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I changed 3 inner-tubes, they keep getting cut right at the bottom of the valve. I sanded down the hole where the valve goes, it had a sharp edge! Flats still happening...

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    What kind of valve? Does the rim have correct sized hole or is it too large? – ojs Sep 19 '17 at 14:31
  • To add to ojs' comment, if you have a Schrader valve hole and you're putting Presta valves in, you really should be using one of those collars to prevent tube pinching (though many people don't use them, and are fine). – Batman Sep 19 '17 at 16:27
  • Thanks a lot! I think thats what I will do. Find one of those collars,that seems like the best solutions... And thanks,to every one for your opinions. I will keep you guys posted! – Juan Melo Sep 20 '17 at 2:07
  • Have you tried changing brand of tube, and supplier? I've heard of bad batches where the quality control isn't good enough. – Criggie Sep 20 '17 at 6:34
  • Did you sand the valve hole flat? Or did you take the sharp corner off the edge? The proper tool would be a countersink bit, with second choice being a drill bit over 9mm in diameter. – Criggie Sep 20 '17 at 6:35
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As Paparazzi said: make sure the valve goes straight through the valve hole, i.e. perpendicular to the tube. This is likely the solution, but I've seen it happen that even after properly aligning the valve it somehow wasn't aligned anymore after a couple of rides. Which resulted in the bottom of the valve getting cut by the edge of the valve hole in the rim. Which means the tube did not (or not completely) stay in place, i.e. at least a part of it shifted with respect to the rim (and possibly the tyre, hard to tell). As far as I know two things could cause this, first one more likely than the other:

  • if your tube diameter is bigger than your rim and you force it in the tyre anyway, there whill be one or more wrinkles in the tube. If this wrinkle is near the valve, it's not too hard to see that it's possible that under stress the tyre can move a bit, resulting in an unaligned valve, resulting in cutting
  • even with a proper tube diameter, if your tube sticks to the tyre around the location of the valve (caused e.g. by lack of some antistick agent like chalk, water getting in, ...) and there is quite a lot of stress on the tyre, the tyre could in theory move with respect to the rim and drag the tube with it. Can, as far as I know, only be caused by skidding or incorrectly landing jumps or so: unless the tyre is the wrong size, it requires sheer force to rotate it over the rim.
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    I've seen it happen that even after properly aligning the valve it somehow wasn't aligned anymore after a couple of rides. Been there. It was hinted to me to change the rim tape and increase pressure, to no avail. Tube diameter and tyres were a match to the rim. In the end only changing the tyres did help. However, the OP might very well try those things too... – Ghanima Sep 19 '17 at 17:14
  • @Ghanima that is interesting. So in your case it basically was the tyre being too loose on the rim, rotating over it and dragging the tube along with it? – stijn Sep 20 '17 at 7:55
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Make sure the valve is properly aligned. It should not be tilted to either side.

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If this keeps happening, it is possible the tubes are too small for the tires, so that the tubes slide around inside the tires and the stem goes askew. I had this happen when I ordered a bike with fatter tires, it took a year of wrong guesses to figure it out.

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