# What is the "C" or "CC" in the rim size?



## hotskillet (Nov 2, 2005)

700C, 650C etc, What is the unit of measurement representing the "C"? I thought it was Centimeters but 700 centimeters is over 22 feet. Does it stand for Circumference in mm?


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## wim (Feb 28, 2005)

*Relic.*

The C is not a unit of measurement, but a relic from an old tire sizing standard. The 700 is the outside diameter of the tire in mm. The C was a tire width code that stopped having relevance many years ago. I don't remember how many letters there were in that old width code, but I'm almost sure it started with A as the narrowest tire.


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## TurboTurtle (Feb 4, 2004)

hotskillet said:


> 700C, 650C etc, What is the unit of measurement representing the "C"? I thought it was Centimeters but 700 centimeters is over 22 feet. Does it stand for Circumference in mm?


It is a size designation, not a unit of any kind. For instance, a 650c rim has a bead diameter of 571mm, a 650b= 584mm and a 650a= 590mm. All modern rims a the c designation. - TF


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## brianmcg (Oct 12, 2002)

Funny, I just happened to be reading an old Bicycling from 1991, and this was a question in their letters.

Here is the jist of their answer.

Wheels used to come in different measurements, 700A, 700B, 700C.

In the USA we adopted the 700C as standard. I am not sure if the other sizes are still available in Europe or not.


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## Kerry Irons (Feb 25, 2002)

*One more*

wim, TT, and brianmcg have answered most of your question. As to the "CC" designation, this is the standard reference to cubic centimeter (aka mililiter) and has nothing to do with tires. Any time you see that in reference to tires, it is strictly a reflection of the ignorance of the speaker/writer


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## JCavilia (Sep 12, 2005)

*It really is a relic*



brianmcg said:


> In the USA we adopted the 700C as standard. I am not sure if the other sizes are still available in Europe or not.


The tire widths were A, B, C, with A the narrowest. The number was nominally the outside diameter of the tire. So a 700C rim (widest tire) was smaller than a 700A, which achieved the same 700mm diameter with a narrower tire. All the tires are now narrower than the old standards, so the letters and numbers are both meaningless. A 700C/23mm tire actually has a diameter somewhere around 667mm.

In the 700 range all but 700C are practically extinct, but in the slightly smaller 650 range, 650A, B and C are all still available. 

There's a new standardized system that uses two straightforward measureable numbers (bead seat diameter and rim or tire width, in millimeters), but the old designations die hard. If you look at the label on your 700C/23mm tires, it will somewhere say 622/23. That first number is the bead seat diameter.

Bike tire sizing designations are among the most confusing areas of measurement, and marketing folks keep thinking up new ways to make it confusing. Witness the recent invention of the "29er" -- a 700C (622mm BSD) rim with mountain-bike width tires.

Here's way more than you want to know:
http://www.sheldonbrown.com/tire-sizing.html
http://sheldonbrown.com/harris/tires/index.html


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## Mark McM (Jun 18, 2005)

*"C" tire size and "C" computer language*



hotskillet said:


> 700C, 650C etc, What is the unit of measurement representing the "C"? I thought it was Centimeters but 700 centimeters is over 22 feet. Does it stand for Circumference in mm?


The origin for using the letter "c" in the tire/rim size (700c, 650c) is basically a parallel to the naming of the "c" computer programming language - it followed "b". Just like 700c tire/rims followed 700b tire/rims, the "c" programming language followed the "b" programming language.


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## Mark McM (Jun 18, 2005)

*E.T.R.T.O sizing*



JCavilia said:


> There's a new standardized system that uses two straightforward measureable numbers (bead seat diameter and rim or tire width, in millimeters), but the old designations die hard. If you look at the label on your 700C/23mm tires, it will somewhere say 622/23. That first number is the bead seat diameter.


I think you're referring to the E.T.R.T.O. (European Tire and Rim Technical Organization) sizing (which isn't al that new).

The standard convention for E.T.R.T.O size marking is WW-DDD, where WW is the tire width, and DDD is tire bead seat diameter. For example, a 700x23c tire, which is 23mm wide and has a 622mm bead seat diameter, would be marked as 23-622 (not 622/23).

Tire manufacturers are often play loose and easy with tire width dimension - a tire marked as 700x23c might actually be anywhere from 21 to 25mm, for example. However, E.T.R.T.O. sizing follows a strict formula. In particular, the width designation is measured by laying the tire casing out flat, measuring the distance between the tire beads, and dividing by 2.5.


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