I have another piece of bike luggage that uses the QL3 system (Ortlieb Vario Backpack/Pannier). My bicycle does not have disc brakes so this may not be of great value to you but... I have a Tubus Logo Evo Rear rack that has the second rail mounting position. Even on a rack with so many tubes available for mounting, I still found it a bit tricky to mount the QL3 system on the rack in a position that way far enough back to keep the pannier away from my heels (have US 13 feet). So for mounting the QL3 is attached a little crooked, but according to the instructions this is perfectly accpetable. What I have found is that, contrary to my intuition, the p-clips to mount the upper part of the QL3 to the rack work just fine on vertically oriented tubing (how I have them now) so long as you are careful to tighten them enough. I regularly carry 20 lbs of stuff in that one pannier and the QL3 system has yet to slide out of position with respect to the rear rack.