Welcome, Gerry. For a good discussion of wheel and tire sizes see Sheldon's page:
Tire Sizing
To give the brief overview, 700C tires are now known as xx-622, xx being the width in mm. 27" are xx-630. Thus a 700C clincher rim will be approximately 622mm in diameter measured to the bead seats. Full size tubular (sprint) rims should have a similar diameter, though it could be a few mm off, as they have no bead seat as such. 700C, at least according to popular myth, were developed in the 70's to have a diameter measured at the braking surfaces which was the same as a tubular rim, thus allowing one to switch between clinchers and tubulars without moving the brake pads. If you have clincher tires, the size is almost always marked on them, either in the old 700C x yy or 27 x 1 x/y format or in the new xx-622 or xx-630 format. Tubulars often lack size markings, but so called "27" and so called "700C" tubulars are the same, just two different ways of referring to the same thing. Any modern tubular other than these will be a distinctly smaller size with a rim diameter significantly less than 622mm. Again, as with most subjects, Sheldon's site gives a much more thorough explanation than I can.
Regards,
Jerry Moos
Gerry and Caroline wrote:
> Hi there folks, I've just joined the list properly after having submited
> an unauthorised posting when I linked in from the home page. From that
> contact I've allready been in touch with Clive in Sydney and Nelson in
> Seattle, so hi again guys and thanks for your thoughts.
> I'm totally new to this scene, having recently been given a mid/late
> '70's Holdsworth, and wanting to find out some stuff about it. It seems
> to have some ok parts on it;Zeus cranks,bb and read mech, and Lambert
> hubs(are they American). It is a sort of purple colour with cream panels
> and is built from reynolds 531. can anyone give me some help on
> age,model and what parts it should actually have. I've not ridden it yet
> as it's got a punctured tub that I have no idea how to fix, I also am
> confused as to how to figure out it's wheel size (27", 700c or even
> 650c), do I measure across the diameter.
> I've been watching how the list works and what you all talk about and it
> seems that it takes a lot of knowledge of classic bikes to know what is
> going on, but obsession is obsession so I'm sure I'll soon pick it up.
> Any tips on good sites to visit or books to get hold of to learn more.
> Hope to hear from some of you soon , bye for now.
> Gerry " yet to find himself a quirky mid-name catchphrase" Burke.
> Cumbernauld, Scotland.