Bicycle Mechanics - Tyre blowout - is my analysis correct?

Was out on a group ride today, and at one point we went down a short (maybe half a mile) but steep (varying between 5-10 % grade) hill. At the bottom of the hill, a sharp bang rings out, and the guy next to me yells "Flat!".

I could see, even before he had come to a stop, that the rear tyre was completely off the rim. I helped him put new tube in, and checked the old. It was split open along its length, for about one fifth of the circumference.

My analysis: He used his rear brake alone throughout the descent. This heated the rim, causing the pressure to increase. This popped the tyre off of the rim, which let the tube out, free to expand unchecked, and then split.

That's a possible scenario but a 1/2 mile hill at 5-10%, even if only the rear brake was used, shouldn't have overheated the rim to that extent. If you were describing a 5 mile, 10% downhill in the Alps, maybe.

i think it's most likely he had the tube partially pinched due to improper installation before the blowout and possibly the minor stress of the decent was the last straw.

That's a possible scenario but a 1/2 mile hill at 5-10%, even if only the rear brake was used, shouldn't have overheated the rim to that extent. If you were describing a 5 mile, 10% downhill in the Alps, maybe.

i think it's most likely he had the tube partially pinched due to improper installation before the blowout and possibly the minor stress of the decent was the last straw.

+1

I find it hard to believe that a half-mile descent could generate that much heat. There are any number of descents like that near me (New England area) that have to be taken slowly due to poor road conditions (hitting a pothole at 40+ mph is not my idea of fun). I've never worried about blowing a tire off the rim on them.

The only time I was really worried about how hot my rims were getting was on a 15 mile long 10% grade filled with hairpin turns. I stopped about half-way down that to let my rims cool for a good long time.