I recently installed cyclocomputers on my bikes, and was faced with the
problem of finding the tire circumferences. I read up on it and found
several methods which seemed to be either inaccurate or cumbersome: for
example, rolling the unloaded wheel one revolution or counting
revolutions while riding the bike with a helper's shoulder for support.
Measurements with a bicycle wheel can be very accurate (the recommended
way of marking off track distances in field sports is to ride a bicycle
with a Jones counter attached), and it seemed a shame not to set the
tire circumference as accurately as possible. The following is what I
came up with. It is accurate to at least a quarter mile in a century,
and could be made more accurate if desired. I'd be interested in any
alternative method of the same accuracy.
First of all, the tire circumference must be that of a loaded bicycle,
ridden in the same way that it is ordinarily ridden. To me this meant
that a bicycle would have to be ridden over a measured course, and I
chose 25 meters, as a reasonable distance -- small enough to measure and
long enough to represent actual riding conditions. I needed to count the
number of wheel revolutions which seemed awkward, but then I realized
that there would be about 12 revolutions in 25 meters, and so the finish
line would be at 12 plus or minus a bit; hence the following:
(1) Mark off a 25 meter distance.
(2) Lock the front wheel with the valve down an place it at the start mark.
(3) Ride the course, stopping near the finish mark, and count the number
of spokes between the ending valve down position and the finish mark.
(4) Repeat this in both directions several times and average the results.
If the finish mark is at 12-3/36 revolutions, then the tire
circumference will be 25000/(12-3/36)=2098 mm. (Of course, you should
adjust this for your spoke count.)
--
Bob Wheeler --- http://www.bobwheeler.com/
ECHIP, Inc. ---
Randomness comes in bunches.