WHAT DOES TIRE SIZE "700 C" MEAN?
February 19, 2005
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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.
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
obscuretho not as obscure as what that "700"
really means. Let me elaborate.
As youve 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 tires 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 tires
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 doesnt,
however, stop at 700-size tires. It seems that you cant
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
It gets worse. The second set of
characters, the width measurement, doesnt 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 diameterin 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. Youd 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 wont usually
get thrown by this amusingly chaotic measurement "system"
unless you want to replace your bikes 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 hyphene.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"
By the way: Some call the ETRTO designation the ISO (International
Standards Organization) designation. Dont ask.