"Chris Hogg" wrote in message
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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?

It is the only distance that matters with the rotation speed of the wheel.
The distance between the axle and the top of the tyre is irrelevant to that.

If you simplify the shape and
call it an ellipse, then you have two radii; quite how many radii
would be needed to describe a tyre with a flat on one side, I wouldn't
like to speculate.

But those distances are irrelevant to the rotation speed of the wheel.

But the circumference, perhaps perimeter would be a
better word, won't have changed significantly. It'll just become
distorted, i.e. no longer circular.

Yes, but that isnt what determines the rotation rate of the wheel.

But that assumes there is a weight pressing down on the tyre, which I
grant you, would be the case for a loaded tyre on a vehicle. But I was
thinking of an unloaded tyre;

Its irrelevant when deciding if the tyre is under inflated.

does the circumference change between
under-inflated and fully- or even over-inflated?

Not by much at all with steel belted radials and any system
that detects under inflated tyres has to work with those.

When I were a lad in
the days when tyres had inner tubes, we used to go to our local garage
and get old tubes that were no longer serviceable, patch them where
necessary, inflate then and use them as super-sized rubber rings for
taking down to the beach (and probably getting blown out to sea!). As
they were inflated, the radius and circumference most certainly did
increase; they blew up like a balloon. But put them into a tyre and
there'd be no significant change in the circumference as they inflated
and deflated. The structure of the tyre wouldn't allow that to happen.

Likewise, I suggest that the circumference (perimeter if you prefer)
of a modern tubeless tyre doesn't change significantly as it inflates
or deflates.

Yes, but the circumference isnt what determines the rotation
rate of the tyre on a moving car.

Every full rotation of the hub must correspond to a full rotation of
the perimeter, regardless of the state of inflation of the tyre,
otherwise serious slippage will be occurring between the tyre and the
rim, which would result in friction heating and fairly rapid failure.

Yes, but the circumference doesn't determine the rotation rate of the tyre.

So as far as speedometers and odometers are concerned, state of
inflation won't make a significant difference.

That's arguable with the very small variations being discussed.

(What amounts to 'significant' as I've used it here, I'm not sure; it
depends on the pressure difference being considered between under and
fully inflated, and the elasticity of the structures within the
treaded surface of the tyre, amongst other things, but others in this
thread have mentioned figures of around 1% for the stretching of the
perimeter as the tyre is inflated. In this context, I would regard
that as not significant).

But someone must have checked that the ABS rotation rate does change
with under inflated tyres otherwise that wouldn't be used to detect
under inflated tyres.