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Very occasionally my chain will come of the small chainwheel on a downshift. It is not at all predictable, except that it always happens when the I'm doing an unloaded shift in preparation for a climb. I don't think it has ever happened under load.

The limit screws seem to be set correctly. Most (>99%) of the time I can shift onto the small chain ring without any problem – both under load and without a load.

I'd appreciate suggestions for what to check or ideas about what could be causing this.

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    Did you try adjusting the limit screw so it limits the chain just a little bit more? Without load the chain is somewhat more loose so it's easier to drop off the chainwheel. Also make sure your cable is properly greased. – stijn Sep 20 '15 at 7:36
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    Check that the derailerueruer is straight - they can perform fine a little crooked, but too much can do this kind of thing. – Criggie Sep 20 '15 at 9:46
  • Do check that your chain is not too worn. – Daniel R Hicks Sep 20 '15 at 12:12
  • @Criggie Front or rear (or both) derailleur? – dlu Sep 20 '15 at 15:44
  • @dlu - i'm referring to the front one. The cage should be parallel to the chain line. They "work" up to 10-12 degrees offline, but when crooked the limit screws don't line up with the cage's location. – Criggie Sep 20 '15 at 22:06
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From the way you describe the problem it seems less likely to be one of the common mechanical causes, like a stiff chain link or bent chainring. I have two guesses as to the cause. One is a subtle mechanical problem that requires the right conditions to set it off. The other is about technique.

  1. The rear derailer may not be taking up enough slack when you're in the small chainring. (This is not always obvious.) If you happen to shift around the time you hit a little bump or dip, or while you throw your weight forward to stand, or while you adjust your cadence, the slack chain gives a bounce and off it comes.

  2. You may be shifting improperly on these rare occasions. If you shift to the innermost chainring while you're in the outermost cog, and the chain line is angled too sharply, off it goes. Also, to reduce the odds of dropping the chain, a front shift ought to be done like this.

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  • Seems like it is #1. – dlu Oct 15 '15 at 20:23
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I would just tweak the limit screw for the low side a bit. See if the problem goes away and you can shift into the low chain ring reliably. Occasional problems are hard.

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  • I had one friend with a problem like this. Limit screws just doesn't fix it like OP wrote. – Alexander Sep 21 '15 at 3:14
  • @Alexander: Could be. This is a free and easy thing to try. My experience says it is easy to think you have the stops properly adjusted and be wrong. If you can't find the sweet spot it is time for more serious measures. – Ross Millikan Sep 21 '15 at 3:50
  • Yet I disagree with you. The last 2 years I've adjusted about 800 deraillerus, so i tell you, this derailleur wasn't adjusted wrong. – Alexander Sep 21 '15 at 5:26

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