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  1. #1
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    Eccentric BB/crank spindle alignment question

    After I rotate an eccentric BB backwards (counterclockwise, when facing it from the drive side) for chain mounting purposes, once the chain is installed and once I get proper chain tension (from rotating the BB clockwise), where should the crank spindle be relative to the seat tube?

    In other words, should the crank spindle be visually as close to centered with the seat tube? By the looks of it, depending on chain tension, it could go up to a cm forward or backwards from center of seat tube, which might affect my fore-aft positioning a bit. That distance from center may be noticeable or negligible, I don't know (yet) until I ride it.

    Any tips would be appreciated. Thanks.

  2. #2
    hors category TandemGeek's Avatar
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    You really don't have a choice in the matter unless you opt to use a half link. The chain uses 1" links and the length of the boom tube determines where the spindle will fall.

    So, if your spindle ends up in the middle, that's not a bad thing since it's pretty neutral / same as your single bike. However, as the sync chain wears you'll need to rotate the eccentic and move the spindle forward to take up the chain wear.

    If your spindle ends up near the front of the eccentric, that's not a good thing for a couple of reasons: a) it means you're saddle set-back needs to be moved forward which also impinges on your bar reach; and b) it also means you won't have much movement left for taking up the slack. Probably best to remove a link so that....

    If you spindle ends up the back of the eccentric you adjust your saddle set-back a bit and have to deal with a little more reach; however, you have plenty of room for your eccentric to move as you take up chain wear.

    Again, the only way to get a spindle that lands well forward or well backward in the eccentric to the middle is to use a half-link.

    Once you have it set, the forward movement to take up chain slack is typically no more than 1/8" 5/32" before the chain needs to be replaced.

  3. #3
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    TG--thanks a ton! You answered my question exactly (I was concerned about fiddling with setback due to spindle position).

    I'll adjust chain length such that spindle ends in middle after setting tension. I'm using a Shimano Ultegra 6600 by the way (on both sides of frame).

    This is the last of the moving parts to get my bike built...finally.

  4. #4
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    p.s. does it matter on which end of the eccentric the spindle/BB cups are (top or bottom of the cylinder)? If I target the crank spindle for centering position relative to top tube, there is more eccentric bb material on bottom or top (depending on which way I turn the spanner wrench), which affects my saddle height one way or the other. I wouldn't think either position has much, if any, effect on performance of the chain (sloping slightly up or down).
    Last edited by JSNYC; 02-12-12 at 01:03 PM.

  5. #5
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    Quote Originally Posted by JSNYC View Post
    p.s. does it matter on which end of the eccentric the spindle/BB cups are (top or bottom of the cylinder)? If I target the crank spindle for centering position relative to top tube, there is more eccentric bb material on bottom or top (depending on which way I turn the spanner wrench), which affects my saddle height one way or the other. I wouldn't think either position has much, if any, effect on performance of the chain (sloping slightly up or down).
    If you ride particularly long cranks, may want to position bb axle UP, to gain ground clearance from pedal strike.

  6. #6
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    after lots of trial and error i got the drivetrain and chainline dialed in just about perfect, it's less than 1/32" of a inch off front to back left to right. works great, and front crank spindle is about 1/8" forward of center of seat tube after tweaking chain length and eccentric adjustments. tg: i tried to will the boom tube to grow 1/8", but no such luck....

    austex, i'm using 172.5 cranks.

    thanks again everyone for the tips.

    i'm glad i put this together myself because now i know exactly how everything works. the drivetrain and chainline, plus eccentric, was a bit intimidating at first.

  7. #7
    PMK
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    Consider too, that if you run the BB in the up or down position will also normally change the stoker position by moving their bars.

    PK
    2006 Co-Motion Roadster, flat bars, discs and carbon fibre fork, size 22 / 19
    2006 Ventana ECDM full suspension mountain tandem
    Some single bikes and a couple of KTM's
    And most important, someone special that enjoys them with me (except the KTM's)

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