Originally Posted by
TonyA
Does anyone know the secret code that translates overall spindle length into information about how much spindle is to the right of the frame centerline?
There is no secret code or handshake. Chainring position is a function of the right side spindle length and the offset built into the arm, which varies from one to the next. Since you're not changing the crank you have only the spindle to worry about.
I know you want a simple answer, but there is none, except that cartridge BBs measure from center like any other, so a 120mm symmetrical BB will measure 60mm from the center of the BB shell, and a 124 asymmetrical with the right 4mm longer than the left will be 62/58 from the shell's center. looking at the sketch you can divide "B" and add half to A & B respectively since you don't care where the bearings are.
You have an existing spindle,
measure it, then purchase a cartridge BB with an equal spindle length, and the same R/L difference, if any. If you can't match it exactly get the nearest one focusing on the right side length. You can also buy yourself fudge room of a few millimeters by using a non flanged bottom bracket like a Phil, which will allow you to float the unit to wither side.
Be aware that even with an, on paper, correct spindle length, there will be small differences in crank position because the distance the crank slides up the taper varies. Allow 1-2mm room for error in chainring-chainstay distance.
BTW- you have another alternative if this is beyond you. Bring your spindle to a bike shop and ask for a comparable cartridge bottom bracket.