Originally Posted by
mrrabbit
Derailleur hanger is aligned in four positions, not two. Top, bottom, and most importantly, front and back. That insures the hanger is aligned properly planar parallel to the centerline.
Theoretically. Functionally only 3 because the 4th one is hard to get to because of the frame. New versions have a swing arm that allows you to get past the frame but older ones don’t. But, again, that has nothing to do with the dish of the wheel.
If you align the hanger against an off dish wheel that is otherwise centered in the chain stays, the hanger will be off alignment either at the forward reference or the rear reference depending upon which side the wheels is off dish.
No, it won’t. A wheel that is off-dish isn’t “off dish” on the front or back or top or bottom. It is off-dish all the ways around. Let’s assume that the valve stem is 12 o’clock. If you put an off-dish wheel in a truing stand, do you move the arms of the stand when the wheel is rotated to 3 o’clock or 9 o’clock? No, the rim is in a plane. It just happens to be in the wrong plane but the displacement is the same all the way around the wheel.
The gauge of a derailer hanger isn’t moved from the measurement top to bottom nor front to back...nor from top to 9 or 3. A wheel that is off-dish will be displaced the same amount of distance from the hanger in all dimensions. If the hub isn’t square in the dropout that would have an effect but the dish doesn’t have the same effect.
What you are describing is a tacoed wheel. The rim isn’t in the same plane depending on the position of the wheel. It
would be silly to align a hanger to a tacoed wheel but I really don’t someone would do that.
I often have aligned the hanger without a gauge for various reasons. I sight to the cogs on the cassette and don’t worry about the wheel location at all. Alignment is done by drawing a line from the cog though the two jockey pulleys.