"Dave Plowman (News)" wrote in message
...
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.

At first I thought that it would be the minimum radius, measured to the
centre of the contact patch, which would be important. But having seen the
diagrams that were referred to earlier in the thread, I now wonder whether
the effective radius is somewhere between the minimum and the no-load
radius, to take into account the fact that the radius varies depending on
whereabouts on the contact patch you measure it.


Part of the problem with all this discussion is that we are talking about a
very small variation between loaded radius and no-load radius, which makes
it difficult to measure in the real world with a crude
tyre-on-a-road-surface measurement. The chances are that if you measure how
far a tyre travels for a given number of revolutions, firstly with
negligible load and/or pumped-up tyre, and secondly with a loaded and/or
half-way flat tyre, you would find it difficult to see any difference that
was larger than experimental error. This assumes that you measure a fairly
small number of rotations to keep the total distance travelled down to a
length that can easily be measured.

I don't really have a feel for how much of a difference there is for a
typical tyre, and therefore how many revolutions you'd need to measure
before you observed a difference that was noticeable.

All you can do is to try to make the difference as large as possible (deep,
high-profile tyre; significantly under-inflated - to maximise the amount of
flat on the contact side of the tyre) in order to demonstrate that there
*is* a difference in distance travelled per revolution compared with a truly
circular tyre.