Originally Posted by Andrew R Stewart

The problem not often mentioned about der alignment (note I don't say hanger alignment) is that it's the der cage and it's movement that counts. Not the mounting bolt several links above the cage. The angle of the parallelogram does effect, to a degree, the indexing. But I will say that from my shop experience the cage, how it pivots, how the guide pulley maintains it's in and out alignment WRT the plane perpendicular to it's pivot has a greater effect then if the mounting bolt is square with the wheel.

I see a lot of ders that when installed on a straight hanger have the cage/guide pulley rotate out of plane. So that when the cage is played out (like when the large ring and meddle to large cogs) the guide pulley is inward of the intended cog's center line YET when the same drivetrain is in the small ring and the middle to smaller cogs the guide pulley (having rotated about the cage pivot) is now running outward of the cog's centers. How does one reconcile this? How does this happen?

It happens when the cage is not flat/square to it's pivot bolt more the when the mounting bolt isn't square to the frame. Why migh the mounting bolt not be square to the frame when the hanger is straight? When the mounting bolt is bent, it's threads are not coaxial to the bolt's shaft or the bolt's B spring plate and retaining clip are not allowing the bolt to seat square when tight. I have seen MANY ders that go through a orbital wobble while threading the upper mounting bolt into the hanger. Happens far more then not. An interesting point is when a hanger is aligned and one der hangs not so straight yet another der does... with no further hanger work.

I have watched other mechanics who have much experience go to the der hanger alignment tool more then once during a repair to solve why the indexing isn't consistent across the full range of the cassette. As though the straight hanger is the end all and they must not have achieved that result even after the second or third time...

This is a long winded post about a topic I have given a lot of time and thought to. I know I am bucking the pedestal that hanger alignment is considered. But indexing is about how the guide pulley maintains it's location below each cog that counts. How that is achieved is the question. I welcome other's thoughts, especially those with years of experience and not just those who read a lot. If you reply please consider offering your years of experience as a reference. Andy.