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I recently started to have a problem where the chain randomly goes from the large front gear to the smaller one.

Ive been trying to work out the reason for this and would like some input on the more likely culprit:

  1. The front derailleur is somehow slipping. in order for it to stay in the position for the large gear there is quite a bit of tension on it. (Its a 105 shimano if it makes any difference)
  2. My front cassette is worn out AND i noticed that the chain angles towards the left as it comes off the front cassette so it naturally wants to slip down to the smaller gear.
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When you say that your front derailleur is slipping, do you mean that the chain is slipping off the chain ring, or do you mean that the derailleur itself is moving inward even though you're haven't shifted it? Also, what makes you say that your front chainrings are worn? It's not very common for them to wear out, especially without having significantly worn the chain and rear cassette as well, usually a couple of chains and rear cassettes. –  jimirings Dec 6 '12 at 2:45
    
i mean it shifts to the easier gear randomly. Front chainset wear is a theory. I dont know what one looks like when it is worn or if mine is. i bought the bike used and have put about 1000 miles on it since. –  mkoryak Dec 6 '12 at 4:34
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1 Answer

This sounds like your cable has stretched. For the front derailleur, you have to apply tension to the cable to get it to move to the right ("harder gear") if the cable lengthens (or slips), by even a millimeter or two, it can cause the front tuning of the gear system to get out of whack with the result being the chain dropping to the easier gear(s).

Shift all the way to the left, loosen the bolt (or nut) holding the front derailleur cable, put tension on the cable and retighten. I suspect this will fix things.

Chainring wear could be at play here, but they would really have to be worn down. The chain angle you see is probably the natural movement when the chain is supposed to be shifting.

Also check alignment: derailleur should be be about 2mm above large chainring, curve roughly matching, and aligned in the same plane as the chainring.

Happy riding.

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