In article ,
PeterC wrote:
My limited experience isn't with car tyres. Now, the circumference
doesn't have to change, only the effective rolling circumference (see
down-thread). (An ellipse, if not too far from circular, has v. liitle
change of circumference, IIRC, so view a softish tyre as a one-sided
ellipse and there's even less change).To me, the rolling circumference
is simply that which is calculated from the 'radius' at the point under
load, so if the axle is 10% lower the rolling circumference is 10% less.

Yes. You are effectively making a triangle with the base being the flat
part of the tyre. And that base is going to be a shorter length than a
similar triangle where that base is a curve.

In other words, the radius at the point of contact to the road determines
the gearing. Whatever happens to the rest of the tyre is immaterial.

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