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700C - What Does It Mean?(5 posts)

700C - What Does It Mean?B2
Dec 9, 2001 9:31 AM
What does 700C represent? I didn't know the answer and did a little homework. Here's what I came up with – Not a lot these days, unless you're riding 700Cx40 tires.

Theoretically 700 represents the diameter of the wheel at the tire tread in millimeters. However for this to be true, you'll need to get up into 700Cx40 tire size for the wheel diameter (at the tread) to equal 700mm.

To calculate the rim diameter you need to know the height of the tire. The "C" in 700C represents the nominal height of the tire. The only problem is that the tire height designated as height "C" is not the real height, but only a nominal measurement. The nominal value of "C" is 1 5/8" or 41.275mm. If you take 700mm overall – 2xC or 82.55mm, you get 617.45mm rim diameter. However since this is based on a nominal tire height, it's not correct. The standard convention today (as near as I can tell) is 622mm rim diameter for a 700C wheel. Bottom line - a siginificant amount of data listed in tire sizes has little to do with reality.

There's a lot more history behind the evolution of tire sizes. Check out http://users.globalnet.co.uk/~pattle/nacc/arc0307.htm for more info.

Bryan
another linkDog
Dec 9, 2001 10:30 AM
http://www.sheldonbrown.com/tire_sizing.html#traditional
700c means 667 mmKerry Irons
Dec 9, 2001 12:41 PM
I've measured a few 23mm tires "in action" and come up consistently with a 667mm actual diamter - in use. In other words, one revolution of the wheel with a rider on the bike is 2095mm. (Divide by Pi and you get 667). That's 26.25 inches, for those who want to convert their archaic gear charts to actual gearing (most gear charts assume a 27 inch diameter, giving numbers that are nearly 3% high).
Yes, 2095mm is the "C" I have measured.mja
Dec 10, 2001 5:38 AM
ghj
the 'C' designation...dsc
Dec 9, 2001 4:03 PM
here's the history lesson that I was given when I asked this same question awhile back. It seems tires did used to be measured based upon their outside diameter. But, since there were varying tire heights, the inside diameter of the tire varied. So there had to be different size rims to accommodate them. There were A, B and C sized rims. This was a pain, so eventually it was decided to standardize on the rim diameter, and vary the outside diameter of the tires. The standard rim size that was settled upon was the 'C' size.

I've been told that you can blame the French for this, as they were the ones that came up with the different size rim diameters, in the first place :-)

-Debi