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  1. #1
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    how do i adjust this? (chain is rubbing)

    On my Trek 3900...for some reason now the chain is rubbing on the thing that moves it when i change gears. Its really bad when I'm in 1 on the front and 8 on the back. (24 speed)

    ~Andy
    Trek 3900

  2. #2
    Ride Responsibly
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    Patient; Doctor, it hurts whenever I do this.
    Doctor; Dont do that.

    there is no good reason to run that combination of gears, and rubbing on the derailure is a good reason not to.
    Instead of staying in the granny (# 1 up front) and shifting all the way to smallest (most outward from center) cog in the rear, when you get to 3 0r 4 in back, shift to the middle ring up front.
    If you do not want to shift the frt then leave it in the middle ring.

    Adjustment is done by cable tension at the barrel adjuster, or by the limit screws on the derailure, but before doing either, take it to the shop you bought it from for a free tune-up.

  3. #3
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    That is called cross chaining, and it's really bad for the drivetrain and the chain. Things will wear out much quicker than they should(chain will stretch fasters, chainrings and cogs will wear down faster)

    It happens when you are in small/small(small ring up front, smallest out back) or big/big(big ring up front, big ring in the back) combination.

    Try shifting to the middle ring if you find you need to use all your rear gears, usually this'll do the trick. There is no way to adjust a front derailleur for it not to rub when cross chaining yet still perform well. I would highly advise you avoid doing so. Your bike may have 24 speeds, but in truth there are a few combinations which give the exact same ratio which would work just as well. Your smallest front ring is probably 22 or 24 teeth, and your smallest rear cog must be either 12 or 13. Your middle ring is probably 32 teeth, and if you choose a cog in the back which has 16 or 15 teeth, it should feel exactly the same.

    I hope this helped you out
    Last edited by snaky69; 05-20-2007 at 08:04 AM.

  4. #4
    dh1
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    what the others said about cross chaining is good advice, but to me it still looks like your front deraileur is out of adjustment by a bit. When you are on the smallest chainring in front and the largest cog in the rear, there should be about 1mm of clearance between the inside guide of the deraileur (nearest the seat tube) and the inner side of the chain. This setting will give you the ability to easily shift to the small chainring and still be able to use most of the rear cogs. It is bad for the drivetrain to use small small or big big gear combinations. From the middle chainring, you should be able to use all of your rear cogs.

    You can download instructions for calibrating your drivetrain from www.sheldonbrown.com

    He has a comprehensive section on drivetrain adjustment. If you don't want to do your own, take it to your LBS.

  5. #5
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    Thanks guys. I kinda figured it was better to use something like 2/3 instead of 1/8 (which probably feels the same)....but I didn't think it should be rubbing that bad. Because it seems to bounce off of it quite a bit even in other gears. I'll just take it up to the LBS and have them adjust it.
    ~Andy
    Trek 3900

  6. #6
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    Quote Originally Posted by 350f
    Thanks guys. I kinda figured it was better to use something like 2/3 instead of 1/8 (which probably feels the same)....but I didn't think it should be rubbing that bad. Because it seems to bounce off of it quite a bit even in other gears. I'll just take it up to the LBS and have them adjust it.
    You figured correctly.

    It should NOT rub. You should NOT cross chain.

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