Thread: Rain Rain Rain
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      03-13-2008, 11:13 PM   #11
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Drives: E90 328xi
Join Date: Nov 2006
Location: Seattle

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Originally Posted by scollins View Post
No, you don't have power going to all 4 wheels unless you are driving perfectly straight with all four wheels having identical traction. The sames goes for RWD cars, they have power going to both rear wheels in equal amounts, except when there is an imbalance in traction.

Or, when one wheel is spinning faster than the other. This happens all the time, to both RWD and AWD cars, no matter how sophisticated the system. Every time you make a turn, the outside wheels will spin faster than the inside ones. This is the reason for the differential, and where it gets its name from. If the wheels couldn't spin at different rates, you'd wear down your rear tires in a hurry. If the front wheels couldn't spin at different rates, you would have a very hard time turning.

But that is getting a little technical.......

The xi system does not have LSDs in the front or rear differentials, just like the i series don't have rear LSDs. It uses electronics to control wheel spin, just like the i series does. But if you defeat DSC/DTC fully and place the car on a surface where one front and one rear wheel have zero traction, the car won't move.

So while the i may be called "one wheel drive", the xi should be called "two wheel drive." The only true "four wheel drive" would be a vehicle with front and rear locking differentials and a locking transfer case. You will find that kind of system on a rockcrawler Jeep, but not in a passenger car.
Thanks for all of the info Scollins. It would be fun to try the "one front one back no traction test" -- I have never heard that. Your knowledge of engineering is clearly better than mine -- but here is what I have read (maybe not correct?). With an xi, power will go to ANY of the four wheels to keep the car moving on the intended path. As I understand it, the power can be applied to any combination, or all of, the 4 wheels. On a RWD BMW with no LSD, my understanding is that power can only be applied via the passenger side rear wheel. If that wheel has no traction, DSC engaged or not, you are not moving. Correct me if I am wrong -- I am sure you will

On a personal experience note -- I have disengaged my DSC before in the snow (just to see what happens) and it sure feels like all 4 wheels are digging.

Below are excerpts from various tech discussions of xdrive found on the web:

In an extreme case, BMW xDrive is even able to fully disconnect the front and rear axle from one another or, conversely, connect them rigidly to one another as one combined drive system. So proceeding from the standard 40 : 60 power distribution front-to-rear, the spread of engine power may be varied infinitely all the way from 100 : 0 or 0 : 100. Without the driver even noticing, therefore, his BMW becomes a rear-wheel-drive-only or – should this be required on account of a very low frictional coefficient at the rear, a front-wheel-drive-only car at least for a few short moments. Should a rigid connection of the two axles be required, on the other hand, xDrive builds up a longitudinal lock, providing permanent all-wheel drive with 50 : 50 power distribution.

Further, when DSC takes corrective braking action on a single wheel, torque is automatically redirected to the wheel opposite it. This means the BMW xi can keep moving even if only one wheel has traction.