I've noticed on a number of different bikes that I've worked on, that when you spin the cranks (without the chain connected), the chainrings wobble side-to-side. It means that the chainrings are not perfectly in a plane when the bottom bracket axle rotates. This side-to-side play can contribute to front derailer rub.
Anyway, there's a number of causal variables, and I'm curious as to which are the most likely (before I go through the long process of switching lots of things to see is it that? no, maybe the other thing.
- bottom bracket taper is not machined perfectly in-line with the axle
- crank is press-fit onto the bottom bracket taper at slightly off angle (not sure if this is possible with square-taper interface)
- the taper interface in the crank was forged/machined/etc. at slightly wrong angle
- crankarm spider arms are not perfectly in the same plane with the crankarm (seems unlikely)
- chainrings are bent (quite possible, but easy to test and rule out)
How often is the taper machined or forged into the crank at a slightly wrong angle? How often is the taper on a square-taper BB at a slightly wrong angle with respect to the BB axle?
Are there any other variables I've missed here? Thanks all.