Originally Posted by
cyccommute
While a tubed tire can blow off, the mechanism is a bit different.
While pressure is pressure, not all pressure vessels are the same. A tubed tire is less likely to blow off the rim, hence the higher pressure rating, because of the way that a tube, rim and tire interact vs the way a tubeless tire and the rim interact. With a tube, there is another, lighter pressure vessel pushing on the bead of the tire to keep it in place. This increases the friction between the tire and the rim which allows it to run higher pressures.
Without a tube, i.e. tubeless, only the air pressure is keeping the tire in place. Without that extra friction, the pressure required for the tire to slip and blow off the rim is lower. The tire has to be made so that it fits more tightly to the rim but it still lacks the friction that a tube provides. And, because the tubeless can't hold as much pressure, the friction between the tire and the rim is reduced.
Additionally, sealant may not be your friend. If there is a leak between the bead and the rim...folded bead, poorly seated bead, damaged bead, damaged rim from running too little pressure, etc...sealant will get pushed into that gap and lubricate the rim/tire interface. Now your friction is far lower than it would be dry and the bead can scoot right off.
This is true, especially since, without a tube, the pressure acts around the tire's beads instead of just having a tube pushing from more of a single direction.
So, without a tube, with air pressure somewhat surrounding the tire's beads, some of that pressure is pushing the beads in the "wrong" direction.
This in addition to what you said about the tube offering a friction-laminated layer that ties the rim and tire beads together as a unit.
I've read that a tire with beads having been severed in several locations will hold tight under pressure, but only if a tube is installed, (and not because of any problem with air simply leaking out at the points where the bead has been cut).
To the OP, I would most definitely hand over these rims and tires to Specialized, so that they can examine them! They should offer you a loaner set for any period of time that it takes them to complete their investigation. I would contact Specialized directly, in lieu of going through any shop acting as a middle-man.