X1 xDrive System...

[iconoclast]

Is the xDrive system found in the X1 electric or mechanical? (Is it more like Haldex or TorSen?)

I cannot find any info on this and contacted BMWUSA. They directed me to their website and I didn't find anything that explains exactly what I would like to know.

[Shock96]

http://en.wikipedia.org/wiki/BMW_xDrive

Wiki seems to have a good explanation of xDrive.

Seems like it is a combo of mechanical (clutches) that are electronically managed.

nstead of a permanent torque split (which is featured in earlier systems), xDrive provides variable torque split between the front and rear axles through the use of a multi-plate wet clutch located in the gearbox on the output to the front drive shaft. This setup allows xDrive to modulate the torque split between the front and the rear axles, which is normally split at 40:60 ratio. If wheel slip is detected by the ABS/DSC system, xDrive can react within a tenth of a second to redistribute up to 100% of the engine power to the front or rear axle.[1] The wet clutch is applied through a high speed electric servo motor turning a cam-shaped actuator disc.

xDrive is connected to the ABS and DSC systems. In the case that wheelspin or directional instability still occurs while xDrive is or has been modulating the torque split, DSC will brake independent wheels to regain traction and improve directional stability without driver intervention.

The front and rear differentials in xDrive vehicles are typically an open differential design, thus relying on brake application by the DSC system to transfer power from the slipping wheel to the wheel with traction.

[iconoclast]

one of the first explanations i encountered however doesn't really tell me if it is similar to a haldex style system.

bmw site wasn't too helpful either:
http://www.bmw.com/com/en/newvehicle...drive.html#t=l

[Twix]

It doesn't use a differential like haled if that's what you're asking. It's completely different than most AWD systems. Like stated above, it uses clutches to engage the front and rear drive shafts. It's more like a clutch type limited slip carrier but electronically controlled by the safety systems on board.

That's why if you press and hold the DSC button until DSC is shown off in your cluster, and you try to drift, the car will act as if those systems were still engaged. xDrive is more intelligent than BMW's DSC and DTC. You will be able to get the rear end to kick out, but the xDrive will move the power around to get you the best traction available.

This is the main reason I want to trade my car in for something different, something rear drive only. The system doesn't allow for me to have enough fun or control over my own car, but it is amazing in snow. Better than my old 4x4 pick up.

[iconoclast]

gotcha... makes sense.
thanks for the help.

[NTTY]

My two cents :-)

I went for this little car mainly because of the transmission. It is based on a transfer case which is located behind the gearbox and transfers part of the power and torque to the front wheels:



Inside the transfer case:



This transfer case acts as a gearbox, sort of, using a multi-plate clutch controlled by the DSC system to engage the front wheels at variable split (see below).

This system is the same on all BMW x-Drive cars to the exception of the Active Tourer which uses a tuned Haldex.

What is important to understand is that this system is based on RWD transmission and adds the front wheels "on demand". The DSC can control the clutch to split torque and power but will never become a pure FWD. There are three basic cases:
- Below 12mph, the split between the two axles will be 50/50 (F/R)
- After that, operating split is 40/60 (F/R)
- Above 110mph, it is 0/100 (F/R)

Of course, the DSC makes plenty of adjustments depending on road conditions, lateral G, brake/acceleration requested by the driver, ..., using several sensors.

Two cases are described in the below picture (understeer and oversteer):



The blue arrows show an example of under-steer. If you loose completely the front, the DSC will transfer up to 100% of the power to the rear wheels. This will generate less stress on the front wheels and therefore has a better chance to give back the necessary grip to the front axle.

The red arrows show an example of over-steer and in this case, you see that the DSC will tranfer up to 50% of the power to the front wheels so that the car will come back to the desired direction (it will pull the front, stopping the over-steer).

Again, as you can see on those two extreme examples, we are still talking about a BMW and therefore a car that will never accept to have more power on the front axle than on the back...

We see that this transfer case acts as a kind of LSD, but an intelligent and very efficient one. The two other front and rear differentials are "standard", we say open differential. Nothing different from any other car. That said, the DSC will use brakes on each axle to compensate an abnormal speed difference between two wheels of the same axle, thus acting as a LSD. Experience proves it is not as efficient as a mechanical LSD (Quaife for instance), but still very good.

Since this transfer case is controlled by the DSC, BMW also offers an option called "Performance Control" (in Switzerland but it might be another name in USA), which tries to make the car more dynamic. This is illustrated by the below drawing:



As you can see, the DSC will accelerate the external wheels in a corner (by slowing the inner wheel) so the car enters the corner in an improved and more dynamic way.

Last but not least, the transfer case of the X1 can compensate its own wear, there is a sensor that tests the multi-plate clutch and will lock a bit more if required. This is also the reason why this system is not badly influenced by a possible difference of front and rear tires diameter. It will adjust itself to reach the requested ratio between front an rear axles. That is another cool feature.

All in all, this is a very good system, and unfortunately BMW will move to the Haldex one on the future F48. It will be a tuned Haldex version that keeps rear wheels always engaged, but it is not technically as sexy as our current transfer case.

I hope this clarifies a bit more.

[iconoclast]

Originally Posted by NTTY

What is important to understand is that this system is based on RWD transmission and adds the front wheels "on demand".

that is exactly what i was trying to find... if it was a rwd system worked into awd or fwd worked into awd and if it was mechanical or haldex style etc.

thank you for the extremely thorough explanation i really appreciate it.

[NTTY]

Glad you appreciated ;-)
I updated my post to correct some mistakes.
Cheers

[iconoclast]

i noticed, thanks for that... i am pretty sure this will serve well in the archives if and when others search for the info.

[matjen]

Great post NTTY. Thank you

[Shock96]

Excellent. That explained it very well. Thanks!