----- Original Message -----
> Jonathan Russell wrote:
> >... Also, I don't know the difference in size of
> >700c and 27". I guess I could divide by pie and figure it out
huh.....<G>.
> >For instance, I want the brake calipers to reach the rims correctly.
Maybe
> >700c and 27" are close enough for that to not be a problem.
>
"Roy H. Drinkwater" <roydrink@mac.com> answered
> 4mm in radius (hub to rim braking surface) with 27" being
> larger.
>
Actually the ISO measurements is called a "bead seat diameter", though I think that's a bad term. If you take a 700c rim (ISO 622mm vs 27" - ISO 630mm) and measure it, the 622mm's ends up being rather at the bottom of the inside of the rim wall, not up at the hook bead where the tire bead sits after inflation. Same with other sizes of rims I've measured (650c - 571mm, 26" - 559mm, 20" - 406mm).
Among same-sized rims, there still seems to be some variance in the actual brake surface height (the ISO standard is for tire fit only), though usually only a few millimeters at most. It's still enough to occasionally have to readjust brake blocks depending on wheelsets.
I've never heard of anyone trying to fit 27" wheels to a 700c frame, but the other way is common. Standard-reach brakes often will allow a 700c wheel in a 27" frame, unless the clearances were already large. It certainly depends a lot on the type of brake you are using, too. Some older canti brakes offer very little height adjustment.
With road calipers, just install the wheel and measure from the brake bolt hole center to the brake track center. That's the reach you need. Check both the front and rear; they won't always match. For cantis, I know of no standardized "reach" rating for brake models, so you'll have to just try the wheel and hope the boss placement was well thought out.
Jon Schaer
Columbus, OH