An electric motor is a very simple machine: In its most basic form it is a stator, made with laminated steel
and magnet wire (as Electromagnets), and a rotor (the spinning part), with permanent magnets to react to the
changing magnetic field of the stator's electromagnets. How this rotational force gets converted into forward
motion on a bicycle varies depending on the application. That said, there are three forms by which these motors
are configured:
- Direct-Drive, brushless: Wire-wound Stator is fixed to the axle, axle is fixed to the frame. Rotor contains
the permanent magnets, and via bearings spins around the axle and is connected to the rim via spokes.
- Geared-Hub Motor, brushless like above, except with internal gearing to give a smaller motor more torque.
- Mid-Drive brushless DC motor is reversed in design whereby the Stator casing contains the wire-wound electromagnets
is fixed to the frame, and the permanent magnet Rotor spins within, like a drill or starter motor at high-speed,
though needs gear reduction to convert speed into torque.
Therefore, Direct-Drive hub motors are the SIMPLEST of ebike motor designs. The ONLY moving parts are the two
bearings at the axle. Because of this simple design, Direct-Drive Hub motors are the most efficient, robust, and
durable motors bar none. It is also partly for this reason that they can take the most power input.
Caveats: DD Hub motors will fail for the following reasons…
- Motor interior gets wet and rusts/Seizes
- Bearings fail
- Windings overheat from excessive current and cause a short
- Hall sensors/wires short or fail. (Can be remedied by using a sensorless controller)
- Motor's Axle may break from impact
Geared-Hubs are similar to DD hubs, except that they use a planetary gear system to multiply torque and reduce speed at the wheel.
In addition to the same failure points as the DD Hub, a geared-hub will have normal wear requiring eventual replacement on the planetary
gears. This is a relatively straight forward repair that may need to be done more or less frequently depending on how much power is
forced through the motor and total hours of use.
Mid-Drive systems are much more complex in nature, and as such these systems require more maintenance and repairs than either DD Hubs
or Geared-hubs. The main motor typically turns at a much higher RPM and must be "reduced" via reduction gears or a set of sprockets
and a chain, and on some set-ups this may involve two stages of reduction. Further, the ultimate method of power transfer must happen
through the bicycle's standard chain and rear sprocket set, which to this day are still designed for Human power, NOT a machine that
is up to 5 times as powerful as an athletic human. Chains and sprockets will wear out and break. And if the chain breaks, there's
no way of pedaling the bike - anywhere!
And of course, with Mid-drive kits, quality makes a big difference:
- The strength of mounting brackets and quality of the reduction gears is paramount to the longevity of the system.
- Buying a poor quality mid-drive can quickly become a moment of regret.
Overall, we have been pretty impressed particularly with the Bafang BBS series mid-drives, as they are quiet, and the reduction gears
are well-made, well-sealed and protected from the elements.