Calculating chainline for bottom bracket axle length

andyp

The second question in my on going "teach myself about fixed stuff by building one" project.

I've measured my chainline using the method Sheldon Brown prescribes. The frame has an OLN of 130 mm and the distance from the inside rear drop out to the centre of the sprocket is 20 mm which gives me a chainline of 45 mm. This seems to tally with the figures given further down the same page above for a Miche sprocket and a White Industries ENO hub.

To calculate bb axle length I presume I need to pick my chainset, a Miche track one, work out that it comes with a 107 mm bb axle to give a 42 mm chainline so for my setup I need an bb with an axle length of 113 mm (calculating that each additional mm of axle gives 0.5 mm of movement outwards).

Does that sound correct?

Thanks.

franklin pierce

andyp wrote:

I need an bb with an axle length of 113 mm (calculating that each additional mm of axle gives 0.5 mm of movement outwards).

Every millimetre of a symmetric axle gives 0.5mm of outward movement. If you could find a 110mm asymmetric (with the right features, obviously can:worms stuff) it would work as well.

The difficulty lies in that manufacturing tolerances, being what they are, lead to the crank seating perhaps a bit further or a bit less so on the taper, which can lead to errors of up to 2mm in my experience. Also, while a "perfect" chainline is obviously desireable, it is not usually necessary nor feasible.

andyp

Thanks for the response. I have a 111mm Centaur axle in situ so it sounds like a sensible option will be to try it first and see how I get on with it.