On Mon, 25 Jun 2018 12:05:03 +0100, Chris Hogg wrote:

snip

and there's no
mention of state of inflation,

But there should be as that's a key variable that affects the
effective 'perimeter length. ;-)

But as others have pointed out in this thread, it's less than 1%. Or
are you saying that 1% is what the argument is about?

I'm not stating anything so specific Chris but yes, that could
possibly be the sort of numbers we are talking about.

How does that relate ITRW. Well, a tyre that is running at a lower
pressure will rotate more time that will relate to a whole mile in
every 100. The point being that 1% doesn't sound like much but a mile
is quite a way (especially if you are walking with a flat spare to a
garage). ;-). ;-)

shape of the tyre or axle to road
distance.

No, but there should be as the shape of the tyre is instrumental in
the calculation.

If the perimeter of the tyre makes one revolution, the vehicle moves
forward by 1.5 metres in my example, regardless of the shape of the
tyre. It could be oval, or even square with each side 0.375 metres,
but only in Michael Bentine's world :-), but the vehicle still only
moves forward 1.5 metres per revolution of the tyre, and with a square
tyre the axle to ground distance would be changing all the time
(giving a very uncomfortable ride!)

Ok, try this (and try to go with the concept rather than the details
for now). Imagine because how a tyre is constructed, steel wires laid
using a parallelogram pattern that 'pantographs' (think pantograph on
an electric train that gets wider as it gets lower) where it touches
the ground and that causes it to widen (width) but because the
material it is widening by has to come from somewhere, it also shrinks
(length = circumference). So, there is a traveling 'shortening' of the
'effective circumference' that is partially dependant on the *amount*
the flexing happens and that is down to the pressure.

On a solid tyre it wouldn't be because you have removed that variable.

Do you also deny the change in tyre diameter on a dragster (and
therefore rpm / mph)?

https://www.youtube.com/watch?v=YViiNxnQTr8

So dragsters use big tyres;

Yes but you are missing how the size (change) is relevant here.

so do earth moving scrapes and dumpers,
even bigger than those on the dragster. https://tinyurl.com/y9krjyv8

See above.

Big tyres rotate more slowly that small tyres, for the same road
speed, except for that dragster where there was rather a lot of
slippage!

There shouldn't be (and the point of having a 'burn out')

(I bet it stank!

Yes. ;-)

Must cost a fortune;

Yes. ;-(

why do they do it?)

To get the rubber hot to give more traction and a better launch.

I
don't see your point.

I know. As others have said, I'm sorry I don't seem able to transfer
the understanding. ;-(


What about this. Imagine some lunar rover module where the 'tyre' is
made up of many completely independent segments.

With the vehicle up on jacks you might measure the effective
circumference of the wheel as the measurement made with a tape drawn
around it.

But put it under load and with it standing on one (sprung) segment.
the RW radius of that segment at that time under that load will be
less. Imagine that reduction in radius being passed round the wheel
segment by segment and you could then see (hopefully) that the
effective circumference would be calculable from the loaded radius and
it *would* be very different from the unloaded one.

Join those segments together on the outside by something plastic, like
say thin balloon rubber and nothing really changes.

Join them with something heavier but with the similar ability to
'give' and you have a pneumatic car tyre. ;-)

But the 'give' in a car tyre, as has been said here by others, is less
than 1% between hard and soft, over and under inflated.

Yes ... ?

Is that the
difference you're considering?

No, I don't think it is, or the iTPMS might not be able to deal with
tyre wear (even though they could from a programming POV etc)..

If so, then OK I agree with you, a
change in tyre pressure will result in a small 1% change in perimeter
length, and will result in a small 1% change in RPM for a given road
speed.

See above (I think it's bigger).

But I got the impression a much bigger difference was being
assumed/discussed.

As long as you are happy with the fact that it does and why, is all
that matters Chris. ;-)

Cheers, T i m