"The Natural Philosopher" wrote in message
news
On 23/06/18 09:34, Jeff wrote:


"Tim Lamb" wrote in message
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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.

Well unless the tyre is slipping on the rim or in the road, there is no
way anyuthing else can.


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.


No, the world is full of stupid people pretending to be clever.

The 'effective rolling radius' is the circumference divided by 2 PI.

Not when the wheel and tyre isnt a perfect circle and it never is
with a vehicle wheel and inflated tyre.

Trying to make a squashed dougnut into a circle doesnt really fit though.




Maybe there is some perceptible scrubbing going on.

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

Excatly. And yet those that say that the radius has altered by 20% and yet
the circumferemce hasn't altered at all, have only one way to make the RPM
rise by the amount they say the radius has rteduced. Introduce tyre scrub.

No need for any tyre scrub, just the reduced radius under the axle.
That is what determines the rotation rate of the wheel.