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      07-18-2012, 04:25 PM   #45
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It's a shame that so much is different between a 135i and a 1M Coupe, because the easiest way to solve this would just be to compare lap times, skid pad and lane change times of a car with a real LSD and one with the stock fauxLSD. I'd guess that the eLSD times would be closer to an open diff car than one with a mechanical LSD, even a clutch type one, not just a geared one.

Does this really relate to my wife in her DD car? Nope, not at all.
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      07-18-2012, 04:42 PM   #46
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Quote:
Originally Posted by GtiGyver View Post
Now, in order to equalize the speed of both halfshafts, the LSD will internally increase friction, potentially all the way to locking the shafts together.
Any friction here is applied to the system that is already in rotation.
Minimal losses.
Ah, you're saying the friction is applied between the two half-shafts. Not the half-shaft and the housing. That would make a bigger difference, of course. Which LSD types work this way? Is it mostly the clutch types?

Quote:
he diff is open, we agree that the total power on both wheels must equal the input, and this is why locking one wheel causes the other to spin twice as fast
Yes. Agreed.
.
Quote:
This friction on the brakes is a pure loss, since it will only result in heat, but in no way will propel the car forward, and thus a loss of power.
But this would contradict what you said above. First of all, any friction is a pure energy loss. The heat generated by a mechanical differential is equally a loss of energy as applying friction through the brakes is. This isn't to say that a mechanical differential would have less friction losses because it is more efficient. But no matter the efficiency, energy is lost.

The bigger issue here is the statement that applying the brakes is pure loss and does not propel the car forward. This is the part that contradicts the statement above.

If we assume a 100% efficient open diff with a 1:1 gear ratio, we seem to both agree on this chart:

Input = 100rpm | left = 0rpm | right = 100rpm
Input = 100rpm | left = 50rpm | right = 50rpm
Input = 100rpm | left = 100rpm | right = 0rpm

If the input shaft in this scenario does 100rpm and you brake the left wheel down to 95rpm using the brakes on that output shaft, would you not agree that the right wheel will spin at 5rpm?

Btw. I'm really enjoying this informative exchange on the efficiencies of mechanical vs. eLSD.
Please let me know if anything in the first post needs fixing/correcting as that is the main purpose of this thread.
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      07-18-2012, 04:47 PM   #47
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Quote:
Originally Posted by BrokenVert View Post
I did explain it already on the first page and GTIGyver added even more to my initial explanation.
It has to do with moment arms and frames of reference.[/QUOTE]

Please let me know if you want to propose any actual corrections to the information in the first post. I'd be happy to make 'em.
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      07-18-2012, 04:52 PM   #48
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Quote:
Originally Posted by int2str View Post
Ah, you're saying the friction is applied between the two half-shafts. Not the half-shaft and the housing. That would make a bigger difference, of course. Which LSD types work this way? Is it mostly the clutch types?
My understanding is that all mechanical LSDs work this way, (gear, clutch or otherwise).

Now, you are correct, a loss is a loss.
And again, you are correct in saying that even hardware LSDs are not 100% efficient.

I guess another good analogy would be that this system *would* be more efficient if the disc from the brakes on the spinning wheel were attached to the opposite wheel somehow (lets call it magic).
In this case, the exchange of energy would be such that even if the brakes were to grab the disc solid, the wheels on both sides would then spin at the same speed. The spinning wheel would pull the other wheel to spin faster by both the action of the diff under braking AND by the action of the disc itself.

Not sure if you see what I'm getting at.

Man, this is so much more fun than trying to figure out why these stupid digital controls are not receiving my data packets correctly... yeah I'm at work.
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      07-18-2012, 04:52 PM   #49
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Quote:
Originally Posted by Red Bread View Post
It's a shame that so much is different between a 135i and a 1M Coupe, because the easiest way to solve this would just be to compare lap times, skid pad and lane change times of a car with a real LSD and one with the stock fauxLSD.
What is it that you're trying to solve?! One more time: This thread was not created to compare mechanical LSD to adaptive braking. What would be relevant to this thread would be to compare those items between a fully open diff and the eLSD.

Quote:
I'd guess that the eLSD times would be closer to an open diff car than one with a mechanical LSD, even a clutch type one, not just a geared one.
I would very, very much doubt that. A 300hp open diff car would be pretty much undriveable, especially on run-flats. That's the part people don't seem to realize.
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      07-18-2012, 04:56 PM   #50
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Quote:
Originally Posted by int2str View Post
What is it that you're trying to solve?! One more time: This thread was not created to compare mechanical LSD to adaptive braking. What would be relevant to this thread would be to compare those items between a fully open diff and the eLSD.
Agreed, a data point for all three would be great, and like I said, my guess would be that the eLSD is closer to the performance of an open diff than a traditional clutch type LSD.



Quote:
Originally Posted by int2str View Post
I would very, very much doubt that. A 300hp open diff car would be pretty much undriveable, especially on run-flats. That's the part people don't seem to realize.

So you do agree that RFT's are bad. Now we just need to get you to come around on the eLSD.
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      07-18-2012, 05:04 PM   #51
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Quote:
Originally Posted by GtiGyver View Post
My understanding is that all mechanical LSDs work this way, (gear, clutch or otherwise).
Just double-checked the Wavetrac again and you're right.

Quote:
I guess another good analogy would be that this system *would* be more efficient if the disc from the brakes on the spinning wheel were attached to the opposite wheel somehow (lets call it magic).
In this case, the exchange of energy would be such that even if the brakes were to grab the disc solid, the wheels on both sides would then spin at the same speed. The spinning wheel would pull the other wheel to spin faster by both the action of the diff under braking AND by the action of the disc itself.
But isn't this kind of what happens?
Once the brakes are applied to the spinning side, the diff action will start spinning the stalled side whilst still spinning the side with brakes applied.

Quote:
Man, this is so much more fun than trying to figure out why these stupid digital controls are not receiving my data packets correctly... yeah I'm at work.


Btw. I edited the first post a little to put less focus on the friction loss differences.
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      07-18-2012, 05:10 PM   #52
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Quote:
Originally Posted by Red Bread View Post
like I said, my guess would be that the eLSD is closer to the performance of an open diff than a traditional clutch type LSD.
You did see the next sentence where I pretty much said that's wrong, right?!
And yes, I have compared my times before/after LSD. And just last week I drove a beautiful 300+hp open diff car and it's hilariously funny. But not in the "safe and fast" way

It's absolutely clear to me that everyone who thinks that BMWs eDiff doesn't work has never driven a semi-powerful open diff car before.

Do I understand the virtues of a mechanical diff? Sure I do! I spent good money on mine. Do I also understand that BMW's ADB gets you 95% there? Absolutely!
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      07-18-2012, 05:22 PM   #53
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Quote:
Originally Posted by GtiGyver View Post
LSD hardware is far more expensive than a software solution so eLSD is definitely saving money for BMW, otherwise they would still install them in all their cars as they once did.
This is why nearly all car manufacturers today feature eLSD in their cars. VW already had it in the early 2000s.
Very little research needs to be involved, you are grossly overestimating the complexity of this kind of software system. A cheap easy solution to a known problem with open differentials.
I don't understand how bolting a $1500 piece of hardware and then making the car cost 1500 more is saving bmw money. I am sure they spread the cost of sw development and testing over the cars they sell however it is surely not a simple thing to do. I do embedded real time systems for a living including sensor processing etc. and I know it always costs way more to do something than people think. I just don't think the complexity of a system which must be calibrated to an individual chasis(tonnes of data collection/ simulation/ and testing) involving many variables is cheap or trivial. I mean why try when bolting up an lsd does exaclty the same thing and can be stamped out quite easily. My guess is safety, before the days of traction control, stability control, and these eLSD hi-po rwd cars were alot easier to wreck. These cars even with traction control off spin like tops with moderate throttle input on a wet road.
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      07-18-2012, 05:30 PM   #54
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Quote:
Originally Posted by FactorX81 View Post
My guess is safety, before the days of traction control, stability control, and these eLSD hi-po rwd cars were alot easier to wreck. These cars even with traction control off spin like tops with moderate throttle input on a wet road.
I think it's the other way around. eLSD is "nearly free" because of local laws. Both the European Union as well as the US have made electronic stability control mandatory. So you already need all the sensors and mechanisms to apply the brakes independently of each other.

With all the sensors and mechanisms in place already because of laws. You might as well save money by plopping in a maintenance free open diff and spend a small amount (compared to developing ESC in the first place) of development time to add an eDiff instead.
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      07-18-2012, 05:45 PM   #55
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Quote:
Originally Posted by int2str View Post
I think it's the other way around. eLSD is "nearly free" because of local laws. Both the European Union as well as the US have made electronic stability control mandatory. So you already need all the sensors and mechanisms to apply the brakes independently of each other.

With all the sensors and mechanisms in place already because of laws. You might as well save money by plopping in a maintenance free open diff and spend a small amount (compared to developing ESC in the first place) of development time to add an eDiff instead.
I dont think ABS or Traction control and stability control were mandated when BMW implemented them and got away from mechanical LSDs. There is a big difference between what eLSD is trying to do and what stability control does. The maintenence thing is a consumer item it doesnt cost BMW anything it costs the consumer money. eLSD wears brake pads faster which will also cost the consumer more in the end. However you are right part of the cost sensors etc is already built into the car these days due to regulations.
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      07-18-2012, 05:55 PM   #56
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Quote:
Originally Posted by FactorX81 View Post
I don't understand how bolting a $1500 piece of hardware and then making the car cost 1500 more is saving bmw money. I am sure they spread the cost of sw development and testing over the cars they sell however it is surely not a simple thing to do. I do embedded real time systems for a living including sensor processing etc. and I know it always costs way more to do something than people think. I just don't think the complexity of a system which must be calibrated to an individual chasis(tonnes of data collection/ simulation/ and testing) involving many variables is cheap or trivial. I mean why try when bolting up an lsd does exaclty the same thing and can be stamped out quite easily. My guess is safety, before the days of traction control, stability control, and these eLSD hi-po rwd cars were alot easier to wreck. These cars even with traction control off spin like tops with moderate throttle input on a wet road.

Hardware is a fixed cost, you need to find a source for the part, you still obviously need to do R&D, and of course failure testing.
The research you mention is a non issue since either solution requires R&D.
This means that the difference between the two is the fixed cost, the hardware. One has $0, the other say...$800 per car.

Why did BMW remove the rubber mat in the door pockets in 2009?
What is that worth? 10c?

Why did BMW no longer pre-wire the cars 2009-onwards for all electrical features like they did previously?
What is that worth? maybe...$30 in wires

Why did BMW remove the power outlet from the tray in front of the shifter in 2009 on most 1-series models?
What is that worth? $5?

Why did BMW stop installing the little metal hooks in the front of the trunk on some 1-series models?

the list goes on...

Bottom line, cost is everything, be it 10c or $800.
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      07-18-2012, 06:00 PM   #57
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Quote:
Originally Posted by GtiGyver View Post
Hardware is a fixed cost, you need to find a source for the part, you still obviously need to do R&D, and of course failure testing.
The research you mention is a non issue since either solution requires R&D.
This means that the difference between the two is the fixed cost, the hardware. One has $0, the other say...$800 per car.

Why did BMW remove the rubber mat in the door pockets in 2009?
What is that worth? 10c?

Why did BMW no longer pre-wire the cars 2009-onwards for all electrical features like they did previously?
What is that worth? maybe...$30 in wires

Why did BMW remove the power outlet from the tray in front of the shifter in 2009 on most 1-series models?
What is that worth? $5?

Why did BMW stop installing the little metal hooks in the front of the trunk on some 1-series models?

the list goes on...

Bottom line, cost is everything, be it 10c or $800.
Yes. exactly cost to the consumer not to bmw. All of those things you mentioned allow BMW to make more profit by making the car for less and selling it to the consumer for the same price. It doesnt cost BMW a thing if they pass the cost to the consumer if anything they make money on it.
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      07-19-2012, 01:29 AM   #58
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Quote:
Originally Posted by int2str View Post
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.
Ok, I wanted to drive my buddies e46 zhp and focus on the diff before I got back to this. A HUGE variable here is that my car has RS3's, so traction (in a straight line) really isn't an issue unless they are cold and his car is on some crappy bridgestones. I will give the eLSD credit in straight line. When my car had v12's, it would break traction at will in 1st and 2nd. I never felt like a 1 tire fire was going on. My buddies ZHP doesn't really have enough power to break 'em loose and I'm not going to pull the shenanigans necessary to get them loose (at least not w/ him in the car ).

Now on corner exits, I did not feel a difference between the two. My car, with linear throttle input, will start spinning the inside tire before getting the ass out. If I am abrupt with the throttle the ass will kick out. If I initiate a slide like that, the car feels pretty solid. When I walk the line of throttle, losing the inside tire and losing the rear end is where the car loses speed. The revs will rise, speed is not gained as quickly and the car is still pointed straight. I felt the same towards the open diff in his car.

You guys are WAY more scientific than me, but I feel as if my experience in the cars can still provide to this discussion, hopefully you feel the same. Just don't ask me to explain what I feel mechanically
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      07-19-2012, 10:03 AM   #59
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Quote:
Originally Posted by Sauce View Post
Ok, I wanted to drive my buddies e46 zhp and focus on the diff before I got back to this. A HUGE variable here is that my car has RS3's, so traction (in a straight line) really isn't an issue unless they are cold and his car is on some crappy bridgestones. I will give the eLSD credit in straight line. When my car had v12's, it would break traction at will in 1st and 2nd. I never felt like a 1 tire fire was going on. My buddies ZHP doesn't really have enough power to break 'em loose and I'm not going to pull the shenanigans necessary to get them loose (at least not w/ him in the car ).
I don't think the eLSD is really helping you that much in a straight line, I've had runs where just because the road wasn't perfectly level made me spin one tire more than the other as could be observed by the single black rubber mark on the road.

I will say this, it is better than nothing, and will provide better cornering control than a purely open diff. Especially around corners when one throttles up at the exit.
I think int2str may be overly optimistic in it's performance, however he is correct in saying that it does make a substantial difference when one pushes the car (slightly) beyond the traction limits.
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      07-19-2012, 10:09 AM   #60
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Quote:
Originally Posted by FactorX81 View Post
Yes. exactly cost to the consumer not to bmw. All of those things you mentioned allow BMW to make more profit by making the car for less and selling it to the consumer for the same price. It doesnt cost BMW a thing if they pass the cost to the consumer if anything they make money on it.
Another thing to remember is that BMW does not offer mechanical LSDs to non-M cars.
And BMW wants their cars to be as reasonably priced as possible (for that luxury segment).
What use is a 135i that costs way more than the competition due to a feature that maybe 1% know about. And they definitely don't want to take sales away from the 1M and M3.
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      07-19-2012, 12:39 PM   #61
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I assumed what my car was doing was due to the eLSD.

Could you help explain to me why my car does what it does? Under hard excelleration in 1st and second, when I'm on it hard enough to spin the tires, like going on an onramp getting on the freeway, my car literally swerves in both directions.
If the diff were open completely, one tire would light up I would think, but my car lurches right-left-right-left. I assumed this was the eLSD applying brakes to either of the rear wheels.
This is worse in the rain, where the tires spin even easier. My car goes anything but straight under hard excelleration.

My car has a tune, DCI, axle back exhaust, Ohlins suspension, M3 bushings, Michelin PSS's, and I know the alignment is good. It has a bit more power than stock, and this issue only happens when I'm accellerationg hard enough to spin the tires.

My friend's M3 with LSD goes straight as an arrow.

I'm pretty sure that a standard LSD would not make my car do what it does. I know a bit about the subject as I've owned and raced a few cars with LSD's. The only thing I could come up with is the eLSD was causing my car to do what it does.



Quote:
Originally Posted by int2str View Post
...


Absolutely incorrect.
If anything, ADB helps you stay straighter at that moment.



*sigh*

This is the kind of FUD I hope to address with this FAQ.
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      07-19-2012, 12:51 PM   #62
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Quote:
Originally Posted by mike the snake View Post
Could you help explain to me why my car does what it does? Under hard excelleration in 1st and second, when I'm on it hard enough to spin the tires, like going on an onramp getting on the freeway, my car literally swerves in both directions.
If the diff were open completely, one tire would light up I would think, but my car lurches right-left-right-left. I assumed this was the eLSD applying brakes to either of the rear wheels.
This is worse in the rain, where the tires spin even easier. My car goes anything but straight under hard excelleration.
Very hard to diagnose remotely...
I'd assume you have traction control fully off? Otherwise that would be your first problem.

From there I'd check suspension setup, alignment and the tires...

This is what it can look like with the electronic diff:
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      07-19-2012, 01:22 PM   #63
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Quote:
Originally Posted by GtiGyver View Post
I don't think the eLSD is really helping you that much in a straight line, I've had runs where just because the road wasn't perfectly level made me spin one tire more than the other as could be observed by the single black rubber mark on the road.

I will say this, it is better than nothing, and will provide better cornering control than a purely open diff. Especially around corners when one throttles up at the exit.
I think int2str may be overly optimistic in it's performance, however he is correct in saying that it does make a substantial difference when one pushes the car (slightly) beyond the traction limits.
I'm just saying exactly what I felt. No speculating. I would lose traction but I did not feel the power all go through the inside wheel.
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      07-19-2012, 04:52 PM   #64
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You can get as technical as you want but the first time I nailed my 135i in first gear I was like, "why the f doesn't this car have a Limit Slip Diff?" The fact that I could tell pretty much sums it up.
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      07-19-2012, 04:58 PM   #65
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Quote:
Originally Posted by int2str View Post

This is what it can look like with the electronic diff:
did you not see how much he was working the car to get it to that point?


See how it came out in steps and not one progressive slide? Thats the notchy action of the eLSD that I was talking about. It even tried to bite him back twice
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      07-19-2012, 05:20 PM   #66
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Quote:
Originally Posted by BrokenVert View Post
did you not see how much he was working the car to get it to that point?
See how it came out in steps and not one progressive slide? Thats the notchy action of the eLSD that I was talking about. It even tried to bite him back twice


Same driver, more relaxed environment:
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