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      08-01-2014, 03:01 PM   #1
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Lightbulb F80 M3 / F82 M4 Active M Differential: Deep Dive and Interview

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Active M Differential: Deep Dive and Interview
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The following comes courtesy of M-Power.com.

Also see:
BMW M3 and BMW M4 Inside (part 1): The new M Servotronic.
BMW M3 and BMW M4 Inside (part 2): The Brake Systems.
BMW M3 and BMW M4 Inside (part 3): The new engine.
BMW M3 and BMW M4 Inside (part 4): Sound and Exhuast System.
BMW M3 and BMW M4 Inside (part 5): smartly light, rigid, precise.
BMW M3 and BMW M4 Inside (part 6): The new chassis.
BMW M3 and BMW M4 Inside (part 7): The Active M differential.


BMW M3 and BMW M4 Inside (part 7): The Active M differential.
Interview with M engineer Bernd Jacob.

August 1, 2014

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Mr Jacob, you are responsible for the locking systems employed in M automobiles. We would like to talk to you today about the Active M differential in the BMW M3 and BMW M4. We wish to begin with the following question: why does a car actually need a differential?

Bernd Jacob: A car needs a differential to compensate for rotational speed differences in the wheels, for instance when driving through a bend. Since the vehicle has a certain width, the wheels on the outside of a curve are required to travel a greater distance than the wheels on the inside of a curve. This means that the outer wheels must rotate faster than the inner ones. To achieve this, a differential is fitted between the wheels of the drive axle – in the case of the BMW M3 and M4, this is the rear axle.


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>>> Run Flash animation: CLICK. Shows how a differential works.


The engine torque is transferred to the rear axle gearing via the gearbox and the propeller shaft. The differential incorporated in the rear axle gearing distributes the torque equally to the two rear wheels and balances out differences in rotational speed.

Although this is not necessarily always an advantage?

That is correct. This equal distribution does harbour one handicap. If you accelerate out of a bend adopting a more sport-oriented driving style, the load on the wheel on the inside of the bend is reduced and in turn it can only transfer a reduced torque to the road surface. The outer wheel would be able to transfer considerably more torque to the road due to the higher wheel load, but the normal, open differential only transfers the torque level of the inner wheel to it.

The effect is the same when the two wheels are rotating over surfaces with different frictional coefficients – an extreme example would be when the car has one wheel on ice and the other on high-grip tarmac.

So is this where the locking differential comes into play?

Yes. If the differential is fitted with a locking mechanism, the counterproductive compensation in rotational speed that would occur in the situation just described is prevented. This can occur to differing degrees with a controlled differential lock. In extreme cases, both wheels will be rotating at the same speed, in which case you would speak of a 100% locking effect.

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Active M differential.


So what you are saying is that with a locking differential it is possible to transfer a higher torque to the road surface. Would the driver be aware of the effect that the differential lock has? Does the car feel any different?

It certainly does. For one thing, in the cases just described, the overall transferred torque increases considerably. The driver is able to drive off or accelerate out of a bend at a much higher speed than would be possible without a differential lock.

The variations in torque between the right and left hand sides result in a yaw moment. If the right rear wheel of the vehicle is subjected to a higher driving force than the left, the vehicle will tend to follow a left-hand bend. The result of this is that the vehicle is more compliant when following the course of a bend. If the driver increases the acceleration even more, after a certain point, there will be an increase in slip when the control systems are deactivated, also referred to as power oversteering. The transition into this range is considerably affected by the triggering of the lock.

In the development and coordination of the Active M (locking) differential, we have attached great value to ensuring that the locking effect is soft, easy to sense, and palpable when it engages. This gives the driver meaningful feedback regarding the traction potential of the rear axle, enabling him to measure out his acceleration at an optimum level.


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Active M differential on the rear axle of a BMW M3 and BMW M4.


But does not the use of a differential lock increase the weight of the vehicle?

Yes, it weighs around 10 kg more than a conventional, open differential – but it is placed at the right position, at the rear and the bottom, which means it has a positive effect on the centre of gravity while optimising the vehicle's traction.

M automobiles have been fitted with locking differentials for several generations but the BMW M3 and BMW M4 are equipped with an Active M differential. What is the 'active' part of the differential?

Active means that a whole range of sensors have been built into the car that are able to identify the road conditions, calculate the optimum locking degree, and activate the lock accordingly by means of an electric motor. The central input variables are the torque, the individual rotational speeds of the wheels, the lateral acceleration and the driving speed. Then there is the steering angle and the yaw rate, plus a few additional factors.


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Active M differential with displacement motor (right) in cross-section.


What do you mean by 'optimum'?

Optimum means that the driver benefits from the best combination of vehicle controllability and traction. Whether a professional driver or a relatively inexperienced amateur, the aim is to enable all drivers to drive as fast as possible along a racetrack while maintaining good control of the vehicle, of course always subject to their individual skill and ability. Moreover, the demands placed on a locking mechanism like this under adverse conditions such as ice and snow are quite different to those on a dry racetrack.

So passive systems are less effective at this?

Passive systems primarily react to one input variable. Depending on their construction, this is either the differential speed of the rear wheels or the engine torque. However, the Active M Differential calculates the locking torque on the basis of considerably more input variables. It is therefore possible to react far more specifically to a certain road situation. In addition, an active system is not only able to react but can itself also act. The Active M differential takes into account so-called pre-steering components, which means that it not only responds to an event but actually sets the locking torque even before an event occurs. This results in enhanced transparency in terms of vehicle behaviour and also in better controllability.

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Easy to control even in drift.


So to a large degree the art consists of attaining the right tuning?

Yes, that is right.

Tuning of the Active M differential is performed hand in hand with that of the chassis and the control systems, such as the DSC. All in all, it is a repetitive process. The responsible engineers conduct their testing activities jointly. The interplay of the individual systems is optimised as they are developed. Our tests are conducted on a variety of tracks under widely differing conditions, such as in Northern Sweden in winter and on the Nürburgring in summer.

In what situations does the differential lock have the greatest effect on vehicle dynamics?

The differential can be felt most clearly when driving in fast or narrow bends. For example, if the car is accelerating out of a hairpin in a pass, you will notice it very clearly.

With the Active M differential, drivers of the new BMW M3 or BMW M4 can not only drive their vehicles faster but they can do so with considerably more driving pleasure behind the wheel.

Mr Jacob, thank you for talking to us.

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      08-06-2014, 09:27 AM   #2
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Meh, I like this series, but this was more like a primer on how locking differentials work for newbies. Was hoping for something like the other articles, with more technical data. More info on the programming.

Oh well, thanks for the read anyway. Appreciate the effort!
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      08-06-2014, 09:44 AM   #3
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Yeah dude... whata "cute" write-up.

Talk about axial needle bearings and the ball bearing with a torsional pressure plates. The electric motor that applies torque to this plate and creates essentially an electronically induced pressure on the clutches.

Don't turn this site into BMW marketing brochure... please...
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      08-06-2014, 10:22 AM   #4
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It's funny how people are excited. BMW used to mount LSD's do most of their cars or it was an option.

Well it's nice to see that's back. That whole e-diff stuff is a joke of course.

Still, motor driven an such. Was nothing bad with the good old clutches (only) LSD.
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      08-06-2014, 10:45 AM   #5
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Pretty neat technology! Thanks for sharing the article
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      08-06-2014, 10:59 AM   #6
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Quote:
Originally Posted by MParallel View Post
It's funny how people are excited. BMW used to mount LSD's do most of their cars or it was an option.

Well it's nice to see that's back. That whole e-diff stuff is a joke of course.

Still, motor driven an such. Was nothing bad with the good old clutches (only) LSD.
All RWD, all with LSD in an ideal world! Unfortunately the modern market environment does not allow for this, it seems.

In any case, the active differential is an impressive piece of technology. I think that as described in the article, the tuning is everything- seamlessly integrating with driver inputs is key, and then predicting what the driver might want, to boot. Should we have lock up on deceleration, or only on acceleration? Should we have an extreme pre-load where a great deal of torque must be applied for lock, or should we have the differential respond to relatively little force?

These questions are ones where "driver preference" and "maximum possible traction" may actually be conflicting considerations. This is where the talent of M engineers must truly lie!
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      08-06-2014, 11:20 AM   #7
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I will say this, my car is very very stable coming out of hairpins even if I put down a lot of power.


I can't believe they test the cars in Northern Norway in Winter. Would have been nice for this engineer to talk about how the Active differential helps in Winter conditions since I often hear people say that having an LSD is just as good as having AWD
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      08-06-2014, 11:27 AM   #8
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First thanks for sharing more info.

Edit: The M3/M4 Technical Manual also has the differential information:
http://f80.bimmerpost.com/forums/att...5&d=1406475520

For those of you that wanted more depth to the operation and construction of the differential which is like that used in the current M5 here is a link to the full M5 technical PDF.
The differential section starts on page 42.
http://f10.m5post.com/forums/attachm...2&d=1360773149

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      08-06-2014, 01:30 PM   #9
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Quote:
Originally Posted by noro View Post
Yeah dude... whata "cute" write-up.

Talk about axial needle bearings and the ball bearing with a torsional pressure plates. The electric motor that applies torque to this plate and creates essentially an electronically induced pressure on the clutches.

Don't turn this site into BMW marketing brochure... please...
You do know they just copy and paste these interviews from BMW marketing, right?
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      08-06-2014, 02:57 PM   #10
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Quote:
Originally Posted by Powaup View Post
I will say this, my car is very very stable coming out of hairpins even if I put down a lot of power.


I can't believe they test the cars in Northern Norway in Winter. Would have been nice for this engineer to talk about how the Active differential helps in Winter conditions since I often hear people say that having an LSD is just as good as having AWD
Good point about winter driving. As i understood if one tire was to have more grip than the other it would fully lock in a "stuck in snow" or slipping around a corner situation preventing tire spinout with the torque control. Will have to test this of course
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      08-06-2014, 06:10 PM   #11
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Quote:
Originally Posted by Powaup View Post
I can't believe they test the cars in Northern Norway in Winter. Would have been nice for this engineer to talk about how the Active differential helps in Winter conditions since I often hear people say that having an LSD is just as good as having AWD
Quote:
Originally Posted by Suave View Post
Good point about winter driving. As i understood if one tire was to have more grip than the other it would fully lock in a "stuck in snow" or slipping around a corner situation preventing tire spinout with the torque control. Will have to test this of course
I'm pretty interested in this too and would like to hear more if anyone would care to comment. I can definitely see how the AMD would help, but as much as AWD????
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      08-06-2014, 06:17 PM   #12
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Sweet!!! Glad I got that option....
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      08-07-2014, 12:39 AM   #13
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Quote:
Originally Posted by M4TW View Post
I'm pretty interested in this too and would like to hear more if anyone would care to comment. I can definitely see how the AMD would help, but as much as AWD????
I doubt it would be as good as AWD with winter tires, but who knows
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      08-07-2014, 07:44 AM   #14
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Sweet!!! Glad I got that option....
Um, it's standard....
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      08-07-2014, 08:17 AM   #15
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Quote:
Originally Posted by Needsdecaf
Quote:
Originally Posted by tooch View Post
Sweet!!! Glad I got that option....
Um, it's standard....
Are you sure?? Hahaha
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      08-07-2014, 08:37 AM   #16
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just wow.
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      08-07-2014, 06:09 PM   #17
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Quote:
Originally Posted by M4TW View Post
I'm pretty interested in this too and would like to hear more if anyone would care to comment. I can definitely see how the AMD would help, but as much as AWD????
That seems impossible.

I mean, it probably has some effects that are as good as AWD, but as any prairie boy will tell you nothing can beat having traction on 4 corners of the vehicle (with weight on each one).
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      08-08-2014, 10:05 AM   #18
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Come someone explain to me how this translates into what we're hearing from owners in terms of a "noisy diff"? Noise in a gearbox usually means either gears are being slammed together or they aren't joining smoothly. Not exactly desirable. Why is it being said it is dependant upon how the driver drives? It's electronic, isn't it - isn't it dependent upon tuning?

Have we seen this in the M5/M6 as well in terms of noise/feel from the diff?

Just to be clear, I didn't notice this at all in my ~5 mile test drive of the car.

Lastly, regarding it's performance in snow/winter, I can share that it will simply perform like any other RWD LSD car.
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      08-08-2014, 10:18 AM   #19
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Quote:
Originally Posted by M4TW View Post
I'm pretty interested in this too and would like to hear more if anyone would care to comment. I can definitely see how the AMD would help, but as much as AWD????

What many mean when they say that an LSD equipped RWD car is just as good as AWD is that, in a RWD car one is able to adjust the attitude and direction of the vehicle easily in inclement weather.

A skilled driver is easily able to control the exact direction and yaw of a RWD car in any weather condition, and this makes RWD preferred to many enthusiasts.

AWD is excellent from a traction and straight line acceleration standpoint, but does not ultimately yield more mechanical grip in a cornering condition- although it does allow you to put a bit more power down while cornering, more easily.

I personally will take the RWD car on any day, in any condition. I prefer being in complete control of my car's behavior at all times.
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      08-08-2014, 11:29 AM   #20
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AWD allows for superior control in a turn than RWD does. With rWd you can correct understeer with throttle or change the point of the nose travel through controlled rear slippage. But you can't control oversteer with RWD nor can you accelerate during a rear end slip.

With AWD you can do those things. You can use the gas to straighten out an oversteering car. You can induce oversteer if desired. Etc.

An LSD is an improvement over an open diff but it doesn't create throttle control at two ends of the car like AWD does.
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      08-09-2014, 07:45 AM   #21
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Quote:
Originally Posted by Ilia@IND View Post
What many mean when they say that an LSD equipped RWD car is just as good as AWD is that, in a RWD car one is able to adjust the attitude and direction of the vehicle easily in inclement weather.

A skilled driver is easily able to control the exact direction and yaw of a RWD car in any weather condition, and this makes RWD preferred to many enthusiasts.

AWD is excellent from a traction and straight line acceleration standpoint, but does not ultimately yield more mechanical grip in a cornering condition- although it does allow you to put a bit more power down while cornering, more easily.

I personally will take the RWD car on any day, in any condition. I prefer being in complete control of my car's behavior at all times.
Quote:
Originally Posted by JoeFromPA View Post
AWD allows for superior control in a turn than RWD does. With rWd you can correct understeer with throttle or change the point of the nose travel through controlled rear slippage. But you can't control oversteer with RWD nor can you accelerate during a rear end slip.

With AWD you can do those things. You can use the gas to straighten out an oversteering car. You can induce oversteer if desired. Etc.

An LSD is an improvement over an open diff but it doesn't create throttle control at two ends of the car like AWD does.
In general I agree with both of what you have said. RWD will be more "fun" in the snow because you have more flexibility to rotate (i.e. induce yaw) the car since you only have one axle (the rear) putting down the power. AWD generally allows you to accelerate more confidently out of corners and would be less "fun" since power is going to both axles and you're less likely to unsettle the car when applying throttle.

I would clarify what Joe said about not being able to control oversteer in RWD; you can - just reduce your throttle

I would interpret the "AWD vs. RWD in the snow" comment differently than Ilia. I think most people are concerned with traction and not getting stuck rather than evaluating handling limits. Although I will concede finding those limits on an empty snow-covered road or parking lot is a lot of fun

AWD vs. RWD vs. LSD (limited slip diff) depends on your situation. Pure RWD or FWD with open differentials will put down the power fine in dry conditions but begin to suffer when it's wet, snowy, or icy. As mentioned in the article, an open differential will allow the wheel with less grip to spin and throw away much of your ability to accelerate. This is where an AWD system or LSD helps.

AWD will distribute power between axles (front and rear) whereas an LSD distributes power across one axle (left and right). If your left rear wheel is on ice, for example, and AWD system will transmit power to the front axle. Most AWD vehicles have an open front differential, so with your left rear wheel spinning, power would be going to the right rear and probably your left front wheel assuming both of those have traction. With AWD and open differentials front and rear, you would essentially have 2WD (two wheels with tractive effort). Without AWD (i.e. RWD with open differential) you would have 1WD (in this case the right rear wheel). And with this 1WD open differential scenario, the power the right rear wheel can put down is only as much as the left rear can put down on its very slippery icy - not much.

An LSD helps in a similar way but across the axle. In the same scenario, an LSD would allow torque transfer to both the left and right rear wheels, allowing you to accelerate if you have grip on the right rear wheel. Again, you have true 2WD with an LSD, or 1WD with standard RWD and one spinning wheel.

As you might expect, AWD + one (usually rear) LSD yields 3WD when one wheel is slipping and true 4WD would be available with AWD and LSDs on both axles.

So, AWD vs. RWD depends on which wheels are slipping and which have grip. It's impossible to predict this for every scenario, of course, but you might expect since the front wheels are farther from the spinning rear wheel than is the right wheel, you would have a better chance of accelerating with AWD than RWD + LSD because those would be farther from the slippery surface plaguing your rear spinning wheel. But in the situation where there is a long icy patch at a stoplight from everyone trying to accelerate, both left wheels might be on ice and so RWD + LSD would benefit you by putting power down to the dry right side rear wheel.

You could also think about weight transfer affecting traction too (more weight over wheels yields more traction). Think about accelerating in reverse up a hill. Your rear wheels are relatively lightly loaded compared to the fronts, so even with a RWD + LSD your traction could be minimal. AWD helps in this scenario since a lot of the weight is over the front wheels giving them more grip, and AWD would transfer some power to these fronts.

Each system has its benefits. On newer vehicles the downsides of open differentials can be partially overcome with traction control (TC) and/or having the brakes "act as an LSD." My suggestion is AWD + rear LSD + TC + snow tires. On bad weather days you will blow by practically everyone. Ask me how I know
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