When a metal fin (typically copper or a copper/aluminum alloy) passes between the rows of magnets, eddy currents are generated in the fin, which creates a magnetic field opposing the fin's motion. The resultant braking force is directly proportional to the speed at which the fin is moving through the brake element. This very property, however, is also one of magnetic braking's disadvantages in that the eddy force itself can never completely stop a train in ideal condition. This effect of magnetic braking can be explained by an example in which the train's speed is halved as it passes through each set of brakes. The train's speed (in any unit) would initially be 40, then 20, 10, 5, and so on. It is then often necessary to bring the train to a complete stop with an additional set of fin brakes or "kicker wheels" which are simple rubber tires that make contact with the train and effectively park it.