If you look at the tyre as it is sitting on the ground, you can see that the
bottom portion is bulged out as the sidewalls have flexed with the load of
the car.
The bottom part of the tyre remains like this ( in a fashion) as the car is
driven.
A more accurate measurement is to measure the rolling circumference and then
calculate the rolling diameter. Measuring the rolling circumference can
easily be performed by placing a mark on the tyre at the lowest point and
placing a corresponding mark on the ground. Roll the car forward for one
tyre revolution and mark the ground again when the mark on the tyre is at
it's lowest point. Measure between the two marks and there you have it. (the
circumference that is). Divide the circumference by pi and you have the
rolling diameter.
Another point to consider, for those very serious about all of this, is the
fact that the tyre becomes more doughnut shaped at higher speeds and the
rolling circumference increases. If you doubt me look at a tyre on a car on
a chassis dynamometer and you can actually see the tyre change shape.
cheers from west oz
John Rowe
>I had to think about this one. Providing the diameter of the tire for the
> computation is just so the program can determine the circumference. The
> circumference, or distance around the tire will remain constant despite
> the
> rolling diameter impact, thus the calculations should be reasonably
> accurate
> when based on actual tire diameters. Am I wrong ?
>
> Mike L.
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