# Re: Unicycle computer calibration [was Re: Fastest speed on a Coker?]

>On Wed, 3 Sep 2003 13:01:15 -0500, Ken Fuchs <kfuchs@winternet.com>
>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.

klaasbil_remove_the_spamkiller_@xs4all.nl (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
>0.18%.

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:

>1. Wobble.
>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
>of 3%.

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
pressure.

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
percentage.

Thank you Klass for your clear comments and insight into the bike
computer calibration problem for unicycles.

Sincerely,

Ken Fuchs <kfuchs@winternet.com>

Re: Unicycle computer calibration [was Re: Fastest speed on a Coker?]

On Thu, 4 Sep 2003 12:48:17 -0500, Ken Fuchs <kfuchs@winternet.com>
wrote:

>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
>percentage.

That is all clear. Of course we have to bear in mind that the
difference between the two kinds of speed is only of the order of 1%,
as pointed out by Ken. So many people would consider the following
argument to be nitpicking; but hey, I like to pick a nit every now and
then

It is a matter of taste what one should consider as the real speed (or
real distance, for that matter). Probably most would agree that road
speed is real speed, or ‘effective’ speed as Ken almost called it.
Yet, contradictorily (SP?), most people seem to calibrate their
computers on a straight-line rollout, sometimes even unloaded.

However, tyre speed (and distance) are equally real in their own
right. For one thing, tyre speed is the real physical speed with which
the tyre contact point travels. Also, tyre distance (as opposed to
road distance) determines tyre wear (combined with a lot of other
factors, including funnily enough wheel wobble in itself). And when
you race between a start line and a finish line 100 metres apart, the
wheel will actually have travelled (say) 101 metres, in a wobbly line
indeed.

Not being able to make a single choice in this matter, the spreadsheet
in which I record my road rides calculates both tyre speed and road
speed.

## Klaas Bil - Newsgroup Addict

I go a sort of ok speed on my Coker… - Roger Davies