Quit fiddling with your rear derailleur's barrel adjuster and address the issue of sloppy shifting at its source. Park Tool's DAG-2. 2 Derailleur Alignment Gauge helps the home mechanic identify and correct a bent derailleur hanger, and - compared to the DAG-2 model - it features a stronger shaft, tighter tolerances, and improved clearance and reach. The sliding arm encompasses wheels from 20 to 29in.