On Wed, 3 Sep 2003 13:01:15 -0500, Ken Fuchs
wrote:

To calculate wheel size, one must ride a carefully measured one turn of
the wheel or several turns / number of turns. Measuring my wheel
without riding resulted in a circumference that was about 2% longer
(2839mm) than measuring when riding (2834mm). Riding appears to
compress the tire slightly, giving a smaller than expected
circumference.

You've probably made a typo in the numbers or misplaced a decimal
point. The difference between those two rollouts is actually about
0.18%.

There is also an unclarity in my mind about the definition of 'speed'.
I can think of two definitions for speed. One is the circumference of
the wheel multiplied by the cadence, I call that tyre speed. The other
is the distance covered per unit of time, I call that road speed. The
two are different for (again) two reasons; for both of these, their
effect is in the same direction:

1. Wobble.
2. Tyre compression.

The first is fundamental, and it is a philosophical question what the
'best' definition of speed would be. The second could be avoided by
measuring the rollout while sitting on the uni. I measured the rollout
of my wheels when unloaded but when I sit on them the tyre compresses
and the effective wheel radius decreases. According to my
measurements, the two effects combined cause a difference on the order
of 3%.

Klaas Bil - Newsgroup Addict
--
I go a sort of ok speed on my Coker... - Roger Davies