Originally Posted by
rothenfield1
It’s this part that is the mystery to me. If no one has a better solution, my idea was to just lay an un-mounted spindle of known length gently across the open BB shell approximately correctly oriented to where it would be if completely installed, then slide the cranks on and run a straight line to the center of the cassette. The problem is, if the straight line is wide or short, it will be just a guess-estimate by how much. Surely there is a better way.
yes, there are two better ways to measure chainline.
Directly - lay a straightedge (if using a flat ruler, use the edge not the surface because only the edge is dependably straight) against the outer surface of the outer chainring on a tangent, and extend it back to the cassette. Allow for the offset from the straight edge to the center of the gap between the rings, (or middle ring of a triple), and that should line up with the center of the cassette.
Indirectly - use a
chart like this one to determine correct chainline range, then measure from the tips of the chainring teeth to the down tube, add 1/2 the DT diameter, and adjust for the offset to center of the chainset as before. If your bike is unique or you want a more precise chainline target, measure from the inside of the dropout to the center of the cassette and subtract from 1/2 the hub's width to know the centerlind distance to the middle of the cassette.