Run a triple chainset with 68 x 113mmm sq taper bb.

Am now running 2×9 with the same bb (triple chainset with big ring removed) and seems reasonably good chainline (not great around 34T + 12T area). However, wonder if I used a 110mm bb would the chainline be better as the whole chainset would be moved in a smidgeon as the original set up was based on a triple chainset?

But with 1×9, I guess I keep the 113mm? Or does it work equally well moving to a 110mm bb?

Any rules of thumb for 3×9 vs 2×9 vs 1×9 (always with a triple chainset and removing outer or outer + inner rings).

Ta

If you're running 2×9 you would want the BB a touch longer, if anything because you will be using the 34 ring all the time on the smaller cogs and the granny mainly on the bigger ones.

When I transferred my chainset from a Cannondale hardtail to my Cotic Soul (using a 2 ring set-up on sq taper, 68mm shell, just like you) I initially used a 107mm BB but under pressure the r/h crank tapped the chainstay (big tyre clearance on a Soul). I tried a 110mm but wierdly the whole extra 3mm seemed to be on the l/h end. When I went to a 113mm, it was all fine so I left it.

I find that my max out-of-the-saddle effort is usually in the 34 ring and one of the middle sprockets. 34:18 or so. That's when I really want my chainline to be straight, which it is with a 113mm BB.

I've been running 1×9 with no problems on a 113mm BB. There was a little bit of rubbing in the 11t but I seem to have got that sorted (I hadn't set the stop properly). Like BJ says, most of your max. power stuff won't be in the top or bottom so it's not really a problem.