DannoXYZ 

Well, friction is a function of normal-force f=uN. The more force you have pressing two things together, the more friction you have. Slide a tyre across a cement floor, easy huh? Now have someone stand on the tyre and try to slide it from under their feet; it's not possible without pushing them off the tyre first. Now multiply that by the inner surface-area of the tyre at the bead times air-pressure and you've got thousands of pounds of force pushing the tyre against the hook-edge of the rim.

There are two opposing forces at work:

1. air-pressure along inside-edge of casing will try to expand the tyre casing outwards.

2. bead resists stretching and translates expansion of casing into lateral movement sideways (the two beads move wider apart).

The bead material is very important for #2. High-pressure tyres with kevlar-beads should always be used with hook-edge rims due to the flexible bead. In which case, the pressure of the tube pushing on the bead underneath the hook generates extreme friction between the tyre and rim.

With straight-edge rims, a steel-bead can pretty much hold any pressure you put into the tyre. You're not going to be actually using any kind of pressure close to where it can stretch a steel-bead; the casing will probably blow through first. Kevlar-beads on straight-edge rim is another matter and would depend upon the size of the tyre (larger tyres have more casing surface-area and generates more stretching and pulling force on the bead).


I think in this case, you may have some issues with pinching the tube under the bead or a tyre that's slightly oversized at the bead-seat or foreign matter under the hook-edge of the rim.