Joined: Feb 2012
Posts: 19,381
Likes: 5,527
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
The lower edge of the inner cage plate needs to clear the tops of the middle ring. So a der with too "deep/tall" an inner cage plate shape could rub an a 42T ring but clear a 39t ring. Also the cage's contours might not match where the chain is running past them if the ring was the wrong size, affecting the chain's lift during shifting. Andy