garage method of checking lsd function

[fredmeister]

Anyone know of a quick and dirty way of checking the condition of the OEM lsd in a 930 4 speed box? Trying to gage if mine is still functioning or not.
I thought there might be a way of jacking the car up on blocks and measuring the forces to turn one wheel while the other is blocked from rotating.
Is this a logical approach, and if so how much force approx. should be expected to turn the unblocked wheel by hand as long as the diff is in good shape?

[JFairman]

There is a specification for that and it can be measured with an inch pound torque wrench turning one wheel hub to measure the resistance while the other is not allowed to turn.
The clutch pack can be shimmed up or 4 new clutch discs installed to raise the limited slip percentage or bring it back to origonal spec if worn. You can also remachine some parts to fit 6 clutch discs and plates - 3 on each side in there if wanted.

I don't know the turning resistance spec but you could find it somewhere or from someone that rebuilds 930 LSD's.

The way the LSD is designed the pressure applied squeezing the 4 LSD clutch discs and plates increases with engine torque.
The spider gearsets teeth are cut at an angle in a way to push away from each other under load while the LSD carrier surrounding them and the clutch discs limits how far thay can push away from each other and thats how the pressure on the clutch discs is increased under load.

It's hard to describe how the ZF LSD used in Porches and BMW's works without referencing to an exploded diagram or having one apart on a bench in front of you.

[mark houghton]

...and while on the topic, I also have LSD (but not the Timothy Leery kind). I've got a weird symptom that has been there since the day I bought my car, and I think at one time I read a diagnosis.

When backing up out of my gargage and turning 90 degrees at the same time, I'll get this little knick-knock sound...only when turning while backing up. Probably a clutch on one side or the other. My knowlege is zipppp on differentials.

Jim, your thoughts....just live with it, or worry about it (or maybe fix it some day)?

[voitureltd]

Fredmeister,,,,,,,,,,,,,,,,,Why not give it a
seat of the pants road test. I own both w/wo LSD. On one without LSD in the lower gears you can definately feel it push the rear of the car to the right ( also lift the left front somewhat) without. On a higher than factory split you can hear a slight knock and feel a tugging when slow sharp turning most noticed in reverse, unless near the upper limit of split, then a knocking noise / rear wheel tug forward or reverse. On a long curve under power with a high split 80/20 the car will push straight unless you break the wheels loose so you must get your entrance speed right as it is difficult to raise it once into the apex of a corner. This is a general fun way I think to decide if you want more posi effect or if it is still working.

[JFairman]

Originally Posted by mark houghton

...and while on the topic, I also have LSD (but not the Timothy Leery kind). I've got a weird symptom that has been there since the day I bought my car, and I think at one time I read a diagnosis.

When backing up out of my gargage and turning 90 degrees at the same time, I'll get this little knick-knock sound...only when turning while backing up. Probably a clutch on one side or the other. My knowlege is zipppp on differentials.

Jim, your thoughts....just live with it, or worry about it (or maybe fix it some day)?

I've never heard a noise like that from my car's LSD while backing up and turning so I don't know what to say on that.
The LSD internals look like they would work the same way in forward or reverse. Rotational torque would just be working on the oposite side of the helical cut ring and pinion gear teeth, and the small spider gears straight cut tapered teeth.

Sure it isn't a CV joint making the noise?

[mark houghton]

Thanks Jim. I guess it could be a CV joint, but they both seemed to be fine last time I had the motor out. Again, it's only when turning while backing up. I thought someone once mentioned that when turning and backing, the wheels are turning at different rpms (which would make sense because of the radius of the turn being different from the inner wheel to the outer wheel) and can result in such a noise if all isn't quite perfect within.

[mark houghton]

Originally Posted by voitureltd

Fredmeister,,,,,,,,,,,,,,,,,Why not give it a
seat of the pants road test. I own both w/wo LSD. On one without LSD in the lower gears you can definately feel it push the rear of the car to the right ( also lift the left front somewhat) without. On a higher than factory split you can hear a slight knock and feel a tugging when slow sharp turning most noticed in reverse, unless near the upper limit of split, then a knocking noise / rear wheel tug forward or reverse. .

So Tony, are you saying that some noises when turning are normal? Or did a previous owner outfit my car with a higher than factory split?

[voitureltd]

Originally Posted by mark houghton

So Tony, are you saying that some noises when turning are normal? Or did a previous owner outfit my car with a higher than factory split?

Mark,,,,,. I have 2 that I know of that have a higher than factory setting and one with a Quaife. My experience is the more split ratio you go the more noticeable it gets. My guess is yours has been tweeked a bit if the rest of the deiveline is trouble free. Most all 930s I've had do have LSD and sport seats (fussy sort of a thing with me) and if stock settings,they do not have the aforementioned characteristics.

[JFairman]

Originally Posted by mark houghton

Thanks Jim. I guess it could be a CV joint, but they both seemed to be fine last time I had the motor out. Again, it's only when turning while backing up. I thought someone once mentioned that when turning and backing, the wheels are turning at different rpms (which would make sense because of the radius of the turn being different from the inner wheel to the outer wheel) and can result in such a noise if all isn't quite perfect within.

There's no way of knowing if a CV joint cage is cracked or deep grooves have worn in the inner and outer slots or races the balls run in without taking it apart and cleaning all the grease out with solvent. If there is a crack in the cage or a deep groove worn in the forward moving direction, then the ball can make a click or clunk while passing over it when the CV is torque loaded in the reverse moving direction.

The wheels turn at differnent speeds on all cars in forward or reverse unless there is a locked diferential in it.
In that case the inner tire will be forced to rotate faster over the pavement it's going over and it will skip and and make short skidding noises as it does it, especially if there is some dirt, sand, or gravel on the pavement.

Try pushing a race car with foot wide race slicks and a locked dif while turning the steering wheel... it takes several guys pushing on the car and it's difficult because the tires want to rotate at different speeds but the locker won't let them.

++++++++++++++++++++++++++++++++++++++++++++++

Try pushing your car backwards in nuetral while the steering wheel is turned. That way the CV's are not loaded with torque to drive the car and the balls would be less likely to click or clunk if they roll over an unsmooth worn internal surface and the differential is just going along for the ride as the 4 wet clutch discs slip in 90 weight gear oil.

I don't know why anyone would bother with the expense or labor increasing the limited slip percentage in a street driven 930 LSD.
It makes no sense at all in a street car. It would increase or cause annoying chattering of the 4 clutch discs when making sharp turns slowly like from a stop sign and would just wear them out faster.

The ZF LSD slip rate raises considerably from static idle state with torque under acceleration and deceleration.

[Fixer]

I bought Jack Lewis's FABCAR 993 with "935 transaxle" I don't like the locked diff at all and will need to get it back to something more manageable

Makes all those noises is reverse, seems to be overkill.

My last 4 speed I bought was used on track with tall first and close ratio, I'm guessing the split was almost modified for track but it wasn't like this FABCAR.

Any advice is appreciated.

[spuggy]

Originally Posted by Fixer

I bought Jack Lewis's FABCAR 993 with "935 transaxle" I don't like the locked diff at all

IIRC, according to Starke, during 935 development, the most controllable/predictable setup (especially on the skidpan) - and preferred by all drivers - was an actual spool instead of a diff. So 100% lockup under both acceleration and deceleration...

I can kind of see it. But I'm also sure that'd feel very different to any street car setup.

Originally Posted by Fixer

Makes all those noises is reverse, seems to be overkill.

Do you have a puck clutch?

Mine *hated* hill starts or reversing up my steep driveway. It would made awful noises doing that. They really are not made to be slipped, and they do not like it...

If it's your LSD making noise, you obviously don't have a spool diff. Maybe ask the PO what is fitted in the box? And hardcore motorsport LSDs - like in real race cars - would very likely be setup very differently to one intended for street or casual motorsport use.

Perhaps talk to Matt Monson?

I've had a couple of limited slip diffs, both setup as 35/65 and ~75-80 ft/lbs clamp. These are apparently pretty typical settings for casual, non-hardcore motorsport use - my use is almost entirely street.

One would often make clanking noises, especially in tight radius turns (parking lots) or when loading/unloading the ramps (changing direction from forward/reverse or vice versa).

The other, (a Guard GT), has never made a sound that I've noticed in the cabin.

Factory ZF diffs don't hold anything like the 80 ft/lbs that motorsport units can achieve - with the earlier ones apparently you'd be doing well to get 30-35 ft/lbs lockup. The late ones are referred to as "snow diffs", because half the plates and half the clamping force.


My own driving experiences in the same car, with/without a LSD (it came without one, and I unexpectedly scored one cheap during the build of a replacement transmission).

Without:

• Loss of traction under acceleration can cause the wheel with grip to steer the car from the back. Abruptly and violently.
• Car turns in just fine.
• Transition from acceleration to constant throttle (or deceleration), rear suspension unloads and the rear of the car "floats" around vaguely for a while
• No tire noise in parking lots

With:

• Loss of traction on one side or the other under acceleration is entirely controllable. Throttle steer, and dial in opposite lock as needed to keep the car pointing where you want to go.
• Noticeable "push" or understeer on initial turn-in. Easy to compensate for, and very quickly get used to it.
• Transition to constant or deceleration - the rear of the car is nailed down and controlled.
• Tight turns in parking lots - and sometimes ramps - you can get tire squeal from the inside tire.

Having driven the car 10,000 miles without and about 5-6 times that with an LSD, I have a very clear preference - so much so that a Guard GT unit was non-negotiable for the G50 conversion.

Driving the car is so much more predictable and controllable with one - and so much less eventful in low-traction conditions.