This has been an interesting dialogue to me, if for no reasons other than that I just found these Weinmann/Gitane 700c clinchers, and the fact that I still have 27" wheels fitted to bikes that I still use for everything from criteriums to CX and mountain biking, and everything in between.
With yours (and Martin's) witness to relatively ancient 700c (622bcd) clincher rims, and given that only the smallest tires (read "sewups") will produce a 700mm overall diameter on such rim sizes, I gather that the 700c designation succeeded the use of 700c tubulars for racing, with the brake track diameter being the only thing they really had in common, and could thus be readily interchangeable. This probably seems more than obvious to you guys, but I'm still learning fast here.
Insignificantly, my 1967 PX10 had been retrofitted with 27" Rigida 13-19's and Tipo hubs when I found it, and they fit fine, but I don't know when that retrofit was actually made, probably around 1979 if I were to guess.
I was surprised to find my local Trek dealer carrying a Bontrager-branded Cheng-Shin 27x1-1/8" tire for just 8.99USD. This one is well-sized to fit the whole range of 27" rim widths while retaining a trim profile at just 25mm actual on a 22mm rim. It is a gumwall, but with tan gum so thin the cords show through, and a thin, true semi-slick tread cap. I was happy to find these, and they feel fast and smooth with only 80psi for my 195lb bike+rider weight. The near-identical Cheng-branded variant, #C-732, has a slightly more pointy, sort-of-ribbed tread and measures the same. I would have figured that inflation and oil costs would already have these tires selling for $15 at a bike shop, so I bought all they had and will be filling a plastic bin liner with 10 or so more for future use.
David Snyder
Auburn, CA
USA
> 700C size rims have been around for much longer than 70 years - it was a
> size that was also called 28 x 1 5/8in and 28 x 1 3/4in in England. The
> bead seat diameter of 622mm was normally used with large section tyres -
> it
> was also a size common in France, the Benelux countries and Italy. In
> France
> it was known as 700C. The Benelux countries normally referred to it as 28
> x
> 1 5/8in. The French system of tyre sizes used the capitalized letters A, B
> and C to refer to the cross section of the tyre combinmed with the overall
> diameter in mm. So hence 650A (26 x 1 3/8), 700A 28 x 1 3/8in etc, B was
> equivalent to 1 1/2in section and C 1 5/8 - 3/4. They also used the size
> in
> mm without a letter suffix to indicate 1/4in section. These were the same
> bead seat diameters as the English equivalents 700 = 28in, 650 = 26in, 600
> =
> 24in etc
> Later in England and especially in France you could buy oversize and
> undersize tyres to fit these bead seat diameters - the full balloon and
> 1/2
> Balloon 650Bs were considerably larger than 26in in diameter. The
> undersize
> tyres were a later invention generally... But you get the absurdity of the
> 650C used by triathletes with 20mm section tyres - originally this bead
> seat
> diameter was designed for use with 1 3/4in section tyres...
> The 27 x 1 1/4in size was invented by Dunlop in England and launched in
> 1935 - it quickly became a new standard but was not linked to any other
> range of wheel and tyre sizes and was a totally bastard size.
>
> Hilary Stone, Bristol, England