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I've been riding on the same rear inner-tube for several months without issue. I topped up the rear tube last week, went on ~10mi ride (including a rest in the middle), and had no problems.

I was about to ride again today, but when I looked at my rear wheel I noticed the tube was completely flat.

While removing and inspecting the tube I found two obvious bumps/bulges/protuberances/knobs on the inner-tube.

These bumps are near a spoke. They are on the inner wall of the tube.

two bumps on bicycle inner-tube bike tube adjacent to the wheel to show where the bumps align

I found the source of the leak. One of two bumps has a sizeable hole/cut. It's not a slow leak. If I try to pump it does not hold any air.

one of two bumps on bicycle inner-tube has a cut

I remember now that last week, after that ride, I had heard an odd noise from my kitchen (where I store my bike) and couldn't find the source when I went to check on it. I thought, "that's odd", but didn't think twice.

In hindsight that noise must have been my rear inner tube very suddenly deflating. It wasn't a bang, but a sound vaguely similar to a bag of rice falling on its side and the grains coming out quickly then slowly then stopping. Sort of like a deflation now that I remember it!

The issue isn't glass or debris, and the tire does not have a bubble and otherwise is not deformed or damaged when I inspect it.

A similar question and answer makes me think that perhaps I overinflated it when I thought I was doing things like normal, but I've never experienced this particular behavior before and was curious. Also, that user did not notice these obvious deformities like I did, so I thought it could be a different scenario.

On feeling the rim tape, I do not notice the spoke protruding out at this spot, but I do notice an obvious dimple/depression in the rim tape at that spoke!

a red circle highlights an area of the bike wheel rim tape

  • What may have caused these bubbles?
  • If poor rim tape may be at fault, must the rim tape be re-done?

Unfortunately I'd have to bring the bicycle to a shop mid COVID-19 lockdown for anything beyond changing the tube, as I'm only really equipped to swap out the tube with my skillset.

For context, the tire says 700x28c, the wheel says 700x18c/23c. That discrepancy between 28c and 23c always bothered me, but I have no idea if it matters. The bike has been like that since I bought it ~7 years ago and multiple mechanics at multiple shops have replaced my tubes and tires with similar ones and never mentioned it. The max PSI is 115 and I usually only inflate to ~100 PSI, if my pump can be trusted.

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    Done, and thanks! I added a photo showing that the bumps are very close to the spoke hole. It's the third spoke from the valve. I also added a photo of the rim tape. I think it may be difficult to see the depth of the depression, but I feel it. You can only see a slight discoloration of the tape in the photo. The depression is in the center of the red circle. – Will Haley Apr 26 at 18:28
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    Sidenote, I think if you can change a tube you can change a rim tape, it's only a matter of taking one off and putting a new one on. That cloth one is stuck down but will peel up easily. They say necessity is the mother of invention, I think – Swifty Apr 26 at 18:30
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    I can see the depression in the rim tape, and also at the edge that it has pulled the tape down from the side, there is a little deviation in the edge of the tape. The one bump certainly lines up with the spoke. I expect the tube was able to push the tape further into the spoke hole when the tyre was on and all inflated up to 100psi, and maybe was chafing. However it isn't anything I've seen before so I'm speculating a little. Hopefully someone can give a solid explanation as an answer – Swifty Apr 26 at 18:35
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    We have rim tape in our terminology index but this is giving me ideas of information we could add to it. – Swifty Apr 26 at 18:38
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    +1 for a well written question, with all the photos and info required to get the right answer straight away. I can also see a lenthwise crack in that innertube, it's past time for replacement, if you can. – Criggie Apr 27 at 4:53
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The culprit is the rim tape. The glue that holds it in place has lost its grip, and when the tube is inflated, the tape is pushed away and lets the tube to extend into spoke hole. Eventually the edge of the spoke hole cuts the tube with the results that you described.

In my experience, this is what cloth rim tapes do. They can be replaced and there are tricks to gluing that may or may not help, but in my opinion the easiest solution is to buy a pair of plastic rim strips and replace the tape with them.

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    The replacement rim tape should be of the high pressure type. They are very stiff and will not be pushed into spoke-holes by the pressure from the inner tube. Cloth tapes should only be used on Dutch bikes running on low pressure tyres. – Carel Apr 26 at 19:03
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    This is a job for duct tape!! – Daniel R Hicks Apr 26 at 20:25
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    I find the idea that there's something duct tape can't fix offensive. – Grimm The Opiner Apr 27 at 8:13
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    This is not my experience with Velox, a high-quality cloth rim tape, even when I ran high pressures like the OP does. It could be the tape was low-quality. I also disagree this is a job for duct tape, as it may not be designed to withstand the pressures involved. – Weiwen Ng Apr 27 at 15:24
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    @IconDaemon That reminds me, I patched a hole in my tire with Gorilla tape a few hundred miles ago. I really need to get around to replacing that tire someday... – Michael Hampton Apr 28 at 2:49
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The bumps/dimples in the tube are due to the tube bulging out into the depressions in the rim above the spokes. As the other posters have said, the primary suspect is the rim tape. It might be stretching under pressure. If you use a gas station or other compressor air pump to fill your tires, it's likely that you overinflated the tire at least once, which could have contributed to the problem. I've also seen properly inflated tires do this when the bike is left out in the sun for a couple of hours. My theory is that the slightest imperfection or concavity in the rim, even with seemingly good rim tape, can have the effect of "sharpness" on the tube under enough pressure.

If this is happening in only one spot, patch the tube (or put a patch on a fresh tube over that spot) and make sure the patch is always oriented over the suspect spoke hole. The patch will not stretch as much as the tube and should prevent another flat. If it is happening in multiple spots, replace the rim tape as suggested by the other posters.

If the tube has a Schraeder (automotive-style) valve, make sure the valve is always at right angles to the rim when installed. Apart from provoking premature tube failure at the base of the valve, the valve being at an angle means the tube has slid to one side and might not be positioned properly anymore (i.e. the patch might no longer be directly over the danger spot.

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Is the spoke corresponding to the pinch location stiff or is it loose? The two bulges shapes are a bit strange, but the suggestion of replacing the rim tape with a rigid plastic one is surely the long-term solution.

For the short-term: duct tape, two or three layers, for a section of 4-5 centimeters over the rim tape, to prevent it from moving too much or being punctured by the spoke. If you cut it in thin stripes, allowing for the tyre to be correctly seated inside the rim, you will be fine for osme time (but sooner or later you will be reminded by a puncture about not having fixed it properly...)

Maybe the tube can work its way through the single fibers of the rim tape.

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    New rim tape is the proper fix. But in the interim, a couple layers of tape will help. I've used paper painter's tape of a suitable width, but even electrical tape will help keep it all together. I suspect the inner tube is getting old and tired too, which would contribute. – Criggie Apr 27 at 4:52
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    @Criggie I agree ... when 100 psi are in the tube, one do not need a strong glue to keep things in place, the tape should only prevent scratching on the tube, so any smooth thing will help. One may even put something "hard" (small section of plastic yogurt container?) under the tape to be sure it is rigid, then the tape to prevent pinching to the tube. – EarlGrey Apr 27 at 6:23
  • The spoke at that location is stiff. – Will Haley Apr 27 at 14:29
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    @WillHaley I was thinking, as a remote possibility, of a nipple failure: bikeforums.net/bicycle-mechanics/… Glad it is not the case. even applying a large tube repairing patch (which are usually a bit more rigid) you should be fine for some time, without changing the rim-tape. – EarlGrey Apr 28 at 12:58

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