I also have size 51 feet. I found that even long'ish chainstays don't provide enough clearance for my heals. My solution was to find panniers with a rear upper hook that could be unbolted and moved slightly forward on the bag and with elastic lower hooks that could also be moved forward. Thereby, alllowing me to move the bags as far aft as possible. I'm don't know what current design fit these requirements. I'm still using an old pair of Madden bags.