# cycle computer on 28" wheel

I have a 28" wheel with a tire that is marked as follows:

700 x 45c - 28 x 1 5/8 x 1 3/4

I bought and installed an inexpensive UDC / Pyramid cycle computer.

The instructions include settings for standard wheel sizes including the following:

28" (700B) — 2237

When I measure the outer diameter of the tire (with a seamstress tape), I get 2500 mm. I know it’s inexact, but should I expect to be off 2.63 cm? Am I measuring the correct thing?

Using 2237 as the C-factor, I just surfed down the pretty steep hill next to my house in the dark at 13.7 mph. My uni has 102mm cranks, and I have a subtle death wish and a great deal of fast twitch muscle fiber. Is there any chance this is accurate?

Knowing you, it is. Also knowing you, you did that w/o any safety equipment. I have heard others on this forum say that your brain is your best piece of safety equipment, but if you were doing 13+mph on your 28 with 102’s in the rain after dusk… I’d say you were completly w/o safety equipment. Were was your wife? You are so in trouble if she starts reading this forum.

C = (28" * 2.54) * Π = 223.43 cm

I measure my tire’s circumference by laying a thin line of water on the dry floor of the parking garage downstairs, riding over it, and measuring the distance between the line of water and the damp dot tracked by the tire. It doesn’t account for wobble, but I guess it’s as accurate as any other method I’ve used.

Thanks JP. My next question is whence 28"? If it’s supposed to be the outer diameter of the tire, I think it’s a little short - at least by my crude measurement method. How much error is there in such a measure anyway? And how does it affect the calculated speed? If the true circumference is greater than the C-factor I’ve entered, I’m actually going faster than the calculated speed, correct? If it’s less, I’m going slower. (Methinks.)

I like this idea. Thanks. I’m going to try it today. Plus now we know for certain that Weeble’s wheel wobbles, but it don’t fall down.

I was wearing my underwear, flipflops and fuzzy earmuffs. (I couldn’t find my aviator’s cap.) What else do I need? I mean, what do you suppose motivates a guy to ride down a \$90 unicycle that way anyway?

A mile is 1609344 mm.

If I use the three C-factors I have from JP’s calculations (a), the cycle computer instructions (b) and my seamstress tape measurement © to calcluate revolutions…

a) 1609344 / 2234 = 720.39 revolutions per mile
b) 1609344 / 2237 = 719.42 revolutions per mile
c) 1609344 / 2250 = 715.26 revolutions per mile

If my outer diameter measure is correct, I rode 4 extra revolutions, making my “mile” about 11.5 meters too long.

(I’ll have to figure out later how to calculate the effect on speed of the extra 11.5 meters.)

All wheel sizes are nominal. A 700c is not exactly 700 mm across. A 20 inch tyre is not exactly 20 inches across. There is also variation according to tyre pressure and the weight of the rider.

Take the diameter, measured between two vertical sticks (or a door post and a spirit level). Choose your units (inches, millimetres, cubits, hands, whatever) and multiply by 3.142.

That will give you the circumference of the tyre at normal pressure with no weight on it.

Knock about a centimetre off the diameter for deformation of the tyre when ridng, if you must.

1 centimetre off a 70 cm nominal tyre size will make 1.4% difference. At speeds of around 10 mph, that’s negligible unless you are obsessive.

Most of the time, you will be measuring against your own previous achievements anyway, and the level of inaccuracy will be exactly constant, and therefore irrelevant.

If you are competing against someone else, variables like their route, the wind speed and so on will make more difference than any honest error in the setting up of your computer.

Well said! Thanks. (To wit: Today I hit at least 15.8 mph.)

Thanks as well for the technical detail. Very helpful.