I'm not a framebuilder but I figure good framebuilders would know the answer to this question better than anyone else.
I ride a '73 Raleigh Gran Sport, Reynolds 531 DB, with 27" wheels. I recently switched from 28mm to 25mm tires and noticed that the bike felt twitchy. Of course lighter tires would do that perhaps and it may have been my imagination because I'd spent time riding an even older bike with more laid back geometry. But the smaller tires create an effectively smaller wheel and that got me thinking about the effect of switching to 700c wheels.
Trail is the distance between the center of the tire contact patch and the point where the steering axis intersects the ground. The rake of the fork serves the purpose of reducing trail. Now, if you switched to smaller wheels the trail will decrease. For a 72deg head angle, for example, the 4mm smaller radius will reduce trail by 1.3mm. That sounds like a lot, and here's why.
Were there no rake, the trail and radius would change in proportion, so theoretically (I would think) the handling of the bike wouldn't change. But trail has already been reduced by the rake, i.e. the rake is a constant subtracted from the radius times the cotangent of the head angle. Numerically the change in trail is a much greater percentage than the percentage change in radius.
I would assume that a certain amount of trail is desirable because too little would make the bike very quick to change direction, in a word. twitchy. So the switch to 700c wheels would seem to change the bike's handling in that direction. In other words, it's a switch one should do only after careful consideration.
Yay or nay? Am I out to lunch? I know the math, but I'm not sure how it translates to bike handling. Is the effect maybe just so small that it won't make any real difference?