>On Wed, 3 Sep 2003 13:01:15 -0500, Ken Fuchs <email@example.com>
>>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
firstname.lastname@example.org (Klaas Bil) wrote:
>You’ve probably made a typo in the numbers or misplaced a decimal
>point. The difference between those two rollouts is actually about
Thanks for pointing out that error. The corrected sentence should read:
Measuring my wheel without riding resulted in a circumference that was
about 2% longer (2839mm) than measuring when riding (2789mm). A
difference of 50mm and not 5mm as previously stated.
>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:
>2. Tyre compression.
My measurement didn’t take into account wobble. I simply pedaled my
Coker exactly one rotation of the wheel on a straight line, next to a
support like a fence to help keep me straight. Thus, I measured the
wheel circumference with rider weighted tire compression, but definitely
wobble was not accounted for. I’m measuring tire speed with
compression; wobble effects should be included as well.
>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
As I measured it, tire compression effectively reduces wheel
circumference by 1.76%. Taking Klass’ total reduction of 3% and
subtracting the tire compression of 1.76% would give us a wheel wobble
reduction of 1.24%. However, this 1.24% doesn’t mean much, because it
is a composite measurement of two riders, Klass Bil and myself.
Tire compression probably varies with rider weight, tire wear and tire
Wheel wobble probably varies with speed, Q factor, rider leg, crank and
pedal mass. (Maybe rider skill can affect it too, but that might
as a side effect actually reduce speed; effort making the wheel wobble
less is not expended in actually making it go faster.)
So to get an accurate wheel circumference, one really should ride a long
accurately measured course in the way one expects to ride. One should
go about the speed one most often expects to go. The Q factor is a
constant for the Coker until the hub is changed. The Wobble mass is a
constant for the Coker and rider until the cranks/pedals are changed or
the rider bulks up (or loses weight in) his legs.
If one gets an accurate wheel rotation count, one can simply divide the
distance by the count to get the effective wheel circumference,
including both wobble and tire compression effects.
Otherwise, one can simply enter one’s best guess of the effective wheel
circumference. Ride the measured course and compare the real (measured)
distance to the computer’s distance. Compute the difference as a +/-
percentage and adjust the computer’s circumference by the same
Thank you Klass for your clear comments and insight into the bike
computer calibration problem for unicycles.
Ken Fuchs <email@example.com>