cyccommute 

Bicycles have had "disc" brakes for many, many years. As long as a bike has rim brakes, it already has disc brakes. It's just a larger rotor. The principle is the same and the mechanism is similar enough to be the same. The only difference is that a hub mounted disc brake has to be set up the way rim brakes should be set up, i.e. close to the rotor and with very little lever movement from off to on. Most rim brakes have been set up so that the lever is half way to the bars before the brakes actuate. You can set up hub mounted discs the same way and they are even worse than the worst rim brake imaginable.