Originally Posted by
JohnDThompson
I suspect the "whippiness" you describe is due the the fact that the load is cantilevered off the seat post with no other bracing. A rack that mounts on the dropouts and seat stays (or brake bridge) doesn't have that problem in my experience, even when fully loaded for an extended camping trip.
I'd also be leery of using a seatpost-mounted rack like that on anything but a really substantial seatpost. Definitely not on any weight-weenie post.