Wow. Thanks to all for the intelligent suggestions. I can only say I am glad to know this is as abnormal as initially I thought. Crude measurement and comparison with other wheels here, doesn't suggest that these rims are in any obvious way undersized, but perhaps this is the only reasonable starting place for solving the mystery-- since yes, the blow outs have ALL involved the edge of the tire bead lifting off and out rather than any of the usual sorts tire failures I know and have dealt with. The two actual blow-outs have slightly shredded the edge of the bead-- so being cautious, I am writing these off altogether. But on two or more other occasions, I watched in horror as tires slowly began to budge at the rim. I leapt and let out enough air to avoid a blow-out in each such instance, but obviously not a confidence inspiring moment, nor the sort of thing to lead me to think under-inflation was my problem. If I myself had done the mounting, I'd have assumed it was purely my own incompetence. But this, like the wheel build itself was actually done by someone of long experience and great skill-- so this is not likely the problem. In any event, I shall confer further with him, and if we eliminate all other possibilities, I suppose I'll replace the rims (ouch)-- if all else fails. But far better this than finding myself under a speeding taxi. Again, much obliged to all.
Guy Lesser
Brooklyn, NY