My son was jumping around the neighborhood on his 2019 Men’s Stumpjumper ST Comp Alloy 29 and noticed his chain sticking on the back of his bike. He was jumping around and noticed that the rear gears and chain were binding. You can see this below:
I took off the derailleur and noticed the notch at the bottom was broken.
You need this little feature to allow the B-tension screw to press on something to keep things from smashing together. Without it’s contact point, the derailleur pivoted too far forward and the guide pulley (top jockey wheel) crashed into the cassette, grinding against the sprockets instead of smoothly guiding the chain. This happened because the notch normally sets the derailleur’s clearance from the gears—without it, the entire derailleur rotated inward, pulling the jockey wheel dangerously close to the cassette. Likely triggered by an impact, a bent hanger, or sudden drivetrain stress, this failure made shifting erratic and risked further damage to the chain and derailleur.
Historically, derailleur hangers were integrated directly into bike frames, making them more durable but also more vulnerable—if the derailleur suffered an impact, the force would transfer directly to the frame, sometimes causing irreparable damage. To solve this, manufacturers introduced the replaceable derailleur hanger, a sacrificial component designed to bend or break under stress to protect both the derailleur and the frame. This removable design made repairs significantly easier, allowing riders to swap out a damaged hanger rather than risk a cracked frame. However, while this innovation improved durability, it also introduced a weak point—the notch that interfaces with the B-screw. If this notch wears down or breaks due to a crash, shifting issues arise, and the derailleur loses its ability to maintain proper chain tension. Today, precise machining and stronger materials have improved hanger longevity, but the fundamental principle remains: the hanger is a breakable safeguard designed to take the hit so your frame doesn’t have to.
stay posted as I get this fixed. . .
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