Update: I just returned from a couple of bikeshops and I have learned that these 26 x 1-3/8" rims are NOT the same diameter as regular mountain bike rims. They're actually slightly larger in diameter, larger than 650c wheels as well. We played around with a 700c wheel and it looks like it might almost fit if I use a skinny enough tire. My goal for my friends bike it to get some decent tires on it that can handle high pressure. The 26 x 1-3/8" tires for these are going to be low pressure 65psi variety because, as one bike shop person pointed out, these are not "hooked" rims. I know the hubs are nice, but I really am not very fond of these WHEELS. I'm going to trial some 700c or even tubular wheels and I'll give an update soon.