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BB for 48 tooth Deore chainset

Started by Md · · Last activity · 3 posts · 573 views

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UK and Europe
Published
23 March 2004
Last activity
24 March 2004
Original author
Md
Posts
3
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Public discussion
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Topics
CranksetsTaper and peaking

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  1. Original post
    Md
    Archived post
    #1

    I recently acquired one of the above items from a classified
    ad, which appears to be in almost new condition. It's a
    Deore 510 26/36/48. I've "roadified" my old MTB with slick
    tyres and rigid forks, so fancied some higher gears.
    Replacing the original Alivio chainset that I've previously
    bent the middle chainring on also seemed like a good idea.

    The downloadable pdf from the Shimano site mentions usng a
    113mm BB. The current one sitting in the frame in a UN40
    73/113 square taper BB that the bike came with when new, so
    I thought there'd be no problem.

    Unfortunately, the chainring teeth started biting into the
    frame when I tightened the crankbolts, as the BB axle seems
    to be too short for the new chainset. It may be a result of
    the bigger chainrings meeting the curve of the chainstay at
    just the wrong point.

    After examining the two chainsets side-by-side, I see
    that on the old chainset, the point where the BB goes
    into the crankarm stands proud of the small ring by about
    2mm, when viewed in profile. On the Deore, it is recessed
    by about 5mm.

    Old chainset New chainset
    __

    Quoted message said:

    ---|---|----< -----___------< ------------------------
    < -----------|---|------------< -----------------------------------
    < ------------------------------------------<

    This appears that I need 2x(2+5)mm difference longer BB,
    which means (2x7) + 113 = 73/127mm BB. Now these are
    available (not in many places, but certainly not from the
    discount suppliers like Chain Reaction, which is a pain.
    £25 for a UN73 is a bit much IMO.

    However, I'm a little confused, as the info from Shimano
    mentions using a 113mm BB for virtually everything, and the
    impression I get is that certainly for MTB cranks, almost
    everything new uses shorter BB axle lengths. I'm a bit
    worried that using a 127mm BB axle will mess up the
    chainline completely as well, although from the way the
    Deore crank interface is "recessed" behind the small ring,
    it may be that it makes no difference at all.

    --

    Regards,

    Mark Davies

    Report
  2. Peter B
    Archived post
    #2

    "MD" <[email hidden]> wrote in message
    "]news:[email hidden]...

    Quoted message said:

    I recently acquired one of the above items from a
    classified ad, which appears to be in almost new
    condition. It's a Deore 510 26/36/48.

    Quoted message said:

    However, I'm a little confused, as the info from Shimano
    mentions using a 113mm BB for virtually everything,

    Older chainsets had straight cranks requiring longer BBs to
    keep rings clear of chainstays. Somewhere in the middle of
    the 90's cranks became bent to allow greater ankle
    clearance, these use a shorter BB. A giveaway would be by
    fitting the 2 LHS cranks alternately and checking the
    relative gaps between the cranks and BB shell, if markedly
    different this would point to the situation I mentioned.
    AKAIK it's a few years since Shimano produced a 26/36/48 mtb
    chainset. Nothing wrong with them, I've still got one on my
    old roadised mtb but it has straight cranks, again nothing
    wrong with them.
    --
    Regards, Pete

    Report
  3. Md
    Archived post
    #3
    Peter B said:

    "MD" <[email hidden]> wrote in message

    Older chainsets had straight cranks requiring longer BBs
    to keep rings clear of chainstays. Somewhere in the
    middle of the 90's cranks became bent to allow greater
    ankle clearance, these use a shorter BB. A giveaway would
    be by fitting the 2 LHS cranks alternately and checking
    the relative gaps between the cranks and BB shell, if
    markedly different this would point to the situation I
    mentioned. AKAIK it's a few years since Shimano produced
    a 26/36/48 mtb


    chainset.

    Quoted message said:

    Nothing wrong with them, I've still got one on my old
    roadised mtb but it has straight cranks, again nothing
    wrong with them.

    Thing is, the Deore 510 (it's stamped on the crank) is the
    current Deore chainset. The 48 tooth version is a bit of an
    anomaly in the Shimano MTB range (everything non budget
    being 22/32/44), and is quite often seen specced on hybrids
    and tourers. The crank arms are bent about the same as the
    old chainset that uses a 113mm BB, and it has a 4-arm compact-
    drive spider. In otherwords, it's a current chainset that
    you can buy brand new, although it can be hard to find.

    So, what's confusing me is that I appear to have a new
    chainset that according to my comparative measurements,
    takes an old-fashioned BB size.

    --

    Regards,

    Mark Davies

    Report

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