Originally Posted by
FBinNY
Before charts came out, the conventional wisdom was tire outside width = 1.5x rim inside width, with plenty of fudge room to either side, especially wider.
If the rim really is 21mm inside width, you can certainly go to 32mm tires, and most likely to 28mm. There are even some who'll go narrower yet, but I won't go there.
Generally, you're good if the tire profile is more like an Omega, than like in inverted U.
Good to know about looking at the profile, that makes sense. I don't think i would try narrower then 28, nor do I really want narrower on this bike, but I am a bit nervous about 28 based on these charts.