Bottom Bracket.

[Roy]

I need to replace the square taper sealed B.B. The question is how do you determine the axle length required and would it make any difference if I went narrower than the one that I have taken out which I think is 118mm. I am using a Shimano 105 double chainset and the frame is a 16 year old 531 Raleigh.

[phantasmagoriana]

Sheldon's list is a good place to start: http://www.sheldonbrown.com/bbsize.html

(Not sure what would happen if you fitted the "wrong" length, though - I've always just stuck with the recommended size so can't help there...)

[Torslanda]

Are we talking a replacement BB with the same chainset?

Axle length is end to end.

Order another one of the same, why would you need a different length . . . ?

[Torslanda]

Yes it probably would make a difference if you fitted a shorter axle but you'd only know for certain when you couldn't shift to the inner ring and/or the chainrings were chewing through the chainstays.

[Kim]

I've always had to resort to trial and error to optimise (that is, finding the shortest length where everything still works) BB length.  But that's because there haven't been any convenient cheat-notes for calculating chainline on the exotica where I've found myself having to care about this sort of stuff in the first place.  If you've got a working bike and you're just replacing a bottom bracket, why change it?  If you're putting a different crankset on and find the chainline is out, measure the difference and change the BB length accordingly.