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


"Chris Hogg" wrote in message
. ..
On Mon, 25 Jun 2018 15:50:06 +1000, "Jeff" wrote:



Yes, but it is the distance between the axle and the road
that determines the rotation rate.

If the length of the perimeter of the tyre is say 1.5 metres, and the
vehicle is travelling at 150 km/hr, that means the tyre rotates 100000
times per hour, or 1666.6 rpm, assuming no slippage between tyre and
road or tyre and rim. There's no getting around that, and there's no
mention of state of inflation, shape of the tyre or axle to road
distance.

Yes, but it is the distance between the axle and the road
that determines the rotation rate.

Why?

Because that's how the physics works. And that should be obvious from
what happens with wheels with no tyre, with different diameter wheels.

Long time since 'O' level maths but I suspect the relationship between the
radius and the perimeter only works for a perfect circle.

Irrelevant to what determines the rotation rate of the wheel.

And it is very easy to actually measure if the rotation rate of the wheel
does in fact vary with the distance between the axle and the road. when
you vary the pressure in the tyre. Not clear if any OBD2 system does in
fact report the rotation rate of each wheel using the ABS sensor on each
wheel, but if they do, that would be very easy to do the experiment with.