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      01-04-2013, 04:32 AM   #32
mlhj83's Avatar
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Drives: '11 E92 M3 DCT ZCP
Join Date: Aug 2011
Location: London

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Originally Posted by anerbe
Originally Posted by mlhj83
Because, like everything, it's a compromise. BMW M probably preferred the way the non-run flats works with their suspension setup. Regardless, the debate isn't about run-flats vs non run-flats, it's about the same model tyre with the same section width but with different profiles. The same argument would also apply if M decided to use run-flats; the lower profile run-flat would generate more grip than a taller profile run-flat, all other things being equal.

F1 use tyres with taller profiles as the car has been designed to use the sidewall flex as a significant part of the suspension travel. If F1 cars used low profile tyres, the cars would effectively have insufficient suspension movement. This is completely different to the suspension design of a road car where there is a lot more suspension travel (excluding sidewall flex) than a F1 car, therefore, increasing stiffness with lower profile tyres for a road car can generate more grip on a sufficiently smooth surface, all other things being equal. Google F1 tyre design and suspension travel if you don't believe me.
Overall idea is correct, but as you also stated, there are reasons that a softer sidewall will also help grip.

Certain tires benefit more than others in outside tread shoulder loading, based off its design. These tires will start to include more of the shoulder area as they flex, increasing the contact patch.

Best way to determine is how the tire wears at the track. If the tire is wearing in the off shoulder area of the tread that is ineffective, you need stiffer sidewalls, more tread grip (footprint or compound) or higher pressures.

Also, it's rare to have any track buttery smooth. Low profiles will cause harsher impacts that will unsettle the suspension and cause momentary loss of grip.

Low profile doesn't always = more grip.
I agree, lower profile doesn't always generate more grip as it very much depends on the setup of the car, however, given that BMW M cars have road suspension which has softer spring rates than a track focused car, a lower profile tyre on BMW M cars will not increase stiffness to a point that the tyre starts skipping over surface irregularities on most tracks, so in this case a lower profile road tyre will improve lap times.

Track biased tyres have much stiffer sidewalls than road tyres, therefore aren't normally available in low profile dimensions.