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  1. #1
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    BB sizing for updating an older bike

    I've got a 1988-ish KHS Montana Sport (totally heavy rigid set up - it rules!), and I need some input on BB sizing.

    I'm trying to replace my bent chainrings, but the spider on my current cranks are a really odd 44 BCD(!) with a really long square taper sindle (125mm!). I've found a used square taper 110 BCD crankset, but have no idea what BB to use. Any ideas on how to calculate this?

    My old chainline, measured from center of the seat post to the center of the middle chainring looks like 44.5 mm, with shimano deraileurs. If I mount the new crankset on the current spindle, the same measurement is about 17.5mm wider. Should I just buy a 108mm spindle? That seems really short...

    And how do I figure out what BB shell size and thread to get? Anyone got specs on my frame?

  2. #2
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    Your bb shell is easily measured with a ruler. It's likely a 68 or 73mm English threaded. The spindle length is associated primarily with the specific cranks you're trying to use, what are they? 44 bcd is something I've not heard of, how did you measure that? Check out http://www.sheldonbrown.com/gloss_bo-z.html for some info on bcd. Did you crank the arms down all the way to determine the chainline is 17.5mm wider? That's a lot, so I started to think your calculation at first made no sense, so I halved it, then thought, no it's right, then realized it goes the other way (thus multiple edits)...how did you measure the spindle at 125 (or is it marked) just to confirm?
    Last edited by Bikinfoolferlife; 12-08-2005 at 10:55 PM.
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  3. #3
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    Quote Originally Posted by Bikinfoolferlife
    Your bb shell is easily measured with a ruler. It's likely a 68 or 73mm English threaded. The spindle length is associated primarily with the specific cranks you're trying to use, what are they? 44 bcd is something I've not heard of, how did you measure that? Check out http://www.sheldonbrown.com/gloss_bo-z.html for some info on bcd. Did you crank the arms down all the way to determine the chainline is 17.5mm wider? That's a lot, so I started to think your calculation at first made no sense, so I halved it, then thought, no it's right, then realized it goes the other way (thus multiple edits)...how did you measure the spindle at 125 (or is it marked) just to confirm?
    Yah, I know that BCD sounds odd. Which is why I'm opting to replace the whole crank, rather than just the chainrings. The current crank is branded 'aero' or 'a.ero' or 'alero' (hard to read). The spider is reall short, with the big ring right up against the base of the crank arm.

    And yep, +17.5mm is really what the new chain line is. I tightened down the crank arm farily firm. The spindle isn't marked - it is a cone-and bearing set up, and I measured from end-to-end of the square taper, not including the threaded ends. Is that right? The whole set of measurements seems so bizzare, which is why I wanted to float it here.

    Right now I'm leaning towards either a 108 spindle, to replicate my current chainline, or 113, to get it up to a traditional 47.5mm. Thoughts?

  4. #4
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    I still wonder about that 44bcd...and here's the link I actually meant to post, grabbed the wrong one http://www.sheldonbrown.com/harris/chainrings.html and am not familiar with that crank in any case.

    The only cup and cone bb I've got is a 125, too, so I didn't think it really odd; I measure end to end on the spindle, but not sure what you mean by the threaded ends, tho. I run 107s on one of my square tapers, and last I looked they were still available in a UN73, a UN72 might be harder to locate.

    As far as that chainline goes, I confused myself last night (okay, some IPA's helped), and if you need to move your chainline 17.5 mm that's a 35mm spindle difference, which would leave you with a 90 or is lack of sleep my problem now? I think I've heard of 102s, but can't think that I've seen shorter...
    Last edited by Bikinfoolferlife; 12-09-2005 at 11:54 AM.
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  5. #5
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    One Idea

    Is the BB spindle symmetrical or offset, ie does it stick out the same amount on each side.
    That was more common on road bikes but it could be something. If this is the case the driveside is usually 5mm longer. Again, pure speculation but that would change the difference to 12.5mm and then if you go to a more standard chainline, either 47.5 or 50mm then you're looking at 9mm or 6.5mm difference. Which would be a 18mm or 13mm shorter spindle...ie 107mm or 112mm...both pretty standard.

    As to the funny bolt circle on the cranks it sounds kinda like the mid 90's STX setup and I think Alivio and some of the lower stuff was similar. Where the inner ring actually held everything on to the crank and the other tings were bolted to that.
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  6. #6
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    Another thought would be to xpost this is the vintage/retro forum...
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  7. #7
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    Thanks for all your input folks.

    So, i've remeasured everying, and indeed, the drive-side of the bb sindle is 5 mm longer.

    I've gotten a better ruler too for the other measurements
    Old chain line = 44 mm
    Chain line with new cranks on old spindle = 58 mm

    So, to calculate for a cartridge-style bb:
    58-44 = 14 mm

    Since newer cartridge bbs don't have an offset:
    minus the 'extra' 5 mm from the current spindle offset on the drive side = 9 mm
    double this and subtract from 125 = 107 mm to match the current chainline

    If I want to take it to a 47.5 mm chainline, I'd get a 113 mm spindle. I think I'm going to stay with the current chainline, as my rear spacing is only 130 mm and I'm not replacing deraileurs, sprockets or hubs any time soon.

    For what its worth, I'm happy to let the mystery of by funky old chainring BCD die. If you are reall curious: remeasured, it was 50 BCD where the big ring attached to the spider, and then the two small rings attached to the big ring at 80 BCD.

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