[sergey8]

Why am I writing about M-xdrive now after more than a year after its introduction and after everyone has extensively discussed it? The answer is that all the discussions I have heard so far turn around four simple facts:

1. It is a sporty rear-biased version of xdrive
2. You can switch the front wheels completely off and get a “true” rear-wheel driven car.
3. It is superior to E63 system in that you are not limited in gear in which the rear wheel drive is available
4. By switching to rear wheel drive you turn the DSC (stabilisation system) entirely off

In other words all the discussions are not far off from BWM press releases and videos of bloggers who are merely conveying the same press releases. We know the BMW (like all other car manufacturers) are usually pretty shallow regarding releasing technical details of their car functioning.

Nevertheless, none of the reviewers actually pointed out that it is not just the rear wheel drive mode when you loose support of the stabilization system. When you select the sport xdrive you are forced to also go into the MDM and loose “half” of the stability system working.

In other words with M Xdrive the amount of working of the front wheels is directly related to the amount of stability assistance you get in a reverse relation: the more xdrive the more stabilisation, the less xdrive the less stabilisation. Why is that?

Furthermore, nobody discussed the fact that when you turn stabilisation completely off you get a choice of three xdrive modes: normal, sport and rear-wheel only. What are these modes for and which one should you prefer and when? For example, which mode is more stable and gives you more performance or fun: full xdrive with no stabilisation or sport xdrive with MDM?

Realising all these questions I decided to describe a little bit how M-Xdrive actually works, the difference between the various modes and the relation to the working of the stabilisation system.
In order to understand the working of M-xdrive let’s compare M5 F90 to F15 X5M.

Why to X5M? Well, because it is the closest vehicle in terms of setup to M5: it has an all wheel drive, it has a rear limited-slip differential, it has a conventional braking system as opposed to brake-by-wire system in new BMWs and, obviously, it has a stabilisation system optimised for performance use.

It does lack the M xDrive, which is the point of comparison. Don’t through stones at me: of course, there are a lot of differences between M5 F90 and X5M, but for the purpose of the point I am trying to make the setup is similar.

So how do actually the traction systems (all wheel drive and limited-slip differential) cooperate with the stabilisation system on X5M?

The simple answer is - they don’t. The xdrive and the diff are distributing the torque between the front and the rear wheels as well as between the rear right and rear left wheels with one single purpose - give you more traction. They work independently of each other and to large extent from the stabilization system. However, by doing so they do give you some extra stability in tough corners.

When you exit the corner the xdrive distributes more torque to the front wheels once they have traction which helps you minimise oversteer. When you enter the turn the limited-slip diff redistributes torque between the rear wheels so that you more easily turn into a curve thus countering the understeer.

Important here is that these systems work completely irrespective off where you really want to go with the vehicle and whether you are in trouble or not. Any help from them in this regard is rather coincidence than a plan.

So the traction systems help you minimise the effects of the physics of the process but they do not correct your mistakes at least intentionally. This is what the stabilization system is for and it does so rather abruptly.

When the traction systems are no longer able to hold you on your course (the intended path does not correspond with the movement path) the stabilisation system comes into working. How does it work?

Well it actually also redistributes the torque between the wheels but it does so by applying the brakes on individual wheels.

The effect of such stabilisation system is that it does correct the path of the vehicle but that correction is experienced but the driver as punishment. You loose not just the traction you actually stop to some extent.

So apparently BMW has for sometime been thinking that it would be great to use torque instead of braking to stabilize the vehicle in difficult situations. This means that when the vehicle understeers the diff is applied to stabilize it more than it naturally would to give traction to one of the wheels. Also when the vehicle oversteers more torque is given to the front wheels together with applying the rear locking diff on the right side to stabilise it.

A lot of this stuff the torque vectoring systems would be doing anyway but now they would work in a orchestrated way and complement each other not just to maximise traction but also to make the vehicle stable.

This is the stablization systems we all have always wanted - the seamless unnoticeable stabilization system with which we actually almost never notice that we make mistakes and even after we have made one the vehicle continues to pull forward.

And this is where M5 differs to E63, in this working of the stabilization system and not in the amount of gears in which the car can be rear wheel driven.

Knowing this we also understand why working of the stabilization system degrades together with the amount of bias towards rear wheels - because M-Xdrive IS the stabilization system. BMW is fair in this aspect - less preference to front wheels means less preference to front wheels even if for stabilization.

So when we switch the front wheels off completely apparently just working of the rear limited slip diff alone is not sufficient to surve as stabilization system and therefore it gets completely turned off.

Why did not bmw implement in this case an old fashioned system with brakes? Well, maybe for several reasons:

1. They think that the new system is so great that nobody would need the old one. After all sport xdrive mode gives you a lot of rear wheel drive sensation
2. It was expensive to implement two stabilization systems on the same vehicle and they decided that one superior would do
3. Both systems are made by different vendors and they simply could not get them working in time for the M5 release and it will come later
4. The new system is way ahead in performance of the old principle and BMW decided to combine both systems when the new integrated braking system (brake by wire) and integrated DSC is available in the newer vehicles like G15

Of course, we do not know. Somehow I think they will combine both systems at some moment in future.

Why did not they implement M X-Drive before. Simply of the computer hardware limitations. In order to handle simultaneously traction and stability you need a lot more computing power than when you do it separately. BMW actually is using what is called massive parallel processing in the XDrive control module. It is the same type of processing which is used for graphics acceleration in high performance computer games.

Of course, a keen reader might have noticed that I haven’t been entirely honest with you. BMW had already for quite some time been working on pairing xdrive and DSC. Traces of this can be found as early as F30 or maybe even earlier. However, this is nothing compared to the degree at achieved with M-Xdrive.

So what happens when you switch the stabilization system in M5 compeletely off and select for example the all wheel drive mode? Well, then you will feel just like in X5M with stabilization off. As long as all wheel drive and limited slip diff (working separately and independently of each other and of your driving intentions) can hold you on your path you are ok, after that you are on your own.

Personally, knowing how the new stabilization system works I would switch it off if only for drifting.

[cfm56d7b]

One of the better write-ups on the subject area. Very well done!

[Ilyam5]

Sergey

Amazing stuff. Good Way to think about it. Current HP wars make it difficult to put power down with just rear wheel drive. We are fighting uphill battle - adding weight to cars and SUVs and giving them more power

I agree - that mechanical stuff should come first (steel springs, suspension tuning, limited slip dif, torque converter). And true BMW driver should be able to disable electronic nannies to drive the car based on physics.
Electronic intervention and stability control - create false sense of security. Like loading the car past its limits on corner entry - and if you push too much - it is going to snap, vs gradually giving up when done without electronic control

I feel the same way about other comfort interventions : Integrated active steering, dynamic roll bar stabilization. they put them in DHP packages on 5, 7 8 series but it only improves the comfort and isolates you more.
There is a reason they don't have those features in true M cars.