Bicycle Mechanics - approx. circumference for 700x26C tires?

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adelie
05-13-04, 01:00 AM
hello all. i purchased a computer (cateye...) for my bike, and its tire circum.(measurement) chart didn't note the 700x26C size.

does anyone own similiar sized tires and have calculated a circumference? i'd really appreciate your help and feedback. thx.

regards,

ben

fujibike
05-13-04, 04:37 AM
The ideal method of determining circumference is to do a rollout. Worth the effort, otherwise, the formula for calculating circumference is Pi x Dia.
Ed Holland
05-13-04, 06:35 AM
Sigma sport manuals for computers are available on-line, and contain reference data for tire circumference. Check out this link:

http://www.sigmasport.com/media/produkte/bikecomputer/alteanleitungen/pdf/anleitung_bc800_00.pdf

It has data for 700x23, 700x25 and 700x28. It is pretty easy to interpolate this and get a useable number e.g. take the difference between the '25 & '28 widths and add 1 third of this number to the 700x25 figure (round up/down as appropriate).
In fact this works out as one or two parts in 2146-ish, which is below 0.5%. According to changes in your tire pressure etc., which affect circumference it is expecting a lot to get computers to read to this sort of accuracy so you would be not too far out just picking the closest tire size. It would be a good idea to compare this to a rollout measurement as fujibike suggested.

Cheers,

Ed

Gonzo Bob
05-13-04, 06:49 AM
A good way to get a reasonably accurate number is to take the bead diameter, add twice the tire width, and multiply by pi. Thus 700x26 is approximately (622+2*26)*3.1416 = 2117mm
Don Cook
05-13-04, 12:19 PM
I have accurately measure 700x25c tyres mounted on Mavic Open Pros. They measure 2096mm with a 163 lb. rider mounted. Since you said approximately, this ought to get you to within a few millimeters.
John E
05-13-04, 01:53 PM
Tyre pressure variations probably affect the effective circumference by several mm.
adelie
05-13-04, 02:29 PM
thx, everyone. your prompt and thoughtful posts have helped tremendously!

ben
Poppaspoke
05-13-04, 06:34 PM
Find a clean level stretch of pavement. Place the front tire with the valve at the bottom of the wheel. Make am chalk mark where the valve section of the wheel touches the road. Ride as straight as possible (don't walk the bike) for ten rotaions of the front tire. Mark the pavement again with the valve closest to the pavement. Measure the distance between the chalk marks. Average several trials and divide by ten.