Originally Posted by
Kimmo
When checking hanger alignment, strictly speaking the wheel doesn't need to be properly dished, only true.
But if it's on a bike with horizontal dropouts, the wheel pretty much does need to be properly dished in order to ensure the wheel is pointing directly ahead...
When you align a derailleur hanger to a properly dished wheel that is centered between the chain stays - the hanger by default is also in parallel to the center line of a symmetrical frame - which is what 99.99% of us ride.
When you align the derailleur to a wheel that has not been properly dished - but is installed centered between the chain stays - all you have done is aligned the derailleur hanger to THAT PARTICULAR WHEEL.
...the hanger is still misaligned - i.e., it is not operating in parallel to the center line of the symmetrical frame. Swapping in different wheel later can cause problems.
If you have a hard time grasping this - read the freely available downloadable version of the Paterek manual....or visit a local master frame builder as I often do.
There's a reason they keep sets of properly dished wheels laying around as part of their frame building equipment.
=8-)