Joined: Feb 2012
Posts: 19,343
Likes: 5,459
From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
What you see is pretty typical. The pivot bolts are not always perfectly made, the threaded portion can be off axis from the shoulder portion. Resulting is an "orbital" motion of either the cage or the upper knuckle. When you threaded the mounting bolt into the hanger did you notice that?
I confirm the der's alignment with the chain on the bike and in a central cog/ring. Best to do two confirmations if a double ring set up, one in each of the rings but with in a middle cog. This tends to average out the cage pivoting.
Remember this is a bike and not a rocket. Andy
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AndrewRStewart