"Jeff" wrote in message
...
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.

True, but the easiest way to visualise the rotation rate for a unit distance
travelled is to divide the distance travelled by the circumference to give
the number of wheel revolutions.

However... it all hinges on what the *effective* circumference is: it is the
circumference of an imaginary circle with the same radius as the distance
between centre of the axle and road surface, which will be less than the
no-load radius because the tyre is slightly flattened where it comes in
contact with the road.

I presume this means that if you took a piece of string and passed it round
the circumference of a tyre this value would be smaller than the no-load
circumference because the part that is in contact with the road will be a
flat rather than a curved profile. This assumes that the radius of the rest
of the tyre doesn't increase significantly when a load is applied to the
tyre - presumably this is constrained by the steel reinforcing belts in the
tyre.


I wonder how much smaller the in-contact radius is than the no-load radius:
what sort of proportion is the reduction, typically? I'll have to measure
the actual distance from the centre of the hubcap to the ground, and then
jack the wheel up until it first touches the road and measure again. Note
that you can't measure to a part of the car body because of compression of
springs :-)