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Chain line - one for you boffins

Ade HAde H
edited August 2011 in General
I've started building up a 2011 surly steamroller singlespeed and trying to buy a square taper bottom bracket is causing me all kind sof head aches. I have a rear wheel with a flip flop hub that has a 41.5mm chain line and a crank that has a 42.5mm chain line. Despite hours of Googling and contradictory emails from various sellers of bottom brackets I'm still non the wiser as to what length BB I need to get a correct chain line. Answers on a postcard please. Help would be very much appreciated.

Comments

  • geoffggeoffg
    comes down to some hand skills Ade...if you have an existing BB then take a fine file and just take a small equal filing off each flat face so the crank slides up the morse taper a fraction more...you only nee 1mm of movement which is about 3 passes on each flat facet. be careful to keep the angle true and take an equal amount off each face.....if you don't have the BB at the moment then someone may be able to direct you to the correct unit
  • tducketttduckett
    Er, i think what Ade is getting at is a question of chain alignment not attaching the cranks to the BB. Im assuming that you are using a single chainring crank and that the rear wheel is not heavily dished in which case you shouldn't have a problem with chain alignment and can use any standard BB as long as its not for a triple. The chain line needs to be good but doesn't have to be perfect, you normally only get problems when trying to put a single sprocket on a dished rear wheel designed for a screw on rear wheel.
    Also you can buy spacers in 0.5 mm increments which you put under the fixed cup, i.e. the chain set side to move the crank out to the suitable position. St John street cycles is good for these sort of bits and pieces

    Hope that helps
  • tducketttduckett
    Should add that the same size spacer should also fit under the rear sprocket if you need to move it out a bit.
  • Ade HAde H
    Thanks guys.
    The problem for me is knowing where to start. I'm trying to achieve correct alignment so the chain runs efficiently and silently (one of the many reasons for riding an SS). Apparently, you can calculate the correct BB length from the chain line figures supplied by the crank and hub manufacturers. I just can't find any useful information on doing that apart from how to measure from an existing BB. I suspect that diligent bike shops make an educated guess on spindle length and then adjust by swapping the BB out or adding spacers.

    This is the wheel I have: http://www.kore-usa.com/products/01wheel_g2fixie.html (chainline is 41.5mm)
    This is the crank: http://www.fullspeedahead.com/products/109/FGimondi (chainline is 45mm - not what I said above)

    Now, the crank needs a square tapered BB and the length is determined by the chainline figure of the hub / crank. BBs can be measured using different standards. Most common is JIS for track type bikes and there are quite a few different lengths of bottom brackets. I appreciate that correct chain line can be achieved with spacers if you have a shorter crank spindle and longer hub. However, if your rear hub is longer than the crank to start with then you're in trouble I think.
    I may have to relent and take the frame, crank and wheel in to a bike shop with a mechanic who is knowledgeable in such matters. Any advice / thoughts would be appreciated.
  • tducketttduckett
    As its a single speed, track chain set, personally i would just get a short BB. Probably 107mm. Depending on the frame you can get problems with the chainring rubbing on the chain stay but this is unlikely and can be sorted with a spacer. As the hubs are designed for ss the chain line is never going to be far out. I doubt you will have any chain line problems with your set up.
  • PeterGPeterG
    have a look here here
  • Richard M.Richard M.
    edited August 2011
    Spacers here

    http://www.hubjub.co.uk/etc/etc.htm
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