Handling and LSD's: questions

Bothari · 12-01-2003, 03:06 AM

Ok, in my on-going attempt to understand handling and its variables, I would like to ask a couple of questions about LSD's.

1- When one says that a LSD is 80% lock-up, does the 80% mean that it allows up to 80% power one way or that it never lets any of the sides go lower than 80%? Or am i missing the point completly?

2- What are the effects of a LSD in acceleration and braking (in the ones which also lockup under braking, of course) and why is it not advisable for the street? Yes, i have a pretty good idea, but i'm somewhat mudled.

3- "true" LSD's and torque sensing (Quaife): How does a quaife work, what is the diference in effects in handling, why are some considered better for the street and others better for track-work.

Sorry, i know it's alot of questions, but i'm trying to get it simple and clean in my mind...

emcon5 · emcon5's Garage

Article on LSDs from POC Velocity a while back.

http://members.rennlist.com/emcon5/velocity_lsd.pdf

Tom

Bothari · 12-01-2003, 10:47 AM

That was a cool article, and it finally explained some things to me.
It also occurs to me that:

1 - Measuring the amount of slip as a percentage is not quite correct, since the factor is actually how much torque the slip plates will take before slipping. The percentages assume a fixed torque number, so It seems to me that they should be termed 100 N/m or 20 ft/lb and not 40% or 65 %... but i digress...

2 - Under both acceleration and braking, the result should be both more traction as well as more understeer (or at least less oversteer).

Now why is this difficult to deal with in a street car? Why are these diffs considered not advisable if the car is not to be used in a track?

Also : So the reason that quaifes are seen as not as good for track as a true LSD is due to their 0% lock under braking. So why are they advisable for fast street driving? And btw, how *does* a quaife work if not with friction plates.

Side-note: Now that i've seen how a LSD works, it seems to me ... unefficient. One can never estimate with 100% certainty how two surfaces under friction will work, due to the huge number of variables involved. I must confess i was expecting some sort of clever gearing, and it doesn't amaze me at all that the article mentions how under high stress the LSD's will slip diferently under the (seemingly) same conditions... Sort of how i felt about 915 sincros when i saw a schematic drawing. btw, does anyone have a good article explaining gertrag sincros?

Bothari · 12-02-2003, 07:22 AM

bumpty-bump..

bsaffin · 12-02-2003, 09:18 AM

How do you tell if you have a LSD? I don't think I have one (not on the options codes), but I've been a little puzzled lately with the wet roads here that my car doesn't 'fish-tail' as I expect when the back tires break loose.

...Bruce

beepbeep · 12-02-2003, 09:22 AM

Bruce:

You car would fish-tail easier with LSD, but forward traction would be better.

Just lift rear and turn one wheel while in gear...if other wheel turns in oposite direction, there is no LSD.

Bothari · 12-06-2003, 12:28 PM

.

HawgRyder · HawgRyder's Garage

What about a Torsen style drive?
They have muliple sets of spider gears that transfer 100% of the power at all times in an infinate ratio divided between the wheels.
Do they make a unit for Porsche?
Bob

Tyson Schmidt · 12-06-2003, 03:59 PM

Quote:

Originally posted by beepbeep
Bruce:

You car would fish-tail easier with LSD, but forward traction would be better.

Just lift rear and turn one wheel while in gear...if other wheel turns in oposite direction, there is no LSD.

Actually, and LSD will also turn the other wheel in the opposite direction if the car is in gear, but it will be difficult to turn.

If it is in neutral, then they will both turn in the same direction.

Also, in a rear engined car, an LSD actually makes the car fishtail less, due to the understeer inducing qualities of the LSD when combined with all that weight behind the rear wheels. Power-on with an LSD tends to lift weight off the front wheels, causing understeer. Unless, of course, you have enough power to spin the tires. Then you're going for a ride.

Bothari · 12-06-2003, 04:27 PM

So, why is this bad in a street car, what am i missing?

Steve@Rennsport · 12-06-2003, 04:32 PM

LOL,..Oh, but its GOOD,.....

You're not missing anything,.....Aside from some initial understeer than can easily dealt with with some swaybar adjustments, there is no downside to an LSD (ZF-type) and there are plenty of benefits on the street as well as the track.

speeder · speeder's Garage

Steve is on the money, (as usual), I switched to a factory LSD @ 40% when I rebuilt motor last winter and don't know how I ever lived w/o it. It took a minute to get used to the additional traction from the drive wheels, but for throttle-steering a properly set-up 911 it's indispensible, IMO. My car absolutely rocks on the track for a (mostly) stock 911, I'm one happy camper!

lateapex911 · 12-06-2003, 10:59 PM

In any car, an LSD won't make the car fishtail (oversteer, or 'loose') more...

In a car with an open diff, when accelerating while still cornering, it's common for the inside wheel, to spin, reulting in little forward acceleration. Traction resumes when the car rolls to level, or the shock allows the suspension to droop. But the tail doesn't slew wildly when the spinning of the inside wheel occurs. It may, if cornering loads are high, drift sideways a bit, as the spinning tire can't provide as much cornering force as it was prior to the loss of traction.

With an LSD, the onset of the slippage is delayed, and when the power actualy overpowers the traction available at both tires, the rear will step out with much more authority, causing a greater oversteer condition.

In a 911, as the guys have mentioned, the weight transfer that you will see due to the greater rear traction will be a major part of the handling.

dickster · 12-07-2003, 12:04 AM

Quote:

So, why is this bad in a street car, what am i missing?

bad?

i have factory lsd and dont notice it most of the time.

speeder · speeder's Garage

Goran, (beepbeep), is correct if refering to driving on snow or ice, where LSD will cause both rears to break traction, sometimes resulting in tail-wagging while trying to go straight. As opposed to a non-LSD car where the one spinning wheel will not upset the trajectory of the car much. To a lesser extent this could be true in rain if you are driving aggressively enough in a powerful car.

The best (2WD) vehicles in snow are heavy and underpowered.

David · David's Garage

While we're on the topic, what type of maintenance is needed on a LSD. My 930 has one and I'll be replacing the clutch soon. Should I plan to replace the LSD plates also?

beepbeep · 12-07-2003, 07:23 AM

Could someone shed some light on 930 LSD's?

One article (http://members.rennlist.com/emcon5/velocity_lsd.pdf) involving LSD's mentioned that Carreras got weaker 40% LSD since 84' as there was only place for two instead of four friction plates inside from that year onwards.

Does this involve 930's as well? Article was more focused on N/A transmissions and PET says -81 930 has "930 332 053 02" LSD and -88 "930 332 053 03" one.

Do -88 930's have same sort of "weak" two-plate diff?

PET pictures are same but -81 LSD shows extra disc #930 332 553 00 called "lamina 1.4mm" on each side of diff.

Our almost new -88 930 LSD had slip-torque of roughly 40Nm.

I really doesn't understand why Porsche called these LSD's for 40% or 70% or whatever...i mean, if it takes 40Nm to make it slip then it's just fitting bigger engine will change this ratio.

dickster · 12-07-2003, 08:02 AM

Quote:

where LSD will cause both rears to break traction, sometimes resulting in tail-wagging while trying to go straight. As opposed to a non-LSD car where the one spinning wheel will not upset the trajectory of the car much

yeah, but then you aint going to go forward either.

dvkk · 12-07-2003, 11:55 AM

David, you don't really need to do any maintenance to the ZF, just change the gear oil a little more often. I can notice a little ZF clutch chatter in as little as 10k miles on fresh oil. That tells me to change the gear oil as often as 10k miles. I'm using the GM additive for LSDs.

Steve@Rennsport · 12-07-2003, 12:58 PM

Goran:

Some misinformation exists on these ZF LSD's,.....

The only 2-plate LSD's are found in the G-50 equipped 993's.

All 911's and 930's up to '94 are the very robust 4-plate units and they hold up pretty well. Porsche Motorsport LSD's are all 4-plate ones, of course.

If you REALLY want the very best LSD, get one of the 4340 chrome moly-cased Guard ones. They are a superb piece of work and thus far, we have not broken one (unlike factory ones)....