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Pseudo limited slip differentials

Zap

Zap

Elite Member
I read an article about the next BMW 3 series having an open differential that used a combination of stability control plus rear brakes to make it work similar to a limited slip differential.

Why don't they actually use a limited slip differential? Is there that much of an efficiency or cost advantage with using an open differential?
 
So now when we are talking about LSDs, we are going to have to specify if it is hardware or emulated in software? Great, as if we didn't have enough problems with the spider gear welding crowd...
 
Correct me if I'm wrong, but the most expensive part of a drive train is the differential, assuming it's limited slip, right?

What I don't get is that BMW is supposed to be a premium brand. Why cut this big of a corner?
 
slugg said:
Correct me if I'm wrong, but the most expensive part of a drive train is the differential, assuming it's limited slip, right?

What I don't get is that BMW is supposed to be a premium brand. Why cut this big of a corner?
Click to expand...

One more electronic component to fail post warranty = big $$ for stealership service center :biggrin:
 
Their m cars still have LSD.

I believe their logic is that in a non LSD setup when one wheel loses traction the other wheel still has traction but loses all power to the wheel that is slipping.

Therefore if the traction control system can accurately apply some rear brake to that one wheel which is slipping, the open diff will automatically transfer power to the wheel that has traction.


IMO this seems like an effective method to handle loss of power assuming the driver doesn't care about turning off TC for performance driving.

I also think it's partially about cost, but also about separating their M cars from the non M. With the 535, 550, 335, 135 the current M cars are a bit diluted compared to a few years ago.
 
The Lotus Elise also didn't come with an LSD up until they had a Toyota engine. Now they have it as an option due to the growing autocross popularity of the cars.

Lotus states that it takes away the balanced nature of the car at speed.

http://www.roadandtrack.com/tests/drives/2008-lotus-elise-sc

Another review even claims the Lotus Elise in particular is at a disadvantage if equipped with an LSD on a racetrack.

http://www.sandsmuseum.com/cars/elise/information/misc/nick/

That said, I am sure cost is 99% of the reason non-M BMWs don't have them, as they aren't nearly as balanced or lightweight as the Lotus for it to have a negative effect on performance.
 
SpatiallyAware said:
IMO this seems like an effective method to handle loss of power assuming the driver doesn't care about turning off TC for performance driving.
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TC is not stability control. They are two separate beasts.

traction control is throttle control, stability control is typically individual wheel braking (along with potentially some other things).

Not sure why you are trying to tie them together here.

Most performance cars let you shut off TC entirely, but rarely let you shut stability control off entirely. They usually let you make it less aggressive, but not turn it completely off.
 
Honestly this doesn't seem any different than most other companies. TC / SC has worked this way for years already. I don't agree with Ferzerps "line" however. I have seen plenty of cars call "using the brakes to control tire spin" traction control. I consider stability control using the brakes to control things like yaw and sliding. Also the speed6 lets you fully turn it off. Nothing more fun than doing 4 wheel burnouts in the parking lot.
 
Sorry, yes, tc can be a combination of power and brakes. Thanks for the correction. They are still separate systems as far as what they do though, as you've pointed out.
 
personally i'm all for it, if they can make the software work as good as a hardware lsd, why not? one less part to break.

open diff is cheaper, more durable, and lighter, stability control systems have been doing this for years, this just goes one step further.
 
MaxFusion16 said:
personally i'm all for it, if they can make the software work as good as a hardware lsd, why not? one less part to break.

open diff is cheaper, more durable, and lighter, stability control systems have been doing this for years, this just goes one step further.
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It also trades a non-wearing part (diff) for a wear part (brake pads). That's not so hot.
 
Ferzerp said:
It also trades a non-wearing part (diff) for a wear part (brake pads). That's not so hot.
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brake pads are consumables.........cheap consumables.

and LSDs do wear out, the bearings inside wear out over time, they are being used everytime you drive the car, whereas in this system, the virtual LSD will only activate the brakes when you loose traction, which is not very often.
 
MaxFusion16 said:
personally i'm all for it, if they can make the software work as good as a hardware lsd, why not? one less part to break.

open diff is cheaper, more durable, and lighter, stability control systems have been doing this for years, this just goes one step further.
Click to expand...

It will never work as good as a mechanical since it's completely reactionary. Secondly, a LSD is shifting power, while the software approach is limiting power. However, if this is just on non-M cars, I don't think the general populous cares.
 
KentState said:
It will never work as good as a mechanical since it's completely reactionary. Secondly, a LSD is shifting power, while the software approach is limiting power. However, if this is just on non-M cars, I don't think the general populous cares.
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I beg to differ, I believe this will work as good or even better than a mechanical LSD.

power follows the path of least resistance

so in an open diff, when one wheel looses traction, that wheel spins freely, all the power goes to that one wheel, even if the other wheel is on solid ground, it doesn't spin.

in a Limited Slip Diff, the diff allows a small degree of speed differential between the 2 wheels to account for turning; the diff locks up once that limit is exceeded, thus force both wheels to turn at the same rate.

with brake assisted/virtual diff, traction control applies braking to the wheel that's slipped, thus forcing all of the power to the other wheel.

with an electronic system, you are able to dynamically control the degree of slip allowed and the amount of power transferred.

This is actually F1 tech, the new McLaren MP4 doesn't use an LSD.

If they can get the software calibrated, this will be better and cheaper than mechanical systems.
 
Ferzerp said:
TC is not stability control. They are two separate beasts.

traction control is throttle control, stability control is typically individual wheel braking (along with potentially some other things).

Not sure why you are trying to tie them together here.

Most performance cars let you shut off TC entirely, but rarely let you shut stability control off entirely. They usually let you make it less aggressive, but not turn it completely off.
Click to expand...


You're trying to argue semantics, and I hesitate to get into it with you.. But in this instance, using stability control to modulate the brake on wheel is essentially the same thing as traction control since TC isn't exactly a defined term of having LSD or throttle control.. Each man'f defines it differently, so we could go on and on.. But you know exactly what I'm talking about and so does everyone else.
 
MaxFusion16 said:
I beg to differ, I believe this will work as good or even better than a mechanical LSD.

power follows the path of least resistance

so in an open diff, when one wheel looses traction, that wheel spins freely, all the power goes to that one wheel, even if the other wheel is on solid ground, it doesn't spin.

in a Limited Slip Diff, the diff allows a small degree of speed differential between the 2 wheels to account for turning; the diff locks up once that limit is exceeded, thus force both wheels to turn at the same rate.

with brake assisted/virtual diff, traction control applies braking to the wheel that's slipped, thus forcing all of the power to the other wheel.

with an electronic system, you are able to dynamically control the degree of slip allowed and the amount of power transferred.

This is actually F1 tech, the new McLaren MP4 doesn't use an LSD.

If they can get the software calibrated, this will be better and cheaper than mechanical systems.
Click to expand...

You might be right. I've only heard the down side of the argument so I'm not sure how well it works in real life.
 
Zap said:
I read an article about the next BMW 3 series having an open differential that used a combination of stability control plus rear brakes to make it work similar to a limited slip differential.

Why don't they actually use a limited slip differential? Is there that much of an efficiency or cost advantage with using an open differential?
Click to expand...

AFAIK, BMW has regularly not included LSD's on their base models and included them as something special on their M series of cars.
 
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