Once is happenstance, twice is a coincidence... I just patched my inner tube, rode the bike a bit, parked it in the sun, and about an hour later --BOOM! the tube exploded tearing a 4" gash near the valve (not near the patch.) This is the second time the exact thing happened. Both times rear, both times after repairing tube. I don't know how old the tubes were, but the second one seems pretty new and the rubber is still very pliable. I tend to overinflate since the roads near me are full of hazards. This time I did not take too much care in seating the tube, but the first time i was very careful and slightly inflated, massaged the tire to make sure there were no folds or twists, and then deflated and inflated. I use a hand pump, and really don't know the pressure but I assume the tire would take any stress from high pressure, and the tube would just be sandwiched in there so high pressure would be no problem. Today was not hot (about 22C), but the sun was on the bike.
OK, here's the twist -- not sure if it matters. The tires are 16x1 3/8 BE, that is an old Japanese standard used on old commercial bikes. (believe it or not, it is my daily ride and I take the beast into the mountains (no gears) 2 or 3 times a week.) The tires have ears that go under the tube and overlap protecting the tube from the spoke ends. They are a pain to change, and although you can not see the tube as you seat the tire, you have to check the tire to make sure the ears are straight and not folded and the tube is not pinched, so this system should not make the tube explode in itself.
Question -- anyone ever have tubes explode in the sun? Any idea what could cause this? Anyone know what happens if the tube is folded funny in tire -- can it form a bubble that is under too much stress? Is there something I can do to prevent this happening a third time? (three times, it's enemy action.)
Update -- (1) I buy the tubes on the net and don't think the stock is old. These delivery bikes were very common in Japan so they are still selling the tubes but not so much locally. (2) no rim strips - the tire becomes the rim strip i will try to attach photo of a tire below. (3) I don't assume the tire will take any pressure, but I assume the tube, when constrained inside the tire will. (4) http://www.raleigh.jp/InfoFAQ/060701_wheel-adv.htm shows a cutaway of the tire system on at the bottom of the page on the right (with "B/E" above the drawing.) (sorry about the Japanese.) You can see the tire is about as thick under the tube as it is on the tread. That is the "ears" i mention above. This also makes it very hard for tire to come off rim, as the pressure of the tube is forcing the bead into the groove. I think D.R Hicks' comment may be a clue, but I can't see how tire could unseat with this system. I did not see it happen, but afterward the tire was slightly unseated at the spot of the gash, but when the pressure is gone it is hard to tell with this system. Again, the pressure hold everything in place. (5) the gash (both times) was at about the 4:00, with the valve stem at 6:00 position.
Update 2 -- here is a photo of a typical B/E tire. (Sorry I can not post photos yet.) http://urekko-cycle.shop-pro.jp/?pid=15948577 The white parts go under the tube. They are white (i think) so you can tell that they are not folded during assembly - this is not a white wall tire. To assemble, you have to put one side on, make sure the ear is laying flat and goes all the way under the tube from bead to bead, then insert the tube and do the same for the other side making sure tube is not poking through. when assembled the white is completely under the tube. You also have to notch the ears for the valve stem to poke through before you use. They used to sell a special tool, but I use an exacto.
Update 3 -- Sorry if i am being wordy. These tires/rims were very common in Japan until about 10 years ago. Almost all delivery bikes used this system and many of the bikes are still out there. My guess is that they used this type of rim from the 60's until around 2000 on most delivery bikes, and some high end mama-chari's.
Update - Final answer -- I think @Daniel and @mattnz are correct and the tire must be coming off the rim. I initially discounted this, since the B/E system makes it seem impossible, but now I am convinced this is the case.
The tire is slipping off the rim at the valve stem because the ears have the notch for the stem and this is a weak point. My guess is that the sun is making the rubber soft and more pliable and the top ear is able to slip past the tube and bottom ear to unseat the bead. This seems unlikely, as the tube pressure is holding the three pieces together, but it is the only possibility.
I researched in Japanese about B/E blowouts, but could not find anything. This is why I initially thought the tube was pinched or folded somehow and in that case it would be potentially a problem for any tire, but now I think the problem is unique to B/E rims.
The solution I will try, is to clean off the rubber and rim really well so that the four layers (rim, ear, ear, tube) all grip when under pressure. This is the opposite of what I planned to do before the discussion, as I thought some talc might help a tube kink straighten out. I may even put a dab of rubber cement between the ears near the valve stem.
Many thanks for the Help