Originally Posted by
capnjonny
I also found 2 different spindle lengths in the pile with different offsets right and left. I had assumed that since all cottered cranks were for double chainrings and 120 mm dropouts they would all have the same profile.
Another variant to be aware of is cranks for touring or utility bikes had more room between the crank and the chainring, to allow fitting a chainguard aka pants guard. Racing-oriented cranks have less room there to keep the "tread" ("Q-factor") as narrow as they can. So for the same chain-line, the race crank uses a shorter spindle.
I think some vintage track cranks don't even have room for a derailer cage between the chainring and the crank, so they can't really be adapted to a 2x drivetrain. Narrow tread was a selling point to racers, especially pre-WWII.