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The tyre is 700 x 25C. Two inner tube sizes were available: "700 x 20C to 700 x 25C", and "700 x 25C to 700 x 32C". Oddly there is an overlap, and so those with 700 x 25C get to choose. I chose the former to reduce the chance of having a kink.

Yet I managed to get a kink anyway, or at least it looks like it. I did ride just fine for 1500 km or so, but a hole eventually developed exactly at the edge of the odd-shaped bulb in the image.

tyre kink

How does one end up with this inner tube kink, and what precautions should one do to avoid it when fitting the tyre? After inserting the first bead, I inflated the inner tube slightly, inserted the stem, pushed the rest working from both directions while ensuring it's not twisted, inserted the second bead of the tyre with levers, applied soapy water in a couple of locations to get a good seating, then inflated to 90-100psi. This recipe failed me in some step. Do you see it?

Somewhat Related:

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    I suspect this is a "feature" of the tube that was there from manufacture. I've seen these several times, in brand new tubes. I don't know precisely what causes them. Aug 12, 2022 at 11:44
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    I’ve seen plenty of good new tubes (Schwalbe and Continental) which had wide and narrow areas when slightly inflated outside of the tyre. They are not at all uniform.
    – Michael
    Aug 12, 2022 at 13:43
  • @Michael The tube (and tyre) in question are Continental. (Grumbling: if we can't depend on brand names being good out of the factory, then what's the point of insisting on brand names?) Daniel: I didn't see any bulge while inflating the tube initially, but I wasn't looking for one.
    – Sam7919
    Aug 12, 2022 at 15:34
  • @Sam In my recent experience, due to the pandemic-related inner tube shortage, tubes manufactured to fill the deficiency are more likely to have defects due to being too hastily made.
    – MaplePanda
    Aug 12, 2022 at 17:54

3 Answers 3

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This has happened to me when I've put too much air in before mounting the tube into the tyre.

This initial inflation can be helpful to prevent you pinching the tube between tyre and rim when you mount the second bead, but because the tube can expand when you're inflating it outside of the tyre, it's easy to over-inflate it and end up with a tube that's a little too big, which can result in a folded-over kink like you've experienced when you stuff it into the tyre.

If you do use the initial-inflation trick, you only need to put a very small amount of air in.

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    Once I have the partially inflated tire mounted, I usually fill it up good and full, let all the air back out, then fill it up again. That should (at least in theory) give the kinks a chance to work themselves out.
    – jimchristie
    Aug 12, 2022 at 12:10
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I've only seen this when the tube is not evenly distributed around the tyre on initial insertion. So it inflates and grips the tyre before the length gets sorted.

The slight-inflation just after mounting the first bead should sort that out, because the tube is not constrained as much.

Another possible cause is a too-long tube in the tyre, but you've got 700c which is 622mm and would do fine for a 630mm rim as well.

Is your tube freshly tested in free air? - I know tubes grow by 20-30% when inflated outside the constraints of a tyre, so if you've just tested it free, then the tube may be still too long. Let it rest for a few minutes before installing, if this is the case ?

Personally I don't bother with the small initial-inflation, but I can't see that causing the foldover.

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Short version: When you pre-inflate the inner tube, use the tyre as a cast to make sure that you don't over-pre-inflate.

Longer version: Thanks Greg and Criggie. The steps do not really allow for any improvisation. I have likely over-inflated the tube. Let me correct what I wrote in the question.

  1. "Put just enough air into the tube for it to hold its shape."
  2. "Install the inner tube inside the tyre."
  3. "Work one bead at at a time on to the rim."

I had switched the order of steps 2 and 3, installing the inner tube after the first bead was in.

By inflating the inner tube to "just enough air to hold its shape without then installing the inner tube inside the tyre, it's easy to over-inflate.

Another detail that's easy to miss is what to do when a bead is not properly seated. Adding soapy water is not useful unless one first deflates the tyre. The bead-to-rim connection may be perfectly water-tight, and soapy water will not make its way to the non-gliding interface unless one first deflates the tyre.

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    When you inflate the tube prior to mounting, it should still be fairly limp. Aug 12, 2022 at 16:45
  • Personally I don't do the little-inflate at all. I put one bead on, then valve through hole, stuff the wet-noodle totally-limp tube in the tyre. spreading it evenly as I go. And then put the second bead on looking to see I don't pinch tube. Whatever works best for your tools, rim, tyre, and preferences I guess. There is no one-true-way.
    – Criggie
    Aug 12, 2022 at 23:16

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