How did these pimple/bubble/bump shapes form on my tube?

Question

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.

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.

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!

• 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.

Answer 1

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.

Answer 2

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.

Answer 3

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.