Originally Posted by
FBinNY
Here's what MAY be happening if my guesses are on target. Consider that your rim is 13mm or so inside width. From that subtract double the tire's thickness at the bead, say 3-4mm, doubled to 6-8mm. That leaves a gap between the beads of only 5-7mm. When you fill the tire, the tube fills the tire taking a round shape, and spanning that gap. As you increase pressure, the section spanning the gap blows down to fill the gap near the rim, stretching to a few times it's original size. This hyper-stretched section is very fragile and doesn't need much provocation to let go.
on a slightly thread hijacking note
regarding the combo of extremely narrow rim, with overly large tire causing this issue
what are some good solutions?
example, had a Sondors BSO electric bike come into shop with this problem, 40mm tires with a ridiculous deep-v 13mm rim. constant tube ruptures due to the rim-side stretching.
I couldn't just tell them to buy a narrow tire that fit's the rim, since the 70lbs+ bike would be constantly pinch flatting at that rate, nor could I tell them to change rims since relacing the motor would be out of budget.
badly speced bike, catch-22...