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I have a road bike (optimo 4). Getting some issue with chain rubbing the front derailleur which I figured was partially due to derailleur alignment. However after I went to LBS and adjusted it I am still getting rubbing, although much less now. Would this be caused be a defect with the Crank chainring or something else? This only happens when set on the biggest ring in the front crank. The LBS said it was due to "soft metal" when pedaling hard. Trimming also helps but noise still there.

Crank is FSA Tempo. Should I buy a new crank e.g. 105 Shimano.

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    When does this happen? All of the time? When hammering? Only with certain rear cogs?
    – Paul H
    Commented May 8, 2023 at 20:47
  • Biggest ring front, smallest ring back to 3rd smallest ring ish. Also it looks like when I'm pedaling the chain doesn't look like its moving super straight when I'm looking down.
    – Bigbob23
    Commented May 8, 2023 at 20:59
  • @Bigbob23 Well, that than actually looks like a slightly bent chainring, also referring to the comment below. If it doesn't constantly rub but only a certain point, it might actually be that. I don't think you need a new crank but only new large chainring matching the crankset (which is cheaper than a whole new crank) - at least I don't think the whole crankset is messed up.
    – DoNuT
    Commented May 23, 2023 at 11:20

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If by "soft metal" your mechanic meant that the frame was flexing, I don't buy it. You said you have an "Optimo 4". I think you mean the Cannondale model. Cannodales CAAD frames should be sufficiently stiff to get any reasonably well-conditioned front derailleur working properly.

To me it sounds like your front derailleur is slightly misaligned, the high-limit screws needs to be backed out just a smidge more, or both.

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  • Note that if you back off the limit screw even a little, don't change under heavy load until you're confident you're not going to drop your chain. But I'd be looking hard at whether the FD is actually parallel to the frame, if it's a clamp-on type.
    – Chris H
    Commented May 9, 2023 at 12:35
  • it looks like only in certain position during the pedaling the chain is hitting the deraileur and it is hitting at that spot every time.
    – Bigbob23
    Commented May 9, 2023 at 16:10
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The issue was fixed after adjust the front derailleur through the barrel adjuster. It seems like there was not a perfectly straight movement from the chain but after shifting the cage slightly outward was still able to get rid of the rub. I just had a hard time using the barrel adjuster so had to reset the tension wire.

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As already mentioned, I think, the FD might still not be fully adjusted. It might be that cable tension is now fixed so that it came out a bit more into the correct position and that reduced rubbing, but the outer limit screw might still be off, so that the derailleur can't come out as much as it should/could. If shifts are fine and you don't have any chain drops, that might be the problem. I don't buy the chainring is untrue/bent, that's something you should see visually when spinning the crank and would be independent of the rear derailleur position.

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