Last Wednesday evening while riding a night-time backwards lap of The Pines at Stony Creek I clipped a tree and fell over. This normally wouldn’t be notable, except someone was following me so closely that as I fell he ran into me, then ran over and fell on my bike. Everything initially looked fine on my bike and neither of us were hurt, but upon later inspection I found the derailleur freshly scratched and it appeared that the derailleur hanger was bent inward, causing a bit of extra drivetrain noise and weirdness.
Tonight I pulled things apart and checked the derailleur hanger, and as shown above it’s bent. Thankfully I had picked up a spare earlier in the year (#82 from DerailleurHanger.com) so I was able to fit it and get things back to normal. Bent derailleur hangers can cause all sorts of strange issues, because once bent the derailleur (which moves and tensions the chain) is no longer working on the same plane as the cogs, so it exerts twisting forces on the chain as it transits between the cassette and jockey wheels. This usually causes frustratingly erratic shifting that’s impossible to adjust away.
Park Tool sells the DAG-2 for aligning derailleurs to wheels (and thus cassettes), but for thick, single bolt derailleurs (such as mine) it’s generally difficult to impossible to properly straighten one. Replacement is really the best option.
For reference, the new, unbent derailleur hanger can be seen with the straightedge (as above) here.