On Fri, 29 Jun 2018 08:26:56 +0100, Tim Streater
wrote:

snip

TNP says if they rotate at different rates the treads fall off..
I say treads do fall of.
You think it means I am agreeing with TNP.

You are.

Aww Bless. The poor goblin is so confused now ... glimmers of
understanding but can't_actually_say_it for fear of upsetting his
goblin master, the great Turnip. ;-)

Just think for a second instead of parroting TNP..
The tread changes length when it hits the road so that bit of the tyre
is rotating at a different rate to the rest so yes tyres do rotate at
different rates to the wheel some bits at one rate some at another and
some at all the values in between.

And over a revolution, the rate must be the same, else the tread would
come off.

Whoosh.

The rate of shrinkage of the tyre is fixed (it's not of course as the
load on the tyre is dynamic in use but that would be a step way too
far for the Streater goblin's tiny brain) for a good (non punctured)
tyre but the location of the shrinkage moves round the tyre. The bit
that is causing the tyre to shrink is the bit sat stationary on the
road so no damage to the tyre occurs.

So, you can have an effective circumference that is smaller than the
unloaded one that does not put the tread under any additional stress.

*That* is QED.

Cheers, T i m