bottom bracket size chain line retrofit

[artfulflounder]

If fitting a 9 speed triple crank to old 7 speed wheel set which bottom bracket should be fitted for correct chain line?

Sheldon Brown lists newer crankset bb in range approx 110-113, while older crankset bb tend to be 116-121.

I've fitted a 110 length which came with lx570 but on large ring front and small rear the chain seems a strange chain run.

Should I change out the 110 for something closer to 120?

 

[jonthefish]

I'm no expert but for a triple crank achieving a straight chain between middle chain ring and middle cog in the cassette/freewheel ought to get you close. Select those gears and see how it looks. If chain line not straight in those gears adjust BB length accordingly. In my limited experience chain line definitely varies between cranks, even retro ones.

There are much more savvy folk than me here that could give chapter and verse (and maybe tell you I'm talking c*%p) but above worked for me when changing cranks on the Erzulie. Shifting and chain line now good.