I use one on multi-day supported tour-style rides. The bag just slides onto the rack via rails integrated into the bag itself; there's a little plastic hook on the front to keep it from sliding back off. Very secure; there's no excess front/back or side-to-side motion to speak of. I've been closing in on 50mph with mine, ridden over gravel roads -- basically toured. It never occurs to me that it's back there. Excellent design. There's a bungee-style weave on top of the bag, so you can stash a jacket on top and leave a little more "interior room" in the bag for other stuff.