xDrive

[asawadude]

Originally Posted by DinanX5

Hey everyone! Long time, no post....

So I was reading about the e70 X5 in last month's Roundel, and it described the functioning of the xDrive system exactly the same as it is in the face-lifted e53, and the X3.
I thought I'd read somewhere (on this forum, probably) that the e70 was going to get xDrive "II", where the electronic diffy could vary torque not just between the front and rear axles, but side to side as well (as opposed to calling upon ABS to limit wheel spin within a given Axle) I guess this new system didn't make it into the e70 (?)

Also, the article briefly mentioned the role DSC and DTC play in the functionality of xDrive... and how it's possible to disengage DSC (which we all know is done by pressing the DSC button on the center console) - which leaves xDrive operating with only the DTC. But apparently you can also disengage the DTC and let xDrive manage traction on its own.
I seem to remember reading (also, probably on this forum) that this can be done in the e53 too... something about holding down the button for 3 seconds maybe (?) Anybody know the details?

The description of xDrive II that you cited is actually a description of xDrive I first introduced on the 2004 X3's and X5's. Here's the quote from Wikipedia describing the 2004 X5:

"For 2004 the X5 was refreshed with new headlights, a few new exterior colors, a new four-wheel drive system and upgraded engines. The exterior touches kept it fresh and gave the X5 a more aggressive look. The grilles were enlarged, as well as their actual slats being modified in a 'flame surfaced' style. In keeping with the E39 refresh of 2001, the 2004 X5's headlights got "angel-eyes" (rings or halos around all front headlamps). The taillights also got an E39-style refresh treatment and the exterior glass went from a "dotted" pattern to a cleaner one of "lines". BMW invented a new four-wheel drive system dubbed xDrive shared both in the X5 and X3 in 2004. Instead of using the previous X5 system which consisted of power being split 60-40 (rear wheels-front wheels) and DSC to brake wheels losing traction, xDrive could variably cut off power to the front or rear axles in a matter of milliseconds, while transferring 100% of the engine power to either, thus allowing the vehicle to regain traction quickly."

I haven't found a formal description of what exactly changed with xDrive II. I'm guessing it might be stronger, lighter, and faster, but from a functional design standpoint, pretty much the same as xDrive I.

[DINANM3]

I hate audi but have to admit there awd system is better than bmws. Quattro is one of the best sytems out there.

[xnsf]

Originally Posted by DINANM3

I hate audi but have to admit there awd system is better than bmws. Quattro is one of the best sytems out there.

i know audi's famous for it...how does the quattro system work?

[DINANM3]

Originally Posted by xnsf

i know audi's famous for it...how does the quattro system work?

Well I believe it has LSDs all around and can transfer 100% of power to the one wheel that has most power not relying on the traction control at all. It can also transfer power form front to back side to side and diagnally if I remeber correctly. My wifes A6 has awesome traction in all conditions. You can feel the difference. But Xdrive is getting closer and im sure Xdrive2 will be as good if not better than quattro but thats 3 years away,

[Wagner]

The DSC button does not disable the X-Drive system components, it simply makes them less aggressive. BMW has a nice (IMO) habit of providing buttons in their rides that change existing functionality. Anyone who has hit the SPORT button in a E46M3 knows what I mean.

I just hope BMW takes a page from Audi on how to design AWD systems....Audi is unstoppable.

[Wagner]

Originally Posted by xnsf

i know audi's famous for it...how does the quattro system work?

Google is your friend: one search on Audi Quattro System

http://en.wikipedia.org/wiki/Quattro...l_drive_system)

[xnsf]

http://en.wikipedia.org/wiki/Quattro...rive_system%29

i wonder why i didn't think of wiki...

thanks

[DinanX5]

Originally Posted by asawadude

The description of xDrive II that you cited is actually a description of xDrive I first introduced on the 2004 X3's and X5's...

BMW invented a new four-wheel drive system dubbed xDrive shared both in the X5 and X3 in 2004. Instead of using the previous X5 system which consisted of power being split 60-40 (rear wheels-front wheels) and DSC to brake wheels losing traction, xDrive could variably cut off power to the front or rear axles in a matter of milliseconds, while transferring 100% of the engine power to either, thus allowing the vehicle to regain traction quickly."

Yeah, you gotta love Wiki...

But the Wiki description you cite is exactly how I understood the current xDrive (and described it above)... it just stops short of explaining how it manages traction side-to-side (either within a given axle, or diagonally). If what I read previously is true, this is what's improved upon in xDrive II.

As for the DSC button... I JUST got back from a romp in the snow to/from the supermarket, and now I wish I'd checked this thread before I left so I could have tried the press-the-DSC button-for-10+ seconds-trick!!
But I have experimented a bit with just pressing the DSC button once to disengage the DSC, but retaining the DTC. My impression is that, on an xDrive vehicle, the DSC is much less invasive than it is on typical 3/5/7 rear drive models (certainly much less so than on my old 328Ci sport). So there's not a huge difference in 'normal' snow driving with it on or off... but when things get really slick, the DSC will cut engine power and engage ABS as it sees fit to stop what it perceives as an over-/under-steer loss of control. For this reason, in the snow I prefer to drive w/ DSC disengaged at all times (with the exception of high-speed highway travel) to allow for a little xDrive-controlled yaw play. The mix of just traction control - which only limits wheel spin - and xDrive is a phenomenal combination for the snow IMO.

But there are times - when things are REALLY deep and messy - when if you don't just let the wheels spin a bit and keep the vehicle creeping forward, you can still get stuck... as I understand it, this is because DTC is cutting wheel spin, so if none of the 4 wheels are getting sufficient traction, then... well, you get the idea.
Now, the part I'm not sure about is: If you hold down the DSC button for 10 seconds and disengage DTC as well, without traction control's influence, would the ABS still intervene to manage side-to-side wheel spin? (Probably... ) Or would the xDrive's eDiff lock-out and let the wheels spin unencumbered? [Not sure, but it sounds like this is what Audi's mechanical diff does (?)... and I think the Cayenne has a manual lock-out control to accomplish this.]
Most likely, I'd think that xDrive would still function in full, managing torque between axles, and calling upon ABS to manage wheel slip within a given axle.
But to be sure, someone should go out in the snow tonight and test it out!

At any rate, I think xDrive is a great balance between dry-road performance (where Audi's heavy mechanical system is losing ground to xDrive - and some of the other new eDiffs out there), and off-road traction. But if xDrive II will in fact add the capability of shifting torque from side to side as well (rather than limiting wheel slip with ABS) then it should be a nice improvement.

[B&DCalgary]

Originally Posted by DinanX5

But there are times - when things are REALLY deep and messy - when if you don't just let the wheels spin a bit and keep the vehicle creeping forward, you can still get stuck... as I understand it, this is because DTC is cutting wheel spin, so if none of the 4 wheels are getting sufficient traction, then... well, you get the idea.

You have a slight misperception about traction. Once a tire starts slipping (by spinning) you lose traction because the coefficient of friction changes. Slowing the rotation of the tire to prevent spinning actually maintains traction, and a non skidding tire will always provide better traction and better acceleration than a spinning tire, the same way that threshold braking will always stop a car faster than if you lock the wheels up. The only time spinning the tire might be an advantage is if the snow is so deep and soft that you can't get traction, then spinning the tires can sometimes burn down through the snow to the point where it hits pavement.

[Mitch P.]

Quattro really is impressive. My old S6 that I just traded in for the 4.8is was unstoppable in the snow with snow tires.

http://video.google.com/videoplay?do...4614&q=allroad

Allroad in this video vs the others up a ski slope. Cool stuff!