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      11-27-2013, 05:57 PM   #1
GWA
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Different Tyre Size Conundrums

Presumably a "standard" X3 has neutral handling - roughly neither over-steer or under-steer tendency. So what happens with the MSport with wider rear tyres than on the front? Does it become an under-steerer?

Secondly, if I switch to standard rims for winter so that snow chains can be fitted is the dealer supposed to reprogramme the electronic stability control stuff so the car " knows" the tyre width is now the same at both ends?
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      11-27-2013, 09:50 PM   #2
brissim
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You may be overthinking this. Why does the car's electronic controls need to "know" the tyre widths? As long as all four tyres have the same rolling diameter and are within tolerance of the OEM tyre's rolling diameter (previous posts on differing tyre ODs have suggested this tolerance may be as high as 3-5%), the cars electronics will be happy.

Tony
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      12-08-2013, 05:07 AM   #3
GWA
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Still confused

The Brissim reply leaves me puzzled - was it a question or a statement of fact that the electronics does not need to know tyre width?
If it is the case that the stability control merely detects wheel slippage then the wheel width might be irrelevant. But BMW literature states that the electronics "predict" what is needed which implies that some measure of something like lateral G force is taken into account so the electronics jump into action to do something. That implies that what it is about to do must need to know that one end of the car is more likely to slide than the other. And the kind of benchmark neutral starting point would surely be that on an MSport the back wheels would have more grip than the front. Fundamentally the question is "how sophisticated and precisely tuned is the stability control?"

And the first query in the original post remains - how neutral is the handling when one has wider tyres at one end? If the MSport has relatively stiffer rear suspension due to springs and/or roll bar then that may "use" the rear tyres more and retain overall neutral handling although the tyres are wider at that end.
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