Converting tire length into mm?

[suhollyberry]

I bought an odometer for my bike, but I'm not sure of the amount of mm to put in for the tire. My tires say they are 700X29c, does anyone know what to put in my odometer? It defaulted at 2085mm so that is what I kept it at. I just want to make sure I'm getting the correct mileage.

Thanks!

[SadieKate]

What computer do you have? The manufacturers usually provide a chart or you can do the rollout method for measuring (which at the moment I can't articulate because the brain is still on vacation).

[SnappyPix]

29c is quite an unusual size for a tyre.
I'd estimate the tyre size to be around 2140 - the size for a 700 x 28c tyre is 2136; next size up is 700 x 32c which is 2155.

Leaving it at 2085 will leave you short on each pedal revolution.

If you want a truly accurate measure, you'll need to actually physically measure the distance covered by each revolution.
Sheldon Brown has a guide on how to do this on his site:
http://www.sheldonbrown.com/cyclecomputer-calibration.html#rollout

[suhollyberry]

I think it's called cyclomaster. They had the layouts, but none of them fit my tires. I guess I can try to do the rollout method. I think they have instructions included.

[suhollyberry]

I have a giant road bike, with the standard tires that came with it.

So by having it set at 2085, that means it is underestimating my miles?

[DeniseGoldberg]

If you don't want to do the rollout, Sheldon Brown also has a calibration chart on his site that you can use as a starting point: http://www.sheldonbrown.com/cyclecomputer-calibration.html#ccc

Of course 700x29 is not included in the chart. That sounds like a very odd size. And wow - this is a first for me. I just did a search on google for
bicycle tire 700x29c
and came up with only 1 document. I've never done a search before that matched only one thing. (If you're interested, the document that matched was a catalog for mountain bike tires (or tyres, as they spelled it) from a company called Geax (http://geax.sostanza.it/vedit/18/immagini/File/geax2006_en.pdf). Of course that doesn't help with your tire calibration, but I thought it was interesting).

--- Denise

[Geonz]

Come on out to ILlinois, where there are scads of places to ride a mile (the standard distance between roads on the prairie) and see how your odometer compares, and adjust accordingly :-)

[HillSlugger]

I wrapped a string around my tire (700x32c) and then got the length from a tape measure. On my first ride my distances were all about 5% higher than what my car odometer had said. I decreased the setting by 5% and ended up short on my next ride. I ended up nearly perfect with a -2.25% adjustment. I don't know why this is necessary. Who can be sure that the car odometer is right either?

[DebW]

I wrapped a string around my tire (700x32c) and then got the length from a tape measure. On my first ride my distances were all about 5% higher than what my car odometer had said. I decreased the setting by 5% and ended up short on my next ride. I ended up nearly perfect with a -2.25% adjustment. I don't know why this is necessary. Who can be sure that the car odometer is right either?

The tire circumference is probably different with a rider on the bike. You should have your weight on the bike when you do a roll-out. That probably accounts for your difference. Of course, the car odo may not be that accurate either.

[ridebikeme]

Sounds like you have had some fun working with your computer!:):)

I have always rolled out the circumference for my computers as well... although there are several different ways to do it, here's what I do..
My computers have always measured the same as the odometer on my car...

Start with your valve at 6:00 and make mark on the floor from that spot... roll the bike in a straight line until the valve is at 6:00 again, and put another mark on the floor. Take a tape measure and measure the distance between the two marks... if you don't have a metric tape measure, you can always use the 2.54cm= 1 inch.

I also would mention to you that different brands of tires(even if they are the same size)measure differently, so make sure that you measure them to be accurate.