Bicycle Mechanics - What circumference are my 26x1.3s?

I have 26x1.3 Continental Sport Contact slicks on my Matrix 750 rims (stock rims on '04 Trek 4300).

Can anyone give me an accurate circumference size given that data? Google has not provided, for once :)

Cheers,
Al.

No one here will be able to give you an exact measurement. Either use a tape measure and wrap it evenly around the tire or mark a point on the tire and measure the distance the tire travels along the ground after 1 revolution.

Yeah, I know how that's done. was hoping someone might be able to tell from the specs, it'll be a while before I can make it to a h/w store to pick up a measure.

Thanks anyways.
Al.

Remember that the tyre is made of a very compliant material so your actual diameter and thus your circumference can change based on internal pressure and loading. The only real accurate way to determine the circumference is to measure the rollout at your expected riding pressure and load.

The incredible Sheldon Brown LINK (http://www.sheldonbrown.com/cyclecomputer_calibration.html) gives the following circumferences:

1913mm - 26x1.0
1953mm - 26x1.25
1985mm - 26x1.5

Based on that 1961 sounds like a pretty darn good guess for 26x1.3, but a roll out is the only way you're going to get a really accurate number.

If you don't have access to a table and you don't want to do a roll-out you can get a very good approximation by calculating...

circumference = (bead_diameter + 2*tire_width)*pi

For 26x1.3, bead diameter is 559mm and width is about 1.3*25.4 = 33mm (measure actual tire width for an even better estimate).

circumference = (559 + 2*33)*3.1416 = 1963mm

Schreiber: I'd seen Sheldons site alright, disappointed 26x1.3 aint on there :) Interesting to see how close that was to Gonzo's math!

khuon: you deadbeef geek... yeah I'll do the roll out asap.

Gonzo: I'm a bike newbie so little lost on some of that bead diameter stuff, and I'm not sure what width has to do with it (is width referring to the distance between the contact part of the tire and the bottom of the rim?). Your math comes out v. similar to Schreiber - hopefully that's good enough for the moment.

Thanks for your help guys.
Al.

Gonzo: I'm a bike newbie so little lost on some of that bead diameter stuff, and I'm not sure what width has to do with it (is width referring to the distance between the contact part of the tire and the bottom of the rim?). Your math comes out v. similar to Schreiber - hopefully that's good enough for the moment.


"Clincher" rims and tires are also called "hooked-bead". The tires have a wire or kevlar "bead" which seats underneath a "hook" in the rim. The bead diameter for some popular tire/rim sizes are...

26" MTB - 559mm
26" road (650C) - 571mm
700C road - 622mm
27" road - 630mm

Many tires have the width and bead diameter printed right on them - e.g. 23-622

You've got to add twice the height of the tire to get the total diameter. The height is usually very close to the width. So adding twice the width is a good approximation.

What it comes down to is that all tires are different. Two tires of identical dimension can be considerably different. The only true way is to hop on and measure the roll out. And actualy, I wasnt surprised it wasnt on Sheldonbrowns website......that place isnt the end all to bicycle knowledge nad Google is well.....just a search engine.

Gonzo...Good math work.
khoun....I'm with your theory.

Does having a computer make you lzy , like a car means you don´t walk ? ( see cnn´s comment that the more you drive the more likely you are to be fat )
I mean , it takes longer to do the seach than to do a roll and measure .