Originally Posted by
3alarmer
It's entirely possible that your 27" rims on a Schwinn take a proprietary tire.
No, 27" tires and rims are all 630mm BSD. Speaking of which, ETRTO/ISO size designations take the confusion out of matching tires to rims. A 27" x 1-1/4" tire will have an ETRTO/ISO size designation of 32-630, where the first number is the nominal width and the second number the bead seat diameter (BSD). Rims can accommodate a range of tire widths, but only the designated bead seat diameter. Match the bead seat diameter and try to get a width that somewhat close to that of the original tire.