A powerful direct-drive hub motor
There are two main types of hub motors currently on the market: geared and gearless hub motors (gearless hub motors are also called “direct drive” hub motors). Due to the lack of gears, direct drive hub motors are the simpler of the two, so we’ll start with those.
Gearless (direct drive) hub motors
In a direct drive hub motor, the axle that passes through the center of the motor is actually the axle of the motor itself, with the copper windings fixed to the axle. This whole axle assembly is called the “stator”. The magnets are mounted to the outer shell of the hub motor. When electricity is applied to the stator a magnetic field is induced that causes the magnets to move. This in turn makes the whole shell of the motor turn and propels the ebike forward.
Inside a gearless direct-drive hub motor
Geared hub motors
Geared hub motors, on the other hand, have their cases connected to the stator through a planetary gear reduction system. For every rotation of the case, the motor inside actually turns many times faster. This allows the motor to work at higher and more efficient speeds, while still allowing the wheel to spin at a comparatively slower driving speed.
simulation of a planetary gear reduction system
How do geared and gearless hub motors compare?
Direct drive hub motors are capable of providing large amounts of torque and power. This makes them especially well suited for higher performance ebikes. Direct drive motors are usually big and heavy which adds to overall bike weight and decreases range. On the upside, this extra mass helps the motor to keep from overheating as easily because the thermal mass of the motor functions as a heat sink.
Light duty direct drive hub motors, like many found on eBay and other auction sites, are usually rated at 500 watts but can usually be operated safely at up to 1,000 watts. Bigger direct drive motors, such as Nine Continent style hub motors, are usually rated at 1,000 watts but can handle even higher power levels.
Geared hub motors are smaller and lighter than direct drive motors, which can help with increasing range, but they are also less powerful and can wear out more quickly. Most geared hub motors are only rated up to 350 watts of power, though the larger Bafang BPM geared hub motor has a rating up to 500 watts and has been successfully used on ebikes up to 1,000 watts, though with a shorter life expectancy (of the motor that is, not the rider).
Some people have had success increasing the lifespan of the plastic gears inside geared hub motors by replacing one of the three plastic planetary gears with a metal gear. This allows the metal gear to take the brunt of the stress, saving the other two plastic gears and extending the usable life of the hub motor. This generally makes the motor a little louder – though this is rarely a deal breaker.
Three plastic gears transfer power inside a geared hub motor
Choosing between a geared or direct drive hub motor usually comes down to two considerations: your power and weight requirements. If you are envisioning your electric bicycle with a lightweight setup in mind, you are pretty much limited to a geared hub motor. If you are looking for a powerful ebike, especially an ebike over 1000 watts, your best option is the direct drive hub motor.
