Originally Posted by
Andrew R Stewart
Trying to get within a mm or two without actually trying the set up is a big expectation. The reality is that how the arm fits the taper, how close to "perfect" dimensions the bike's BB drive side from frame centerline is, where along the BB shell does the chainstay reside, how the chainstay is indented (or not) are all variables that one has a very hard time really nailing down without actually trying a set up. When in doubt go for more stay/ring clearance.
This is one of the situations that working with your LBS can make things go far quicker. Andy
+1
On both your posts in this topic (and most others
).
Except Shimano, who state the required axle length for their cranks, specifying chainline, most other cranks are trial and error, for me at least. Keeping various axle length BBs, starting with the one I
think is most likely to fit, then going size up, or down, depending on the results after test fitting the cranks. If there is a way to know up front as accurately as possible, I'm eager to learn. Also any rule of thumb charts for best possible starting point would be great. The best I have found so far is Sheldon Brown's
Bottom bracket size database.