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Crankset replacement question - BB length

ketsbaiaketsbaia Posts: 1,718
edited October 2010 in Workshop
Hello all-knowing oracle.

I need to replace the crankset on my bike. Current BB length is 113 mm, but the crankset I want to replace it with has a recommended crankset length of between 107 and 109 mm. Assuming the BB on the bike is slim enough to cope with the smaller BB, would it be fine to go ahead and use a smaller BB.

Alternatively, what are the implications of using a BB longer than that recommended for use with the crankset?

Posts

  • The BB length in combination with the crankset will determine the Chainline which again will determine the amount of flex in the chain when on the extremes of gears (small/small or big/big). The more flex exerted on the chain the sooner it will wear out.
    The ideal chainline will put the middle of your chainset (middle ring on a triple or gap between rings on a double) in line with the middle of the rear cassette.

    Personally I would first put the crankset on the old BB and see how the line looks. If it looks too far out (beyond the line of the centre of the cassette) then check you have sufficient clearance between the chainrings and the chainstays were the crankset to move in by 2-3mm and then if so you should be fine to change the BB.

    Hope that makes sense, it's clear in my head!
    Scott CR-1 (FCN 4)
    Pace RC200 FG Conversion (FCN 5)
    Giant Trance X

    My collection of Cols
  • John.TJohn.T Posts: 3,698
    All BB shells are the same length. Either 68mm (English) or 70mm (Italian). BBs are made in these 2 sizes. The threads differ so you will need to get the right one for your frame.
    You are refering to the length of the axle. You need to get one that has the length specified for the crankset. If you fit one that is too long you will have chainline problems. If too short you will have these and also the inner ring may catch on the chain stay. If using a triple you may also have difficulty getting the front mech to go far enough in to change on to the granny ring.
  • ketsbaiaketsbaia Posts: 1,718
    For reference, it's a single chainset and single sprocket. And it's a 68mm BB shell.

    The current axle is 113mm, but I'm fairly sure I can get away with something smaller. Will obviously measure up and see how the chainline is affected, but I'm reasonably sure I can get away with it. Just wondering if the axle length has any bearing on how the cranks operate, but it doesn't appear so.
  • John.TJohn.T Posts: 3,698
    If you are running a single speed (especialy on fixed) then it is important to get the chain line right. Cranks differ in where to chainring lies in relation to the square taper on the axle thus needing different lengths to get the ring in the same place. Why they could not have standardised this I do not know. We would have only needed 3 lengths then. One each for single,double and triple.
  • ketsbaiaketsbaia Posts: 1,718
    Blimey, that's a pain, isn't it?

    OK, so now I need to find a single chainset that will fit on a 113mm axle.

    I'll just find that needle in this haystack first, it'll be easier.
  • Butterd2Butterd2 Posts: 937
    I would get the chainset you want then buy the correct BB, they're cheap enough. Chainline is more important on a FG/SS as the chains are not designed to have sideways flex the way chains for geared bikes have to.
    Also think at this stage if you want 1/8th" chain rather than the 3/16ths standard for geared.
    Scott CR-1 (FCN 4)
    Pace RC200 FG Conversion (FCN 5)
    Giant Trance X

    My collection of Cols
  • ketsbaiaketsbaia Posts: 1,718
    This is a single speed bike. Or rather, it's a hub geared bike, so the chainline is reasonably important I think.

    It's a minefield out there. I can't find a single crankset that uses a 113mm axle. They all use smaller axles. I could get a double or triple crankset to fit the axle, but I don't want/need one of those. What I really want to do is buy a crankset and just use the longer axled BB that I have, but I'm not sure if that'll censored something up.

    It's all very irksome and time-consuming.
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