There were never any 27in tubular tyres it was a bastard size introduced by Dunlop in the late 1930s. What we know as 700C tubulars have always been the most common size though various sizes of 26in tubular were popular at one time. 700C sized rims were made to take 28 x 1 3/4in tyres originally but were somehow adopted along the line for other cross sections too. They did become really popular with narrow section tyres until the middle/late 1970s when Mavic introduced the original E2 rim and Michelin, the Elan tyre. You should not need to adjust the brake blocks when changing between 700C sprint rims for tubular tyres and 700C rims for wired-on tyres but a change in the width of the rim can sometimes affect your brake block position. Hilary Stone
Steve Freides wrote:
> There was a time when clincher bicycle rims and tires were usually the
> larger 27" size rather than the smaller 700C size. At that time, were
> sewups also 27" and not 700C? I'm curious to know if there's a
> period-correct way to put 27" rimmed sewup wheels on a bike that came with
> 27" clinchers (and not have to deal with different brake reach, etc.)
>
> Let me observe, as someone who rides sewups in the year 2001, that at least
> some 700C sewup rims seem larger in diameter than 700C clincher rims - I
> often find myself having to adjust my brakes shoes to the very tops of their
> slots. I believe I recall a Campy wheelset catalogue from a few years back
> where they list the ISO rim size; the number for their sewup wheels was 10mm
> larger than for their clinchers, 632mm vs 622mm if memory serves correctly.
> Of course, bead seat diameter means nothing for sewups, but the numbers were
> there nonetheless. Perhaps because 700C sewup rims are already larger there
> was/is no need for a 27" sewup rim size?