Originally Posted by Chrome Molly

No worries, was just handing back some of that tongue-in-cheek razzing that I got above...

You are correct that it is the bead shoulders keep the tire from moving off. Having a really tight circle around a closely sized circle inside of it is intended to prevent movement. Therefore, having the tire centered when starting with inflation is key to keep problems from starting. The comment about "good tires" just means that given the tolerances of the two circles needs to be so close, having one of the two circles poorly spec'd will cause a problem. It's not always that tires improve the situation, some rims are better than others in terms of sizing (but most are more consistent than the sizing of tires). At the other end of sizing extremes, the tires can become hard to mount because they are "too tight" for that particular rim. It can go wrong both ways.

If you put 75 in, and it holds for a ride, then you are probably out of the woods. 85 is about the max for those rims though. Straight wall rims rely on the relative sizing of the tire to the rim (and not super high inflation) to hold together. A hooked rim wall, has a little "bead" toward the top of the rim wall that uses the pressure from the tube to hold the tire from blowing off. You have straight walls, so it's just more sensitive to pressures and the rim/tire relationship, whereas a hooked wall rim uses the pressure of the tube against the tire to keep the tire from blowing off the rim wall.

Again, no razzing you and your difficulties, just defending my man-cred (or trying anyway)...