On Sun, 24 Jun 2018 09:17:03 -0700 (PDT), wrote:

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.

True ... so, it's either witchcraft, or your assumption that the
*effective* circumference (how far the vehicles travels per
revolution) must be wrong?

Because of the way the steel wire is laid up in the tyre they tend to
cross and form a parallelogram. As the tyre deflects the shape of the
tyre changes and therefore causes an effective change in circumference
(more accurately, how far the vehicle travels per wheel revolution
(down to pantographing)).

The smaller the distance the smaller the effective circumference (or
effective rolling or load radius as it's known) and to the faster the
wheel will spin for a fixed ground speed of the vehicle.

Cheers, T i m