Originally Posted by
TejanoTrackie
Dropout spacing has no impact on chainline if you start with a centered (un-dished) track wheel, and add an equal number of axle spacers on both sides, because the position of the cog/freewheel will not change. All frames regardless of rear dropout spacing will have both dropouts equidistant from the plane of the frame. Of course, this assumes you are using a track crank with a single chainring and a compatible bottom bracket. If you use a road double crank and either the inner or outer chainring, then your chainline will be slightly off.
Thanks for the explanation, I think I'm getting closer to understanding.
Assuming track crank and compatible bottom bracket, it seems, as you say, a 120mm hub with centered un-dished wheel, given spacers (if needed), and enough axle, just works, with any dropout spacing ... is this (at least partly) because bottom bracket shells are (usually) always 68mm?
Assuming you only have a centered un-dished 126mm wheel, I imagine getting proper chainline on a track crank could be achieved with 3mm bottom bracket spacers (one on each side)? Or do I have that wrong?
If not, I wonder why people don't do this more, vs. dishing and re-spacing the axle, it seems simpler (and would result in a sounder wheel similar to a 120mm one)?