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  1. #1
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    Another BB (length) choice question

    Hi guys,

    This is my first foray into Italian thread BBs, which means my earlier approach of test fitting stuff until it lines up isn't working for me, as I haven't got any parts apart from the take-off to tinker with. And what has come off is a bit suspect for various reasons.

    Anyhow, the crank I want to use is a Shimano Nexave FC-C600 (http://techdocs.shimano.com/media/te...9830608872.pdf). The tooth count would fit the speed requirements of the bike's rider.
    (I'm guessing I might have to cut out that cam thingy on the inside of the chaingring spindle, which I'm guessing has something to do with front shifting.)

    According to herehttp://techdocs.shimano.com/media/te...9830613207.pdf) a 117.5 mm BB would give me a 52.5 mm chainline.
    But according to Sheldon Brown (http://www.sheldonbrown.com/chainline.html#adjfront) for a 2x 9 road bike I should shoot for a 43.5 mm front chainline.

    9 mm off seems quite a lot.

    I've found Italian thread, square-taper BBs here:http://www.chainreactioncycles.com/s...t/rp-prod71369 in: 107, 110, 113, 115 mm.
    But which one to go for?

    107 would put me closest to the general recommendation. But then there's that "special" feature of the crank, maybe it has more than average inside protrusion? Perhaps the 110 is a safer bet?

    Any hints would be appreciated.

  2. #2
    slow as I ever was Ex Pres's Avatar
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    My first question was what you addressed last - will the crank interfere with the BB cups on a shorter spindle. I don't have that answer. But between the 107 & 110, oddly enough both of those have the same drive side length and the 3mm difference is on the non-drive side.

  3. #3
    me have long head tube TallRider's Avatar
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    While there is some variation in symmetry of BB's (as Ex Pres noted, Shimano 107mm and 110mm have the same drive-side placement), generally decreased spindle width will be split between drive-side and non-drive-side. So to decrease chainline by 9mm, you'd want a BB that is 18mm shorter. The narrowest square-taper BB's made are 103mm, so not quite 18mm shorter, but close.

    You might try to find a 103mm square-taper BB in Italian threading. I'm sure that Shimano made 7400 (8-speed) Dura-Ace bottom brackets in Italian threading. But I doubt they're common.

    On the other hand, this seems like a lot of trouble (and potentially cost) to get an low-end crank to work on an Italian-threaded bike. Have you considered buying another double crankset for cheap, that works with a more common bottom bracket size?

  4. #4
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    Quote Originally Posted by TallRider View Post
    On the other hand, this seems like a lot of trouble (and potentially cost) to get an low-end crank to work on an Italian-threaded bike. Have you considered buying another double crankset for cheap, that works with a more common bottom bracket size?
    This was the cheapest double crank with a suitable tooth count I could find at the time of purchase. Don't remember how much I payed for it any longer, but it wasn't much. Came off Ebay.

  5. #5
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    The 103 wont work, too short IMO specially for that crankset. Remember Shimano made 104s and were used for track, never for a crank like this one, doubt you will find one now a days anyways, too short.

    From 107 to 110 you have 3 mm, 1.5 mm per side. Doubt will be actually noticeable to cause any problems with the chainline. What was normal for shimano was to send some of their cranksets with a 109 in some dura ace and 600 series, 105 sc were sent with a 112 or 118 I believe (105 sc works better with a 109 just in case). Again the same situation, from 109 to 112 are 3 mm that means 1.5 mm per side, will make an impact? probably not.

    I was you I would try the 112 (113) to play safe because no idea how that crankset seats (crank spider relationship with the stays). The other thing is that that crankset apparently was mated to a 73 mm BB shell bike, maybe a hybrid or something, if you want to put that in a road bike then you have to go shorter, but from 112 to 117 are just 2.5 per side, not that is a lot you know, I'm assuming a symmetric BB aswell ok? If that thing was designed for asymmetrical then the situation will change a tiny bit.

    I would try 113... the next is 115, again 2mmm... 1 mm per side.. .if the 112 doesnt touch the stays you should be ok but w/o actually installing the thing in the bike there is no way to know if it will work, seen guys doing all the math and crap but if the stays are wider or the cranks are longer the math sometimes doesnt work as good as actually trying, specially if you want to put hybrid stuff in a road bike or the opposite. You have to actually test.

    By the way the chain line is just an estimate, some guys are really anal with the chain line but if you are off 1 mm or 2 the thing will work just fine. I imagine the 52.5 mm chainline shimano is talking about is using maybe a 135 mm hub or maybe a nexus hub or something where the cog is seat close to the drop out and not in the middle??? Campagnolo track hubs are wider than shimanos, zeus track hubs were not wide so the cog was set closer to the center line, the chainline using those hubs was different and even using a spacer in the back of the cogs the chainline was different than with campagnolo stuff, just giving you an example.

    Hope this helps and dont get you more confuse but no idea what bike are you going to put this into aswell you know.

    Good luck.

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