Originally Posted by
woodcraft
Are you saying that you trust your eye over what the tool shows? That's an outlying position.
No, I said I confirm the results with my eyes. This is coming from 45 years of doing this stuff, owned a Campy N tool since 1979... my eyes are well trained to see stuff many don't. But the point i didn't press was that an aligned hanger doesn't always result in a der cage that hangs coplanar to the cogs. Here's 3 reasons why that are not uncommon.
One is that the der mounting bolt (the "B" pivot) might not be machined axial and/or the various plates and clips not sit square. As one threads the mounting bolt into the hanger the hex wrench often is turning in a wobble path (using a T or Y wrench w/o a ball end makes this easier to see. At the point of complete install the bolt is at some slight off angle from the hanger's threads/face.
Two is that the cage pivot bolt can also be not made perfectly. Or the cage not mating with the bolt squarely.
Three is that the cage can be bent. This overlaps with example two to a degree. We see cages/pulleys that are planar to the cogs at one cage rotational set but shift into other combos so the cage is rotationally different and watch the pulleys loose that coplanarness to the cog set.
This is why I set the hanger to a point when the cage is at it's middle of the range of play out/take up it is as parallel to the cogs (generally the middle of the cog set) as possible. To average out any tolorances best possible.
I will certainly agree that in a LBS shop setting using DAG is a vast time saver and establishes a base line for fine tuning if that should be needed. But I will also disagree that it is a needed tool for the critical thinker at home. Not that having nice tools is wrong
Andy.