Why do I keep getting cut looking flats?

Question

My flats are causing some deep confusion.

In the last year I've replaced at least 12 inner tubes because I keep getting flats. They are not puncture or pinch flats, they look like cuts to the tube, sometimes small incisions sometimes long gashes. They're always the back tire on the side facing the hub and they're usually close to the valve.

Tubes only last a couple of rides and it seems to be getting worse. I took the wheel to a shop recently hoping to gain some insight into the problem and the guy replaced the rim tape and put a new tube in and it went flat on the walk home. I tried a heavier duty 'hybrid' bike tube, and that only lasted one ride. What happens is I'll ride it once or a couple of times and either during the ride or more often at home later, after the bike has been sitting for some time, I'll hear a hissss and it's flat. When I examine the tube there's a hole that looks like a slice near the valve.

Anyone have any clue what could be going on or what to do about it? I'm thoroughly confused.Could it be a poorly designed rim? I thought it had something to do with the rim being too narrow and that putting excessive pressure on the tube. Only thing is I can't figure out why it would pop when the bike is at rest.

Answer 1

Is the cut always the same distance around the tube from the valve?

If so then you have something wrong in your rim. Use the punctured tube to measure where the damage is coming from... that will give you at most two positions, one in front of the valve hole and one behind.

I think you'll find a cause in this very small sector of the rim.

BTW - had you patched your tubes rather than replacing them, the punctures might have been reduced by the extra thickness of the patch in that spot. And if they punctured again though the patch, then you'd know straight away something's wrong there.

Answer 2

I could swear there's already an answered and highly upvoted post here that features this exact issue, but I can't find it.

This is a really infuriating and mysterious problem. I've run into it a couple times. It's hard to figure out and is an under-publicized issue. The only bright side is the symptoms are always the same - repeat flats, always the same inexplicable cut/gash near the valve, rim side, no burrs, no irregularities, no protruding spokes, rim strip in good condition, replacing rim strip anyway doesn't help. In other words, it is not any of the usual causes of repeat flats from the rim side.

It happens due to this combination of factors:

1. Most of the inner tube is one single, supple layer of rubber that expands easily to fill whatever cavity it finds inside the tire. The part that isn't quite like this is the reinforced area around the valve.
2. Some rims, from what I've seen ones with kind of a funky cross-section that narrows at the bottom more than usual, keep the reinforced area from being able to lay flat against the rim under pressure without introducing a stress riser into the tube rubber. This is what causes it to burst, sometimes very quickly after inflation.

I've heard the notion that by tugging down on the valve aggressively as you inflate it, you might be able to mitigate the problem. I'm highly dubious but it may be worth trying.

I've been able to fix it seemingly for good by layering pieces of rim tape and/or tube at the valve to build up the area there, essentially creating a more normal face for the valve to interface with. I used electric tape to hold them in place and I layered several lengths of them to create a little bit of a taper effect to smooth the transition. A normal rim strip then goes on top of all that. I believe I cut the valve hole on each of the layers first. Note that doing this on most rims could easily create problems with the tire bead not wanting to seat evenly near the valve, but from what I can tell this issue tends to come up on rims that are odd-shaped with deep well sections.

Answer 3

Sounds like a spoke is a little too long to me. It could also be a bur of metal sticking up inside the rim. Take the tire and the rim tape off the rim yourself and carefully run your finger around inside the rim. Use care not to cut yourself. If you find a sharp edge or spoke that is too long, well? Sand it or file it or recruit someone to help you. You may also want to consider just getting a new wheel rim. They are not prohibitively expensive.