Bottom Bracket Sizing

Discussion in 'Cycling Equipment' started by Nlee1875, May 4, 2003.

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  1. Nlee1875

    Nlee1875 Guest

    May 4, 2003

    I am trying to replace my old Dura-Ace double crank (model 7402 ) on my road bike with a mountain
    bike Shimano XT triple crankset.

    The current bottom bracket is stamped 68x113.

    The Shimano XT crank allegedly also uses the same 68x113 -- but the chain line doesn't line up
    correctly. The positioning of middle chain ring on the XT is where the outer chain ring of the
    Dura-Ace use to be.

    Should I be looking to replace the bottom bracket with a 68x107 to make my new set up work on a
    road frame?

    BTW.. The XT crank works with 68x113 bottom bracket on my mountain bike frame just fine. Is this
    because the rear triangle spacing on the mountain bike is wider than a road bike?

    Much obliged. Nick Sparks, NV
     
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  2. NLee1875 wrote:

    > I am trying to replace my old Dura-Ace double crank (model 7402 ) on my road bike with a mountain
    > bike Shimano XT triple crankset.
    >
    > The current bottom bracket is stamped 68x113.
    >
    > The Shimano XT crank allegedly also uses the same 68x113 -- but the chain line doesn't line up
    > correctly. The positioning of middle chain ring on the XT is where the outer chain ring of the
    > Dura-Ace use to be.

    Thats more or less as it should be. Stock Shimano spec for road chainline is 43.5 mm. This is
    measured halfway betwixt the two rings, so the outer ring would wind up at about 46 mm.

    Standard chainline for a "mountain" triple is 47.5 mm, only 1.5 mm different.

    > Should I be looking to replace the bottom bracket with a 68x107 to make my new set up work on a
    > road frame?

    Certainly not. This would likely cause the small chainring to hit the chainstay, and you might not
    be able to get the front derailer to move far enough inward for good shifting.

    See: http://sheldonbrown.com/chainline

    and: http://sheldonbrown.com/bbsize

    Sheldon "Where's The Problem?" Brown +-----------------------------------------------------+
    | Life is the art of drawing sufficient conclusions | from insufficient premises. --Samuel Butler |
    +-----------------------------------------------------+ Harris Cyclery, West Newton, Massachusetts
    Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
    http://captainbike.com http://sheldonbrown.com
     
  3. nlee- << The current bottom bracket is stamped 68x113.

    The Shimano XT crank allegedly also uses the same 68x113 -- but the chain line doesn't line up
    correctly.

    << The positioning of middle chain ring on the XT is where the outer chain ring of the Dura-Ace
    use to be.

    On the triple or double, the middle of the cogset should line up on the middle of the crank
    rings...to preserve spacing, chain stay spacing, etc, the big ring of the triple gotta go
    somewhere...

    Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
    (303)440-3535 http://www.vecchios.com "Ruote convenzionali costruite eccezionalmente bene"
     
  4. On Sun, 04 May 2003 16:23:33 +0000, NLee1875 wrote:

    > Should I be looking to replace the bottom bracket with a 68x107 to make my new set up work on a
    > road frame?

    That's what I do, and I always use mountain-bike cranks in order to get better gearing options.
    >
    > BTW.. The XT crank works with 68x113 bottom bracket on my mountain bike frame just fine. Is this
    > because the rear triangle spacing on the mountain bike is wider than a road bike?

    yes, of course. The cassette width is the same, and the axle is 5mm longer, which means that the
    cassette on a mountain bike is splayed to the right 2.5mm compared with a road bike. OK, a 107 cuts
    a bit more off than the needed 113, but it's close enough, especially if you use the mountain bike
    cranks as a double.

    --

    David L. Johnson

    __o | Do not worry about your difficulties in mathematics, I can _`\(,_ | assure you that mine
    are all greater. -- A. Einstein (_)/ (_) |
     
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