Learn me up about LSDs
 

Another thread starts the discussion about whether a Quaife (torque biasing) or ZF diff is better.

As I understand it, the Quaife has variable lockup (through some kinda internal gearing) depending on how much the inside wheel tries to turn, while the ZF works with internal clutches with a specific percentage breakaway.

Quaife only works on acceleration, ZF goes both ways which sounds worse to me as it would enhance understeer.

But, that isn't what I have read elsewhere (see below)...

Anyone got comments! Please give them in relation to a 911 and in relation to a front engine RWD car (eg, oh, I dunno, a BMW 2002 race car which spins the inside wheel A LOT).

And yes, I could read this article again, but I would rather have subjective experiences. Even better if you have driven both.

Thanks, Cam

LSD, yeah, man, wow. Pop this acid.

Cam? My only subjective experience is when I asked the guy going through my gearbox if Quaife was "better" than the limited slip my car came with. His response was that he'd gladly install a Quaiff in exchange for my factory unit...and not charge me a dime extra for labor OR parts. I really like this mechanic...a lot! Now you know why I bring him a bottle of his favorite adult beverage from time to time.

The ZF allows more engine braking on decel..... you can use the compresion of your engine as an engine brake......

the quaife distributes the power only when it is applied.. if you lift or decle the quafe "unlocks"

Cam,

Thanks for reminding me that my car needs another $1-2K thrown at it!


Matt Holcomb
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I have another LSD question. My 86 has a factory limited slip in it's 915. Could I swap that diff for the non-limited slip diff in my 73S? I know the output flanges would have to be changed, any other problems? I know I would use the limited slip more in the 73 so I've thought about doing the swap if possible...

There was a pretty informative & thorough article about LSD's in the Feb 2001 POC <i>Velocity</i> club magazine.

I scanned it for posterity...posted without permission but hopefully author/mechanic Greg Brown doesn't mind:

http://www.nosubstitute.org/articles/LSD_Velocity_Feb01

Cheers,

Keep it coming guys - I haven't seen anything compelling ;)

- I don't see the benefit of engine braking
- I do see the benefit of same LSD effect on lifting off as accelerating (in terms of stability), but also see a negative??

And Matt - US$1200 or $1500 - that's A$ 2-3... NZ$ boo hoo.

I'm outa cash, so it will have to wait for me. I'll be the guy funneling 250+hp through 205 rear tyres with NO LSD... But I'll save my pennies and go f'glass bumpers at the same time.

So if I had 120000-150000 pennies, which is better...

The benefit is not with engine braking but trail braking. This allows you to go into the corner deeper and not get passed by the 25 cars behind you *in theory*. With the heavy rear weight bias of a 911, you need all the help you can get when trail braking and this is where the Limited slip comes in. Trail braking is not taught in DE's until you are in the higher run groups. Now that I do it I can't imagine just braking in a straight line and then turning in, with trail braking I'm able to conserve more of my momentum in the early part of the corner with out scrubbing off my speed and sliding. IMO, the Quaife unit is not as well made as the factory unit too. Remember, your transmission is a very precise, expensive part of the car and having anything grenade in there would be catastrophic.

Cheers, James

problems with Quaife?
 

I was asking around for advice and general experience with adding Quaife to the 915 last year when I was upgrade hunting.

Quaife was thought to not be as reliable a unit as factory LSD, and when the Quaife unit goes it is not gradual-- rather abrupt and terminal.

I wish I had more specifics, so hopefully an experience veteran will add to the thread.

Basics, on braking the Quaiffe opens up and you tend to loose directional stability. If you are not tracking the car heavily, that's OK. For hardcore racing (or DE) you want a ZF type. For AutoX you want a Quaiffe since it opens up minimizing understeer and thus allowing for the tight radious turns.

PWD's mechanic is very smart. I'd do the trade with you anyday also!

If you must go the torque sensing way, I'd get the GT (Guard Transmission) torque sensing instead of the Quaiffe. My personal opinion anyway......

I've been running a Quaiffe in my 71T for the past 4 years in both DE & Racing. My decision was based on the lack of availability for 901 trans, and I was told the Quaiffe 'wastes' less power. (no clutches ot overcome?) and my stock 2.2 doesn't have an abundance of power. I trail brake often, and have not noticed any 'directional instability'

It also still works. 4 years later. IMO a factory limited slip would have required new clutches by now.

I agree with Juan and Alan.... for track racing go with the factory ZF, for AutoX go with the Quaiffe or GT. They each have their strengths and weaknesses - but durability isn't an issue for the Quaiffe. The factory ZF will require maintenence - the Quaiffe in my autox car hasn't in four years of hard use.

So last question...

... for the 911 - it is a street/fun car with occasional auto-x and track day use - hence GT ro Quaiffe torque biasing seems best bet????

And for the BMW 2002, it is a race car and a factory ZF one is best (except I need to find one! Very difficult in New Zealand)?

Thanks

Cam

That's what I'd say. FYI, I've heard that the gears used by GT are made in New Zealand or Australia.

Food for thought.

In NZ or Australia you say! Tell me more...

Actually, I believe that it is the other way around. Why?

Keep in mind that "wasting power" is another way of saying "convert into heat".

1) Quaiffe's (and other torque biasing diff's such as Torsions) having parts sliding against each other as long as they are open. They lock up because the difference in torque between the left and right axles causes a set of screw gears to try to be turned by their adjacent gears (I forgot the specific name for the adjacent gear), which doesn't work. Screw gears like to turn their mating gears, but don't like to be turned by them. When a diff such as a Quaiffe starts to lock (especially partially), that is when heat is being generated. Unfortunately, this is also when the car is trying to generate maximum HP to the road, so this heat constitutes "lost HP". To be honest, I doubt it is a lot compared to the amount of HP which would be lost by a spinning tire.

2) Salsbury Diff's (AKA: Plate type diff's) actually generate less heat when they lock because they are locked and no clutches or anything are rubbing against each other. Plate type diff's generate their heat when they are running "open" which is generally while the rear end is not attempting to transmit maximum torque to the road.

In either case, neither unit should be generating a lot of heat while accelerating in a straight line.

So as far as HP loses are concerned, I'd give the nod to plate type diff's as being more efficient. But in all honesty, the "inefficiency" of the Quaiffe style is fairly trivial compared to the "one size fits all" benefits which it provides in many situations. When my car is converted to full time race car for track use, I'll be using a plate type diff.