Electronically Controlled Limited Slip Differential

[aesthetect]

Good, bad or ugly?

Originally Posted by wikipedia

Electronic limited slip differential system uses Anti-lock brake sensors and hardware to electronically monitor wheel speed. If one of the wheels on an axle is rotating faster, computer briefly applies brakes to it, slowing the spinning wheel down and causing the wheel on the opposite end of an open differential to start spinning and gain traction. This is an opposite to the anti-lock brake application, when a locked wheel is electronically released.

One advantage of this system over mechanical is that the vehicle steering and control is less affected. It also generates less stress on the drivetrain compared to a mechanical locking device, making it particularly suitable for the vehicles with independent suspension. It can also be tuned for specific applications off- and on road and a different speeds.

A disadvantage is that it is less predictable when going over an obstacle, as the system needs time to react. Also, the wheel with traction will only have half of the available torque applied to it.

Just for discussion's sake

Opinions? Does anybody have any experience with electronic (simulated) LSDs? What diff is going to be on the M3? To the best of my understanding i'm not impressed with the potential. An open diff will let one wheel slip for days and not even move the other one if the friction is right (or lack thereof) so in essence you have a system that will stop one wheel from spinning out of control but does nothing to aid in gaining additional traction.


It can't really be replacing this..

Variable M Differential Lock

Every car has a differential to compensate for the different distances covered by the inside and outside wheel when cornering. Performance-oriented cars have used limited slip differentials for many years. The limited slip differential limits loss of traction by locking the planet gears when one of the two drive wheels threatens to slip, for example, on a wet patch of road. But in more extreme driving conditions, the benefits of the differential lock are limited.

The Variable M Differential Lock recognizes the differential speed building between the driven wheels and generates pressure in an integrated shear pump. This pressure activates a multiple plate clutch via a piston, and conveys drive forces to the wheel with better grip, according to the difference in wheel rotation speed. In extreme cases, the entire drive forces may be transmitted to the wheel with a better frictional coefficient.

Once the difference in wheel rotation speed reduces, pump pressure is reduced accordingly and the locking action decreases. The pump system is maintenance free.

[iDiaz]

I wouldn't call the traction control system a "limited slip differential", exactly. A limited slip differential generally transfers power across to the other wheel with very little loss of energy, whereas applying the brakes converts that motive energy to heat. Sure, some heat is generated in a real limited slip, but not nearly the amount of heat generated in a traction control system.

DSC includes the capabilities you're describing, and I've used it on the track. It works pretty well if you already know how to modulate throttle for minimal wheelspin, but if you go too far and it can't stop wheel spin with just a brake application, it will cut back power from the engine. If anything, it serves more to preserve your tires than to make you go much faster.

[atr_hugo]

My concern would be how progressive (or rather, not 'jerky') they make the application of the brakes on the 'free' wheel.

[aesthetect]

diaz, i am not referring to the traction control system, although it does have some of the same functionality now that you mention it. but because the 135i will not have a true mechanical lsd (which we would all like, and would do as you say, transfer power away from the slipping wheel as opposed to simply wasting torque production) they have mentioned incorporating an electronically controlled limited slip differential (being a speed sensitive lsd, like viscous fluid unts)

[FinInk]

it's not going to replace the real deal. Just like the 335i it seems we'll have to go after market to get a good LSD.

[iDiaz]

Originally Posted by aesthetect

diaz, i am not referring to the traction control system, although it does have some of the same functionality now that you mention it. but because the 135i will not have a true mechanical lsd (which we would all like, and would do as you say, transfer power away from the slipping wheel as opposed to simply wasting torque production) they have mentioned incorporating an electronically controlled limited slip differential (being a speed sensitive lsd, like viscous fluid unts)

It IS the traction control system. It's a function of DSC, and it's been incorporated into BMWs for years. They used the same "electronic limited slip" marketing jibber-jabber when they launched the MINI years ago.

[Mahlzeit]

The problem with the electronically controlled limited slip differential system is that it relies on traction control. So, typically if someone wants to have some fun w/out having power cut by the traction nannies, the first thing a driver needs to due is turn it off. It remains to be seen if the turning off the traction control (DSC) also turns off the electronically controlled limited slip differential. If it does, then you're back to a 100% open diff.

With any luck the 135 will let you disengage the traction control, while still getting some benefit from the electronically controlled diff.

I suspect however, that nothing beats a mechanical LSD and 135 buyers will have to add an aftermarket LSD to their tuning lists. Funny thing is that BMW started welding something on the diff housing (not quite sure how this works) to make it difficult to simply swap the diff guts. Which increases the cost of the swap, and requires core charges... wonder why they did that!?!

-Mahlzeit

[kurichan]

Originally Posted by iDiaz

It IS the traction control system. It's a function of DSC, and it's been incorporated into BMWs for years. They used the same "electronic limited slip" marketing jibber-jabber when they launched the MINI years ago.

You're just flat out wrong on this one. Rewind a bit and check it out - it is different. DSC retards engine output. ELSD brakes the spinning wheel in an open diff which causes the opposite wheel to receive torque.

My ZHP had DSC. When it kicked in, the engine was retarded and the car slowed. No torque was transferred. It sucked. The 135 ELSD is different and works even when DSC is turned off.

[kurichan]

Originally Posted by Mahlzeit

The problem with the electronically controlled limited slip differential system is that it relies on traction control. So, typically if someone wants to have some fun w/out having power cut by the traction nannies, the first thing a driver needs to due is turn it off. It remains to be seen if the turning off the traction control (DSC) also turns off the electronically controlled limited slip differential. If it does, then you're back to a 100% open diff.

With any luck the 135 will let you disengage the traction control, while still getting some benefit from the electronically controlled diff.

Already stated as such in official published BMW materials.

[gotgemini2000]

What would happen if one attempted to do a burnout in a E-LSDed 135i with the traction control turned off but the E-LSD still turned on? It seems to me that such treatment of the E-LSD could result in very very expensive repair bills very quickly.

[grant]

Originally Posted by gotgemini2000

What would happen if one attempted to do a burnout in a E-LSDed 135i with the traction control turned off but the E-LSD still turned on? It seems to me that such treatment of the E-LSD could result in very very expensive repair bills very quickly.

No, just disatisfaction and wear on the brakes. A real LSD is a must with one of these cars...

[iDiaz]

Originally Posted by kurichan

You're just flat out wrong on this one. Rewind a bit and check it out - it is different. DSC retards engine output. ELSD brakes the spinning wheel in an open diff which causes the opposite wheel to receive torque.

My ZHP had DSC. When it kicked in, the engine was retarded and the car slowed. No torque was transferred. It sucked. The 135 ELSD is different and works even when DSC is turned off.

Err, I beg to differ:

Originally Posted by BMW World

The ASC + T system will reduce engine output until the vehicle can move or or until acceleration can occur without the wheels spinning. If this reduction is insufficient, each drive wheel is braked individually until both wheels have optimum traction.

This makes it possible to drive on snow, wet roads or other slippery grounds without the wheels spinning. Even if the back wheels suddenly lose traction in a bend during acceleration, ASC + T intervenes so quickly that the vehicle is stabilized before it can oversteer and swing out of the bend. The instrument console informs the driver of any control intervention and the relevant situation.

The DSC computer constantly calculates an ideal driving condition that is compared with the current status. If the vehicle oversteers or understeers, the calculated ideal deviates from the measured status, and DSC will intervene within a few milliseconds via the engine management system by reducing the engine torque and applying the brakes on individual wheels where necessary.

http://www.bmwworld.com/technology/dsc.htm

Originally Posted by Yahoo Autos

BMW’s standard DSC system can reduce power and even brake individual wheels to ensure the car benefits from all available grip.

http://motorshow.yahoo.net/a_637_IT_13683_6.html

Originally Posted by BMW World

The normal traction-control function not only applies the brakes of individual wheels as they start to slip, but also reduces engine power.

http://www.bmwworld.com/models/z4/dsc.htm

[iDiaz]

Originally Posted by BMW 135i Press Release

BMW’s Dynamic Stability Control (DSC) also includes a Dynamic Traction Control (DTC) function that provides electronic intervention to prevent loss of vehicle control, but at a higher threshold before activation. This allows the driver to experience more spirited driving on dry roads and offers more flexibility when driving in more challenging conditions such as in the snow. If the driver desires, both DSC and DTC can be disabled entirely.

DTC, which is part of the DSC system, is what generates the braking force and/or bogs the engine. It's a pretty poor substitute for a true limited-slip, due to the reasons I cited above.

[mikeo]

The ELSD cannot be disabled, AFAIK, and still works when all the other electronic drive control doodads are turned off. I'm actually beginning to warm up to this technology, but I'll just have to wait and see.

[kurichan]

Originally Posted by iDiaz

DTC, which is part of the DSC system, is what generates the braking force and/or bogs the engine. It's a pretty poor substitute for a true limited-slip, due to the reasons I cited above.

What you're totally missing iDiaz, is the fact that on the 135i, the DSC system can be completely disabled but the ELSD will persist.

In other words, there will be no power retardation with this system if you turn off the DSC but the ELSD will assist.

This is different than the systems to date and the configurations you cite above.

The result will be a semi effective ELSD despite the open diff configuration that will send up to HALF of the available (and UNRETARDED!) torque to non spinning wheel.

Sure, it's not a full mechanical LSD, but it's certainly vastly better than nothing!

I am in La Costa - you want to sit down, have coffee and discuss it?

[iDiaz]

Originally Posted by kurichan

What you're totally missing iDiaz, is the fact that on the 135i, the DSC system can be completely defeated but the ELSD will persist.

In other words, there will be no power retardation with this system if you turn off the DSC but the ELSD will assist.

This is different than the systems to date and the configurations you cite above.

The result will be a semi effective ELSD despite the open diff configuration that will send up to HALF of the available torque to non spinning wheel.

Sure, it's not a full mechanical LSD, but it's certainly vastly better than nothing!

Hm, okay... I suppose I got confused in the application BMW uses for the MINI. On the MINI, there are two types of traction control available. One is DSC, the other one is ASC+T. DSC includes the functionality of ASC+T, but also adds yaw and steering position sensors to control the dynamics of the car, not just traction at the drive wheels. ASC+T does exactly what is described as the E-LSD in the 135i. I tried to find the pages where I read about it before buying my MINI, but they're long gone, having been replaced by R56-generation MINI information. It would seem that the BMW models haven't yet included this variation on DSC/ASC+T, then.

[kurichan]

Originally Posted by iDiaz

Hm, okay... Pardon my ignorance, but I have a question for you:

If I'm not mistaken, some of the sportier BMW models have independent control over DSC (Dynamic Stability Control) and DTC (Dynamic Traction Control), where DSC can be disabled independently from DTC, or both can be disabled altogether. Is this correct?

I've never heard of that but I am not an authority.

I'm quite certain the ZHP, a very sporty BMW model had three assist modes -

(1) power retard ON/brake retard ON, ABS ON (DSC)
(2) power retard OFF/brake retard OFF, ABS ON (DTC)
(3) all assist off

[edit]

I checked. It appears the DTC mode (#2) simply raises the limit for retardation to allow some slip, for example, in snow conditions.

[iDiaz]

Originally Posted by kurichan

I've never heard of that but I am not an authority.

I'm quite certain the ZHP, a very sporty BMW model had three assist modes -

(1) power retard ON/brake retard ON, ABS ON (DSC)
(2) power retard OFF/brake retard OFF, ABS ON (DTC)
(3) all assist off

[edit]

I checked. It appears the DTC mode (#2) simply raises the limit for retardation to allow some slip, for example, in snow conditions.

Yeah, I changed my post to reflect having read that about DTC a moment after posting that. I'm pretty sure ASC+T still eases back on engine power. Maybe the 135i traction control won't be as intrusive as previous BMW iterations, but regardless, I would still call this "traction control" and not "E-LSD", which is a bit misleading.

[kurichan]

iDiaz, sorry to come back at you again, but "Traction Control" is a very generic word that simply means any management of wheel spin via power retardation or braking that serves to return traction. DSC is traction control.

BMW's ELSD appears to be specifically designed to function much like an LSD by specifically causing torque to be sent to a wheel that isn't turning enough. I'll let it rest here, but I think the term is quite appropriate.

Probably best to wait and see it work in practice. It might be great, and it might be a total crock. :iono:

Experience is so much better than conjecture!

[iDiaz]

Originally Posted by kurichan

iDiaz, sorry to come back at you again

All good, mang. :smile:

Originally Posted by kurichan

but "Traction Control" is a very generic word that simply means any management of wheel spin via power retardation or braking that serves to return traction. DSC is traction control.

DSC is stability control, not just traction control. Maybe we've got a syntax problem, because I consider "traction control" to be a way reduce wheelspin off-the-line, or in a situation where it's more an overabundance of power (where the ELSD would come in) than it is of cornering behavior. That's where "stability control" comes in. DSC will brake specific tires when you begin to oversteer or understeer to attempt to correct yaw rates, not just traction under power.

Originally Posted by kurichan

BMW's ELSD appears to be specifically designed to function much like an LSD by specifically causing torque to be sent to a wheel that isn't turning enough. I'll let it rest here, but I think the term is quite appropriate.

But I contend that it's really not anything new in the BMW line, and dumping power through a brake rotor as heat is not what I would define as an LSD. An LSD is capable of transferring substantially more power to the outside wheel without losing much in efficiency at all. The main benefit you'll get from something like the ELSD is you might save your tires a bit from the reduction in wheelspin if you fail to modulate the throttle accurately.

I've noticed my MINI controlling traction under power without throttling back the engine at Buttonwillow. I modulated the throttle enough for a hair of wheelspin on exit coming over the hill after the Cotton Corners, when I forgot to turn DSC off and noticed something different and the DSC light flashing out of the corner of my eye. It just braked the wheel to stop it from spinning the tire, converting that kinetic energy into brake heat instead of using it to propel me forward, something a real LSD would have done by actually biasing torque effectively.

Originally Posted by kurichan

Probably best to wait and see it work in practice. It might be great, and it might be a total crock. :iono:

Agreed, but either way, I'll be putting money into a true limited slip, instead of blowing it on factory options like bigger OEM wheels (racing wheels), fog lights (135i drops these for brake cooling ducts, doesn't it?), or iDrive. I've driven cars with and without limited slips, both front and rear-wheel-drive, and cars with traditional limited-slips don't waste power like a brake-based system does.

Originally Posted by kurichan

Experience is so much better than conjecture!

Agreed.

[coolguy]

I am not sure if this helps:

From Edmunds Insideline

http://www.edmunds.com/insideline/do...hotopanel..1.*

We would prefer a proper mechanical limited-slip differential to the SRT-4's traction-control-based arrangement. With the so-called Brake Lock Differential, a wheel speed sensor detects wheelspin and applies the brake to that side to force the transfer of power to the other side. As a rule, we think that applying the brakes to make forward progress is the wrong approach.

[Powerman]

Originally Posted by kurichan

iDiaz, sorry to come back at you again, but "Traction Control" is a very generic word that simply means any management of wheel spin via power retardation or braking that serves to return traction. DSC is traction control.

BMW's ELSD appears to be specifically designed to function much like an LSD by specifically causing torque to be sent to a wheel that isn't turning enough. I'll let it rest here, but I think the term is quite appropriate.

Probably best to wait and see it work in practice. It might be great, and it might be a total crock. :iono:

Experience is so much better than conjecture!

I would prefer they call it something else besides ELSD. Brake Traction Assist or something.

To make things completely confusing, and I'm suprised we have not landed here yet, is BMW's new, electrically actuated, mechanical limited slip they are introducing in their x-drive cars. It is an actual mechanical LSD that uses electrically actuated clutch packs in the LSD to work.

Every instance I have seen of the above discussion of E-LSD, eventually gets confused with the ELD (electronic locking differential) that I'm talking about. I have no doubt that the brake management scheme will work better than an open differential for 70% of drivers in the majority of normal driving situations. I have a hard time beleiving it will perform any where near a true LSD in sport driving applications. We shall see.

I believe Land Rover and other 4X4 applications have used this braking scheme as well. It works, but does not replace LSDs, or locking differentials by any means. It is just better for the "suburban wheelers" than open diffs. I assume BMW is trying to do the same for the "suburban racers" as well.