This is great!
 

Witness: the power of the internet. engineers debating across continents effortlessly in their spare time.

I very much enjoyed reading the posts. But I still must hark back to the original question: What is the harm of using a DSC system with a low threshold of intervention on an SAV if - if - it can be changed for the dry conditions via a switch? The M cars have an alternate traction mode "M dynamic" which allows for more wheel spin and sliding. Why can't the X cars have the ultra sensitive DSC from a 3/5/6/7 series rear drive car? Call it "winter dynamic mode" right?

I don't understand the need to allow the 4 wheel drive system the opportunity to regain traction? I'm sure there's more to it than I am aware of. You don't want the traction light going off because you hit a bump in the corner while accelerating. I understand that traction/stability control can be too sensitive in dry conditions (a complaint of many AMG mercedes drivers), but what's the harm in offering a winter mode?

Example: your heading down a secondary road at 70 km/h on packed snow in your E70 and you need to make an abrupt turn onto the next side street. You brake and then turn, but you're going just a bit too fast and the front end (only) looses traction and the arc of your turn increases enough to put your vehicle onto the other side of the road facing oncoming cars. At no point in my example has the accelerator been used. Therefore, the X-drive is not being used either. If you were in a 530i doing this, the DSC activates and brakes the inside rear wheel and brings your car back into the correct lane. With an E53 X5 you slide into the next lane unless you exaggerate your turn by cranking the steering wheel into the turn which "wakes up" the intentionally dormant DSC. Then the car somewhat overcorrects due to your exaggerated steering input. I'm guessing that the DPC on the X6 does much of the the job done by the aggressive DSC on rear drive BMWs info far as correcting slide in a corners in concerned?

Can anyone defend BMWs decision to calibrate the DSC this way on X cars?

Thanks again for the great input.

DRP

Have you checked out this thread:

http://www.xoutpost.com/x5-e70-forum/...itzerland.html

I think it comes down to the fact that fwd/rwd/awd vehicles have different cornering dynamics, especially with limited traction. Your example assumes that the cars are all driven the same, and that is not what should happen. It is an example of driver error. The awd benefit is only there if your foot is in it. In your example, the driver went too fast for the conditions, and then drove through the corner without throttle. Two errors that compound. If he went slower into the corner in your example, and then accelerated to let the awd system do what it is designed to do, this wouldn't be an issue.

I think that if you want the awd system to work as designed, you need to allow the nanny devices enough tolerance to let the drive systems work, ie not correct for a situation if in fact it is just a case of the drive system doing what it is supposed to do.

There was a thread awhile ago where a member posted that his new X5 didn't seem surefooted on snow, and he wasn't confident in the cornering of the vehicle. I suggested that he keep his foot in it (within reason) and let the system work. His response confirmed that the approach worked. I think the DSC is just doing the same thing, letting the awd work. Since BMW isn't about to add throttle automatically in those situations, that action will remain a driver responsibility, and rightly so.

That isn't intended so much as a defence, as much as a comment on why the different programming may exist.

Hmmm, I had actually decided to enjoy the Friday evening with doing just nothing but you are right, what is better than debating technical details across the globe with clever people? :iagree:

Feel the need to throw in some sticks to the fire. Actually my reasoning with the center of gravity is based on dynamic calculations (Lagrangian mechanics to be specific, which together with chassie dynamics was my field of study - well concluded JCL :thumbup:) and not statics.

To clarify my reasoning, a low center of gravity is always preferable for the handling, holding all other parameters constant. Between E53 and E70 there are however some major differences/improvments made which IS changing the parameters:

- The body is stiffer
- The track is substantially wider (68mm front, 74mm back)
- The dampers are faster (Flexray)
- The wheelbase is larger (113mm)
- The DCS is improved (ver. E7x)

To this should be added that according to the spec the weight is only around 15kgs higher for equivalent model at E70, while the center of gravity (CoG) has increased from 678mm to 680mm (i.e. with 2mm).

There are other improvements made which would play a role on dry like the active anti-roll bar. The active anti-roll bar does limit understeer behavior and is tuned by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front will increase the proportion of the total weight transfer that the front axle reacts and decrease the proportion that the rear axle reacts. This will cause the outer front wheel to run at a higher slip angle, and the outer rear wheel to run at a lower slip angle, which is an understeer effect.

All in all, the physical improvements between E70 and E53 are a lot more substantial than the deterioration of the weight and CoG (unsubstantial). It should be noted that it is the relation between the track and the CoG that is important, not the CoG itself. Weight is an insignificant factor for handling if the corresponding parts are upgraded (tires, brakes etc). As an example, a light bicycle with four wheels is not able to perform higher cornering speeds than a big SUV due to insufficient friction from the tires. :wtf:

The above reasoning is valid mainly for dry conditions but the question was actually performance in snow. In slippery conditions the CoG has significantly less importance since the curve speeds are lower due to lack of friction (type of tires are VERY important). Consequently, the horizontal force and corresponding roll is less than on dry.

In the DTC mode (one push on the button), the system allows more spin/skid before the "safety belt" (DSC) is activated. This is perfect if you want to perform highest possible curve speed without risking doing a 360 on the road...!ouch:! :loco:

the DPC in X6 is working both on- and off throttle. DPC uses an electromechanical system of two planetary gearsets, a multiplate clutch, and an electric helper motor to divide torque between the rear wheels. The DPC thus supress DSC to a maximum with improved drivability in the curves (and traction on the winter road). So, as a matter of fact BMW DOES add throttle automatically in the situation Pex5 is describing. :bounce:

Here are some interesting documents I found on the X5word's homepage(OK, a bit simplified descriptions):

Kungen:

You note that the CoG is only 2 mm higher. I didn't have the specs, but the E70 is 50 mm taller, so I would have assumed it was more than a 2 mm difference in the CoG.

You note 15 kg more for the E70, and that same number is in the BMW training material you link, but published figures for both vehicles show a 350 lb/160 kg difference. That is probably due to the standard equipment in the North American market, as compared to the bare vehicle. The weight increase is also consistent with the performance reduction of the 3.0, even though it went from 225 hp to 260 hp or thereabouts.

I don't disagree with your comments on dry road handling, but with respect to the snow handling performance of each vehicle, I think it still is pretty much 98% about the tires.

The DPC system (in the X6, not the X5) does vary the torque split between wheels, but I didn't know that it added throttle. I would have said that it rebalanced the throttle. I wasn't referring to simply backing off the power to one axle by varying the torque split, but to actually powering through a corner.

Good discussion. :thumbup:

Our X5 E70 is a beast in the snow with the stock set-up it was delivered on. By far the best riding/handling vehicle I have ever driven in bad weather. I look for snow/ice to "play" in when given the chance.