I used this to move my rack mounts out of the way so that I could convert our tandem's rear brakes from cantilever to V brakes without interfering with the rear rack. Nice, clean and stiff solution. Be aware that this does not mount on the seat post, rather it replaces the seat post clamp that goes over the bicycle frame's seat tube, so the diameter you want is the diameter of the frame seat tube, not the seat post diameter.
I had to modify this to make it work as I was confused as the the dimension to order. I ordered to the seat post size and not the seat tube. It worked out because I removed the inner upper lip with a Dremel tool and that was all I really needed. I kept the original clamp in place to manage seat height and this one went just above that simply as a point to mount my rack. (no eyelets on the upper seat stays)