Square Taper axle length question

thepainandsuffering
thepainandsuffering Posts: 44
edited August 2016 in MTB workshop & tech
I have a basic Giant Rock MTB (triple chainset) in which the bottom bracket is finished.

I have removed It and found it is a cup and cone type with a 124.5mm axle which is asymmetric (longer on the drive side by around 3-4mm).

When I go shopping It seems most new cartridge type BB's are symmetrical and have shorter axles. Could anyone advise which length cartridge BB I should replace it with please. I have asked Google the question and read a number of threads on forums which have just served to confuse me more. Sheldon Brown's site says 133mm which seems very long and I have yet to find one.

I should add this bike gets very little use so only looking for something cheap to replace it with. Also it's a 68mm shell.

Many Thanks. TP&S

Comments

  • poah
    poah Posts: 3,369
    just buy a new deore crankset and it comes with a hollowtech BB. easiest way to go about it.
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  • cooldad
    cooldad Posts: 32,599
    BB size depends on the chainset, so it really depends if the correct one is fitted anyway.

    Assuming it is, see how much clearance you have between the drive side crank and chainstay. You might be able to get away with a little shorter, or if not just get a symmetrical one wiyh enough clearance both sides, maybe 127.5 if you're lucky.

    Doubt you'll notice a few mm more on the non drive side.

    Main thing is to have a reasonable chain line and enough clearance so the granny ring or cranks don't hit the frame.
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  • cooldad
    cooldad Posts: 32,599
    POAH wrote:
    just buy a new deore crankset and it comes with a hollowtech BB. easiest way to go about it.

    That's like £50 or so against a tenner or less for a BB.
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  • thepainandsuffering
    thepainandsuffering Posts: 44
    Ok, thanks

    Chainset is a Shimano fc-tx71. Shimano info I can find indicates a BB-TY30 bottom bracket but no mention on axle length, however I can't find one to buy and some pictures show this BB as an octalink which is not what I need???

    So a bit of trial and error required with a 127mm-ish then?

    New chainset would be a last resort as am after a cheap fix.
  • The Rookie
    The Rookie Posts: 27,812
    Measure up, what is the current chainline, how much is the offset, how much clearance do you have to the chainstay.

    Alternatively look at getting a decent used BB and better chainset off eBay, can be had for about £15-20.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.

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