front mech rub

shiznit76
shiznit76 Posts: 640
edited July 2018 in Workshop
Hi, my front mech starts to rub when on big chain ring at front and largest three cogs on rear. I know not suppose to cross chain, but surely shouldn't be issue with three sprockets on rear. Is there a quick fix for this?

Comments

  • StillGoing
    StillGoing Posts: 5,211
    Presumably you're trimming it when in the big ring and larger sprockets? Is it a braze on or band on front derailleur? A band on can sometimes twist on the seat tube, but its usually rear end inwards causing rub when in big and the smallest sprockets. Try re-indexing the front derailleur with the chain in the inner ring and on the largest sprocket. There should just be enough clearance that the chain isn't rubbing on the derailleur when in this gearing. The derailleur should line up with the big ring of the cranks.
    I ride a bike. Doesn't make me green or a tree hugger. I drive a car too.
  • shiznit76
    shiznit76 Posts: 640
    yes, when I trim it the third biggest sprocket starts running fine but two largest ones still rub.
    It is a braze on front mech.
    My mechanical skills are limited but dont really want take the bike to a shop when it could just be the case of turning a screw.
    Do the high / low limit screws have anything to do with adjusting it?
  • keef66
    keef66 Posts: 13,123
    No, they just limit the very extremes of movement. Cable tension is what determines where the mech sits when trimmed. Do you have a downtube or inline barrel adjuster for the front mech?

    If not, it may just be a case of setting it up again from scratch. What shifters / mech are we talking about? Shimano Tech Docs have step by step instructions for setting up / indexing all their stuff. You shouldn't need a bike shop, just an allen key.

    I fitted inline adjusters when I recabled my Scott so I could get the front mech spot on. Which was handy as the cable detached itself from the clamp 30 miles into a 60 mile ride last week. Easily fixed / adjusted at the roadside.
  • shiznit76
    shiznit76 Posts: 640
    its a shimano 5800 105 11sp groupset. I do have inline adjusters, what should I do to adjust them?
    Sorry, I must sound like an idiot
  • keef66
    keef66 Posts: 13,123
    First make sure the mech cage is absolutely parallel to the chainrings and not skewed at all. Then wind the adjuster in a bit to reduce the cable tension till the rubbing is eliminated. Then check you still have sufficient tension to fully shift back up into the big ring.

    You may not be able to completely eliminate chain rub at the extremes of cross-chaining, it depends on the size and geometry of the frame.
  • shiznit76
    shiznit76 Posts: 640
    wind in the front mech adjuster? Is that clockwise or anti?
  • keef66
    keef66 Posts: 13,123
    Clockwise I suppose (depends on where you're looking from...)

    With my inline Shimano ones you can see the thing getting shorter as you wind it in. Easier to do with lowest tension in the cable, so shift into the small ring first, then shift up and trim to see if it's worked

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