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On Sunday, 24 June 2018 16:16:29 UTC+1, DerbyBorn wrote:
The hight of the axle is reduced - therefore the effective radius is
reduced and therefore its effective circumference. The part of the tire
not
in contact with the ground is irrelevant.

I DO get what TNP is saying. Consider a fully inflated tyre with a mark
on the tread area. That mark will always be at the same relative angle
to the wheel, otherwise the tyre would be sliding on the wheel rim.

The wheel rotates one revolution. Measure the distance between the
first and second points where the mark touches the road.

Now partially deflate the tyre so that the distance between the axle
and the road is much less. Repeat the measurement. The distance
along the road will be almost exactly the same as before because the
circumference of the tyre has not changed significantly due to all
the steel wire in it and the tyre is not scrubbing against the road
surface.

In both cases the wheel rotates exactly one revolution - the tyre is
not slipping on the wheel rim - and the distance traversed in that
rotation has hardly changed.

So the "effective radius" has hardly changed, even though the radial
distance from axle to road has changed considerably. It really
is the circumference (or perimeter) of the tyre that matters, not
how much the axle has dropped relative to the road.

And yet if you consider a solid wheel, no tyre, it is the distance
between the axle and the road that determines the rate at which
it rotates at the same speed of the car over the ground. So when
a wheel with tyre sees a reduction in the distance between the
axle and the road due to a lower pressure in the tyre, you get
the same change in the rotation rate of the wheel which is
easy to measure with the ABS sensor on that wheel.