A mass-production mid drive uses a small, high-rpm motor running
through some sort of gear reduction. The motor gets to use the
cogs on the cassette to alter the gearing and aid in climbing.
Having the motor in the front poses no real problems if the power
is kept to modest levels. It can help the weight distribution if you
have the battery mounted on the rear rack.
A direct-drive hub motor like this one from BionX is pretty simple:
magnets are mounted on the inside of the hub, and the copper
windings are up close to the magnets.
at a little higher rpm for more speed but has less torque.
Direct-drive motors are usually silent. The main problem
with hub motors is that all the turning force is generated
right at the center of the wheel. The motor is pulling huge
amounts of power at low speeds and with high loads.
Going up a steep hill with a load on the bike, the motor
is nearly stalled and can’t generate much torque. Huge
power in and not much torque out means the motor is
getting really hot. Pulling a really steep hill can heat a
motor up to the point of “letting the smoke out of it” and
ruining the motor.
Geared hub motors may look identical to a direct-drive
motor where the hub is the motor, but the geared hub
motors have a smaller motor inside that turns internal
gears so that the motor spins faster than the wheel. This
allows for more efficient power use at low speeds and
under heavy loads. Geared hubs may be a little heavier,
but when comparing similar power output, they are usually
lighter. They may also have a lower-rated power output
because the motor is smaller. They are generally noisier
(with a mild gear sound) and certainly have more moving
parts than a direct-drive motor. The other advantage to
geared hubs is that they often have an internal “freewheel,”
which lets you pedal the bike with the motor off without
having to turn the non-working motor against magnetic
Aftermarket hub motors that bolt onto a standard
bike are growing in popularity because they’re the easiest to install. Buy a hub motor laced into a wheel and
bolt it in place—done like dinner. Companies like Currie
Technologies ( www.currietech.com) and BionX ( www.
bionxinternational.com) can sell you a complete kit that
anyone with a set of common tools can install in an afternoon and be rolling. For relatively short trips on fairly level
terrain, they’re a good option.
The mid-drive design seems to be enjoying the biggest
surge in popularity. There are several advantages to a mid
drive where the drive system is frame-mounted and the
power from the motor goes through the bike’s chain to the
rear wheel. As the name implies, typically, these motors
mount in the center of the frame, either above or behind
the bottom bracket and crankset, or actually taking the
place of the standard bottom bracket. The mid drive is
more complex than a hub motor, requires some specialized parts, and is naturally more expensive. These come in
fabricated designs like the Stokemonkey
http://clevercycles.com/cleverchimp-stokemonkey-human-electric-hybrid-drive), aftermarket units like the Bafang 8fun that replace the crank in
the standard bottom bracket, or the latest generation
like the Bosch ( www.bosch-ebike.de), where the frame
must be designed for the drive from the beginning with a
motor mount rather than a bottom bracket. Kits like the
Stokemonkey are complicated, require skill to mount and
must be set up correctly, but you can use your bike’s
gears to match the power output to the load and terrain. The Bafang is a little involved to mount, and like the
Stokemonkey, you must find a mounting spot for the controller. Probably the biggest game-changer in electric bikes