I note that there is a bit of variation in tire fit, differences in tires and differences in rims, but this sounds like a too-severe case. I would start with a bit of air in the tube, letting air out (as will be usually be needed) during installation. This helps to prevent the tube being trapped under the bead, as Mike mentioned. I put tube in tire before putting tire on rim, and get the tube centered within the rim before starting to fit the second bead. My Kevlar Paselas do seem particularly easy to install. I run only 60psi in a generously-sized tire like the 1-1/4"-sized Paselas. I have known wire-beaded Kendas to push over a plain rim's sidewall at just under 90psi, but have run other tires (not the Paselas) at that pressure.
A narrow rim necessitates a bit higher pressure to prevent pinch flats with the same tire. A wide tire on narrow rim may need help with the valve stem coming up during inflation, so I give it a tug before inflating in ernest, lest the tube tear in the valve stem vicinity as the tube expands into the rim cavity.
I have also seen (I have a pair) Sun rims with 700c label that are really 27". Threw me for a loop.
David Snyder
Auburn, CA usa
>
> At a shop I worked at we got a pair of Sun CR-18 rims that were marked
> 700,
> but actually were 27"...
>
> You shouldn't get blow-off at such low pressures unless you are trapping
> the
> tube between the tire and rim....
> Mike Fabian