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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

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I generally never repair a tube. If I get a puncture I'll chuck it. I can do this because my wheels are a standard size, I can readily buy replacement tubes and they cost next to nothing. But how true is this for your wheels? Can you just walk into a bike shop and buy replacements? Sheldon seems to indicate that this size is pretty rare. –  PeteH May 12 '13 at 10:27
    
The tubes are available, but tubes are expensive in Japan and these cost about $8.00 each so I generally patch. This size is hard to find now but was very common back in the day for the delivery bikes. –  tabun tadashii May 12 '13 at 10:58
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I'm just wondering whether because of the rarity, the innertube has been sitting on the shop's shelf for years –  PeteH May 12 '13 at 11:00
    
Do you have rim strips? I think you should have them no matter that your tires already have "ears". –  Mladen Jablanović May 12 '13 at 12:37
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I recall this happening to several guys on a ride 20-odd years ago. They were riding old (new then) racers with full pannier loads, meaning the wheels were way overloaded. Laid the bikes down in the parking lot on a sunny day and within 10 minutes -- "BOOM". I saw one of them go: The tire bead slipped from the rim and the tube was bulging a couple of inches before it popped. Dollars to donuts your tires are slipping off the rims. –  Daniel R Hicks May 12 '13 at 12:55
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closed as not constructive by Neil Fein, jimirings, Benzo, zenbike May 17 '13 at 19:16

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3 Answers

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@Daniel has said it many times in this QA - The problem is the tire, not the tube. You need to remove any discussion of tube from this problem.

Either tire is coming off the rim, or it has a hole in it big enough for the tube to balloon out. The most likely scenario is, as said by @Daniel, The tire bead is coming off the rim.

This can happen if the tire is over inflated - too much pressure for it to hold together, under inflated while riding - rolling off the rim, The rim is damaged - visual inspection will confirm or if the tire bead is broken. If the pressure is correct (Below max printed on tire) and the rim not damaged, suspect the bead is broken and the tire needs to be replaced.

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Also, it can be a simple mismatch between tire and rim. These are weird tires, and may not be properly matched to the rims. –  Daniel R Hicks May 13 '13 at 0:09
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I have heard of tubes failing in situations like yours. If you inflate the tires to the maximum rated pressure on the sidewall of the tire, the pressure can rise over that when warmed by the sun or the friction and heat from the road. The occassions I have seen this are with high pressure road tires. Since you don't have a pressure gauge there is the chance that you over inflated the tube. As you state everything is sandwiched between the tire except at the point the valve goes through the rim. Verify there are no sharp edges near the valve hole and inflate it to the correct pressure.

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But a tube won't fail from overpressure unless the tire fails too. If the tire is not being (apparently) damaged then it must be slipping off the rim. –  Daniel R Hicks May 12 '13 at 12:57
    
It can if the valve hole is damaged or oversized. –  mikes May 12 '13 at 13:20
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It wouldn't produce that large of a rip. –  Daniel R Hicks May 12 '13 at 14:52
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The expansion of air at normal temperatures between sun and shade should have no effect if the tube is inflated to its recommended running pressure, even going above by 10% should not have any effect as the tube will be built with significant tolerances well above that. For example, i have tyre's rated to 110psi but run them at 120psi and i weigh 95kg. I think the best thing for starters is to get a track pump with a guage, particularly if you are using a pump with a high psi which can easily go well above the rating for the tyre or tube. All of this said there can be many reasons - it'll perhaps be a case of eliminating them one by one.

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But place a wheel in the sun so that only one side of the tire is getting sun and there will be differential expansion. This will exacerbate any tendency of the tire to slip off the rim. –  Daniel R Hicks May 12 '13 at 14:54
    
Perhaps, but expansion of air in a contained space will be even and distribute equal pressure across the whole internal surface of the tube. This is one of the 'gas laws'. The internal ambient pressure would probably need to be well above the maximum recommended for a catastrophic failure to occur. Rubber will also expand and flex under heating to accommodate increase in pressure. –  velo-oldster May 12 '13 at 15:28
    
The OP states that he rides them into the mountains. I'm don't ride in the mountains, so don't have any experience on this, but when going up to higher altitude, the lower pressure on the outside would result in a higher (relative) pressure on the inside of the tire. Since there is less atmosphere pressing back on the outside of the tire, the inside air presses harder against the tire and rim. I'm not sure how much of a difference this makes in the actual scheme of things or if its enough to make the tire explode. –  Kibbee May 12 '13 at 17:13
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@velo-oldster - Yes, the air expands evenly, but the metal and rubber don't. The tire is creeping off the rim. –  Daniel R Hicks May 12 '13 at 19:10
    
velo-oldster -- Sorry, I may have been misleading, but by "mountains" I mean a couple hundred meters up behind the city so not enough to affect the pressure I think. –  tabun tadashii May 12 '13 at 21:56
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