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Old 04-30-2003, 01:57 AM
rrf rrf is offline
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Join Date: Jun 2002
Posts: 15
Is true wheel diameter setable in the DAS software?

Hello, maybe a dumb question, but probably worth a thought,

W203 2002 C-class

Is there an internal DAS setting to accomodate the EXACT tire circumferences for the BAS/ESP system?
For example when tires from different manufacturers are used or when slightly different sizes are installed.

Let's use a basic C320 as a reference:
The C320 offers a range of tires as options from the factory options list.

From the MB website for just the C320 :

Style B 225/45/17F and 225/45/17R
AMG Style III 225/45/17F and 245/40/17R

The diameter of the 245 is 629mm the 225 is 637mm so there is a 1% physical difference.
(Add in some normal tire wear and these differences could get even higher).

I see there are some possible answers to how the BAS/ESP might accomodate these "small differences":
1) The C320 BAS/ESP system doesn't care about small 1-2% differences in circumference
2) The C320 BAS/ESP system is changed/replaced/different whenever optional tires are chosen
3) The C320 BAS/ESP system is reprogrammed or "told" about the new tire option

#1 may be true, which is likely, and apparently is everyone's assumption. (Nobody touches ESP when they go to aftermarket wheels/tires). However, even when when optional FACTORY tires are selected, we know circumferences are different. My feeling is the computer will be a little more "eager" to use the ABS than it should be.... it has no choice but to be thinking there is a little slippage going on, which in its mind is OK, but after a certain point it must start to intervene. right? Won't its calculations be "off" by the % error above?
#2 seems a little crazy, and I have never heard of anyone replacing the system with tires.
#3 seems logical, but never mentioned by anyone, either

Is there such a setting as #3, and which DAS menu would it be available in?
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Old 05-02-2003, 07:33 PM
rrf rrf is offline
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Join Date: Jun 2002
Posts: 15
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Old 05-03-2003, 04:18 PM
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Join Date: Sep 2002
Posts: 2,486
Hi rrf, interesting question. Your correct to assume that the traction system "doesn't care" about acceptable differences in tire diameter. This is why you can get away with having two new tires and two old tires on the car at the same time. As far as version coding goes, tire information isn't needed. You must remember how ESP works. Lateral/longitudinal acceleration, steering angle, wheel speed, etc, are taken into account to determine if any intervention is needed. ABS only looks at wheel speed. ESP is used to slow a spinning wheel ( by selective braking ) to prevent the vehicle from getting "out of shape". The spinning wheel doesn't necessarily mean it's lost traction though. That would be a function of ASR. ABS is used to prevent a wheel from locking up by reducing/regulating braking pressure. As far as ABS being "too eager" to activate, this is a function of the ECU looking at wheel speed, so the actual tire size doesn't come into play. Lastly, BAS is something altogether different and deserves it's own Shop Forum Topic!
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