Originally Posted by pierce

there's more leeway going the OTHER way with modern rims and tires, like a 23mm tire on a 23mm OD rim is actually a fairly common racing configuration as it leads to a ride more like tubulars.

a 14mm ID rim is probably 16-17mm OD ? I don't think I'd want to go over 35mm (about twice the OD of the rim). you'll likely find a really wide tire on a skinny rim will feel 'squirmy' on turns. if you picture whats happening with the inner tube and the tire bead, the sidewall of the tire would be bent quite sharply, and the inner tube will be bent into a 'keyhole' shape. since a tire as wide as a 40mm typically runs rather low pressure, I'm not sure there would be sufficient pressure to hold the bead snugly, especially if leaning over hard so the tire is pushed to the inside side...