What caused this puncture in my tube?

Question

Here's a photo:

This is confusing me because of several things:

• This is clearly on the inside of the tube, so it's not something I rode over
• This happened while the bicycle was standing untouched on my balcony
• You can see two bumps in the rubber, of which one has ruptured
• There are no other bumps like this on the tube
• The tips of the spokes are all covered with a rubber tape which seems to be unpunctured
• No tips of spokes stand out more than others (or any other irregularities), including the ones that would correspond with this puncture
  • However the location does match one of the spokes, so it seems likely there might be a relation
• This has happened before, but doesn't happen all the time (maybe once or twice per year).

Any ideas what's going on here? Do I need to replace the rims? They are pretty old anyway...

Update, a little later:

So, I changed the tube one evening, and the next day was a bit hotter again and - poof! The next one failed too. In the exact same fashion, in the exact same place. I didn't even get the chance to ride it!

So this time I thought of taking the advice from here to heart and a few days after that went out and got two things - more new tubes (this time from a different brand, although I think it's a cheaper one) and a patch kit.

Then, later yet, I tried to patch it. It seemed to work, so I reassembled the tire and pumped it up to my regular 3.5bar (around 50psi).

It looked good, for about 10 seconds. Then with a loud hiss the patch failed.

You can see a bulge in the middle of the patch and where it came open on the side.

Now, I suspect this might just be me not knowing how to patch right. I've never done this before. I did try to follow the provided instructions, but maybe I didn't sand the place down well enough, or didn't apply the glue thoroughly enough, or didn't let it cure long enough. Who knows.

Anyways, I went back to what I know and put in a new tube. This one seems to be holding - so far. Haven't had any hot days though.

Before putting it all back together though I did one more thing - I took a picture of the rim at the place where the break was. And... best I could tell, it was BETWEEN two spokes, not on top of one.

Here's the suspect area, with and without rim tape:

And... I still don't see anything special there. The tape and the spoke tips are the same as everywhere else.

I did however notice something else - the bumps on the broken tube are on both sides of what seems to be some sort of a rib. Could that have something to do with it? Do tubes have a maximum pressure rating? I didn't see any, and the tires say "2.7 to 4.4 bar", so I think I'm within bounds, but...

Answer 1

Your tube has punctured because of a lack of support in that spot.

I bet if you examine the inside of your rim, the one spoke access hole at that spot will either have a sharp edge, or the rim tape has moved and is revealing an open space.

Your tube holds pressure all the time. When the black tyre is exposed to heat from hot sunlight, it will expand and will push harder on the tyre and rim.

In this case, the tube probably herniated out through the gap into the rim, and popped like a balloon.

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You may need to replace the rim tape, perhaps yours is not quite wide enough. Secondly, there may be a burr or sharp edge exposed, though this tiny peak is indicative of an unsupported spot.

The tube can probably be patched and reused well enough. If you can't replace the rim tape, then several layers of painter's tape or electrical insulation tape will help. Just be careful to get a good width so it covers the spoke holes, but not so wide that the tape interferes with the tyre bead seating itself into the crochet hooks on the inside of the rim.

Answer 2

I had an issue like this and it was due to rim tape. Tyre could be inflated and would be fine for a while but then would randomly fail. Rim tape is pretty cheap and in the end my local bike shop replaced it for me for free, so it probably cost less than one or 2 new inner tubes. This was on a road tyre at 100 psi.

Answer 3

Addressing temperature specifically:

The formula is PV = nRT where n is the amount of air and R is a constant.

This means is that pressure * volume is proportional to temperature and the temperature is absolute, as in (celcius + 273)

So if you go from 20->40 C, the pressure * volume increases by (40+273)/(20+273) or about 6%.

A bike tyre has between 2400 cc and 44,000 cc of VOLUME inside depending on size and width. (source: http://thebicyclewizards.com/index.php/2016/05/12/tiresizeandairvolume/ )

6% increase is not a lot and is unlikely to be the sole cause here

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Thing is, your black tyre in the sun will be getting substantially hotter than ambient temperature.

It's winter here for me so I can't do any empirical testing. However last summer I did have a bike parked outside in direct sunlight and mid-afternoon the whole tyre popped to the point it tore the tube for a length of 20 cm, and the pressure damaged the plastic fender.

I'd suggest borrowing an IR thermometer and use it to measure the tyre's temperature on your balcony on a hot day. You might be surprised how hot it gets. If the tyre got to 60 degrees C then that's a ~14% pressure increase from 20 degrees.