I have the Arkle Randonneur Rack and bag. It attaches to the seat rails and then cantilevers to the seat post,but does not clamp to, but does strap on for stability. I have a carbon seat post and works fine with it since the weight is supported by the seat rails. This does mean no seat bag or seat light. The system works well, but it is expensive and you do feel the weight that high when climbing out of the saddle.