Wheel hub motors are electric motors installed inside a wheel hub, directly driving the wheel. The electromagnetic field of the wheel hub motor is provided to the motor's fixed winding. The wheel connected to the motor rotates as it follows or attempts to follow those fields externally. In brushed motors, energy is transferred through brushes that make contact with the motor's rotating shaft. In brushless motors, energy is transferred electronically, eliminating physical contact between stationary and moving parts. Although brushless motor technology is more expensive, most are more efficient and durable than brushed motor systems.
Wheel hub motors are typically designed in one of three configurations. The axial flux motor, in which the stator winding typically sits between magnet assemblies, is considered the least practical. The other two designs are radial, with the motor's magnet adhering to the rotor; one is an internal rotating motor with the rotor located inside the stator, similar to a conventional motor. The other is an external rotating motor with the rotor located outside the stator and rotating around it. The application of wheel hub motors in vehicle use is still evolving, and neither configuration has become a standard option.
Wheel hub motors can of course be installed on any wheel and the differences give rise to some other unique advantages and disadvantages. Front hub motors can help balance weight better because more weight usually sits on the rear wheel. Front wheels also typically receive less plain because they often kick up road debris and prepare for the rear wheel. Front hub motors keep the rear wheel clean for easy replacement of inner tubes and tires. The lighter weight of the front wheel of a bicycle means less traction, so sometimes more powerful front hub motors can cause burning when you hit the accelerator pedal. Also, the front fork is not as sturdy as the rear, so powerful front hub motors can even damage the front fork over time, although this can be mitigated by installing a device called a torque arm. A 750 W or higher power wheel hub motor is often best placed at the back of a bicycle.
Rear hub motors have better traction and stronger frame mounting advantages. They also don't produce a slightly strange gyroscopic effect when turning at higher speeds. Perhaps my favorite thing about rear hub motors is that they give you more of a motorcycle push feel than the pull of a front motor. However, once you reach a stable speed and drive along a straight line, the difference between front and rear hub motors is barely noticeable.