Helical LSD vs Clutch LSD

[GBPaterson]

So I'm looking at replacing the LSD in my M Roadster (as the current one is whining a lot as I accelerate / decelerate and when I turn corners I get intermittent groaning).

One of the LSD units I'm looking at advertises itself as Helical vs what I believe standard in the M Roadies is clutch + spider gears?

Is there any feeling as to which is better / preffered, is there a definitive answer to this or is it one of these subjective things.

If the latter can someone surmise the pro's / con's

Thanks,

Gareth

[GBPaterson]

The reason I landed on the LSD is that I just spent the last couple weeks swapping out both the rear wheel bearings, rebuilding the CV's (replacing one axle stub entirely), even put in new wheel hubs and whilst I was busy doing this I had the diff in to a specialist to replace all the bearings plus change out the final drive. I think the only thing left that I can think of is the LSD itself.

I know I can get the clutch discs if I opt to go that route (from bimmerdiffs.com) but if the gears are gone or going are they replaceable or is that the time to look for a new LSD unit?

[JoshS]

Most all modern diffs no matter if non possi or possi or lsd or torsen have helical cut ring and pinion and all have spider gears.

Don't mean to be pedantic, sorry, but that's "posi", not "possi." Comes from the old Positraction, which was GM's brand name for their clutch-pack LSDs.

Helical diffs, like Torsen and Quaife, don't have spider gears.

As to the original question: the general tradeoffs go like this:
clutch-pack LSD: "stronger" effects, especially in a corner when the inside wheel is unweighted, but as you noticed, they wear out
helical LSD: behave like an open diff when one wheel is totally unweighted, but "never" wear out, and generally are quieter and have easier driveability

My personal take is that for a street car, helical is pretty much always superior, mostly due to the noise and maintenance.

[GBPaterson]

cool, thanks for the info. Sounds like you know your diffs, a couple of follow up q's.

So if I aim one day to track the car what's your take on helical v's diff in that scenario?

Second, are the symptoms I have typical of a LSD failure?

[JoshS]

Clutch-type is better for the track, because they better handle the situation when the inside wheel gets in the air, which is possible with a stiff rear swaybar or with curb hopping.

And I think that that the symptoms are fairly typical, although the more normal failure mode is that the clutches just wear out and slip without any noise.

[GBPaterson]

I did do some foolish things with mine that may or may not (read as definitely did) have contributed to this predicament. When fitting the impulse wheel that goes along with Randy's subframe kit I at one point tried knocking it on with a hammer (sending shock through the LSD). In retrospect this makes me feel somewhat silly. The fact that I latter in my haste filled the diff with 75w90 and no additive instead of the 75w140 I had before probably also did not help. Overall I have done some foolish things to that diff.

[rdsledge]

Yes the diff with clutch type LSD need special fluid.if the inside rear wheel is popping and or you hear banging back there the clutches may be sticking, with GM LSD's there was a conditioner ("whale oil"?) that we would put in that would smooth it right out.Many people thought they had big problems when all they needed was the conditioner.

[GBPaterson]

This morning I switched it back to the proper 75W-140 using RP which claims to have the additive already there but the noises whilst slightly less remain.

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Would it be excessive to add the additive on top of this?

[Randy Forbes]

I did do some foolish things with mine that may or may not (read as definitely did) have contributed to this predicament. When fitting the impulse wheel that goes along with Randy's subframe kit I at one point tried knocking it on with a hammer (sending shock through the LSD). In retrospect this makes me feel somewhat silly. The fact that I latter in my haste filled the diff with 75w90 and no additive instead of the 75w140 I had before probably also did not help. Overall I have done some foolish things to that diff.

I doubt that had any consequence (though I haven't seen the size of YOUR hammer...) or else I would've gotten A LOT of phone calls by now

Did it whine & growl before it went to "the specialist" because that's a bit of a mixed art, setting up a ring and pinion; it's a lot easier to get it wrong than to get it right.

[gmushial]

I doubt that had any consequence (though I haven't seen the size of YOUR hammer...) or else I would've gotten A LOT of phone calls by now

Did it whine & growl before it went to "the specialist" because that's a bit of a mixed art, setting up a ring and pinion; it's a lot easier to get it wrong than to get it right.

Pretty much on topic: as diffs wear they begin to whine - I assume this is because of the ring/pinion gap/freeplay becoming excessive. My question is: if one reduces that gap back to spec, does the whine typically disappear again? Or, once the pair have worn into a pattern, it's there forever even with a reset gap? As always: many thanks.

[rf900rkw]

Sometimes they will get quieter when they are re-setup, but most times they get louder.

[GBPaterson]

Yeah, it whined and growled since the moment I reassembled it from putting in the impulse wheel and at the same time put in the wrong oil. Hence why I was thinking its something I had done.

[fire2]

Don't mean to be pedantic, sorry, but that's "posi", not "possi." Comes from the old Positraction, which was GM's brand name for their clutch-pack LSDs.

Helical diffs, like Torsen and Quaife, don't have spider gears.

As to the original question: the general tradeoffs go like this:
clutch-pack LSD: "stronger" effects, especially in a corner when the inside wheel is unweighted, but as you noticed, they wear out
helical LSD: behave like an open diff when one wheel is totally unweighted, but "never" wear out, and generally are quieter and have easier driveability .

Thanks for corrections, did I get 1 out of 3 right? lol, dang it might have been 2 out of 3 if I took out one of the S's in posi. lol.
Here is a good write up from some BMW guy on differential differences, including lsd and posi. I always just lumped LSD and Posi into the
same category and I always thought Quaife (I ran in my TR6) and Torsen were the same thing. For me this is a real informative article, kind
of a long read and must read slowly to digest all the info. (it is a little outdated though, 1999)
http://www.318ti.org/notebook/diffs/

LoL, I even had to look up what the word pedantic means, guess I didnt get enough schooling.
https://www.vocabulary.com/dictionary/pedantic

[Randy Forbes]

Yeah, it whined and growled since the moment I reassembled it from putting in the impulse wheel and at the same time put in the wrong oil. Hence why I was thinking its something I had done.

Were the spacers for the side covers, and the sides covers themselves, returned to the same side of the case they were removed from? I go so far as to match up the rust stains on the spacers, so they're even oriented bolt for bolt, as originally assembled at the factory.

I guess what I'm getting at, is I don't see how putting another impulse wheel on would've ruined the limited slip mechanism. Careless handling, inflicting damage to one of the bearings, maybe, but still a stretch.

What do you think Randy (rf900rkw)?

[Z3racer701]

LoL, I even had to look up what the word pedantic means, guess I didnt get enough schooling.
https://www.vocabulary.com/dictionary/pedantic

Hey Steve,

Do you mean that your pedagogical experience to this point has been insufficent for excessively verbose exchanges of information?

[GBPaterson]

pfft logophile! Such a bombastic response is surely nothing more than a pompous display of circumlocutory grandiloquence.

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Oh and I brought my car into the guy who rebuilt the diff and he told me to stop worrying and add a bottle of friction additive (which I later did and do seem to have found improvement in the cornering growls). Apparently of the two high pitched whines the one of acceleration / deceleration he said is fine and the howling type one at 65 he said can probably be fixed by shimming / reseating the gears. Meh...there goes my excuse for shelling out on a cusco.

[Telecomtodd]

I'd find your situation a perfect reason to go 3.73 and get it over with. I made the change and couldn't be much happier. Oh, and a 40% kit from Jonathan helped quite a bit, too. Just don't ask about the process for which the final (there was more than one) diff was procured and rebuilt...the end result is two thumbs up and a pat on Jonathan's back. I think I ended up contributing well to his Mindy fund, LOL.

[mcoupemindy]

Oh and I brought my car into the guy who rebuilt the diff and he told me to stop worrying and add a bottle of friction additive (which I later did and do seem to have found improvement in the cornering growls). Apparently of the two high pitched whines the one of acceleration / deceleration he said is fine and the howling type one at 65 he said can probably be fixed by shimming / reseating the gears. Meh...there goes my excuse for shelling out on a cusco.

I would highly suggest you find a different differential guy. I won't rip apart into this statement, but there is more than one concerning item you've said.

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I'd find your situation a perfect reason to go 3.73 and get it over with. I made the change and couldn't be much happier. Oh, and a 40% kit from Jonathan helped quite a bit, too. Just don't ask about the process for which the final (there was more than one) diff was procured and rebuilt...the end result is two thumbs up and a pat on Jonathan's back. I think I ended up contributing well to his Mindy fund, LOL.

Well, thank you. I still have the victim sitting as parts in my garage. One day I'll rip it all apart and decide to either rebuild it or, more likely, scrap it for parts.

[GBPaterson]

I'm running 3.91, I did that on the first rebuild

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I would highly suggest you find a different differential guy. I won't rip apart into this statement, but there is more than one concerning item you've said.

Please do rip apart! I don't have many people around here who claim to be differential specialists (he is the only one near me I could find). I was already looking at your clutch packs and having tried to get the diff back in once now am seriously considering those alternate mounting bolts

Please elaborate as much as you can / like on his erroneous statements! Information is power