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  1. #1
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    Reducing chainline on XTR front derailleur below 50mm

    Shimano recommends a chain line of 50mm for their XTR front derailleur. My current setup runs a SKF BB 118mm spline length that makes 51mm chain line on a 3x9 crank. The rear chainline is 45mm on a 9 speed Shimano cassette w/ 135mm rear spacing.

    While the bike shifts fine, its not as crisp or quiet as I like it to be. Would changing the BB to a 113mm thereby moving the front chain line to 48.5 improve things? What would be the effect on the XTR front derailleur?

    As an asside, I've been considering going the route of a 2x9 using Middleburn Duo cranks. I've been informed that they keep the chain line the same so their effect should be non-existant. Right??

    Thanks.

  2. #2
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    Assuming there no physical limitations (ie the derailleur can move in far enough w/o hitting the seat tube of anything else) then a 48.5mm chainline will work fine.

    If your going small/middle 2x9 its probably best to stay at a 51mm chainline. If you going middle/big 2x9 then the 48.5mm chainline will be better.

  3. #3
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    mtnbiker72,

    Can you please explain why small/middle - 51mm vs. middle/big 48.5?
    The Middleburn Duo centers the two rings around the chosen chain line. I would be running a 40/27 with a 11/34 cassette.

    Thanks.
    Boris

  4. #4
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    With a traditional three ring crankset...there are some inherent limitations to using all 9 gears in back (such as small/small or big/big) due to cross chaining.

    Most riders who use 2x9 like to use the whole cassette in either gear. By moving the chainline around, you can lesson the cross chaining issues. In your case, you are likely to want to use a 40 front/34 back combination. This would be an extreme cross chaining with a 51mm chainline, but not so much with a 48.5mm chainline.

  5. #5
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    Thanks for the explanation. Just thinking about my current setup. Since the XTR front derailleur is capable of moving from small to middle chainrings using the 118mm spindle with a 3 ring crank. The motion should be the same give or take 1mm with a 2 ring crank on a 113mm spindle.

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