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CRASH + now front derailleur rubbing

parkinpantsparkinpants Posts: 53
edited September 2014 in MTB workshop & tech
My mate's had a bit of a crash and I'm trying to fix it. There were two problems:

- front derailleur was hitting the tyre in lowest gear
- the chain rubbing on the front derailleur, mainly in the hardest gear/smallest sprocket at the back

I started from scratch (aligned the deraillleur with front chainrings, loosened the cable, set the lower limit on largest sprocket rear + smallest front, tightened cable, moved to largest chainring on front and set higher limit).

I've fixed the first problem, derailleur no longer hits the tyre, but I'm struggling to eliminate the rubbing no matter what adjustments I make with the limit screws and barrel adjuster. I think the problem is something other than patience + minor adjustments though - when I adjusted the lower limit to allow minimum clearance I could see the chain moving to and from the derailler by 2mm or so depending on where the cranks are (pedal down seems to pull chain away, pedal up seems to push it towards derailleur).

Can anyone suggest why that might be? I'm thinking the crash has bent something to make the chain move left/right when the cranks turn.

These might be red herrings but just in case it might be worth mentioning that:
- its an old 8 speed Cannondale MTB
- it has 3 clicks on the shifter, but only 2 cogs at the front
- derailleur is quite high above the top chainring. I know they normally say it should be 2mm above but I'm guessing that doesn't apply here with a 2 cog system? I can send a pic if it helps.


  • It sounds like the chain rings are a bit bent.

    Take the chain off the chain rings and spin the cranks. look directly down onto the chain rings. They should run true and not wobble.

    Also, the front mech should still be 1-3mm from the outer chain ring as the dérailleur should be designed for it. Otherwise you're risking the chain dropping over the 2nd ring. If it's a triple dérailleur on a double. Get a new front dérailleur.

    The other thing is a lot of cheap chain sets don't even run completely true from new, so it may be perfectly normal that there's a little with of sideways movement in the chain that causes it to tap the front mech. It just depends how much. Maybe 2mm is too much and something is bent.
  • Okay, thanks. I'll start by checking the chain rings tomorrow, sounds a likely culprit. It's hard to see because there's one of those guard's in the way to stop the chain going over the large cog but I'll figure it out, I'll use an old school pencil + elastic band as a tru-ing gauge if need be.
  • supersonicsupersonic Posts: 82,708 Lives Here
    Is the wheel central in the frame?
  • It might be that some chain links are bent or some links are stuck solid. If so bend back, remove, use penetrating oil, take apart and whatever else works to fix it
  • I'll have a good look at all suggested possibilities tomorrow, thanks for your help.

    Supersonic, that's a good question. The wheel definitely isn't in the centre of the frame. It's quite difficult to get it centre for some reason. I can't visualise how that would make the chain move side to side though, or are you saying that might be a red herring and that the wheel alignment might be the real reason for the derailleur rub?
  • supersonicsupersonic Posts: 82,708 Lives Here
    I think it could be a combination of factors - though I not you actually fixed the tyre rubbing issue.

    If the chain is moving like that (in all front chainrings) then either the bottom bracket axle is bent, or the chainset spider is bent. But many are out of alignment slightly anyway.
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