Limited Slip Differential

[Candide13]

Track season is dawning on us here on the East coast, and in preparation I've been pondering some technical issues and practical goals such as how to become more aware of what the car is doing (especially with nannies off) at any moment and at all four corners.

One issue that I would like to understand better is what it feels like when the mechanically actuated LSD in our cars is active under hard cornering. Should the driver be aware of it doing something? What does it feel like?

I know the mechanical and physical fundamentals of the LSD, and I understand the role of the LSD for burn-outs (perish the thought), and for getting the car going when one wheel is on ice, but how does the LSD come into play on the track?

For example, how would you know if your LSD is not working on the track?

Pointers welcome, thanks.

[Richbot]

Do a few rain sessions with everything turned off and you'll have no more questions about what it feels like, rain tends to slow everythign down enough to be able to tell what's going on out back. The stock diff is relatively slow to act and fast to lock up once it locks, so you'll feel a little inside wheelspin followed, probably, by the tail trying to kick sideways once it locks, in general. IMO it doesn't do anything near what the brake force distribution system does on decel so impact seems negligible to me, maybe I'm wrong that's my butt dyno readout says I have it right here

You'll know if it's starting to wear if you start getting more inside wheelspin in 2nd and 3rd gear corners. Mine still makes elevensees at 91,000 miles but it's pretty clear to me that it doesn't react as quickly as it used to when it was new, so just being able to leave two stripes isn't enough info

[Candide13]

Originally Posted by Richbot

Do a few rain sessions with everything turned off and you'll have no more questions about what it feels like, rain tends to slow everythign down enough to be able to tell what's going on out back. The stock diff is relatively slow to act and fast to lock up once it locks, so you'll feel a little inside wheelspin followed, probably, by the tail trying to kick sideways once it locks, in general. IMO it doesn't do anything near what the brake force distribution system does on decel so impact seems negligible to me, maybe I'm wrong that's my butt dyno readout says I have it right here

You'll know if it's starting to wear if you start getting more inside wheelspin in 2nd and 3rd gear corners. Mine still makes elevensees at 91,000 miles but it's pretty clear to me that it doesn't react as quickly as it used to when it was new, so just being able to leave two stripes isn't enough info

I do like rain sessions, so your suggestion is another reason to look forward to them!

However, I don't understand what you mean by IMO it doesn't do anything near what the brake force distribution system does on decal. Are you referring to the stability management (eg MDM level)? Or is that something that goes on even with all nannies switched off?

[roastbeef]

the guys that get aftermarket LSD's always report a drastic difference in traction and braking. kinda makes me think about how good the oem differential does its job. or maybe the aftermarket performance diffs lock up sooner? either way, it seems to be a drastic difference.

[dogbone]

I did an extensive write up on differentials on my build thread. It starts on Page 8 and is 3 parts. ( http://www.m3post.com/forums/showthr...1158445&page=8 ) Using data from my AIM Solo DL that's tied into the CAN bus, I was able to analyze how the rear wheels spin in relation to each other. I used this data to compare the behavior of the OEM diff to a custom setup OS Giken LSD on my E90 M3. I think my analysis will give you some of the info you seek.

[Mvez]

+1 for Richbot and Dogbone, all good info.

I say this all the time....an LSD doesn't magically lower lap times, what it does is give you more options on how to drive (and rotate) the car more quickly. The handling dynamics are increased, because now you have lock on accel and decel. This allows for both confidence and the ability to better perform (and consistently) things such as trailbraking, threshold braking, and allows for better on-throttle modulation of the car's attitude.

Plus, when you want to be a hoon, it's just way easier to drift a car with a good locking LSD.

I run Giken in my miata and 987, and I can tell you without a doubt, I can do things in my 987 that would be damn near impossible with an open, or stock diff. Mid engine cars don't like to be sideways, that's just not their inherent trait, so my goal was to make it as progressive as possible. The Giken achieves it. Mid engine cars are pussy-cats until you get to 9-10/10ths, and then you better be up on the wheel....there's a reason you don't see mid-engine drift cars, they change direction way too fast. The Giken is superb in controlling it's change in direction, and allowing me to do so.

Giken is dynamite in the low powered miata as well. Again, gives me options on how to rotate the car. You can't go flat fast enough.

I ran a Guard 4-clutch (60/40 ramp) LSD in my RS, and the difference over the stock unit was night and day. Go to throttle sooner, better control off the corners, rock solid stability under braking, and the ability to lay down awesome stripes when you feel frisky.

Money well spent, but it's something you can't really appreciate until you get to a point with the car where you really are in tune with it. These M-diffs really aren't that bad, they do lock and are very reliable (very much like Torsens in that regard), but once you go to a really high performing unit, you get ruined, and typically don't go back.

IMO, based on the wear from both my Guard and Giken, and how they work (which is slightly different), it seems the Giken should offer a longer service life, which I like. What I learned is that at the highest levels (pros in Porsche world), some didn't like the Giken as much, simply because it was a bit more difficult to tune because of how it works, and the way it uses gear oil as part of it's locking factor, especially in transaxle applications (think Porsche 911 and Cayman).

For us DE and club race guys, I think it's a fantastic diff, with a more buy-it-and-forget-it use. Because they can use virtually zero pre-load, it also reduces heat and wear of the internals...a lot of other clutch-pack diffs use more pre-load as part of the equation. The more pre-load, the more the diff is still somewhat locked in off throttle (coasting) situation....basically at turn-in. It's all a tradeoff. For my RS diff, I specified low pre-load, so that car would be free-er off throttle and on turn-in, so less understeer, but the tradeoff with less pre-load is that when the diff locks, it locks faster as the ramps do their magic....and, pre-load always lessens over time, as the clutch plates wear...so it means the non-Giken type diffs lock even harder/faster. I can say certainly this was the case in my RS, thus requiring a rebuild at some point.

In a BMW or miata, for example, you don't have to worry about gear oil weight because it's not a transaxle, you run what is specified for the LSD, or how you want to Giken to lock. If you run lower weight than the specific Giken oil (80W250), the diff will lock faster and harder....which takes away from the progressive nature. We always use Giken fluid in our miatas, it just works better than anything else.

FWIW - I will very likely put a Giken in my E92, for these reasons. The Diffs online LSD's are great too, no doubt, but I value service life a lot, and this is where I think the Giken shines. This is where the progressiveness of locking is preserved, via the lack of internal clutch plate wear. Part of that voodoo, black box magic of the Giken I suppose. Just my .02.

[roastbeef]

Do you guys see aftermarket LSD's as a tool that can help a driver develop? Think it's a good investment for a dual duty street/track car?

[GeorgeA]

The LSD in the M3 is one of the main reasons why I love this car, and a major factor in any M car. When you are accelerating out of a corner, the power will transfer to the outside wheel to prevent the inside wheel from spinning. It will also help rotate the car during cornering, kind of a steer with the throttle effect. On top of that, when you toss the car into a slide, the LSD will give you angles and control (precision) while sliding the car that an open diff can't match.

I have owned cars with "viscous" LSDs and they are absolute rubbish, they don't lock fully and tend to never work. The M diff on the other hand always kicks in and tends to split the power beautifully.

Upgrading the M's differential won't be a worthwhile mod since BMW spent a lot of time on this particular differential and to be completely honest, M diffs are the best in the business from what I have tested. Improving your driving will improve your times far more than upgrading the diff in the M3. Just my two cents.

[Richbot]

Originally Posted by GeorgeA

The LSD in the M3 is one of the main reasons why I love this car, and a major factor in any M car. When you are accelerating out of a corner, the power will transfer to the outside wheel to prevent the inside wheel from spinning. It will also help rotate the car during cornering, kind of a steer with the throttle effect. On top of that, when you toss the car into a slide, the LSD will give you angles and control (precision) while sliding the car that an open diff can't match.

I have owned cars with "viscous" LSDs and they are absolute rubbish, they don't lock fully and tend to never work. The M diff on the other hand always kicks in and tends to split the power beautifully.

Upgrading the M's differential won't be a worthwhile mod since BMW spent a lot of time on this particular differential and to be completely honest, M diffs are the best in the business from what I have tested. Improving your driving will improve your times far more than upgrading the diff in the M3. Just my two cents.

That first paragraph is just describing how an open diff and any half-decent LSD compare. The GKN unit BMW has used since the E46M is a completely passive diff designed for good road manners and that means compromises for absolute performance. That last bit is demonstrably false based on a variety of users' data, some of which has been provided in this thread, and my own experience.

I agree Viscous LSD's are indeed usually crap. But they're also the bottom of the barrel and bear no resemblance to the OS Giken and other motorsport solutions, which have the advantage of performing as well or better than many of the street car active diffs out there, at the expense of some road manners and service life. tanstaafl

[Candide13]

That's a lot of great input from everyone! Here is my short take-home synthesis.

The stock M3 differential is a solid effort aimed at a balance between road driving and track duty. As a clutch-based system it is superior to the simplest, viscous systems, that are always on to some extent, deliver less clear locking results, and eventually wear out.

But because the clutch actuation is based on wheel speed differential, the M3 diff must necessarily operate with some time delay, so if turning a corner at the track the inner wheel must first lose traction and spin up for the clutch to close, and when it closes there will be a more or less sudden load variation at the rear wheels as you exit the corner and put the power down, which could lead to instability. This can not be eliminated and requires a sensitive right foot. The time delay inherent in the stock diff locking process is as inevitable as turbo lag is for engines.

So therefore the M3 diff is a notch below dedicated performance diffs, which are torque-sensing (cf dogbone's excellent write up and the embedded video) and therefore act instantaneously and predictably; they also act under braking, though I don't imagine that makes that much of a difference given the heavy front-bias under braking. Still, as dogbone points out, it adds confidence to the driving, which allows you to edge towards the limit of performance. A small drawback of the performance diffs is that they too are always on to some extent, and therefore they wear out very slowly as well.

All this sounds good to me; I'll be watching out for the LSD action this season. I think I have already encountered it in a positive way on snow and ice in freezing temperatures, where it is all good, but on the track it will require more finesse and right-foot sensitivity. But that's fun and a big reason why we do this!

[Mvez]

One other very important factor to note. Everyday drive-ability.....if you daily, or drive the car to and from the track, going with a super high performance, high locking factor LSD of any kind, can be down right diabolical in rain conditions on public roads.

My Guard LSD was incredible on dry track, but if it was raining on public roads, you better have two hands on the wheel, it tries to kill you.

This again, is why I prefer low-pre-load LSD's...and perhaps the Giken most of all for dual purpose cars, because it gives you good DD capability, in the event your car isn't just a track whore.

If you feel your stock LSD isn't locking enough, before buying a new unit, perhaps just run some non-LS gear oil, and you should feel it grab faster and harder than before. Our diff's don't have external coolers, so the gear oil gets plenty hot. Most gearbox builders will tell you for track applications, to just skip the "LS" oils, because at high temps they make the clutch plates too slippery and you lose lock. You can go as far as even mixing 50/50 with LS and non-LS to tune it better. Cheap DIY LSD tuning.

Just something to keep in mind as well.

[roastbeef]

so what are racecars running during rain races?

[Mvez]

Originally Posted by roastbeef

so what are racecars running during rain races?

Whatever they have. I'm not talking about race cars, they would typically run a more aggressive setup.

I'm just saying whatever lsd you go with, if it's a street driven car, I wouldn't go with a high preload, high locking factor setup for an LSD.

[roastbeef]

i was just curious because it would be something they would have to deal with too, right?

[bigjae1976]

Originally Posted by roastbeef

Do you guys see aftermarket LSD's as a tool that can help a driver develop? Think it's a good investment for a dual duty street/track car?

No. A clutch type diff is going to be more of a handful than the OE LSD. You need excellent throttle control because the car is going to do exactly what you tell it.

The OE unit is softer (road manners) so you can chop the throttle and do all sorts of whatever and the car won't spin.

I would compare it to R-comps to street tires. There is a small reward for the novice but a much more significant reward for skill.

[Richbot]

Heh. Yep most people are telling their cars to do dumb stuff especially with the throttle

[driftflo]

Originally Posted by GeorgeA

Upgrading the M's differential won't be a worthwhile mod since BMW spent a lot of time on this particular differential and to be completely honest, M diffs are the best in the business from what I have tested. Improving your driving will improve your times far more than upgrading the diff in the M3. Just my two cents.

so what else have you tested so far?
and why does bmw motorsports use a drexler clutch type lsd in the m3 gt4 and z4 gt3 (until my2012... and xtrac from then on)?

people who do not recognise a massive difference/improvement in handling when upgrading to a (properly set) clutch type lsd are so far away from the cars limit or do have so few "car control" that an open diff would be good enough for them as well.

i drove all clutch type units available for the e92 m3 in my own m3... most important is a proper setup... no matter if you use drexler, titan, bacci, osgiken...

and no manufacturer really has any "magic" in its lsd cores... any clutch type lsd works in the same way and has the same parameters that can be set. all lsd units do wear... how fast depends on various factors...

i wrote my findings in an own thread here:
http://www.m3post.com/forums/showthread.php?t=788992

... and until today i have (re-)built +700 lsd units with individual setups.... drexler, giken, titan, zf, gkn, kaaz... for porsche, bmw, amg, nissan, honda, ford, ferrari, ...

[Richbot]

Originally Posted by driftflo

so what else have you tested so far?
and why does bmw motorsports use a drexler clutch type lsd in the m3 gt4 and z4 gt3 (until my2012... and xtrac from then on)?

people who do not recognise a massive difference/improvement in handling when upgrading to a (properly set) clutch type lsd are so far away from the cars limit or do have so few "car control" that an open diff would be good enough for them as well.

i drove all clutch type units available for the e92 m3 in my own m3... most important is a proper setup... no matter if you use drexler, titan, bacci, osgiken...

and no manufacturer really has any "magic" in its lsd cores... any clutch type lsd works in the same way and has the same parameters that can be set. all lsd units do wear... how fast depends on various factors...

i wrote my findings in an own thread here:
http://www.m3post.com/forums/showthread.php?t=788992

... and until today i have (re-)built +700 lsd units with individual setups.... drexler, giken, titan, zf, gkn, kaaz... for porsche, bmw, amg, nissan, honda, ford, ferrari, ...

OP You can use this guy's username to find some of the best threads on the topic on this forum too

[GeorgeA]

Originally Posted by driftflo

so what else have you tested so far?
and why does bmw motorsports use a drexler clutch type lsd in the m3 gt4 and z4 gt3 (until my2012... and xtrac from then on)?

people who do not recognise a massive difference/improvement in handling when upgrading to a (properly set) clutch type lsd are so far away from the cars limit or do have so few "car control" that an open diff would be good enough for them as well.

i drove all clutch type units available for the e92 m3 in my own m3... most important is a proper setup... no matter if you use drexler, titan, bacci, osgiken...

and no manufacturer really has any "magic" in its lsd cores... any clutch type lsd works in the same way and has the same parameters that can be set. all lsd units do wear... how fast depends on various factors...

i wrote my findings in an own thread here:
http://www.m3post.com/forums/showthread.php?t=788992

... and until today i have (re-)built +700 lsd units with individual setups.... drexler, giken, titan, zf, gkn, kaaz... for porsche, bmw, amg, nissan, honda, ford, ferrari, ...

Didn't know OP was racing gt4, I was just talking about casual track days and daily!

I've tested AMG, Nissan, Ferrari (360) and a few other cars and from my experience the M diff has performed the best. Again if I'm racing in LeMans I'm sure there are better units that are designed for racing applications. What LSD would you recommend for street/canyon with a bit of track?

[roastbeef]

Originally Posted by driftflo

so what else have you tested so far?
and why does bmw motorsports use a drexler clutch type lsd in the m3 gt4 and z4 gt3 (until my2012... and xtrac from then on)?

people who do not recognise a massive difference/improvement in handling when upgrading to a (properly set) clutch type lsd are so far away from the cars limit or do have so few "car control" that an open diff would be good enough for them as well.

i drove all clutch type units available for the e92 m3 in my own m3... most important is a proper setup... no matter if you use drexler, titan, bacci, osgiken...

and no manufacturer really has any "magic" in its lsd cores... any clutch type lsd works in the same way and has the same parameters that can be set. all lsd units do wear... how fast depends on various factors...

i wrote my findings in an own thread here:
http://www.m3post.com/forums/showthread.php?t=788992

... and until today i have (re-)built +700 lsd units with individual setups.... drexler, giken, titan, zf, gkn, kaaz... for porsche, bmw, amg, nissan, honda, ford, ferrari, ...

i'm interested in hearing more of your input. what would you recommend for a street driven m3 that spends about 8 days a year at the track? i would be looking to improve the feel of my car, but its a little while before i can make this jump.

[driftflo]

Originally Posted by roastbeef

so what are racecars running during rain races?

if the lockup is too high and there is no time to change the diff/lsd they add some fm additive to the oil.

[driftflo]

Originally Posted by GeorgeA

Didn't know OP was racing gt4, I was just talking about casual track days and daily!

if he was racing a gt4 there would be no need to change the lsd... but it needs to be rebuilt quite often. after one season of racing the performance has dropped massively. but once opened you can make it more durable easily.

Originally Posted by GeorgeA

I've tested AMG, Nissan, Ferrari (360) and a few other cars and from my experience the M diff has performed the best. Again if I'm racing in LeMans I'm sure there are better units that are designed for racing applications. What LSD would you recommend for street/canyon with a bit of track?

amg has a very soft throttle response on the m156-powered cars... it may feel that the diff locks lately but this is due to the torque that comes with a little lag (but then it comes massively making it a little harder to control the car with the lsd spinning both wheels). furthermore, only since the new generation with turbo engones cars are equipped with an lsd in stock config. so if you have driven a c63 w204 (even with performance package) it may have had no lsd at all. and then the car drives horribly indeed!

nissan has even worse viscos in most of their s-chassis, z-fairlady and skyline cars :-(
the bmw m-variable is indeed better then that. an upgrade is strongly recommended for these cars as well.

ferrari 360 should have worked quite well but you really have to push a mid-engined ferrari quite hard to judge on the lsd. you can hardly test this on the street. and it depends on the condition of the lsd of course.
as i wrote above these units do wear... so if the car had some miles on it and was driven spirited it may simply be worn out.
ferrari always did a good job on the lsds used, even in the older cars. and i also like the new e-diffs pretty much. i do a lot of track driving with the new 488 and the car performs great on the edge... no matter if you go fast or sideways. and although i prefer driving it unfiltered with everything turned off, the side slip control (ssc2) that also interacts with the lsd is great for non professional but ambitioned drivers. it really helps you exploring the limits and even go beyond but safes you if anything goes out of control.