Chain rubbing outer plate of cage on front

Discussion in 'Cycling Equipment' started by lol168, Mar 3, 2013.

  1. lol168

    lol168 New Member

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    Just got a new bike from LBS: Trek Lexa SLX C (compact), and I got a question: When chain is on the largest cog in front, and smallest cog in rear, and it was rubbing the cage. Actually, the chain will start rubbing the outer plate of cage when rear gear was on 7th gear,... and this is not "cross chain", and am I correct? Already went to my LBS, after 20min., they said it's been fixed.... Only to find out the issue still the same after I got home. Is it something that I can learn to DIY to fix? Any advise, and comments are appreciated!
     
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  2. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Your front derailleur needs some small adjustments to the outer limit screw, the overall cable tension or both. It may or may not also need some small rotational tweak, IOW when viewed from above the derailleur cage plates should be parallel to the chainrings and not rotated in or out very much if at all.

    This is easy DIY stuff but the LBS should really take care of you on a new bike. If you want to tackle it yourself, surf over to the Park Tools website for good information, if you take it into the shop ask if you can watch them make the adjustment so you learn a bit about bike maintenance. If you take it in, also have them check rear shifting as it's pretty normal for cables to stretch or cable housings compress when new bikes are first ridden and if your front cable has slack (not sure it does, but not uncommon) then your rear may as well.

    http://www.parktool.com/blog/repair-help

    -Dave
     
  3. vspa

    vspa Active Member

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    thats the unwritten law of checking the bike repairs before leaving the bike shop,
     
  4. lol168

    lol168 New Member

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    Thanks guys! 1) Learned my lesson. Next time will check before leaving. 2) No cable slacking, and cage is parallel. So, here's decided to go ahead play around with the limit screws..... And YES.... I fixed it by simply adjusting the outer limit screw! ;-)
     
  5. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Good deal, just shift the front derailleur up and down a bunch of times in different rear cog combinations to make sure you don't overshift and drop the chain to the outside. That's the basic risk of adjusting the outer limit screw but find the point where the chain doesn't rub in at least the smaller cogs and the chain doesn't drop off the big chainring during an upshift and you're good to go.

    -Dave
     
  6. lol168

    lol168 New Member

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    Oh, and by the way...... According to Park Tool: The gap between the teeth of the outer chainring and lower edge of the outer cage plate should be 1-2mm" Is that just a very general rule of thumb that doesn't apply to ALL types of bike?? Cause when I look at my bike, it's definitely higher than that,... I can almost fit in my index finger. Having said that, there's nothing else wrong with my bike. Shifting is fine..... So, is there anything I need to look into further?
     
  7. daveryanwyoming

    daveryanwyoming Well-Known Member

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    For the best shifting the gap should be very narrow as outlined on the Park website. A front derailleur will still typically shift with a larger gap but the shifting performance usually isn't as good.

    Any chance your bike was originally built up with a standard crankset (e.g. 53x39) but you had them swap in a compact (e.g. 50x34)? If so it's likely they didn't drop and then retension the front derailleur to close up that gap.

    If your frame has an integrated (aka braze on) front derailleur hanger then it's possible your current front derailleur cannot be dropped low enough but if your bike has a clamp on front derailleur hanger or the derailleur can be slid lower or in some cases (e.g. SRAM Rival or Force front derailleurs) there are two mounting holes in the derailleur body to support high or low positions for just this kind of reason. If it's possible to lower your front derailleur to close the gap, you should (or really you should insist that the shop does it as they should have done that when assembling the bike if at all possible) drop the derailleur till you close down that gap and then you'll have to re-tension the front derailleur cable to take up the excess slack.

    It's not absolutely critical if you're happy with the shifting, but if it were my bike I'd sort that out to ensure the best possible shifting.

    -Dave
     
  8. lol168

    lol168 New Member

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    Thanks Dave! *my bike was originally built for 50/34. I didn't swap anything. *It's a clamp on front derailleur I'll watch some video to see if I can lower it and tighten up the cable tomorrow :)
     
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