FAQ: BMW eLSD - Not your dad's open diff!

[int2str]

I'd like to dis-spell some rumors and clear up some misconceptions regarding BMWs electronically controlled differential or as some call it the "electronic LSD".

The BMW 1-series as well as many other recent BMW models feature an electronically controlled Automatic Differential Brake (ADB) system, that in conjunction with a normal open rear differential works to emulate a traditional mechanical limited slip system.

Below are some questions I've compiled based on forum posts I've found here. Feel free to ask more questions and I'll update this FAQ periodically.

All these answers are from my personal experience and to the best of my knowledge. I reserve the right to make corrections as better information becomes available.

----

Does the 1-series (or other recent BMWs) have an open rear differential?

Mechanically, the answer is yes - there is a completely open rear differential between the two rear wheels. This open diff is however assisted by an automatic differential braking system that will apply resistance to one wheel to equalize traction between the two wheels.


What does that look like in action?

Here's a graph showing the on-board telemetry of a 135i during an autocross run going around a left hand corner:


The light blue line shows braking force applied to the left (inside) rear wheel and the yellow line shows braking force applied to the right (outside) rear wheel. Dark blue shows the throttle pedal position and the red line show the brake pedal.

As you can see, without any brake pedal applied, the car applies some light brake pressure to the inside (less loaded) rear wheel to prevent wheel spin.

Also note that when the brake pedal *is* actually pressed, the car still manages the left/right brake balance to keep the car as neutral as possible under braking.

This car was equipped with a mechanical LSD as well - so even with a mechanical LSD, the system still provides additional inputs.


If I loose traction, will only 1 wheel spin?

No. The system automatically applies brake pressure to the wheel with the least amount of traction to get both wheels spinning.


Does the system reduce engine power?

No. Brake pressure is applied to one wheel, the engine power is not affected.


Will the braking force lock up the wheel or slow me down?

No. Only light brake pressure is applied to the spinning wheel this does not slow the car down. Mechanical limited slip differentials perform the exact same function, though instead of using the brakes to apply friction on the spinning wheel, mechanical friction is applied inside the differential.


Does this system interfere even if I'm not smoking tires?

Yes. The electronic differential braking acts even if you're not lighting up tires to help stabilize the car in a variety of situations.


Can I turn this system off?

No. Even with all traction and stability control (DTC and DSC) turned off, differential braking is still enabled.


Can I drift with the only the electronic differential?

Yes, absolutely!


Do I still need a mechanical LSD then?

Maybe. This is a tough question to answer and a personal one at that.

The electronically assisted differential works rather well actually. It puts power down to both wheels, allows drifting and generally provides a positive driving feedback and control.

However, the electronic system has it's limits. Once traction loss is too great, the system can be overwhelmed, causing a more sudden loss in traction than what a mechanical differential would do. In other words, you might spin more suddenly than with a mechanical diff. Also, the electronic control does not provide the same feedback to the driver and the same confidence that a mechanical differential does. Thus controlling the car is more difficult than it would be with a mechanical diff.

Ultimately, there is no huge difference in traction availability between the mechanical and electronic differentials. The biggest difference is in the way the car "feels". This is mostly evident exiting corners under heavy throttle. This is where the mechanical differential shines.

Because the differential system uses your brakes to aid traction, excessive heat and wear may be generated on your rear brakes during track driving. A mechanical LSD can help reduce this.


Do I need a mechanical LSD for....

... daily driving? No.

... "spirited" canyon driving? No. If you're regularly overwhelming the electronic differential on public roads, please start autocrossing or track driving your car instead.

... drag racing? Maybe. The electronic differential is very good providing straight line traction. A mechanical LSD is likely not going to make a big difference here. On the highest levels of the sport, maybe an LSD would make a difference. At that point you may be better off with a locking one instead though.

... track/autocross? Maybe. BMW's differential braking system works rather well even for autocrossing or track driving. Don't be afraid to track/autocross your car in stock trim. A mechanical differential will make a big difference in controllability and confidence here so there is a definite benefit for track junkies and autocrossers.

... drifting events? Most likely. More control = better drifts.

---

Revisions:
2012-07-16 Added comment on brakes
2012-07-18 Removed friction comparison statement

[BrokenVert]

The problem with the eLSD - or at least my problem with it - is thats its a reactionary system. Despite how sensitive the sensors are or how quickly and accurately the rear brakes can be applied it still takes time.

The eLSD also changes the handling balance of the car by applying braking force to slow a wheel rather than clutch friction on a shaft. Using the eLSD vs not using it will result in a different attitude (ie roll or squat angle) for both cases.

It all makes a difference.

[Red Bread]

Yep, nearly worthless system. When running the Ortega Hwy, Mullholland or GMR, the 1er was constantly roasting the inside tire, something my M Coupe with an honest to god LSD rarely did, even with an E30 rear suspension and a much shorter wheelbase.

[int2str]

Originally Posted by BrokenVert

The problem with the eLSD - or at least my problem with it - is thats its a reactionary system. Despite how sensitive the sensors are or how quickly and accurately the rear brakes can be applied it still takes time.

While that is technically true, the system also anticipates. In the graph I posted above, the brake was actually applied without the inner wheel braking traction at all. But as mentioned above, this is where the mechanical LSD provides more "positive" feedback to the driver and thus feels more connected.

The eLSD also changes the handling balance of the car by applying braking force to slow a wheel rather than clutch friction on a shaft.

The reasoning here is not correct. The brakes apply friction to the outside end of the drive shaft. A mechanical LSD applies friction to the inside end of the drive shaft (or better half shaft). Same difference in regards to handling.
The bigger difference come from how much friction (and when) is applied.

Using the eLSD vs not using it will result in a different attitude (ie roll or squat angle) for both cases.

Please elaborate further as it's not clear what you mean here.
Also note that you do not have an option of "not using" eLSD.

[int2str]

Originally Posted by Red Bread

Yep, nearly worthless system.

This is not very helpful and misleading.
Please back up statements like this with better data.

When running the Ortega Hwy, Mullholland or GMR, the 1er was constantly roasting the inside tire,

Again, misleading. If you were "roasting" the inside tire, your outside tires was spinning, too. Also, if you are "roasting tires" on those roads, consider attending more track days instead. Also, suspension setup has a lot of influence here - and throttle control.

[Red Bread]

I've done plenty of track days, thanks. I'll continue to do them, in cars with LSD's, not my wife's 1er, it's just not much fun and between the lack of a mechanical LSD and the copious factory understeer, I just don't enjoy tracking the 1er.

I think the eLSD is severely lacking in the manner in which it allows power delivery. The combination of throttle lag in the N54 (the N55's I've driven seem worse) and the lack of an actual LSD make for an unsteady and unpredictable power delivery. I'd actually be curious to drive a 128i hard to see how much of my dislike of the eLSD is perhaps related to the turbo engine.

I did mistakenly say inside tire, when I should have said outside tire, entirely my mistake.

In my opinion the 135i is a hot mess of handling that is further hindered by the eLSD. An E9x is noticeably better sorted, but still exhibits lots of the same weaknesses from the eLSD, but at least the suspension is better tuned towards neutrality. The 1M and M3 completely fix the suspension issues and magically rock an actual LSD too.

[Bimmer-Bob]

Any idea if the 128i has the e-diff or not? I've read some press releases that seem to imply that it is only included on the 135i (along with more sport-oriented traction control logic, maybe?), but I haven't come up with anything definitive.

[ZIPNBYE]

The eDiff can put you into a spin if you ride the speed curbing through a series of esses. Basically, it can cause a wheel which is momentarily in the air to land with the brake fully applied, which results in a nasty surprise.

A very experienced fellow BMW instructor nearly took his car into the wall at Calgary's Race City Speedway like that. The car was on R-compound tires (higher grip levels than street tires), which of course contributed somewhat, but the system is not flawless.

A proper Torsen LSD won't put you off track backwards with one locked wheel, nor will it see you wear out your rear brake pads faster than your fronts.

While a decent system, it's not by any means a replacement for a proper mechanical (clutch or geared) limited slip differential.

I also find that the eDiff is pretty useless in winter conditions.

[NYC6]

"Any idea if the 128i has the e-diff or not? I've read some press releases that seem to imply that it is only included on the 135i (along with more sport-oriented traction control logic, maybe?), but I haven't come up with anything definitive."

_____________________________________


The 128 also has the system. Dont have any proof at hand but have read so from reliable sources. Someone will confirm, Im sure.

[BrokenVert]

Originally Posted by int2str

While that is technically true, the system also anticipates. In the graph I posted above, the brake was actually applied without the inner wheel braking traction at all. But as mentioned above, this is where the mechanical LSD provides more "positive" feedback to the driver and thus feels more connected.

I understand the way it works. But

Originally Posted by int2str

The reasoning here is not correct. The brakes apply friction to the outside end of the drive shaft. A mechanical LSD applies friction to the inside end of the drive shaft (or better half shaft). Same difference in regards to handling.
The bigger difference come from how much friction (and when) is applied.

Two words - Moment arm.

The force applied by an LSD is centered on the halfshaft. Braking pressure is not centered on the shaft. This creates a positive moment on the entire system - a moment that isnt matched on the opposite side of the but why would you want your suspension to absorb a force that it doesnt need to?car. One that is more than likely fully absorbed by the suspension, granted. That brings me to your final point.

You can PM if you want more details on any of these principals. But like I said. Its not just a force - but where and how you apply that force on a system that makes all the difference.

Originally Posted by int2str

Please elaborate further as it's not clear what you mean here.
Also note that you do not have an option of "not using" eLSD.

[/QUOTE]

Im fully aware of how the car ive owned for four years and put 75 miles on functions mechanically.

By braking a wheel youre effectively changing the brake bias of the car - bringing it more to the rear and to a corner. Changing the way the car loads the suspension which changes the center of gravity in a corner.

If you want to experience it first hand, albeit in an extreme case, go find someone with an ActiveE (If you were local id let you try it on mine) and let the energy recover system kick in on a fast and tight turn. and note the way that the corning behaviour of the car changes relative to you having a balanced throttle.


Through personal experience I can tell you that the eLSD is a very jerky system - infact when its on at full bore you can hear it pulsating similar to an ABS system - next time youre at an autox pay attention for the sounds. In a slide you constantly have to correct for that pulsing - leading to a lot of work that you simply dont need to do.

A mechanical LSD is more smooth and progressive - its just a better way of doing it from the standpoint of driving dynamics.


Like I said earlier. PM me if you want some more clarification. Also sorry if my words are muddled a bit. I woke up at 4am today to drive from Boston to NYC and went straight into my job after a very busy and tiring weekend.

[int2str]

Originally Posted by Red Bread

I did mistakenly say inside tire, when I should have said outside tire, entirely my mistake.

Good, so the eLSD is working well.

Originally Posted by Bimmer-Bob

Any idea if the 128i has the e-diff or not?

As far as I can tell, all recent BMWs have adaptive differential braking, including the 128i.

Originally Posted by BrokenVert

A mechanical LSD is more smooth and progressive - its just a better way of doing it from the standpoint of driving dynamics.

Thanks for the insightful explanation and information. We basically agree on the benefits of the system.

I'd still maintain that they are largely the same for most drivers, controllability and "feel" aside.

Through personal experience I can tell you that the eLSD is a very jerky system - infact when its on at full bore you can hear it pulsating similar to an ABS system - next time youre at an autox pay attention for the sounds. In a slide you constantly have to correct for that pulsing - leading to a lot of work that you simply dont need to do.

I'll have to disagree on this one though. Neither the wheel speed and brake force data acquired, nor videos I've collected back this up. I could not see any pulsing or uneven pavement marks.

[int2str]

Originally Posted by ZIPNBYE

Basically, it can cause a wheel which is momentarily in the air to land with the brake fully applied, which results in a nasty surprise.
...
The car was on R-compound tires (higher grip levels than street tires), which of course contributed somewhat, but the system is not flawless.

Lifting a wheel completely, even on rough surface streets is the biggest challenge for BMWs traction control as well as the ADB system, I agree.

[BrokenVert]

Originally Posted by int2str

Thanks for the insightful explanation and information. We basically agree on the benefits of the system.

I'd still maintain that they are largely the same for most drivers, controllability and "feel" aside.

Im not denying that - remember, 80% of 1 series owners dont know which wheels so much as get the power!

But those on this site typically arent considered "most drivers" and value things like driving dynamics and controlability.

Originally Posted by int2str

I'll have to disagree on this one though. Neither the wheel speed and brake force data acquired, nor videos I've collected back this up. I could not see any pulsing or uneven pavement marks.

Maybe they fixed it on the newer systems. But back in 2008 and 2009 it was a very noticeable and jerky system. Then again I refuse to update my SW - the new versions are terrible engine-wise.

[Red Bread]

Originally Posted by BrokenVert

Maybe they fixed it on the newer systems. But back in 2008 and 2009 it was a very noticeable and jerky system. Then again I refuse to update my SW - the new versions are terrible engine-wise.

I'd like to hope this is some of it. My old system is pretty jerky, and perhaps somewhere along the way they made it better.

Yet it remains that the only good thing the eDiff has over a realDiff is that the eDiff is cheaper for BMW.

[doublevanosrc]

I don't track my car so I think the eLSD is a great feature.

[Diver]

Great information, especially with respect to efficiency. Perhaps all the moaning about mechanical LSD should stop, but it won't.

[BrokenVert]

Originally Posted by Diver

Great information, especially with respect to efficiency. Perhaps all the moaning about mechanical LSD should stop, but it won't.

It's not moaning when it's science.

[Sauce]

OP, why are you such a big supporter of the eLSD? It doesn't work. Plain and simple. I think I have seen you post up autoX videos, never road course. Besides the fact that the higher the speed the less effective it will be, it cooks your brakes. I'd much rather have my brakes in better shape than the supposed traction it gives you. Every single corner that requires finesse with the go pedal, one of the tires gives up before the car will get sideways. At low speeds when you know the rear is going to step out, the car is so unpredictable. It steps out, the brakes come in, you lose some angle, it gains the angle back, and so on until the car grabs.

We haven't even gotten into the effects under braking and turn in.

[Sauce]

Originally Posted by BrokenVert

It's not moaning when it's science.

Great explanations in here man.

[int2str]

Originally Posted by Sauce

OP, why are you such a big supporter of the eLSD?

What makes you think I'm a "supporter"? Hint - my car has a limited slip.
I'm simply trying to refute some misconceptions and overall misinformation that's being proposed throughout various posts on this forum.

It doesn't work. Plain and simple.

This is exactly the kind of baseless and misleading statements that I'm trying to refute. Sorry to be so brash.

Besides the fact that the higher the speed the less effective it will be, it cooks your brakes. I'd much rather have my brakes in better shape than the supposed traction it gives you.

There's no substance here to argue against.
I'll simply ask a question: Have you driven a semi-powerful (let's say 150hp+) BMW with an actual open diff on track before?

At low speeds when you know the rear is going to step out, the car is so unpredictable. It steps out, the brakes come in, you lose some angle, it gains the angle back, and so on until the car grabs.

That's an incorrect account of how the system works. The brakes neither "come in" (the brakes are applied long before traction is lost; see graph above), nor do they pulse on/off, nor are the brakes applied to lock up.
You must think that a digital feedback loop is measured in minutes, not microseconds.

We haven't even gotten into the effects under braking and turn in.

Feel free to do so.

[Sauce]

Originally Posted by int2str

What makes you think I'm a "supporter"? Hint - my car has a limited slip.
I'm simply trying to refute some misconceptions and overall misinformation that's being proposed throughout various posts on this forum.

I do agree about the misconceptions. People make out the open diff to be dangerous, blaming crashes and spins on it. This cracks me up

This is exactly the kind of baseless and misleading statements that I'm trying to refute. Sorry to be so brash.

I'm basing it on my experience with the car. You know what, I personally can't back it up with "science", but that is because I have not spent the time to really understand how it works mechanically, only how it feels.

There's no substance here to argue against.
I'll simply ask a question: Have you driven a semi-powerful (let's say 150hp+) BMW with an actual open diff on track before?

Probably, I have driven tons of peoples e30's on track. It's hard to tell on someone else's swapped car that you only drive for 5-10 min, what is causing wheelspin. I have driven many other cars w/ open diffs on track. Don't notice any difference between an open diff and the diff in our car.

That's an incorrect account of how the system works. The brakes neither "come in" (the brakes are applied long before traction is lost; see graph above), nor do they pulse on/off, nor are the brakes applied to lock up.
You must think that a digital feedback loop is measured in minutes, not microseconds.

Again, I don't have a great understanding of how it works mechanically, but I do have a great understanding of how it works in the real world. Mind you it was much, much more pronounced on the stock sups. I don't feel this unpredictability quite so much anymore, but it still just feels odd.

Feel free to do so.

Ok, well since you obviously have a good idea of how the stock diff works, will you explain to me how the system reacts under various braking and throttle situations on turn in before I try to point out anything? i.e: trail braking, trailing throttle, etc.

[int2str]

Originally Posted by Sauce

I have driven many other cars w/ open diffs on track. Don't notice any difference between an open diff and the diff in our car.

Honestly, I find that quite shocking. Unless they were modern cars that like our BMWs have electronic aids in addition to the open differential.
I recently drove an older, open diff 528i again and the difference to even a stock 135i is shocking. I would find it hard to believe that the difference isn't obvious. With a completely open diff, the inside wheel lights up like a Christmas tree around pretty much every corner.