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I'm a bit confused about what's going on with my front derailleur (which is PROBABLY a Shimano Acera, but the bike is 13 years old and the name has long since rubbed off). The chain has started rubbing against it when it's in a hard gear on the 3rd (largest) chain ring. This is the 8th gear.

If I adjust the barrel adjuster until the chain just stops rubbing, then it will invariably rub in the 5th gear. Previously, the bike was able to work without the chain rubbing until I got into the 3rd gear. If the barrel adjusters aren't working anymore, what might be the issue? Is it just a cable tension issue that's too far gone for the barrels to fix?

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    You should follow ww.parktool.com/blog/repair-help/front-derailleur-adjustment – Batman Jul 6 '16 at 15:40
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    Maybe your chainring had bent a little, so in some position it will rub the chain on upper gear, and in other position rub in low gear. – Alexander Jul 6 '16 at 18:48
  • Could also be the high and low screws need a tweak. – mattnz Jul 6 '16 at 20:03
  • Some crucial information is missing here. Which side of the chain is rubbing on which side of the cage, in each case? – andy256 Jul 7 '16 at 1:30
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    Usually when you have this problem it is because the derailer is either bent, or has become twisted on the seat tube. Aligning the twist of a front derailer is a bit of a black art. – Daniel R Hicks Jul 7 '16 at 1:47
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If I were you, I would first get new cables and maybe even housing for the derailleur cables. Put the barrel adjusters in sort of the middle and change the cables. If it's still having an issue, then the problem might lie in the upper limit of the derailleur. Check the limit screws on the derailleur and loosen the upper one.

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