Had an over-the-bars moment that has messed with your shifting? Spent countless frustrating hours trying to index your shifting, but even when you follow the procedure, your shifting is still terrible? It could be the alignment of your rear derailleur hanger.
BIKEHAND Bike Bicycle Frame Rear Dropout Derailleur Hanger Alignment Gauge Tool
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This simple piece of metal that attaches the rear derailleur to the frame can become bent. This will then set your jockey wheels out of alignment with the cassette sprockets. If this is the case, the alignment can be adjusted and corrected using a Gear Hanger Alignment tool.
In all honesty, the average home mechanic probably wont have this tool in their personal arsenal, but they should. Proper alignment is crucial to accurate, crisp shifting, and this can only be measured with one of these tools.
Even a newly-fitted hanger can be out of alignment, so being able to measure and correct this is a vital skill for sorting drivetrain frustrations. This video shows how to use a derailleur hanger alignment tool to straighten a bent derailleur hanger.