How Does the DAG-2.2 Derailleur Alignment Gauge Work?

The Park Tool DAG-2.2 Derailleur Alignment Gauge is used to check and correct the alignment of the rear derailleur hanger, the critical interface between your frame and rear derailleur that takes the brunt of impact in a crash or wheel removal. The gauge mounts to the derailleur hanger thread and uses a long arm with a reference indicator to measure deviation from true vertical and lateral alignment relative to the rim. Precise hanger alignment is the foundation of clean shifting, and even small bends in the hanger cause indexing problems that cable adjustments alone can't fix.

A bent derailleur hanger is often the first thing to check when shifting becomes erratic or a bike stops indexing correctly after a crash or transport incident. The DAG-2.2 takes the guesswork out of hanger alignment by giving you a direct visual reference for how bent the hanger is and confirming when it's been fully corrected.