Chain catcher
thingiverse
The derailleur stopper is a dog-fang style chain catcher that keeps the chain on track by preventing derailment relative to the bike. This item fits only 28.6 mm seat tubes. It comes as two separate parts that are joined together using a strong zip tie, approximately 0.20 inches wide. I installed one of these on my bike once, which had a triple crankset. Unfortunately, the fang was too short outboard to effectively catch the chain when it started to derail. Instead of modifying the stopper, I decided to fix the front derailleur so that the chain would not drop inboard.
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