Originally Posted by
Marvelousmarkie
Lots of interesting information. Maybe I am doing this wrong, but my approach has been simple.
i usually put the crank on with a BB I already have. I adjust the derailleurs, measure the chain line and crank arm clearance on both sides.
If something is off, I calculated how much more/less clearance I need and whether I need spacers.
Then in I buy the same model BB I tested, but with the appropriate spindle length.
It has always worked well for me, but all the square taper cranks I use are JIS.
At the risk of stepping on your toes, as they say, you can leave test-adjusting the derailleurs for much later.
All you really need to do is thread on one of those $10 cartridge BBs hand-tight (don't bother with the NDS cup), place the crankarms on the spindle and slap the mounting area with your palms to get them started. From there, you can gauge the chainline and clearance and subtract a couple mm to account for the cranks moving inward when they would be torqued down. Obviously, if things are almost touching with a particular BB, you know it'll be too short with everything properly installed, and conversely, if everything is hanging way out there, that it'll probably be too long.
The nice thing about this method is that it won't put any wear/squishage on your crank tapers. And it lets you figure out the ballpark BB length and then remove everything by hand to try again, or install everything for real.
