On Saturday, 23 June 2018 12:48:16 UTC+1, AnthonyL wrote:
On Sat, 23 Jun 2018 18:34:59 +1000, "Jeff" wrote:



"Tim Lamb" wrote in message
.. .
In message , Jeff
writes


"The Natural Philosopher" wrote in message
news On 23/06/18 07:08, 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.

This seems to be a perpetual urban myth.

It can't be given that Andy and others have had a number
of warnings that have turned out to be accurate every time.


It tyre pressure sensors are using this, it has to be a very very
complicated bit of software to detect - say - less than 1% change in
RPM relative to the other wheels.

Yes, but that isn't hard to measure when its relative to other wheels.

A piece of online research
[https://one.nhtsa.gov/cars/rules/rul...prmonsys.html]
showed that relative wheel rotation was pretty crap at detecting low
tyre pressures especially in all 4 wheels or 2 wheels on the same side
(= I am going round and round in circles!)

Hence the move to in wheel sensors.

I am notr syoing it doesn't work, just that it relies on some pretty
iffy interpreation of very small differences in road speed.

Tyre tread belts do strech under inflation, but not by much. The radius
is completely irrelevant as a road weheel is not, in use, round and
does not have a 'radius.'

Of course it does at the only place that matters, between the axle and
the road.

I've been thinking that but... unless the effective circumference is
changed, the tread in contact with the road surface will not alter. Steel
bracing etc. as mentioned above.

I'm not convinced that it is the circumference that determines
the rotation rate of the wheel. And those who actually measured
it talk about the rolling radius, for a reason and must have done
the most basic tests of watching the rotation rate as the tyre
pressure varies, and see that it does vary by enough to measure.

Maybe there is some perceptible scrubbing going on.

Why should there be any significant scrubbing with say a 20%
lower pressure in the tyre ?


From my distant past of mechanics of motion the centre of rotation of
a wheel is its point of contact with the ground and the rotating
radius/diameter will be less the more the tyre is squashed. Beyond
that and trying to find my books I cannot clarify other than it worked
in getting my degree.

Before some of you roll your eyes think about it, if the centre of
rotation was anywhere else the tyre would be scrubbing. The point of
contact is instantaneously stationary.

--
AnthonyL

https://en.wikipedia.org/wiki/Centra...flation_system