Originally Posted by
jonathanb715
Nope. If you took a tape measure all the way around the outside of the tire, it will give the same distance whether the tire is loaded down or not (within reason - the material obviously does stretch in the real world, but not enough to matter).
We don't care what the rest of the tire is doing.
Originally Posted by
jonathanb715
This is the circumference.
No, this isn't the "circumference" because the loaded and low-pressure tire
is not a circle!
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Let's try it this way.
1)
Remove the tire. Do the rollout. That's the
smallest possible radius or circumference (*). This represent a wheel without any pressure and/or a very large load.
2)
Put a completely rigid tire on (one that does not deform). Do the rollout. That's the
largest possible radius or circumference. This number represents a tire that at maximum pressure without any load.
A real tire with any particular pressure and load (weight of the cyclist and bike) will yield some radius or circumference
between the two above values.
If you have less pressure or more weight (more deformation), the radius or circumference will be closer to the smallest radius or circumference.
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* (This number is actually too small because it doesn't include the thickness of a completely uninflated tire.)