Geared or Direct Drive Hub Motors?
There are two basic types of motors in wide use today for electric bikes, direct drive and geared. The direct drive has no internal gears or other moving parts except the actual case which rotates around the axle on sealed bearings. The coils are wound around an assembly that is fastened to the axle and remains stationary. The outer ring of the case has a ring of magnets that rotate in close proximity to the electromoagnets formed by the coils. As the coils are energized in a specific pattern by the motor controller, the magnets are attracted and repelled causing the wheel to rotate. The outer case directly drives the wheel of the bile. The geared motor has the same basic configuration, but does not directly drive the case. Instead, there is an intermediate gear assembly driven by the motor. This consists of a freewheel and three planetary gears which transfer the rotation to the outer case and wheel. The gear ration is typically 4:1.
Geared motors are smaller and lighter. Due to the gear ratio, they have good torque, even at low speeds. The gears make a little noise but usually not very noticable. Due to their small size and the physical strength of the gear assembly, there is a limit on the maximum power that they can handle. The internal freewheel isolated the wheel from the motor so there is no resistance when turning the wheel. This prevents the motors from becoming a generator, so can not be used for regen.
Direct drive motors are bigger and heavier but can take more power. Their larger size, bigger magnets and coils, allows them to dissapate more heat. They are totally silent. There are less parts to go bad as they do not have the internal gear assembly. Depending on the motor, there may be some drag due to the magnetic attraction. DD motors turn into generators when turned so they can be used in regeneration (regen) applications.
Which should you choose?
For riders who want assist but still want a light bike with the least amount of change in how it rides, the geared motor is best. It can get you up most hills with the right power level, but has an upper limit.
For high speed cruising over low to moderate hills, the diect drive works well. It can go faster than the geared motors and take lots of power. If you can live with a heavier bike, you can load up with batteries and carry a heavy load very far and very fast. If you wwant regen, you have to use the DD motor