| Question: why is 700 smaller than 27?
Speculation:
I think that the problem might have arisen from the fact that the
wheel size was quoted as the outside diameter of the tire. If the
tire was more balloon like, then the same outside size would mean
a smaller rim.
Gospel:
When matching tires with rims, the dimension that really matters
is the bead circumference. Most new tires are marked with the new
ISO standard designation of the form XX-YYY, where XX is the width
in mm and YYY is the bead seat diameter in mm. For example, a
narrow 27 inch tire might be 25-630, and a wider 27 inch tire
might be 30-630. Sutherland recommends that the diameter be
within 1mm, or you risk blowouts.
It might be better if everyone just ignored the "traditional" names
for wheel sizes. The old markings vary strangely from country to
country. I cribbed the following table from Sutherlands but I
left out the ISO width value because it doesn't matter as much for
fitting the rim.
Bead ! ISO !
Circum. ! Mark ! British ! French ! Italian ! other
--------+--------+----------+----------+----------+-----------------------
2032 ! **-647 ! 28x1-1/4 ! 700 ! !
2016 ! **-642 ! 28x1-3/8 ! 700A ! 700x35A ! 28x1-1/2 Swedish
1994 ! **-635 ! 28x1-1/2 ! 700B ! 28x1-1/2 ! 28x1-5/8 Swedish
1978 ! **-630 ! 27x1-1/4 ! 27x1-1/4 ! 27x1-1/4 !
1955 ! **-622 ! 28x1-3/4 ! 700C ! 700C ! 28x1-1/2 Canadian
1876 ! **-597 ! 26x1-1/4 ! 650 ! 650x32 ! 26x1-3/8 Schwinn
1854 ! **-590 ! 26x1-3/8 ! 650A ! 26x1-3/8 ! 26x1-1/2x1-3/8 Dutch
If you find that the national markings are confusing, it is
because they ARE confusing. Notice that the 700C is actually a 28
inch wheel. Notice also that if someone hands you a 28x1-1/2 tire,
it can be just about any size. I left out some of the more
obscure sizes.
-Jeff Bell
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