4

Here's a photo:

Punctured tire

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.

Failed patch

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:

Rim photos

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...

4
  • "This has happened before" - at the same spot? Aug 13, 2023 at 21:44
  • @leftaroundabout - I don't know. But similar bumps. It was a good while ago, so I don't have any records/pictures...
    – Vilx-
    Aug 13, 2023 at 22:45
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    How hot was it on your balcony ?
    – Criggie
    Aug 14, 2023 at 1:19
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    @Criggie - It can get hot there. Hmm... last Monday was pretty hot. It got to 30°C outside, so maybe 40°C on the balcony? I don't know, I don't have a thermometer there.
    – Vilx-
    Aug 14, 2023 at 7:26

3 Answers 3

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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.


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.

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  • Well, when I removed the tube, the rim tape looked just fine. There were no sharp edges or other irregularities (compared to other places on the rim). That's why I'm confused. Also, a new tube costs mere 4€, so I don't bother repairing them.
    – Vilx-
    Aug 14, 2023 at 7:30
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    @Vilx- And a patch costs 25p which is why I DO bother repairing them. So, try measuring off from the valve stem hole around the rim and it will identify one single spoke hole to look at closer. Try poking at the rim tape with 50 PSI of finger pressure (that's really about as hard as you can press) and see if the tape moves. Worst case, add some tape there as reinforcement over that one single spoke access hole.
    – Criggie
    Aug 14, 2023 at 10:31
  • Well, I suppose at the rate they've been failing lately, maybe I should start considering patches too. :D I need to get a new set of tubes now anyway, so I will ask about patches and rim tapes while I'm there.
    – Vilx-
    Aug 14, 2023 at 12:11
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    The good thing about patching the tube is, it might by itself be enough to prevent this particular issue from happening again because a patch is much stiffer than the tube wall and therefore won't work its way through a rim tape deficiency as readily. Aug 14, 2023 at 15:24
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    @Criggie ... or just a patch made of an old tube. I use them a lot because they drop the price from 25p to just like 2p for the glue, and so far all of them have held up perfectly.
    – MaxD
    Aug 14, 2023 at 18:50
1

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.

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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


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.

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  • 2
    I actually have an IR thermometer myself, so I'll check the next time it gets really hot. But I'm not sure if it was really getting a lot of direct sunlight. 60C would be so hot that it would be difficult to touch, but I don't recall the tires ever being that hot. I do however pump the tires close to the limit (it says 4bar on the tire, and I pump to 3.5), so maybe the little difference is enough. But I too feel like there's more pieces to this puzzle.
    – Vilx-
    Aug 14, 2023 at 12:09
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    @Vilx- rubber being a good insulator it won't necessarily feel hot to the touch, metal burns you because its a good conductor so can quickly dump its energy into you. There could certainly be a big difference between the internal and external temperature of the tyre Aug 14, 2023 at 13:45
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    @AlanBirtles - Perhaps, perhaps. Although in that case I'd guess that an IR thermometer also won't show much.
    – Vilx-
    Aug 14, 2023 at 13:56
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    @Vilx- 60°C rubber will show up very nicely on an IR thermometer. Rubber (including synthetic rubber) has good emissivity, which is what matters. It will feel warm to the touch rather than hot because it has low thermal conductivity - your hand cools it quite effectively before you have time to notice it heating your skin.
    – Chris H
    Aug 14, 2023 at 14:21
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    It's about 20°C here and intermittently sunny, and I have access to a thermal camera. So I went and had a look. The bike is in the shade of the overhanging building, unfortunately, and probably has been for the last few hours
    – Chris H
    Aug 14, 2023 at 14:32

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