"pamela" wrote in message
...
On 17:01 23 Jun 2018, The Natural Philosopher wrote:

On 23/06/18 13:42, Chris Hogg wrote:
On Sat, 23 Jun 2018 17:04:38 +1000, "Jeff"
wrote:



"Chris Hogg" wrote in message
...
On Sat, 23 Jun 2018 16:08:44 +1000, "Jeff"
wrote:



"The Natural Philosopher" wrote in
message news https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

But the radius clearly does vary significantly.

Does it?

It clearly does with the distance between the axle and the
ground.

I think we're at cross-purposes here. Obviously, as you say, when
a tyre is flat, the axle is nearer the ground. But is it
reasonable to regard that as the radius of the tyre?

Not if you have any understanding of mechanics. The whole tread,
up to and including a caterpillar track, goes round once per
revolution of the track or tyre.

Surely the circumference changes because the area in contact with the
road is a chord and not an arc.

Nope, not when the circumference doesn’t stretch and it doesn’t
with steel belted radials.

The length of the chord increases when the tyre is underinflated on
account of compression of more of the former arc in contact with the
road. The circumference is reduced.

Not with steel belted radials.

Only if all the compression of the former arc occurs at the leading
edge of contact would the speed be unaffected.

The circumference is irrelevant. What determines the rotation
rate of the wheel is the distance between the axle and the road.

THAT is what determines the RPM/speed relations ship.

What happens with tyre pressures is quite clear. The tread
stretches slightly under higher pressures. How much will be a
function of the tyre construction. And this is what the sensors
rely on.

Since no wheel is circular using radius as a concept is plain
wrong. At best you can calcualate a '*radius it would be if it
were round*,' from the actual circumference.

For it to be any other way the tyre must slip on the rim or on the
road, substantially.