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E82 / E88 1 Series (2008 - 2013)
BMWs throw back to the iconic 2002, with a renewed form and function. The smallest car in BMW's line up but still packs a punch. Available in coupe or convertible, powered by either an inline 6 in the 128 or the twin turbo rocket sled 135.

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  #1  
Old 12-27-2013, 06:43 PM
Chunner Chunner is offline
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Lack of mechanical LSD. Why hate it? I do not get it...

Why does everybody complain about a 135i's lack of a mechanical limited slip differential (LSD)? When we have DSC?

I do not get it. I read about it so much on this forum, I started to think that way too, until .....

From what I have researched ... people complain that without a mechanical limited slip differential (LSD), too much power is lost through the rear wheel on the inside of the bend which is carrying the smallest part of the load. Resulting in a loss of power mid-corner.

However, since 2008 all 135i's has a modified DSC function program that "simulates the action of a mechanical limited slip differential (LSD)". When the vehicle is powering through a bend the DSC function applies the brakes to the spinning (inside) rear wheel, torque is then transmitted to the outside wheel which is carrying most of the load, optimizing power and maintaining vehicle speed through the corner.

In comparison with an actual mechanical limited slip differential (LSD), the advantage of the modified DSC program function is more than just the fact that it is substantially more cost effective. Whereas a mechanical limited slip differential lock is permanently active, the DSC function only comes into action only when required. Nor does it add to the vehicle weight and fuel consumption. Something a mechanical limited slip differential (LSD) does.

Comment as you see fit.
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  #2  
Old 12-28-2013, 04:50 AM
mr_bean mr_bean is offline
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The problem with the electronic system is that it is reactive to controlling wheel spin. It senses when the inside wheel has lost traction and then applies the brakes to that wheel. At this point, the car has already lost its ability to accelerate while the driver waits for the electronics to bring things under control. Furthermore, the system doesn't precisely know when to let off the brake so even if the inside wheel has enough traction some power is inevitably lost.

A mechanical LSD is proactive at controlling wheel spin. Torque is transferred from the inside wheel to the outside wheel before the tire loses grip. More power is being used to accelerate the car so it will go faster through the turn.

Granted, the electronic system works very well for most street driving. Only the most spirited drivers and one's racing on a track would fully appreciate the subtle differences. Considering this and the additional cost of an LSD, it's no surprise that it's not available. BMW knows that people who want to race their cars and need an LSD will have an aftermarket unit installed.
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  #3  
Old 12-28-2013, 06:48 AM
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Chris90 Chris90 is offline
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Why buy a rear drive car with 300 lb-ft of torque if you're not gonna occasionally try to drive it sideways through a slow corner? LSD makes this easy and controllable.

I don't think anyone hates the car cause it doesn't have an LSD, and there are many options for those that want one, so I don't see the big deal.

BMW agrees, the M235i now has an optional LSD.
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  #4  
Old 12-28-2013, 07:46 AM
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Zeichen311 Zeichen311 is offline
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Re: Lack of mechanical LSD. Why hate it? I do not get it...

Electronic differential braking will apply a brake while exiting a corner to transfer torque, whereas a mechanical LSD manages torque without involving the brakes. This means that compared to a true LSD, the simulation robs the brakes of a fraction of a second of cool-down time between corners. On the street, this should never matter. On the track, it may mean the net cooling time is insufficient and heat will build up in the brakes, lap after lap, leading to that exciting moment when the pads fade or the brake fluid boils. This is a Bad Thing.
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  #5  
Old 01-02-2014, 06:49 AM
JimD1 JimD1 is offline
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Another thing the electronic LSD/Stability Control System does is to reduce your throttle input. You can switch the setting to a middle position or turn it completely off but then you have no LSD action. I was dodging a daughters car on our admittedly steep driveway with one wheel off in the wet grass and couldn't climb the hill due to the LSD. A mechanical locker would have just locked the wheels up and I would have climbed the hill. I had to back down and hit the problem area with enough momentum I could get out of the driveway.

Using brakes to simulate a limited slip differential is a neat software trick but not really a replacement. A true limited slip differential does nothing until the rear wheels want to spin at very different speeds and then it locks up the rear end. That is much better under extreme conditions. The electronic version keeps you from fishtailing or spinning but may not let you go anywhere. A mechanical LSD doesn't remove control of the car from the operator, it just gives you more traction. The electronic version only removes power. The mechanical version helps put power to the pavement.
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  #6  
Old 01-10-2014, 04:23 PM
gchin gchin is offline
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Would both of those also work in reverse gear to get out of a patch of icy road?
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  #7  
Old 01-15-2014, 10:52 AM
Texan Engineer Texan Engineer is offline
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Because computers will never beat a good mechanical (fill in the blank). Electronics can enhance, but never replace.

Physics is physics, you can only do so much by reducing input (braking or cutting power) before you get nothing. BMW M (and Porsche, and Ferrari, and...you get the idea) still use LSD's in everything. To borrow Porsche's tag line, there is no substitute.
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  #8  
Old 02-06-2014, 11:15 PM
Geekenstein Geekenstein is offline
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I have to disagree with some comments here. When DSC is off, BMW's simulated LSD or "e-diff" doesn't cut power. A mechanical LSD redirects torque the same way the simulated LSD does, by creating friction. Even a clutchless torsen diff does it with friction. The gears press against the housing. So I think the e-diff can work just as well as an LSD in theory.

However, in practice the e-diff has some problems. On wet pavement you can hear a "ratcheting" effect where the inside rear brake is being rapidly applied and released, so it appears to lack the smoothness of a mechanical LSD. The brakes overheating I find is the most compelling reason to favor a mechanical LSD. At the track overheating brakes is always an issue, so you want to minimize their use as much as possible.
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  #9  
Old 02-09-2014, 04:50 AM
mr_bean mr_bean is offline
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Quote:
Originally Posted by Geekenstein View Post
I have to disagree with some comments here. When DSC is off, BMW's simulated LSD or "e-diff" doesn't cut power. A mechanical LSD redirects torque the same way the simulated LSD does, by creating friction. Even a clutchless torsen diff does it with friction. The gears press against the housing. So I think the e-diff can work just as well as an LSD in theory.

However, in practice the e-diff has some problems. On wet pavement you can hear a "ratcheting" effect where the inside rear brake is being rapidly applied and released, so it appears to lack the smoothness of a mechanical LSD. The brakes overheating I find is the most compelling reason to favor a mechanical LSD. At the track overheating brakes is always an issue, so you want to minimize their use as much as possible.
You are correct in that the two diffs function in similar ways by creating a braking action on the wheel with less traction in order to direct more power to the other wheel. However, the way in which this braking action occurs on the e-diff inherently creates more frictional loss and robs power.

The e-diff brakes the slipping wheel. The LSD brakes the slipping wheel relative to the other wheel. Consider this example:
The car is going around a sharp turn at 30 mph. The inside wheel is spinning a little slower than the outside wheel. Let's say 28 mph and 32 mph, respectively.
The driver plants his/her right foot and the inside wheel begins to slip. In a car with an e-diff, the brake is being applied to keep the inside wheel spinning at 28 mph. Let's say for example that it requires 100 ft-lb of braking torque to slow the wheel down to this speed. The amount of power loss is 28 X 100 X some conversion factor.
Consider the same scenario in a car with an LSD. The diff "brakes" the output to the slipping wheel via friction against the non-slipping wheel. So the relative speed at which this braking action occurs is mush slower. The braking speed is 32 mph - 28 mph = 4 mph. The power loss in this case is 4 mph X 100 ft-lb X the same conversion factor.

That's a pretty big difference in power loss.
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  #10  
Old 02-09-2014, 10:12 AM
Geekenstein Geekenstein is offline
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That's not quite how it works, though. It doesn't brake hard enough to eliminate all the inside wheelspin, it just reduces it by some degree. And if you consider what happens when friction is generated, kinetic energy is converted to heat, kinetic energy is lost in either system. I don't see any reason a mechanical LSD should fundamentally have less power loss.
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  #11  
Old 02-10-2014, 04:03 AM
mr_bean mr_bean is offline
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My previous example is only meant to be illustrative. While the e-diff may not eliminate all wheel spin the basic principle is the same.
You are correct that kinetic energy is converted into heat. But the fact is that the e-diff converts more kinetic energy into heat compared to an LSD and thus less kinetic energy is used to accelerate the car.


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  #12  
Old 02-10-2014, 08:18 AM
Geekenstein Geekenstein is offline
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I don't see how you can know it consumes more kinetic energy. Depends on how much the system brakes.
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  #13  
Old 02-11-2014, 02:40 PM
mr_bean mr_bean is offline
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Quote:
Originally Posted by Geekenstein View Post
I don't see how you can know it consumes more kinetic energy. Depends on how much the system brakes.
My masters degree in mechanical engineering and a thorough knowledge of automobiles; that's how.

Ponder my example above.
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