Coherent explanation of AWD system on E70s
 

I haven't read a good explanation, anywhere, of the AWD system on the E70s.

What kind of center differential do they have? What about front and rear? Is the traction control strictly brake-based, or do they have electronically modulated differential lockup? If it is electronically modulated, are they hydraulic, clutch-based or mechanical?

If it matters, I have an 09 xDrive35d.

Well, I'll give it a try.

The centre differential uses wet clutch plates with an external electrically operated actuator (which applies the clutches) to divide the torque front/rear. A control module montors wheel speed and controls the actuator. Torque is 40/60 front/rear (from memory), but will vary from that depending on measured slip front or rear (using the ABS wheel speed sensors)

The above varies from earlier (non x-drive models, prior to 2004) that don't have clutch plates in the center differential, but rather just an open center differential.

At each of front and rear there is an open differential in the axle, with no mechanical anti-slip function. All side to side traction control is achieved via brake application on each wheel, through the DSC (Dynamic Stability Control) system, which includes traction control.

There are further systems that manage vehicle stability and can apply an individual brake, but I would refer to those stability systems as opposed to traction systems.

The BMW training materials from when the E70 came out in 2007 covered this. Read through this link for more information on the E70 powertrain.

http://www.bmwmotorsports.org/pdf/e7...Powertrain.pdf

The following link includes information on Longitudinal Vehicle Dynamics, which covers the use of brake application to increase traction. Note that BMW refer to it as Automatic Differential Brake, but that is the effect it achieves; it actually works by applying a brake on a single spinning wheel.

http://www.bmwmotorsports.org/pdf/e7...%20Systems.pdf20Dynamics%20Systems.pdf

These and more are available under the section titled Online References at the top of the page. Select PDF, then E70.

Jeff

IIRC, no center differential per se only transfer case. Transfer case is multi-disc clutch with electronic stepper motor engagement with a multi-link chain connecting the driveshafts bathed in TC oil.

Front and rear diffs are conventional and open with no limited slip differential, only DSC 'brake the spinning wheel' traction control.:thumbup:

Beat me to it Jeff! Props on the detailed explanation!:cool:

I thought BMW had some sort of system that could over-drive the outside rear wheel to reduce understeer? Is that a different model, or different manufacturer completely?

So, the E70 wouldn't be able to transfer power just to one wheel?

2011 X3 seems to be a better system, as shown in the video
2011 BMW X3 xDrive AWD capabilities demonstration - YouTube

IMO, by braking the spinning wheels, you in fact 'transfer power to the 4th wheel'...semantics.

And I agree that video is a manipulation... if the driver feathered the accelerator in te X3 as he did in the others, the car would have sat longer.

That system is called Dynamic Performance Control. It applies only to the rear wheels, and produces a form of torque vectoring by the use of a planetary gearset with clutch plates within the rear differential. By applying the clutches one wheel can be 'overdriven' based on commands from the controller. This system is only available on the X5M, X6M, and X6.

Didn't they say they were doing away with DPC now? In favor of something else?

I also read that this fancy stuff has always been available with other makes, like the Acura/Honda SH-AWD.

How would you guys rank the XDrive system compared with others like Subaru or Audi? I've seen youtube vids that show how some systems can handle one-wheel incline better than others. The BMW videos show an E70 with one side of wheels on a patch of ice and still be able to climb in a straight path. Judging by that, it has to be better than a lot out there, but are there better?

I think this is the video I watched: Subaru AWD vs. competitors - YouTube

Not always, I believe the prev. generation of RL (= Honda Legend for the rest of the world) produced since 2004 (?) was the first and only mass produced car with torque vectoring; later it was added to MDX also.

I can't remember which, but either BMW or Honda/Acura's vectoring is rather binary, i.e. the torque to the outside rear wheel gets either 1 or 1.05 multiplier.

I think it is an academic point with limited real-world impact.

Once you go to a full time awd system instead of a part time awd system, and if you include some type of variable torque control front/rear, then everything after that tends to have a smaller difference IMO. Tires matter more than anything. It is possible to make a video demonstrating that just about any vehicle is superior to another in some specific and sometimes artificial circumstance, and most manufacturers have done it. While entertaining, it doesn't mean that there is any real-world advantage.

Given my interest in reliability, and having less complexity in a vehicle, I would go for an awd over a fwd or rwd if my application required it, and would accept a centre differential with some type of control. That is where it would stop, I wouldn't want DPC or whatever they call it next. I would spend the money I saved on a set of winter tires and wheels.