I've got a Giant Cypress with 700X40C tires on there now (stock). Since I am 99% on the road, I want to go with a smaller width on these tires. The rims are not branded, but Rainman has referred to them as "Hoopsters" and they are 21cm measuring from inside to inside width of the rim.
Sheldon Brown's article recommends 35cm as the smallest:
but his source, written in German, (which I don't read so well !), seems to leave some fudge down to 28 or 32cm .
Does anyone have experience with this? I'm just trying to avoid as much trial and error as possible, and still have a fast safe tire