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  1. #1
    bcs
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    Chain line question--new XT BB

    I have a Specialized 2002 Enduro Comp. It has the stock LX front and XT rear derailleurs. Today I upgraded to a XT bottom bracket/crankset assembly and I replaced the cassette with an XT cassette. I am installing a new Shimano HG93 chain.

    First question: I understand the new chainrings, cassette and chain I installed are all intended to be compatible, per Shimano's web site. This stuff has a 50mm chainline. But I can't tell from the tech info what was the chain line on the original equipment. It looks like the chain is now a bit to the right compared with before. Should I leave it and try to adjust the front derailleur to compensate? The frame is 68mm wide, so I have 2 2.5mm spacers on the right and one on the left. What if I just shifted one from right to left? That would move my chain to the left 2.5mm. Any drawbacks to this?

    Second question: what length should I cut the chain to? Looks like it should be 108 links. I don't think I can go shorter when I'm on the big chianring + sprocket. But when I go small-small, there is a bit more slack that what I would like to see. anyone know what the right length is on this bike? My chainrings and cassette have the same number of teeth as stock. Thanks!
    Well, I didn't expect the Spanish inquisition...

  2. #2
    bang
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    the old chainline was 47.5mm. you can try moving a spacer over, just make sure the crankarms and chainrings clear your chainstay. note that the crankset will be off center by 2.5mm.

    the standard chain sizing procedure is to loop the chain around the largest chainring and largest rear cog (without routing the chain through the rear derailer), and then add one inch in length. it doesn't really matter how much slack you have in the small-small combo, since that combination is never used. the large-large is also never used, but it could spare your drivetrain if you somehow manage to shift into that combination.

  3. #3
    bcs
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    Thanks, Cycodude.
    I was working on this last night, and got tired so I didn't really take a careful look at the chain line. Overall (I mean in relation to the cassette, the chain line may not be bad. The real problem is I am a novice at bike mechanics and obviously the fron derailleur is gonna need some real adjustment unless I mess with the spacers.

    if the chain line (chainrings in relation to the cassette) is good as is, is my old LX front derailleur gonna adjust out far enough to compensate for the extra 2.5mm chain line? Or am I gonna need to upgrade to a new XT front derailleur?

    If the chain line is not so good (i.e., the chain rings are too far to the right) is there any other alternative than messing wiht the spacers?

    If I move the spacer, and the cranks are 2.5mm off center to the left, is that a difference that most riders would notice?

    EDIT: After closer inspection, it is clear that moving a spacer to the left side is not a good option--it would reduce the number of threads on the BB that would engage the frame. Shimano has some good tech documents on their web site, including a "compatibility chart". So although I bought a crankset, chain and chainring that are all compatible, I didn't check the front derailleur (because I didn't know the model number). After inspecting it, and finding the model number, Shimano's chart shows that its not compatible. So I guess the best solution is just upgrade the front derailleur. Off to the bike shop again...
    Last edited by bcs; 02-25-2007 at 01:53 PM.
    Well, I didn't expect the Spanish inquisition...

  4. #4
    bang
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    i would try to adjust the front derailer first. it may or may not work, but it won't cost you anything to try.

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