On Tue, 26 Jun 2018 19:57:42 +1000, "Jeff" wrote:



"Dave Plowman (News)" wrote in message
...
In article ,
Jeff wrote:
No. Once yopu put a non circular tyre on, you vannot talk about radius.

The only distance that matters is the distance between the axle and
the road. That is what determines the rotation rate of the wheel.

Yes, it is better not to call that the radius.

Radius between centre of wheel and point of contact to the road surface.
That is the only relevant part as regards how far the car travels per
rotation of the wheel. Not any theoretical amount.

Yes, but you also have a problem with explaining why it isn't the
circumference of the tyre which doesn’t change much isnt what
determines the rotation rate of the wheel.

Looks like we have a problem explaining it to Turnip so I think he is
just trolling us all now (he asks the question but he neither wants
nor cares for the answer).

Let's agree on some things we can probably all agree and consider to
be facts because of what works ITRW (iTPMS).

1) The unloaded circumference of a car tyre is irrelevant to anything
unless you are just about to take off. ;-)

2) The loaded peripheral length must be different to the unloaded one
and different again under low pressure conditions (for iTPMS to work).

3) The tyre isn't slipping on the rim nor 'skidding' (in the
traditional understanding) on the road whilst in normal use.

4) Steel belted tyres are constructed with the wires set on the
diagonal (as they zig zag across the tread, around the bead and back
again) and form a parallelogram (like a electric train pantograph or
pop rivet gun).

5) One of the FACTS about how a parallelogram works is as it gets
WIDER it becomes SHORTER ... and as it gets longer it gets narrower.

So, as a (car) tyre rotates, it flattens out when it is in contact
with the road and the tyre construction pantographs (because as it
flattens it gets wider) causing the tread to *shrink* longitudinally
(around the periphery) as shown on here ...

http://the-contact-patch.com/book/ro...-contact-patch
Figure 12
Longitudinal compression or bunching of the tread

.... and therefore cause the tyre / wheel to rotate faster than it
would if you compared it with it's unloaded circumference.

So, within practical constraints, from the typical correct running
pressure to something no more than 25% less (to conform with the TPMS
regs), the tyre shortens is peripheral distance (let's call it the
'effective circumference' and of course, an 'effective circumference'
would have an 'effective radius') so that iTPMS can do what they do.

No (in significant terms) scrubbing, no slipping of the tyre on the
rim, no magic, just plain mechanics and physics that even I can fully
understand!

Cheers, T i m