good point. I use them on a couple of bikes for the top of the rack at least. I wouldn't use them for the bottom though. For that, I build something to go off of the axle.

Bikes that are not designed for racks tend to handle pretty poorly with racks put on them. A race bike is going to have pretty lively handling, and putting racks and weight on the rear makes them a handful. A bike that is designed for racks (and has mounting points) is much, much easier to ride carrying a load.