If it's a carbon fiber Roubaix, you're making a huge mistake by attaching a rack to the seat stays with anytype of "retro fitting." I would recommend that you use a Seatpost Quick Release system made by carradice available at peterwhitecycles.com. However, I would not recommend that for a carbon fiber seatpost. The Roubaix is a racebike (or performance road bike) and if you need to carry 25+ lbs of gear then it's probably not an appropriate commuter unless you are willing to carry it on your back in a backpack. The beauty of the SQR is that you can remove the bag in about 15 seconds and use the bike for it's intended use - road rides.