Join Date: Jul 2009
Location: Central Coast, California
Bikes: Colnago C-50, Calfee Dragonfly Tandem, Specialized Allez Pro, Peugeot Competition Light
Most owners manuals will say not to (they're written by lawyers). I've used one for years, since about 2003, on a carbon seat post. In fact, I've done 6 1200km randonee's using that setup. It can be done safely if you are careful and take a few precautions. First, make sure you have a plastic schimm between the metal rack and the seatpost. Sharp edges are a bad thing. Second, don't over tighten it!!!! Tighten it just enough so that it doesn't move easily, about the same torque as your seat clamp should be safe. Finally, don't put too much weight in your bag.
If you are at all worried about it go buy a cheap aluminum seatpost and problem solved.