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I have a recurring flat problem in my rear tire. I can never find the obvious cause and it does not always occur in the same position on the tube. My rim is a replacement and may be the root cause. I have noted for some time that putting a repaired tube with outer tire back on the rim is ridiculously easy. I do not need tire levers; my fingers will do. Indeed, it practically a finger tip job!

I believe that putting a tire back on my original rim was not so easy; that I really did need a tire lever. But it is so long ago that I am not sure my memory is accurate and that I am not confusing my front and back tire repairs.

So my question to others is simply whether this is plausible and whether most cyclists can expect a little resistance when they try to replace a tire on its rim.

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    Where are the punctures? If they are on the inside diameter of the tube then likely the rim strip is missing or there are some spokes that poke too far through. Otherwise, I'd suspect under-inflation, or damage to the tube from getting it pinched between tire and rim.
    – Daniel R Hicks
    Sep 15, 2019 at 18:54
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    Agree with @DanielRHicks. There is some margin for error in manufacturing, and it is possible that your new rim is at the small end of that margin (it also might just have a deeper center trough), but I have a hard time seeing how a small-ish rim in itself would be causing flats.
    – Adam Rice
    Sep 15, 2019 at 19:09
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    This is the sort of problem where 2 minutes of eyeball time would be way quicker to spot the cause. Another thing you can do to isolate cause is to always put the tyre on the same place, by lining up a tyre logo/side with the valve hole. Then when it flats, compare the hole's offset (around the rim from the valve) with the same distance on the rim from its valve-hole. That will give an area of ~50mm to check closely rather than the whole rim.
    – Criggie ♦
    Sep 15, 2019 at 19:28
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    If the rim was too small the flats would be obvious - blowouts or rolling the bead causing snakebites on the 'side' of the tube. Where are the punctures and are they snake bites or blowouts. If the hole on top edge of the tube from things sticking though, there are just punctures.
    – mattnz
    Sep 16, 2019 at 2:31
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    Your post is a bit unclear. Could you answer the following questions in your post: - If you have a picture of the wheel, please include it in your post. - What are the sizes of the rim, inner tube and tire? - How old is your inner tube and have you been patching it up yourself? - How old is your tire and have you checked for glass or other sharp objects that could cut the inner tube again if it has been mounted in a different place? -Is your rim tape in good shape? (The plastic strip in the middle of the rim covering the spoke nipples.)
    – JLT
    Sep 17, 2019 at 7:20

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I don't believe there are any common rim diameters where the next size down would be close enough for the difference between tyre bead diameter and rim diameter not to look wrong. The tyre would blow off the rim with very little pressure.

When finishing mounting a tyre, it's common to pull the already-mounted part of the bead into the middle of the rim where the well is deepest, to give enough slack to easily get the last part of the bead over the lip of the rim.

With narrow rims and single-wall rims, the rim well may be almost flat, leaving very little slack when mounting the bead. A thick rimtape can make this even worse.

If the new rim is a wider dual-wall rim, or you now have a thinner rimtape, you may have more slack even with matching tyre and rim sizes.

Given the recurring punctures, your rimtape is possibly inadequate or missing.

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