I've managed to upgrade the tandem from 24 speed to 1 x 12 ( well 11 really as I've had to limit smallest rear cog ) but after changing front triple to one my chainline is a bit extreme in 1st/ 2nd gear etc , I have plenty of room to bring the crank in but if I just buy a shorter square taper bottom bracket this will also pull in the crossover crank on the left side making that out of line , is it possible to get a square taper with different length on each side
I thought they already were?
If there’s still clearance on the left hand side, isn’t the solution to fit narrower, matching bb’s on both cranks?
(Ps I know nothing about tandems)
Thanks , yeah that's possibly an option but just checked and there's less than a cm on each side ( front)
Assuming you have reused the triple crankset, have you tied mounting the single ring in the middle rather than the outer ring position, if that is not far enough in you could use extra length chainring bolts and spacers to adjust the chainline in?
Wheels Manufacturing Chainring spacer (sjscycles.co.uk)
No you don't get square taper BBs with variable lengths on each side, square taper bottom brackets are basically symmetrical, you can usually get away with spec'ing a shorter axle to reduce q-factor or adjust chain-line as long as the cranks don't interfere with the BB cups, only way to find out is trial and error. As goldfish24 wrote you'll need to change the pilot and stoker BBs to maintain the timing chain alignment, or change both cranksets and run the timing chain on the drive side.
is it possible to get a square taper with different length on each side
If you have a 68 mm BB shell, you could fit a 73 mm BB and use spacers to fine tune things.
"less than a cm" clearance is clearance, BBs come in 3mm axle length increments, i.e. 1.5mm per side, so plenty of scope for adjustment.
Have changed triple for single specific crankset , might have a measure up with front to see if a shorter axle at rear will line up...thanks for help