Pelican Parts Forums - LSD experiences

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- - LSD experiences (http://forums.pelicanparts.com/porsche-911-technical-forum/129567-lsd-experiences.html)

Thanks, James, I was looking for that article but couldn't find it. Old Timers disease is hitting me hard.:(

We call it CRS syndrome( Can't Remember *****) in the engineering field, My wife has some other clever acronym for it too(figures, she's a nurse).

Cheers, James

I have KRS. That's "Keith Richards Syndrome"

Here's a link to another LSD thread LSD discussion - round 2 .

If you scroll down a ways in that thread you'll see where I posted what Greg Brown said about the Quaif and the newer Porsche diffs i.e. "trailing throttle lockers".

Quote:

Exerpt from that thread:
The new generation of differentials that change the locking effect were
created as a direct result of the early European Cup Races. They were not
(repeat not) used to "get around the understeer issue I mentioned earlier."
When you drive a 911 into the corners real deep (Which ABS and trail braking
techniques allow you to do), the rear end on a 911 tries to step out.
(Oversteer) The increased locking effect of the "trailing throttle lockers",
which is what the factory called these units in the beginning, increases the
locking effect and drives both rear tires at the same rate. This keeps the
rear end going in a straight line and helps with the oversteer under braking
problem. Of course, the more the locking effect, the more horsepower needed
when accelerating through a corner. (Anyone tried to push a race car with a
locked rear end with the wheels turned?) So, as you get back on the
throttle, the locking effect decreases, which is what is desired.

-Chris

Just as an FYI, people generally describe LSD's in two fashions -- as % 's or by ramp angles. As far as I can tell, Porsche has always used %'s and everyone else talks ramp angles. 80% DOES NOT equal an 80 degree ramp angle, in fact the opposite is true, as ramp angles approach 90% they clamping forces become less and less.

So let's be clear about terms, OK?

While we are being clear about terms...

I understand that 80% is 80% of torque. It must be with reference to a specific engine, ie the maximum for the engine the transmission is attached to originally (as in 80% for a 964 is gonna be about 95% for a 912). It really should be referenced in ft lbs?

Am I right on this??

Mr9146, I live very close to Santa Cruz. Know what mean. Window payne, World, etc.,etc. LoL

Cam;
I believe that the %'s prefer to the % of torque sent to the non-spinning wheel. Actually an open diff will proportion out the torque to each wheel based on the speed. Unfortunately once one of the wheels spins, 100% of the torque is sent to that wheel which keeps it spinning. With a LSD or a TBD, a certain percentage of the torque is sent to the non-spinning wheel. Based on the ramp angles and the clutch set-up, a LSD with 80 locking will basically send at least 80% of the axle's torque to each wheel. Keep in mind that when the car is advancing straight ahead and not spinning the tires, 100% of the axle's torque is being sent to each wheel. The neat thing for setting up a race car is that this can be set to a different value for acceleration versus over-run situations.

A TBD will on the other hand send to each wheel the amount of torque that it can handle. In the case of a Quaife or torsion diff, it is done via screw gears arranged so that they take advantage of their ability to transmit force in only one direction. This is why they only work under acceleration, but not when braking.

I don't think that a LSD really cares how much torque the engine puts out except that higher torque engines will apply more force to the ramps and thus clamp the clutch(es) harder.

I think...

higher torque engines will apply more force to the ramps and thus clamp the clutch(es) harder

All that makes sense. Buuuut, I have been told the other way (which is why you put more clutches in and "shim them up" to get more lockup).

Anyone know?

My understanding is that there are 3 adjustments in an LSD:

1) Ramp angle on acceleration and overrun. Generally these are machined as part of the cage. I mentioned the affects earlier. This factor influence lock-up, but not pre-load.

2) Spring Pressure (can be changed by either increasing the thickness of the clutch pack or installing stiffer springs (generally bellview (?) washers).) This also affects lock-up AND pre-load. If it goes too high you wind up with basically a locked diff.

3) Clutch arrangment. If you alternate the clutches (IOIOIO where I = input and O = output) it increases the surface area of the gripping surface and as a result the lock-up will be sharper, just like a triple plate clutch between the engine and transmission is much chrisper then a single plate clutch. If you re-arrange the clutchess so that there is only one gripping surface (IIIOOO) then the lock-up will be more gradual.

So I guess there are lots of ways to play around with the LSD's charactoristics.

TrustMe, I went to UC Santa Cruz for a year so I know EXACTLY what you're talking about. :D

I think (not sure though) that Obrut will be using the race car mainly for tarmac rallys; would that change anyone's recommendations? Somewhere in between street and track settings?