One of the big draws to electric vehicles is their relatively maintenance-free nature. A new patent application from BMW shows how future electric motorcycles could use braking technology that’s more commonly found on trains and power tools to eliminate pad and disc wear.
Normal disc brakes work by taking the kinetic energy of a motorcycle and converting it into heat using the friction between the discs and the pads. As the bike slows, its motion is turned into heat and dissipated into the air. Wasteful, but effective. On some electric bikes, the motor can be turned into a generator by using regenerative braking, turning that forward movement back into electricity and returning it to the battery. That’s a much more efficient way to operate, as you recoup at least some of the energy spent getting the bike up to speed in the first place, but it has drawbacks. Not least because when the battery is already full it can’t accept the energy generated during regenerative braking—so you still need to have conventional friction brakes as well.
BMW’s new patent puts forward the idea of using a different braking technology—eddy current brakes—on the front of an electric motorcycle. These work a bit like regenerative braking by turning kinetic energy into an electric current, but instead of feeding that current into a battery it’s turned into heat and dissipated into the air, like a conventional friction brake.
The idea is that whenever a conductor is moved through a magnetic field, it generates electric current. In the case of a generator, that conductor is a coil of copper wire, and the electricity goes into a circuit. In an eddy current brake system, the conductor isn’t a neat coil of wire but a lump of conductive material (the brake disc) and the electric current that’s generated has nowhere to go. It swirls around inside the brake disc, hence the name eddy current, and heats it up. By using electromagnets mounted near the brake disc to generate the magnetic field, the magnetic field can be turned on or off, and its strength can be modulated, to control the braking. Because there’s no direct contact between the electromagnets and the disc, there’s no wear.
The idea behind the BMW patent is to eliminate the need for a rear brake on an electric motorcycle fitted with regenerative braking. Even if the battery is too full to accept the electricity coming from the regenerative braking, the current can be redirected to the electromagnets in the eddy brakes at the front, dissipating it as heat, so no mechanical rear brake is needed. The eddy brakes at the front also have potential benefits because they can react faster than conventional friction brakes. With a normal disc brake, it takes a moment for the brake fluid pressure to move the pistons, pushing the pads into the disc’s surface, and then another fraction of a second for friction to build up. Eddy current brakes have no such delay and can switch instantly from zero to maximum braking force.
The patent shows there would also be a conventional friction brake to back up the eddy current brake and provide braking even when the bike is switched off. However, by using the eddy current brake for most braking events, the wear to discs and pads would be kept to a minimum.