Bottom bracket sizing problem

Discussion in 'UK and Europe' started by Rob, Jan 31, 2006.

  1. Rob

    Rob Guest

    I have an old steel specialized rock hopper that I use as my about town
    bike. The bottom bracket (not the original) is now about to fall apart.
    It is a UN72 for 73mmm shell and the spindle is stamped 110E. Measuring
    with the vernier claipers I get 111mm, so I guess its about right.

    About a year ago I purchased what was probably one of the last square
    taper LX chainsets as the specialized chainset had finally died. Since
    then I have noticed that the lefthand (non drive) crank is approx 5mm
    further away from the chain stay than the righthand crank. That is 15mm
    gap for the left and 10mm for the right when measuring from the inside
    of the crank along the middle of the peddle spindle to the chainstays.

    What can I do to make my cranks my symmetrical?

    Do I buy a 107mm spindle or a 113mm spindle?

    How do I know that the length variation will only affect the left hand
    side?

    Are there any decent engineering drawings that show the dimensions of
    shimano BB's?

    Thanks.
     
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  2. Clive George

    Clive George Guest

    "Rob" <[email protected]> wrote in message
    news:[email protected]
    >I have an old steel specialized rock hopper that I use as my about town
    > bike. The bottom bracket (not the original) is now about to fall apart.
    > It is a UN72 for 73mmm shell and the spindle is stamped 110E. Measuring
    > with the vernier claipers I get 111mm, so I guess its about right.
    >
    > About a year ago I purchased what was probably one of the last square
    > taper LX chainsets as the specialized chainset had finally died. Since
    > then I have noticed that the lefthand (non drive) crank is approx 5mm
    > further away from the chain stay than the righthand crank. That is 15mm
    > gap for the left and 10mm for the right when measuring from the inside
    > of the crank along the middle of the peddle spindle to the chainstays.
    >
    > What can I do to make my cranks my symmetrical?
    >
    > Do I buy a 107mm spindle or a 113mm spindle?
    >
    > How do I know that the length variation will only affect the left hand
    > side?
    >
    > Are there any decent engineering drawings that show the dimensions of
    > shimano BB's?


    The 73mm and 68mm shimano BBs are almost the same - the difference is in the
    shim which goes on the end and an extra 5mm on the LHS spindle.

    I've used this to my advantage on our tandem - different LH and RH cranks
    need different spindle lengths, and fortunately it's the LH one which needs
    to be longer.

    Anyway it sounds like you've got a similar thing here. If you put a 113mm
    long 68mm BB in with the same shim as you've got now your LH will move in
    4mm and your RH out 1mm, and all will be about right.

    cheers,
    clive
     
  3. "Rob" <[email protected]> wrote:

    > I have an old steel specialized rock hopper that I use as my about
    > town bike. The bottom bracket (not the original) is now about to
    > fall apart. It is a UN72 for 73mmm shell and the spindle is stamped
    > 110E. Measuring with the vernier claipers I get 111mm, so I guess
    > its about right.


    > About a year ago I purchased what was probably one of the last
    > square taper LX chainsets as the specialized chainset had finally
    > died. Since then I have noticed that the lefthand (non drive) crank
    > is approx 5mm further away from the chain stay than the righthand
    > crank. That is 15mm gap for the left and 10mm for the right when
    > measuring from the inside of the crank along the middle of the
    > peddle spindle to the chainstays.


    > What can I do to make my cranks my symmetrical?


    > How do I know that the length variation will only affect the left
    > hand side?


    John Barnett's exhaustive bike repair manual has a table of "relative
    chainline", giving the 107mm "MM" axle as a reference. As luck would have
    it, the relevant chapter (a 400k .pdf) is available for download from his
    website:

    http://www.bbinstitute.com/BM5 chap 10.pdf

    page 10:7

    According to this table, the 107mm and 110.5mm axles have the same relative
    chainline, which must mean that the difference is in the amount of axle
    overhang at the left of the unit. In other words, a 107mm unit should bring
    the left crank in by 3.5mm without affecting your chainline. I think that's
    as close as you can reasonably expect to get to symmetry.

    > Are there any decent engineering drawings that show the
    > dimensions of shimano BB's?


    Shimano used to supply one with the fitting instructions for their bottom
    brackets, though I haven't seen one in a long while. Perhaps a local bike
    shop might still have one kicking around.

    James Thomson
     
  4. "Clive George" <[email protected]> wrote:

    > If you put a 113mm long 68mm BB in with the same shim as you've
    > got now your LH will move in 4mm and your RH out 1mm, and all
    > will be about right.


    I think you've got that muddled. The critical difference between a 68mm and
    a 73mm unit is that the axle in the latter case is shifted leftward 2.5mm
    relative to the RH flange, so in the case of identical axle lengths,
    installing a 68mm unit in a 73mm shell will shift everything rightward by
    2.5mm. Going to the 113mm axle will then add a further 3mm to the
    chainline, so the right crank ends up 5.5mm further right.

    James Thomson
     
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