"The Natural Philosopher" wrote in message
news
On 24/06/18 22:18, Jeff wrote:


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.

Yes.

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.

No. Once yopu put a non circular tyre on, you vannot talk about radius.

What counts is the distance round whatever shape the tread is. Al;l of
that has to be in contact with the road during one wheel revolution unelss
the tyre slips on the rim

But if you were to label part of the tread with a line continuing radially
up the sidewall to the centre, that line would not remain a straight line;
it would curve one way as the labelled part approaches the road surface and
then the other way as it leaves it; this happens because the plies of the
steel belt move towards or away from each other as the tyre rotates. Not by
a noticeable amount; probably by an amount that is hard even to measure. The
tyre is no longer a perfect circle but a circle with a flat on the side
which is contact with the road at this precise instant. The distortion of
the sidewalls, both rotationally and as the bulge "travels" around the tyre
as the tyre rotates, is the reason that tyres get hot. The lower the
pressure, the more distortion and the more heating - to a point that the
tyre softens, melts and bursts.