On Thu, 28 Jun 2018 19:24:48 +1000, "Jeff" wrote:

snip

So is relying on 'experts' who are equally ignorant.

But it is very easy to measure both a fully inflated tyre
and one that is say 20% under inflated and see if the
rotation rate of the wheel is in fact determined by the
distance between the axle and the road or the very
minimal change in the circumference of the tyre and
that must have been done when designing the ABS
system for detecting under inflated tyres.

The thing is one is a function of the other (cause and effect).

eg, If you have a solid wheel then the arbitrary circumference is the
same as the rolling circumference (so wouldn't work with an iTPMS
system). ;-)

Once a tyre can deflect under load and especially when it does so as
per of it's design, you no longer have a linear circumference (or even
a circumference in the true sense) but you do have something that
relates to the original circumference by some factor. The *exact* same
applies to the real radius (solid wheel) and an effective / loaded /
rolling radius when you don't have a real circumference.

Now, I haven't stated the RW relationship between the distance between
the axle and the ground on a loaded pneumatic tyre / wheel and the
'effective circumference' other than there is likely to be a link
between the two.

The key factor here is *how* does the speed of a deflating tyre change
for an iTPMS system to work (and they do work obviously). Now, we
don't need to define the situation by saying 'assuming no slip between
the tyre and the rim' as anyone who has tried to remove a tyre will
attest, nor do you have to state 'with no slip between the tyre and
the road' because if there was it would nearly be impossible to
predict anything. We certainly wouldn't need to mention anything about
the sidewall winding up because that will simply unwind again as the
torque (acceleration / braking) is removed.

So, we are down to *something* happening to effectively shorten the
effective circumference and about the only think logical (to any right
brainer that is) that at the point of the cord where the tyre flattens
out and the sidewall bulges where the tyre sits on the road, there is
a shortening of the tread length across this cord and hence a produce
a shorter 'effective rolling circumference'.

So, the left brainers deny this being even likely but without offering
any better solution themselves (and Turnip never will now of course,
not now he's backed himself into a very dark corner. Or he might, but
it will be that he has now come up with the answer all by himself).
;-)

Cheers, T i m