Various instructions of front derailers with clamp want me to ensure that the derailer cage is parallel to the chainrings. What is the preferred way to do this?
Since the derailer cage has quite a complex three dimensional shape I'm not entirely sure which lines to use as guides. Should I use the inner side of the outer plate? Do you look from above or from behind? I'm looking for some best practices.
It depends on the derailleur.
Most or perhaps all Shimano FDs intend to have you use the top forward part of the outside cage as your reference surface. Where the logo is. Their published procedures show this. This is especially true of the knuckle type road models of the last few years.
SRAM Yaw FDs have you use the marked lines down the center of the cage in the back and front while looking down. Other SRAM ones use the same forward top surface.
Sometimes you don't know and have to intuit it. Usually you should avoid looking at the inside cage as your surface because it's typically stamped and manipulated all over the place. There are of course FDs, usually older, where the sides of the cage are parallel, but usually those are easy to align. (They're the sort most likely for you to need to angle the cage.)
Various instructions of front derailers with clamp want me to ensure that the derailer cage is parallel to the chainrings. What is the preferred way to do this?
Visually, by eyesight.
Don't worry though if you don't get it perfectly straight.
A front derailleur in my touring bike is visibly unaligned. The plates are not parallel to chainrings. I clearly see it at great distance by just glancing at the front derailleur.
I have never bothered to fix the misalignment, because it simply does not matter. The front derailleur works perfectly even though it is slightly misaligned.
If you align it as closely you can with your eye, your alignment will be far better than mine, and since my alignment works perfectly, yours will work even more perfectly!
One can take a long, L shaped Allen key, placing a flat side on the flat aspect of the large chainwheel's teeth. Use the top, right side of the chainwheel and have the long part of the key facing back toward the front der. Rotate the chainwheel/Allen key back so the Allen key's long part will move across the flat, forward aspect of the derailleur's outer plate. You must keep the Allen key flat against the chainwheel teeth. This way you can determine parallelism of the der by gauging any difference in the gap of the key to the outer plate along the length of the derailleur cage. Correction is made by rotating the band clamp accordingly. Also, the proper lateral alignment (flat front aspect of outer plate to large chainwheel teeth) is gauged as well. Good alignment for me is when the Allen key just barely brushes that aspect of the outer plate but is not at all deflected from staying flat along the teeth of the chainwheel. Manipulate the Low limit screw to adjust the lateral alignment of the derailleur so the outer cage is parallel with the outside plane of the chainwheel's teeth (which is what the Allen key is projecting when held as described).
