Originally Posted by
bones_mcbones
An older 27" road bike frame will fit pretty much any 700c tire.
You'll be able to put 42s on your 23 rims, but the ride is going to suck, I think they call it "fold over" or "roll over". Basically you have no stability in corners because of the balloony shape of the tire squeezed onto a smaller rim.
General rule of thumb is 1.5-2.0 x inner rim width. I have some 32 tires on 17 rims and it's OK. But you are right, exaggerated light bulb shape will make them feel really weird.
Stu2. FYI, be sure to check for vertical clearance too. I measured a fork once to make sure 35mm tires would fit and there was plenty of width, all the way to the top. When I tried to put the finished wheel into the fork, it was about 3mms too tall. Ooooops. It was an easy fix though, since I caught it before painting.