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Old 01-09-2006, 11:05 AM
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DSC Operation in Action

Contributed by: TurnAround

DSC Operation in Action


I finally got around to taking my X5 in to ask about the "getting stuck on the driveway approach" issue (see below for a brief explanation). They duplicated the problem, then checked with corporate. BMW claims that this is normal and that the wheels have to be able to move / spin loosely like this in order to handle off road driving.

I really like my service department, and I fully realize they were just passing along BMW's information they were given. But of course, we all know that's silly. To me, the design situation is clear. In any four wheel drive system you have to have something to relieve the binding that occurs from driving on dry pavement, when the vehicle is turned. 4 different wheels, all rotating at slightly different rates. Full locked diffys will drag a tire or two some where, and the components need to be tough enough to not be torn apart by these resistance forces. Fully "open" diffys (like on the '03 and earlier X5's) relieve this stress by allowing one of the wheels (per front or rear axle) spin freely. This is necessary on the X5, because the transfer case is fully fixed. It doesn't any ability to "slip" and relief dry pavement stresses (like an Audi or Subaru coupling would). Limited slip diffys are an excellent compromise, but for some reason BMW didn't use them. Cost? Maybe the skid forces were still too high, and limited slip technologies couldn't bleed off enough of the dry pavement binding?

My personal belief is that it's a combination of those two factors, plus, it was paramount to BMW that the X5 drive like a sports car in performance driving situations. That, she does! I wouldn't trade this aspect of the vehicle. To have some binding and thus dragging wheels.

So why won't the DSC stop the wheel spin? BMW states that the DSC in these models is calibrated for "some" pavement contact. In other words, the system, or it's programming, aren't quite sophisticated enough to handle a tire that is NOT in contact with the ground at all... and it allows this tire to spin freely (open differential causing this). Bummer. Add a line of programming dudes. "Wheel all out spining = clamp down on that one brake".

The '04 X-Drive system adds variability to the transfer case, for front to rear axle power ratio. This, in my opinion, isn't very important. Side to side power variance is much more important. The only way to provide for this, would be some kind of limited slip differentials, and or, improving the DSC programming to catch the spinning wheels. I'm about to find out if this is the case. I'm gonna test drive an '04 on my driveway next week. We'll see if it looses traction like my '03. I'll let you know.


- TurnAround



Brief History: My street is flat. My driveway takes off from there at a 30 degree up angle. If you approach the driveway head on (at 90 degrees / perpendicular to the street) the car goes up fine. As you increase the approach angle of the vehicle you are asking the suspension to compress on opposite sides (twist) and keep all four wheels in contact). THis works up to about a 30 degree approach angle. Past that (so now you're coming at the driveway at a 40 degree angle or more), and the suspension on the X5 (I have a non sport by the way) is stiff enough, that one of the front wheels leaves the ground slightly, and one of the back wheels on the opposite side leaves the ground slightly. Presto! You're stuck. Those two "off ground" wheels spin. We're talkin' full out spin. Puffs of burnt rubber. NO power goes to the two other wheels still in contact with the ground.

Question at hand; SHouldn't the differentials or the DSC overcome this problem?



Message Title: The solution of your Problem
Posted by: RobRoy on 2003-11-07 at 14:15:14 (147 views)
(posted from: Host: ppppay132-157-tn.aknet.it IP: 213.21.132.157)
Message:

When one wheel lost ground contact, you MUST turn of DSC.
DSC first reduce the power of the engine, to prevent a spinning wheel, in a second step it applys the brake on the spinning wheel, but the reduced engine-power will not be able to move the car.
When you turn of DSC, the computer will apply the brake on the spinning wheel without reducing the power. So the car will move with one or even two wheels without ground contact.
BMW calls this function ADB - Automatic Differential Brake.
I checked it with my 4.4i. When the DSC is off, i hade no problem offroad, even when a wheel leaves the Ground.

The '04s have an open rear-differential, but they haven't a center-differential anymore. They use a electronically controlled multi-plate clutch with a variable power-distribution from 0-100 to 50-50 (front-rear %)


BMW Article




Message Title: DSC slip-Up-date. RobRoy where are you!
Posted by: TurnAround on 2003-11-10 at 11:30:39 (104 views) (134 thread views)
(posted from: Host: risxlr3.ris.lane.or.us IP: 199.79.41.254)
Message:

In the loudest bawdiest Scottish accent I can muster.. "rrRRoobRRRooyyYY! YEARRR A FOOOKING GENIUS !!!"


That RobRoy was dead on right. Counter intuitive as it is, you have to turn the DSC OFF, of all things.

It worked perfectly. I'd get the X stuck on the drive.. 2 wheels a spinnin'. One front and one rear (opposites). Goin' no where fast.

Reach over, "plink" off the DSC. Give it some gas, and presto, the right front brake and the left rear brake, only, clamp down, and bashoomp... right up the drive I go. No problem!

I tried it many times. Every time, same thing. I tried it in reverse, backing in... same thing. Get stuck, click off the DSC, and up she went.

You can clearly hear the ABS hydraulics working over time.... once the DSC is off. It's very weird, and as I said, counter intuitive... to get stuck with the DSC on, then turn it off, and THEN the brakes start pumping and grunting like crazy.

So.. Automatic Differential Braking (ADB-X) is a reality. Check out the link RobRoy provided (and here's the whole thread);

DSC Not working? Open differentials

So it appears (to me) that BMW went with open differentials to relieve the dry pavement stresses caused by four wheel drive, and decided to rely on the braking system to prevent the inevitable wheel spin caused by open diffys. DSC is apparently calibrated for wheels that are all in contact with the ground, and (this part is significant)... are moving. As in, the ADB-X system is only for slow, to no, speed manuevers. I'm fully confident that the DSC is working well with the open differentials, while moving (see my story on carnering hard on a wet mountain road).

What I find interesting is;

1) Couldn't this be integrated into the DSC programming so that no driver intervention is needed? Vehicle not moving very fast, or not at all, plus full wheel spin (ground contact loss) detected on one or more wheels = kick in ADB-X mode. I wonder if the '04 X5's have these functions combined?

2) The manual just barely makes mention of this, and it's very thin. ADB-X isn't mentioned at all, and it basically just says, you might want to think about turning off the DSC in sand or deep snow. No explanations or guidelines.

So there you have it. I'm impressed. It turns out that our X's are really smart and fully capable after all. It's an unusual set of circumstances that I have a house / Driveway where the conditions are so easy to duplicate over and over so that I can test this subsystem. After what I've seen, I can tell you that it works brilliantly!!

I'm not really complaining that I have to press a button. After all, it makes me feel like I have some control over the vehicle's modes. Kinda cool. And, I still don't have to get out and "turn the hubs" like an old Willies!

If you get stuck - Try the DSC Off.


- TurnAround

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