Trail on older bikes: The more the fork is raked (usually in conjunction with a shallow head angle), the more trail is affected by tire size. For example, the 1952 Herse shown in Riv. Reader 26 had a very fast "criterium"-like handling when first equipped with 30 mm wide tires (good quality 650B tires that are wider are a bit hard to find). The downside was a slight instability - I found myself touching bars with the guy riding next to me.
Changing to 37 mm wide tires solved the problem, and imbued the bike with the handling it should have: stable, yet responsive.
So by changing tire size, you can play with the handling of a bike. Or mess it up...
A more complete discussion of this topic will be in a future issue of Vintage Bicycle Quarterly (with illustrations, etc.)
Jan Heine, Seattle
(Makes one wonder about bikes that supposedly do it all, from criteriums on 20 mm tires to fire roads on 45 mm tires!)