Tires vs. Rims
 

I am presently in the process of repainting an old J.C.Higgins cruiser bike from the 50s. The rims are exactly 1.5 inches wide. I would like to put balloon tires (2.125 or 2.25) on it. Would those tires be too wide for rims? What is the ratio for recommended tire width vs. rim width? I appreciate any knowlegeable input...........Pix

Sheldon has a pretty good chart on that topic. Note that many mountain bike rims are technically too narrow for the tyres, but they seem to work. (Or do they?? Several times, I have had to scrap a mountain bike tyre because its sidewall was getting weak and tattered, even though its tread was still excellent. In contrast, I usually scrap a road tyre when its tread wears out.)

More importantly, what is the fork and chainstay clearance?

As a general rule , you probably can if there is enough clearence, as Alex suggested. But what is your intended use? If you're keeping it a cruiser, 1.75's with a road tread would be a better choice.

Use the size YOU want to, no problems as far as the rim is concerned.
Ride Your Way
Pat