jrgordonsenior

Congratulations Matt on the birth of your daughter. We'll forgive you this one time . Hope you and your exhausted wife can catch a few hours of rest....

Bill Verburg

Matt, congratulations!

But the snipers should have contributed more than innuendo and their concerns ought to be addressed.

I for one would be happy to hear any corrections to my posts that may be warranted.

J richard

yes please...I thought Bills operational description was one of the clearest I've seen to date, too often these are treated as a black art and "voodo"...

GTgears

Bill,
I suspect Bernie was talking about things in Steve's original post, like the part where the sales rep for his Camaro's LSD told him those numbers were breakaway test numbers and that going more aggressive would be the way to tune out understeer. I don't immediately see any glaring mistakes in your post that jump out demanding significant correction. I dunno though since I'm not Bernie...

Matt Lane

Not looking to derail the original topic, but Matt (or Bill, or others with 964 or 993 stock LSD) - can you share your experience on how long a stock Porsche LSD would last with moderate DE use? I have heard "not long at all", just curious as to your experience.

I will check the torque on mine this winter and send out for a rebuild as necessary. My driving experience is not at a level where I can effectively evaluate any issues...

Best,

Matt

Bill Verburg

My stock one was ineffective at 60k, but it had seen a lot of track use.

The Porsche Motorsports lsd in my /30 has ~7k track miles and another couple thou of street use. My feeling is that it has lost it's edge and is currently being evaluated by a good track shop. I'm having the trans freshened and the lsd is certainly on the list.

sig_a

[QUOTE=Bill Verburg;9035592]
(RE. clutch disks)

".........inners are splined to the transmission, outers are splined to the axles.........."


---------------

Pardon me Bill, I see this as backward.

The outside tang disks fit into the inside diameter splines of the LSD housing. The inside tang disks fit into the outside diameter splines of the side gears.

I think this is how it moves energy. Pressure rings transfer power from the transmission R&P to the axles where they are splined to the inside diameter of the housing and engage against the cross shaft where its ends ride inside the four ramps.

See below description of salisbury differential

http://intothered.dk/simracing/differential.html

Kaaz USA has a well laid out web page. Ray is very helpful.

http://www.kaazusa.com/tech_intro.html

Mvez

Just had Matt build me a Guard 40/60 with a bit lower pre-load than their "default" amount, to help free up the car mid-corner and under partial throttle applications.

Alot of variables to consider. I'm such a nerd, I literally had a couple sleepless nights trying to decide on my build setup. Can't compliment Matt enough regarding his help and input.

Bill Verburg

[quote=sig_a;9040053]Yes my bad, I didn't think it through and got it backward

here's the full stack, from a 993 25/65


and the empty case which accepts it. The crown wheel bolts to the case

amso3

Interesting thread. I've always thought that LSD installed would make the opposite wheel rotate in the same direction, when up in the air. This summer we were campaigning a Cayman in the Grand Am Continental series. We could not make the car handle until we reduced the lock up to 20% and reduced the preload on the disc (thanks to Matt for the guidance). When we did that, the opposite wheel rotated in the reverse direction when in the air, like an open dif does. Oddly enough, I was reading an article about the Cayman R a few weeks later and they stated the dif was at 22%. This change significantly improved the car! I wish I understood these better, threads like this will help. Great forum.

Bill Verburg

the wheel turning observation was great for the older symmetric ZFs w/ 29-58#s of breakaway torque but it doesn't always works as expected on the modern low preload asymmetric lsd's.

w/o preload the diff works like an open diff. The ramps force the plates to load up which causes the lsd function to increase. On a car w/ a low preload lsd one wheel needs to be held stationary, then turn the other to feel the lsd work. On a bench one axle side is held fixed and a torque wrench is applied to the other axle side, 5-15mm is the spec for a modern GT3 lsd

sig_a

---------------------

Bill, Could you take a minute to explain this. What does 5 to 15 mm have to do with it? I'm not sure of the difference between "initial breakaway torque" and "preload".

Does varying the amount of breakaway torque increase or decrease the threshold where the discs begin to slip?

Does varying preload increase or decrease the reaction time of the discs to begin limited slip?

Appreciate your thoughts.

GTgears

The Cayman has been a very interesting experience. It will serve as a good example to emphasize a couple of the points Bill made previously. In this instance, some remakrs on tires and how they effect set up. The GA spec Conti is very different than other tires being used on Caymans elsewhere in racing. In Interseries and ITC we set all the Cayman diffs at 45/65, 50/80 or 50/50. But in GA those standard settings with regular preload and alternating steel plate (SP) and friction disc (FD) doesn't work. All of our LSDs are generally set up SP, FD, SP, FD.

We've ended up playing with two different approaches in the GA cars. Either we use the 45/65 ramps with SP,FD,SP,FD and moderate to low preload and a larger internal gap OR we use the 50/80 ramps, switch over to SP, FD,FD, SP( like an old air cooled 911 ZF LSD), again using moderate or low preload. IIRC, the Nobel car is using the latter option that you describe. Interally we refer to it as 25/40, following Porsche's historical nonemclature.

Here's something else I find interesting. Below is Porsche's factory Cayman LSD, that they call 22/27.
Take a good look at that LSD. It's got really steep ramp angles. Furthermore, it's "old skool" too, with the SP, FD, FD, SP ordering. Lastly, it's got no belleville washers in it. It's got zero preload and measures the smallest internal gap (like .2mm) of any LSD I've ever seen. For reference a factory 8 plate Cup Car LSD runs a +/-.7mm internal gap, so the Cayman diff is really really tight.

This is in direct comparison to the 50/80 ramps Bill posted earlier. Our 50/80 ramps run 45 degrees on 50% and 30 degrees on 80%, so clearly WAY more laid over and active than Porsche's. But we're calling it 25/40 in a "2 plate" configuration while they are calling their's something simliar with ramps that aren't anything like ours.

So, what's the point? The point is that there's no universal language or configuration "rules" here. Knowing who you are talking to and what their own terminology and configuration is is paramount. The Japanese don't even talk percentages most of the time. They use a 1 way, 1.5 way, 2 way language and do this funky thing with assymetrical pins that can be rotated above and beyond the ramp angles to manipulate what they call it. In short, there's very little consistency across brands.

sig_a

In answer to my own question, I found this here: http://www.jeepfan.com/tech/differentials-explained/

Does varying preload increase or decrease the reaction time of the discs to begin limited slip?

"in a situation where there is little or no traction available to either one wheel or the other, the amount of power that can be transmitted to the other wheel which has traction is dependent on the friction or “preload” in the clutch plates. High levels of “clutch preload” will result in good torque transfer but some chattering of the clutches during cornering may occur. Lower levels of preload results in minimal chatter but reduced levels of torque transfer to the wheel with traction."

So increasing clutch preload transfers more torque quicker. A bit like a hair trigger on a pistol moving less distance to discharge.

Bill Verburg

That should have been NM.

The more you preload the disc stack the more torque it takes to differentially move the sides. Brake away torque is how much torque is needed to deferentially turn the axles, preload is just the static tension in the assembled disk stack, it is varied by using different thickness of internal or external discs and by choice of belleville washer

The older ZFs tended to use a higher preload to get a more effective coupling between the axle sides, the 29-58# number was for a street 40% diff, the 80%'s were much higher. Driving my car which has the ZF 40% you notice these much more in typical street use than when driving my car w/ a Porsche Motorsport lsd, even though the PMS diff is ultimately more effective under load. The modern lsds use leverage from the ramps to raise the pressure on the disk stack and so tend to use much less mechanical preload from stack thickness and belleville washer choice, as a result when they aren't loaded as on a lift the tires may rotate as w/ an open diff.

Adding preload raises brake away torque at all times, the older 80% ZFs were great coming out of a corner but could be unpredictable in a sweeper because the lsd was over effective for the conditions

using ramps to load the disk stack only raises brake away torque when you need it which is basically under heavy braking and hard acceleration out of tighter corners, In other less load intensive situations you don't want or need the lsd to be interfering a whole lot, though you can still steer w/ one by getting on/off the throttle