February 19, 2005

    What does the “c” in 700c stand for? My first guess would be, of course, centimeter, but a wheel with a diameter of 700 centimeters would be nearly 23 feet across! Thanks.

Chris S.

    The "C" formerly indicated the width of the tire. 700-size tires come in a variety of widths and "A" once meant the narrowest while "D" meant the widest. Nowadays, the letters more correctly indicate tire diameter, for reasons that remain obscure—tho not as obscure as what that "700" really means. Let me elaborate.
    As you’ve obviously seen, bike tires have printed on their sidewalls a sequence of two sets of characters separated by "x". The first set, usually a number, tells you the tire’s diameter (measured, allegedly, at the bead, the circular piece of metal or plastic embedded into the tire that holds it into the rim). The second set of characters indicates the tire’s width, sometimes followed by an extra letter at the end to indicate, uh, the true diameter (I swear I did not make this up).
    It turns out that a 700 C tire actually has a bead diameter of 622 millimeters. So why do they call it 700? For the same reason a 2-by-4 piece of wood has neither 2 nor 4 as its dimensions; it really measures 1.5 by 3.5 inches, but "2 by 4" rolls off the tongue better, and probably sells better too. To complicate matters, a tire labeled 700 B has a diameter of 635 mm and a 700 D tire has a diameter of 587 mm.
    This bit of subterfuge doesn’t, however, stop at 700-size tires. It seems that you can’t trust any of the inch diameters, either: A tire labeled 26" might actually come in at around 24 ¼ inches (it varies by type of tire, believe it or not), while a tire that claims 27" might measure roughly 24 ¾ inches.
    It gets worse. The second set of characters, the width measurement, doesn’t cause much trouble if it appears in millimeters, as in 700 x 32 (usually shown as 700 x 32 C, with the C, as I said before, indicating the true diameter—in this case, 622 mm).
    The mischief arises if the width appears in inches. It sometimes appears as a decimal, as in 26 x 1.75, and sometimes as a fraction, as in 26 x 1 ¾. You’d think that a 26 x 1.75 tire has the same size as a 26 x 1 ¾ tire. In reality, the first one has a slightly larger diameter.
    Fortunately, you won’t usually get thrown by this amusingly chaotic measurement "system" unless you want to replace your bike’s wheel or install a tire having a different width. In that case, look for another designation on your existing tire, a designation having two numbers separated by a hyphen—e.g., 32-622. This designation, established by the trusty European Tyre and Rim Technical Organisation (ETRTO), tells you, in millimeters, the width followed by the true diameter. Most new tires come with the ETRTO designation, so when replacing a tire or wheel you can ignore the "x" designation.
    By the way: Some call the ETRTO designation the ISO (International Standards Organization) designation. Don’t ask.
Mr Bike

Back to questions