What Size BB for Single Speed?

myopic
myopic Posts: 692
edited January 2011 in MTB workshop & tech
Just bought a new crankset for SS conversion and found when it was delivered that it did not include a BB. It says ISIS drive on it, and the BB shell on the frame is 68mm, so that takes care of those aspects, but what dictates the spindle length I will need? Will it be dicated by frame geometry, number of gears or something else? I was thinking with a single speed set up I could get away with 108mm.

Frame is a Marin Bobcat Trail 2000, Cranks are Bontrager Earl.
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Comments

  • warpcow
    warpcow Posts: 1,448
    If you're using a standard cassette hub, you'll have more leway in what axle length you can get away with. Still, it's possible 108mm might be too short. You won't really know until you try to fit it. Axle length is pretty much dictated by the cranks themselves (and a bit of personal preference), but you can usually play around a bit. 113mm is the most common length.
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  • myopic
    myopic Posts: 692
    Hmmm, its an XT hub, but I have a DMR single speed kit and spacers to play around with so hopefully that should give me some flexibility, but ideally I'd like to be sure I'm getting the right thing up front rather than finding out when I try to fit it! Maybe safer to go with 113?
    You don't need eyes to see, you need vision
  • nicklouse
    nicklouse Posts: 50,673
    what crankset is it? and what does the makers info say?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
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  • myopic
    myopic Posts: 692
    crankset is a Bontrager Earl - no manual and can't find anything on Bontrager website
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  • nicklouse
    nicklouse Posts: 50,673
    originally a 3 ring crankset?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • myopic
    myopic Posts: 692
    nope, it came as a single speed - here's link to CRC http://www.chainreactioncycles.com/Mode ... elID=59278
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  • nicklouse
    nicklouse Posts: 50,673
    lbs time and try both the 113 and the 108 it depends which the crank was designed for.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • myopic
    myopic Posts: 692
    :( thanks Nick. What am I looking for regards fit - enough to clear the frame and then some? How much do I need?
    You don't need eyes to see, you need vision
  • nicklouse
    nicklouse Posts: 50,673
    depends on your rear chain line and if it is adjustable.

    EG the gusset SS kit put the cog in gear 5 position or if the spacers are reversed in gear 4.

    5 is the normal for a 50mm front chain line.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • myopic
    myopic Posts: 692
    so would it be an option to go for an external bearing ISIS compatible system with a 51 mm chainline? Or is such a thing not possible? I was looking at the Truvativ Howitzer XR, but I can't tell if it is ISIS compatible or not.
    You don't need eyes to see, you need vision
  • nicklouse
    nicklouse Posts: 50,673
    sorry there is no external ISIS BBs.

    ISIS is ISIS. Howitzer is Howitzer they are NOT interchangeable.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • myopic
    myopic Posts: 692
    OK, thanks, Nick.

    Another approach. Is there any kind of measurement I can take from the cranks that would allow me to predict the BB spindle length I would need to achieve a 50 mm chain line?
    You don't need eyes to see, you need vision
  • The Rookie
    The Rookie Posts: 27,812
    Stock BB on my commuter was 113mm (square taper), I swapped to a 103mm when I went 1x9 and that aligned the chainwheel (in outer position) quite nicely with the gap between 4th and 5th gears.

    Simon
    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.
  • myopic
    myopic Posts: 692
    OK, measured it up. There is a 3 mm gap at the base of the chainset before the splines start. The splines themselves are 15 mm long, and the chain ring is 9 mm from the base of the chainset. That looks like it could work with a 68 x 108mm ISIS? 113 might be safer, but then that might put the chain ring too far over. I can't easily get ot a shop to check out options as Nick suggested, so hoping measurements will help!
    You don't need eyes to see, you need vision
  • The Rookie
    The Rookie Posts: 27,812
    1/2 of 108mm is a 54mm chainline (wheel aligned with end of axle)

    Your numbers put the chainwheel (centre?) 6mm out from base of splines, less the 15mm spline gives a chainline of 45mm, or 47.5mm for a 113.

    I think I'd go with a 113!

    Simon
    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.
  • nicklouse
    nicklouse Posts: 50,673
    The Beginner wrote:
    1/2 of 108mm is a 54mm chainline (wheel aligned with end of axle)

    Your numbers put the chainwheel (centre?) 6mm out from base of splines, less the 15mm spline gives a chainline of 45mm, or 47.5mm for a 113.

    I think I'd go with a 113!

    Simon

    sorry wrong. that presumes that the chain wheel is at the end of the axle.

    again it all comes down to which length 108 or 113 the crankset is designed for.

    get bot and see and return the wrong one.

    or call CRC and ask them.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • The Rookie
    The Rookie Posts: 27,812
    Not sure how that presumes the chainwheel's at the end of thje axle at all as that would be 54mm when I calculated 45mm, read what I wrote again please Nick!

    Simon
    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.
  • nicklouse
    nicklouse Posts: 50,673
    but the problem is you are presuming to much.

    like the measurements are something like correct.

    myopic
    have a read http://www.isisdrive.com/isisdrive/ISIS ... t_revD.pdf
    the drawings at the End will show tou what you need to measuse to see what BB you need for a given chain line.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • The Rookie
    The Rookie Posts: 27,812
    Indeed I presumed, hence
    The Beginner wrote:
    Your numbers
    as a clear and unequival caveat!

    Simon
    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.
  • myopic
    myopic Posts: 692
    I had missed that last exchange between Nick and Simon. (and no need for any presumptions, my measurements were correct! :wink: )

    I had worked out 113mm shoud be the right one, ordered it up, tried it and it gives a straight chainline run with the largest DMR spacer on the rear hub behind the cog.

    Thanks for all the help and feedback!
    You don't need eyes to see, you need vision

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