ZF LSD

[SECTOR]

I am trying to find information on how to test a LSD to check on its functionality. I believe the item fitted is ZF item, when I purchased the car several years ago it was described as having a Motorsport 80% locking LSD. After a few years of competing with the car I feel it’s not operating as it used to do. I am lead to believe there are clutch plates installed and these may wear. If I jack the rear of the car up with both rear wheels of the ground and rotate one wheel forward, the wheel takes up about 25mm of slack then the opposite wheel rotates in the same direction. I understand this to be normal for a LSD but how do you test the strength / condition / operation of the LSD.
Thank you

 

[SECTOR]

Hi,
The basic test is to jack up one side at the rear leaving the other wheel on the ground. Chock all the wheels on the ground, handbrake off, box in neutral. Now use a torque wrench on the hub nut to turn the free rear wheel backwards, and by trial and error figure out the "breakaway torque" - that is, how much force is required to turn the axle. if you set the wrench for 50 ft/lb and it turns the axle before the wrench clicks, adjust the wrench down, if it clicks before the wheel turns adjust it up and keep working until you have a figure where it just turns without clicking to get an approximate figure. Then repeat on the other side and you should get a figure which is very nearly the same. I'm not an expert, but this should tell you if it's working reasonably well. Sorry I don't have a figure for what it should be - if you find out please let me know!

 

[SECTOR]

Thank you Steve
I will let you know if I find anything out.
cheers

 

[scam75]

I believe a standard turbo LSD is 40%. The only test method, other than what you have described, is to mash the throttle on a grass surface and you should end up with 2 similar skid marks which would indicate the presence of a functioning LSD. Not very exact admittedly but maybe a starting point!

 

[Sprint924]

I rebuilt the LSD on my Turbo. A standard "40%" Porsche ZF LSD. The LSD has IIRC 4 plates in either side which is less than on my Elite Racing Transmission LSD in my sprint car. The above mentioned test only works if there is pre-load on the plates, but the lack of pre-load doesn't mean that the diff is badly defective. You need to understand how a plate LSD works. The real clamping force on the plates derives from the engine (or deceleration) torque pushing the spider gear pins up the ramps on either side. Racers utilise different ramp angles to give different characteristics. Typical angles are 45 degrees under acceleration and deceleration. But some people use 90 deg on deceleration (i.e. no diff locking at all) to keep things stable under braking and flatter angles to exert more clamping force on acceleration. To answer OP's question, I would have expected the motorsport LSD to have a certain amount of preload so with one wheel on the ground you should see a level of torque to get the other wheel to turn. On a road car the plates will wear more quickly and will need to be replaced from time to time. Thicker plates are (or where) available but I had to get from ZF in Germany. I also had a cracked thrust washer. You may also need to replace the Belleville washers. For a road car, if you get both wheels spinning when one is on less grippy ground than the other and you accelerate away OK then I would suggest that you are fine.