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I was riding recently and kept hearing a noise similar to a rock stuck in a tread, but I have no tread. When I got off and inspected the tire, I found these weird bubbles had formed. I have replaced the tire (the bubble doesn't telegraph through to the inner wall), and the tube appeared just fine. I can't figure out what caused this though. I'd really like to know, so if it's something I'm doing, I can keep from doing that.

It was my rear tire, and the tire only has about 1200 miles on it.

Tire Bubble

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New one on me. I'd guess that something bad is happening to the threads inside, but the plural that was used "bubbles" has me leaning toward a manufacture defect. Would love to see this tire in person. –  Ken Hiatt Jun 27 '12 at 23:11
    
One might expect to see this on a tubeless tire, but it's odd on a tubed tire (unless the rim is for a tubeless). –  Daniel R Hicks Jun 27 '12 at 23:52
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3 Answers 3

I suggest taking it back to the shop you got it from, as it looks like a manufacturing defect. Since the bubble isn't visible on the inside it's clear that the layers of cloth inside the tyre have separated, and that's not supposed to happen. You should be able to get a warranty replacement.

Sheldon Brown has an explanation of how the tyre is made that may help you picture what's going on: http://sheldonbrown.com/tires.html

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Definitely take it back. You may want to avoid riding it back to your shop to keep those bubbles from tearing open. –  WTHarper Jun 28 '12 at 2:08
    
Yeah, I tried that... Apparently, Trek pulled out of that shop a couple of months ago due to the owner not paying his bill. The new tire didn't have any warranty info on its packaging, and I bought the bike a couple of years ago, so I just kind of assume that's out of warranty. –  baka Jun 28 '12 at 2:16
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You could try a different shop, or talking to Trek directly if you felt lucky. But if it's 2 years old they may say it's not their problem. –  Kohi Jun 28 '12 at 3:24
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This happens commonly on vehicle/motorcycle tires, and for similar reasons. Non-bicycle tires use woven metal 'bands' that form the structure, with rubber molded around them. Bicycle tires use cloth/nylon/some other type of non-stretchy material with the rubber molded around it. You can usually feel them as the lines inside the tire.

When bands break and/or separate, it allows air pressure to push out the now unsupported rubber around it out farther than normal. You may not even notice the inner part following, because they're not really one piece, more like two pieces of bread surrounding the meat (the bands). Further, they don't need to make both sections airtight, so usually just the outside is (even if you don't see holes, the rubber can be quite permeable (indeed it usually is to a small extent, even airtight ones).

If you're asking what causes the breaking, it could be poor manufacturing, extreme flexing, folding, extreme heat, sitting flat on the bead for a while (or getting ridden on while flat), over-inflation, or any number of things that stress one section of the tire more than the rest of the structure.

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Yet this is not a tubeless tire, and reportedly there is no sign of the defect on the inside. –  Daniel R Hicks Jun 28 '12 at 11:25
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I can think of 4 causes for the "bubble":

  1. Somehow air pressure from inside the tire has worked its way through the inner cords and pushed out the outer tread. But for this to happen the rim would have to be essentially airtight, and no rim is that tight unless designed for tubeless tires.
  2. Some sort of gas created from within the tire collected in the bubble. One might expect this on a fairly new tire, due to some manufacturing process problem, but an older tire (more than maybe 6 months old) should have "outgassed" everything.
  3. The defect is actually a bruise/rupture in the cord, allowing the tube's pressure to press out the tire. But if this were the case the bulge would go away once the tube's pressure were removed, and there would be a soft spot inside the tire that you could feel with your fingers if you pressed.
  4. The defect is due to a bruise/rupture, but the bulge results from the ruptured cord "bunching up" under the tread. One would expect such a defect to be less regular looking, though. Additionally, the bulge would be hard when pressed, whereas the other cases would result in a bulge that could be compressed with your fingers.
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