LSD rebuild and ratio change

[gareth]

pm on it's way

[Dave_M3]

And replied

[N00b]

pm on it's way

Same here please, Gareth.
I'll be looking for a 3.7 slipper in a few months, and I'd like to ask you a couple of questions.

1) Can you give me a rough idea what you'd charge to overhaul one of these things (the pics in your thread above look fantastic BTW)

2) What can you tell me about modifying the slipper, and what extra costs would it incur? Your sig used to say something about a 40% lockup mod. Can you give me more info on mods like this and what their effect on the car would be.

PM me if you prefer, mate.

[Dave_M3]

Gareth is extremely reasonable on price for ramp angle mods and even building the diffs now too it seems. I was just inquiring on parts costs as I want to do it myself as I have access to the right tools etc and "I think" I know how to use them

Otherwise, If I was in the UK or didn't have a clue what I was doing I'd be sending him a few diffs

[N00b]

Gareth is extremely reasonable on price for ramp angle mods and even building the diffs now too it seems. I was just inquiring on parts costs as I want to do it myself as I have access to the right tools etc and "I think" I know how to use them

Otherwise, If I was in the UK or didn't have a clue what I was doing I'd be sending him a few diffs

I'll be happier just removing the whole unit and sending it to him, then refitting when it comes back. I was wondering about improvement options (like a higher lockup rate and probably others I don't know about) while the thing was in bits, but I'll need to hear a few opinions on the effects of any mods before deciding.

[gareth]

cheers for the positive comments gents

in a nutshell, i'd go for a 40% lockup on a road car then also add a 4 plate (rather than two plate) upgrade if you're going to be giving the diff some sustained abuse. eg, drifting, loose surface rallying etc.

a 40% lockup transforms the diff and removes the vagueness, making it a lot more predictable.

a 4 plate upgrade will provide more static and ultimate locking as well as spreading the loads over more plates so it'll withstand sustained abuse better. the downside is a theoretical increase in tendancy to understeer at lighter throttle loadings, like more accelerating out of high speed corners.

the other stuff i do is all about replacing and setting the diff bearings and shimming up properly so it's quiet and the gears mesh properly. without correct setup, it'll self destruct in little time. then there's new seals so the oil stays inside (the best place for it) and a thorough cleanup and repaint so it looks nice and you can show your mates while they pretend to be interested

N00b, PM on it's way

[N00b]

^
Thanks Gareth, received read and (almost) understood, lol.
Increasing lockup rate was something I was wondering about as I have no idea what it will do to the cars behaviour. My car is first and foremost a road car. It will attend the odd track day/'ring trip, but most of its life is on public roads. Sorry to be a pain, mate, but how does the increase in lockup make the car feel/behave?

EDIT:-
It sounds like this amounts to a full overhaul. Roughly speaking, how long will it last before needing a refresh/service again?

[gareth]

in terms of 'feel' it locks up harder exiting corners, preventing wheelspin on the inside wheel but to a greater extent to a standard LSD. it's all a lot more predictable and controllable when the back does go, which makes it a whole load more fun

regarding life span, if you have all new seals, oil, bearings, LSD plates etc, there's no reason why it wouldn't last another 20 years. the only remaining 'wearing' parts would be the gears and unless the oil is totally lost or the bearings fail in spectacular fashion and the car is still driven there's nothing that will damage them. i keep seeing the gears coming out with their original machining marks just still visible that's BMW engineering for you

[N00b]

in terms of 'feel' it locks up harder exiting corners, preventing wheelspin on the inside wheel but to a greater extent to a standard LSD. it's all a lot more predictable and controllable when the back does go, which makes it a whole load more fun

regarding life span, if you have all new seals, oil, bearings, LSD plates etc, there's no reason why it wouldn't last another 20 years. the only remaining 'wearing' parts would be the gears and unless the oil is totally lost or the bearings fail in spectacular fashion and the car is still driven there's nothing that will damage them. i keep seeing the gears coming out with their original machining marks just still visible that's BMW engineering for you

Is this a good argument for buying a worn/noisy diff to overhaul?
Noisy examples change hands for less money than good ones, and if you're gonna change almost all of the internals anyway then why pay top dollar for a good one to rip its guts out?

[Brianmoooore]

So how exactly do you assess a LSD for wear from the outside? Anything you can do with a torque wrench?

[Dave_M3]

It'll feel rougher to turn for a start anyway if the bearings are on the way out compared to a good one.

If the plates are completely fooked, the output shafts will rotate in opposite directions as the teeth will have been ripped off. Only have seen this happen in cars given alot of abuse or E36 3L M3s etc.

Then you might have alittle too much lash then too if its in real shite condition or has been rebuilt poorly.

[Brianmoooore]

I don't mean obviously knackered ones - I mean ones that are still working, but not up to new spec. I was thinking along the lines of holding one output shaft solid, and putting a torque wrench on the other. Would that give any meaningful result?

[gareth]

i've only seen them in 'fine' and 'trashed' states. once they start to fail, the amount of friction within the LSD pack drops off at a rapid rate. one i saw acted like an open diff in the 'jack rear, put in gear, turn one wheel' test as the plates were totally fecked! a rebuild with new clutch plates sorted it a treat. it could have easily been mistaken for an open diff and binned

as far as a scientific test goes, the only torque figures i've seen are those posted ages ago by demlotcrew. 80lb.ft should hold without slipping, after that it's in need of a rebuild. i dont know where his figures came from but have no reason to disbelieve them and they 'feel' about right to me. obviously this only applies to a standard diff as a 4 plate modified version or one with altered ramp angles will have a different breakaway torque.

if it's gone, it'll easily spin an inside wheel pulling out of a wet 'T' juction for example as once the locking and preload are reduced, there's little resistance (grip) for the ramps to push against and lock the diff up harder. for this reason, a ropey LSD will often pass the 'burnout' test as a similar load is put on each wheel and it doesn't need to lock up hard to spin both wheels.

incidentally in perfect circumstances, an open diff will spin both wheels. they will only spin one wheel when one has less grip, starts to spin and has less grip than the other (gripping) wheel so can't regain traction.

i personally prefer to get surgical on them and find out for sure whie also checking the rest of the diff (bearings etc) over

in almost all LSD's i've had a look at the score is usually, worn clutch plates and usually the input bearings on the way out.

[Dave_M3]

one i saw acted like an open diff in the 'jack rear, put in gear, turn one wheel' test as the plates were totally fecked!

I've seen three "open" LSDs... that'll be the Irish diffing brigade for you

The clutch plates weren't much difference in appearance to the spring plates

[gareth]

one i saw acted like an open diff in the 'jack rear, put in gear, turn one wheel' test as the plates were totally fecked!

I've seen three "open" LSDs... that'll be the Irish diffing brigade for you

The clutch plates weren't much difference in appearance to the spring plates

that sounds familiar