may well be toast....will certainly be worn, whay do you think it might be worn,,,what did our responses not answer??...if not sure about mot test effects, then don't let them run it on the rollers...easy to be cautious there, like most of us.
 
Clubsport,

Car has 41000km (26000 miles) - at least 1/2 of the mileage on track by previous owner. I have tried spinning a wheel with both rear wheels of the ground. In neutral only the side I am turning moves. When I engage 1st gear (engine off), and turn a wheel, the other wheel turns in the opposite direction - presumably this means the LSD is gone. Since in neutral only one wheel was spinning I wondered if the brake test in MOT can be done on the rollers without damaging the LSD (this is for future reference more than anything else). I guess I may have had the LSD toasted without realising it as I had it tested previously on the rollers but the car was in neutral at the time, hence I just wondered about testing the brakes in neutral would lead to LSD damage. Sorry I wasn't more precise in my questioning.
I've seen people talk about 'open' diff - what does this mean?
Cheers.

Harry
 
Harry

If your car has a lsd, then it can not and should not have its rear brakes tested on the rollers for its MOT. I've just had mine MOT'd (964RS), in order to check the rear brakes it should be road tested.

Damen
 
The LSD limits differential rotational speeds between your 2 rear wheels (ie limited slip), having the box in neutral makes no difference at all (BTW an open diff is a diff without an LSD)

The mot tester should use a tapley meter which is basically a G meter in the car, but that means they have to drive your car.

As the rotational speeds of the brake tester is low, then it won't tend to damage your LSD, more likely is that the tester won't give a good reading. My old EVO 5 had 3 LSD's and could not be tested on a brake tester, they tried and it does not do any damage. Remember that the LSD recieves much more torque transfer under power out of a bend than it does on a brake tester.


Harry, thats normal, your LSD will be fine. Modern diffs are torque sensing (Such as Torsen and Suretrac, the old friction type are the spawn of the devil) and therfore need some torque to operate, turing by hand does not do this and it is essentially 'open' at this point. Turning one wheel just turns the half shaft and the prop shaft. When in gear, the prop can't turn so it turns the opposing wheel backwards, totally normal.
 
My 964rs with lsd has been roller brake tested every year for the last 9 years with no ill effects. The lsd on these cars is only a mechanical clutch that is lubricated & designed to slip so there should never be any problem.

 
BTW, did the 3.2 Carrera have an LSD anyone.

A bit more about LSD's

Most road diffs can transfer torque at about 5:1. This means it can put about 5 times more torque on one side than it does on the wheel with limited grip. However this means that the wheel with the low grip is still receiving some torque. If the wheel is on eg ice, then you can't apply any significant torque to it and as 5 x 0 = 0 so you get no significant torque on the other wheel. So with a torsen or suretrac you can't test the LSD by putting one wheel on slippery mud and not expect it to spin.

Plated diff's can be set for total lock up, so in this case they will play merry hell with the brake tester as they may try to lock and spin the stationary wheel. This is probably where the tales of problems come from. I doubt the 993 has a diff that locks fully.

The FF type diff (usually used as a centre diff) uses silicone. As the differential rotational speeds increase, the silicone becomes thicker and acts to limit the differential.
 
ORIGINAL: Gary C

The LSD limits differential rotational speeds between your 2 rear wheels (ie limited slip), having the box in neutral makes no difference at all (BTW an open diff is a diff without an LSD)

The mot tester should use a tapley meter which is basically a G meter in the car, but that means they have to drive your car.

As the rotational speeds of the brake tester is low, then it won't tend to damage your LSD, more likely is that the tester won't give a good reading. My old EVO 5 had 3 LSD's and could not be tested on a brake tester, they tried and it does not do any damage. Remember that the LSD recieves much more torque transfer under power out of a bend than it does on a brake tester.


Harry, thats normal, your LSD will be fine. Modern diffs are torque sensing (Such as Torsen and Suretrac, the old friction type are the spawn of the devil) and therfore need some torque to operate, turing by hand does not do this and it is essentially 'open' at this point. Turning one wheel just turns the half shaft and the prop shaft. When in gear, the prop can't turn so it turns the opposing wheel backwards, totally normal.
Click to expand...

The Porsche diffs are not Torque sensing and are friction type with an initial amount of pre-load setup for the amount of locking. This dictates the amount of locking under power and the diff pressure ramps up on the over-run to 100% in some cases depending on model.

Harry it does sound like you need new friction plates in your diff as there should be some resitance.
 
Hum

I cannot believe that the 993 has a 1950's style friction diff !

Are you saying that its like the old Opel diff where you hang a weight from a rim and measure the weight that causes rotation to determine pre-load !

These diffs are useless on a performance car. Ok for mud/snow but little use for much else.

Plated diffs with clutch packs are a slightly different matter. Ramps allow the clutch pack to close up as the differential rotational speed forces the follower up the ramp. These are mainly used in racing as the progression can be altered by changing the slope of the ramp(s) and can provide total lockup (usefull of you break a 1/2 shaft in a race apparently), however the ones I have seen are set 'open' at zero differential speed.

I imagine when you say friction, you mean a plated diff ?
 
993 RS diff does run a clutch pack which was the basis for this topic......I am due to have the plates changed on my own 993 and the parts sent from porsche motorsport are from the 997 GT3RS which will fit the 993....don't think these diffs are too bad if Porsche are still using them on the latest cars.
 
So its a plated diff rather than a 'friction' diff

Terminology is a wonderfull thing.

The old 'friction' diffs had a friction ring that rotated when the wheels rotated at different speeds, it did not vary torque transfer in relation to diff speed and provided basically the same lock up at 0 diff speed as at 100% diff speed.

I still would be suprised if this plated diff was set up with some static locking other than a small amount of static friction ?

Take that back, I only have a little experienc with a FWD plated diff and that had no static preload, but just read that RWD applications tend to have preload
 
OK,

The topic has become too technical for my poor brain [:D]. When I turn the wheel manually, there is slight resistance. So does the roller damage LSDs? I guess if it has to be driven to test brakes then I will insist on driving my car as they won't be insured on it [:-] God knows what they will do - remeber the topic about an OPC driving customer's RS like a bloody idiot!

Harry
 
It is a plated diff with ramps for varying lock up as required.
I run the motrorsport plates,fitted myself on the bench,easy job and around 2 hours max.
Harry its very difficult to work out what the plates are like from just spinning a wheel etc.
The new plates also come with differant thickness spring washers etc,so you need to be carefull to get this part right.
The original plates also break down with time,as the oil attacked the friction material more than actual wear some times.I would recomend changing for the new plates,and you will find a differance straight away.
My GT2 plates were knackered with only 6,000 miles on the car,because of the miss match of material vs oil.The gearbox also goes a nice black colour,like a teflon coating!!
Colin can help/ fit/advise on this.
 
Originals did nt exactly fall apart before ur eyes , but lets say never last long ...........usual step is to go for 996 cup plates from the Motorsport dept , simply an evolution and they re tougher .........nothing lasts for ever , reasons in earlier post by Paul, but these do last longer and to me dont feel much different
 
ORIGINAL: micksummer

What's the difference between the standard plates and motorsports plates?
Click to expand...

I understand the friction material is more durable and therefore lasts much longer.
The m/sport plates do not appear any more expensive than the regular plates...not a guess either as I have a set of motorsport supplied 997GT3RS plates waiting to be fitted ....hopefully next week now [:-]
 
Anyone have a part number for the longer lasting Motorsport plates? When I had mine relaced a year or so back they were replaced with the OE 993 RS part. If, as it seems from comments, they are made of cheese it would be useful to have the Motorsport part no. on hand to be able to spec those next time.

Thanks
 
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