was disappointed by the one seatpost rack I tried and wound up DIY'ing vertical supports to keep it from swinging left/right as I pedaled. that movement also pivoted my seatpost. also, I tend to overpack so I wound up with a heavy load. eventually switched to a conventional rack. I like that the rack you showed has some kind of additional support, looks like to the saddle rails. fyi - there are ways to mount traditional racks using p-clips aka compression clamps