LSD suggestions for 915 gearbox

[haasad]

In reality (apart from the undeniable fact that having an lsd is racey in the pub bragging) they are a bit pointless in a purely road car.

But wheelspin in corners and under hard acceleration is limited by preventing ensuring the power is transmitted more evenly across both driven wheels unlike an open diff which allows the path of least resistance to take all the power. One should notice better acceleration and harder corner exit as power is not "spun away". Very tight diff's do make the car feel a bit understeery with normal road corner entry speeds, you have to be quite brave or perhaps stupid on the road to exploit the capability it gives to adjust the cars attitude in a corner using the throttle. Did I mention they make the car more stable under braking

[fetuhoe]

I am not sure that an LSD 'kicks' the back out.

It will help produce power oversteer if that's what you want but unless deliberately provoked it is unlikely to cause a problem.

The most important benefit is that it helps to significantly reduce Trailing Throttle Oversteer which can be helpful in many circumstances.

Plate type LSDs also help to reduce locking of the inside rear wheel under heavy braking and if you have a bias pedal box will allow the bias to be moved backward and hence reduce stopping distances.

They do have a negative impact on turn in and can introduce understeer so this needs to be dealt with a small change in steering input.

The Torsen type diffs such as a Quaife have no effect on trailing throttle behaviour as they behave identically to a conventional diff in this mode. The Wavetrac claims to has a small amount of preload on the coast side but looking at the detailed design I would have thought the impact of this feature would be negligible.

In general I am not sure there is much to gain in normal road use. the situations which give benefits tend to be associated with 'on the limit' driving and certainly around town the noise and understeer that can result from using a diff with too high a preload can be a real PITA.

If a diff is well set up and preload kept to a reasonable level the influence in normal conditions may be hard to detect and the question then becomes whether or not the cost is justified.

[Lightweight_911]

I agree with the last 2 posts - I don't think that most people would reap the benefits for a pure road car.

Typically an LSD is added at the same time as other suspension/brake/transmission work so it's difficult to know what effect it would have had in isolation.

I think any 'perceived' improvements are more often than not psychological.

[sladey]

Mine is predominantly road use but I'm really glad I got the LSD.

Previously it would hang on more in corners before losing grip, but when it did finally lose it, it was very sudden and hard to catch

With the LSD installed
I have more rear grip through the corners (it must have been slipping before but I was unaware). This in turn meant it under steered more but I corrected that by stiffening the rear anti-roll bar

Now when the rear does let go it's controllable and catchable - to the point where I can provoke it on wet tight 2nd gear bends (should I so desire!)

I step the back end out more than I used to now - but that isn't a fault of the LSD - it's a consequence of the LSD giving me more faith in my cars handling

HTH

[Brewster]

And there's the rub. Sladey pretty much describes exactly what I'm after. What type of diff are you running?

I have visions of a plate type diff catching me out on a greasy roundabout and I thought the Quaife might be a decent halfway house. It's also another "while he's in there" job. I didn't have any drivetrain shunt and I imagine my open diff would be good for another 150k miles if I ever made it that far, but I can get my hands on a brand new Quaife for £880 and it might just increase my enjoyment a little. Would be a shame to spoil the build for an ha'porth o' tar. Or £900 of tar. Whatever.

[sladey]

Mine's a Quaife which some people are quite sniffy about (but usually racer/rally people who need to use them in anger).

I don't know whether or not it provides any stability under braking or not, but as I say, I really like it

[gridgway]

Well one of the many things I am not an expert in is suspension and handling which is why I posed the question. Anyone care to describe why sladeys car does what it does?

I can understand why you'd get more grip at the rear under power coming out of corners and then reduce it again with a stiffer rear arb. But why doors that make the rear break away more progressively? Is that actually just the effect of a stiffer rear bar?

Graham

[fetuhoe]

The Quaife is a Torsen so when you lift of the throttle they have no effect and behave just like a conventional diff. Thye also 'open' up when one wheel lifts which can be a pain on some surfaces.

They won't affect rear wheel lock up behaviour under braking and have no influence on Trailing Throttle Oversteer.

KAAZ and CUSCO would describe this as a 1-way diff.

They do improve traction and are relatively smooth and quiet in operation and don't wear very quickly but they don't give the performance enhancements you get with plate diffs but they generally bias more torque than a plate diff depending on the angle of the invex gears.

They don't need any friction modifiers in the oil and this can be an issue with plate diffs in 915 transmissions. With a Torsen you can just use a non-LSD transmission oil without concern.

Normally Plate type LSD Oils have a friction modifier to smooth out stiction between the plates and stop them chattering at low speed in sharp corners. The type of plate also has an impact on the oil required. Steel to steel being noisy and having a poor transition behaviour compared to a moly coated plate such as that used in a ZF.

Friction modifiers are not great news in 901/915 Porsche transmissions as their use tends to affect the synchro and can increase the time needed for the gears to match speed. If you have an oil with a great deal of friction modifier you can end up with a quite LSD and a slow gear change.

I think it is better to use a non-LSD oil and then add a friction modifier. You can put in the minimum amount to eliminate plate chatter and this helps with synchro performance.

[IanMcLeod]

...With the LSD installed
I have more rear grip through the corners (it must have been slipping before but I was unaware)...

Don't believe you.

...This in turn meant it under steered more but I corrected that by stiffening the rear anti-roll bar. Now when the rear does let go it's controllable and catchable - to the point where I can provoke it on wet tight 2nd gear bends (should I so desire!)...

The LSD is forcing more power to the inner wheel out of the corner than it needs. This has reduced the effectiveness of the front suspension because the car now wants to go straight on more so causing the car to understeer. So you have reduced the effectiveness of the rear suspension to compensate by stiffening it .

...the LSD giving me more faith in my cars handling...

You happened to have arrived at a handling situation you like better which is the most important factor in any discussion on the merits of a particular approach. The car is not as effective through the corners now so it slides at a lower speed and is more progressive and easier to catch. Which gives you more confidence in the car and makes it more enjoyable to drive. I am not posting this to be controversial but to suggest you could have got there without fitting an LSD.

Ian.

[Darren65]

....90% of the stuff I'm fitting to my car is pointless!

[haasad]

[quote="Darren65"]....90% of the stuff I'm fitting to my car is pointless! [/quote]

Me too.... long may it continue...

You should hear the row my gripper diff makes .........awesome..

[IanMcLeod]

....90% of the stuff I'm fitting to my car is pointless!

:- D

[scalino65]

....90% of the stuff I'm fitting to my car is pointless!

Such as the accelerator pedal.....




Sorry dude, couldn't resist!

[Brewster]

Thanks Sladey, that's made my mind up; Quaife is going in. As I said I have no interest in lap times. I drive the car purely for fun. If I go around a tight bend 5mph slower I don't care, as long as the car is easy to have fun with and provoke if I want to.

Much of what is being fitted to my car is overkill. My 95mm 3.0 P&Cs are worn out. I could have had mine machined/lined or bought direct replacements, but I decided to go for brand new 98mm Max Moritz short stroke 3.2. Will the extra 20bhp being noticeable to a hamfisted driver in a road car? Maybe, a little. Whether it'd be enough to justify the outlay is another matter.

Any of these tiny improvements here and there in isolation would give a negligible effect, but add up to a whole lot more when put together.

Maybe I should start a thread in the Restoration forum. At least then people could see what I'm trying to achieve and congratulate or scoff as appropriate.

[Darren65]

Maybe I should start a thread in the Restoration forum........

......that's a must, do it!