A
bicycle hub motor is a drop in replacement for a regular bicycle wheel,
and can be installed and uninstalled quickly and easily. They have been
in mass production since the early 90s, and there are literally
millions of them in use around the world. They are cheap, reliable, and
simple. Where's the motor? A stealthy build using a rear geared hub motor | A front DD hub motor on Amberwolf's home built "crazy bike 2" |
Besides
front and rear, Hub motors come in two varieties direct drive (DD) and
geared. While both are single speed motors, there are no gears in
between a direct drive hub and the wheel. When the wheel is turning the
motor is turning. When not powered, the motor is effectively a
generator. This creates a drag that makes them harder to turn compared
to a freewheeling geared hub or a normal bicycle wheel. The upside to
this is that DD motors can do regenerative braking while geared motors
can't.
Geared hubs are about half the weight (5 - 7lbs) and size than DD hubs. Their gearing allows them to make more of
their smaller sizes. They can also have a bit more torque.
The downside to their smaller size, weight,
and materials cost is power and reliability. With a DD motor you're
only power limit is how much heat the motor can take and the strength,
or lack there of, of your bike frames drop outs. The gears on geared
motors don't take kindly to over volting, high powers, and heavy loads.
With the exception of a few, expensive, higher powered models geared
hubs are for lower powered "pedal assist" ebikes. A "peanut buttered" gear from a geared hub. The brake was held while the throttle was engaged.
The
primary ratings for hub motors are watts and voltage. The voltage
rating on geared hubs means something, but DD will take a wide range of
voltages (the accessories that come with a kit won't however.) Wattage
is almost meaningless. Essentially a motor is limited in power by the
amount of heat it creates. Too much heat and it smokes and dies. The
problem with ratings is that unscrupulous (or clueless) sellers often
give the "peak" power rating. This is supposed to be the amount of power
a motor can take for a short amount of time, but this is misleading as
well because any motor can take almost any amount of power for short
bursts. A continuous power ratings is the amount of power a motor can
take for any amount of time with out over heating, and is totally
appropriate for our uses. Use your common sense though; if you see a
motor rated at 2000w that looks identical to one rated at 500w, assume
it's been over rated by the seller.
On
the other hand, the very popular nine continents DD hubs are rated at a
very Conservative at 600w and many, including myself, regularly run it
stock at 1500w continuous. With mods like larger wires, thermometers,
and ventilation it can be hot rodded to run at greater power levels. A hub drilled for ventilation
Hub
motors typically come laced in machine built wheels. Like pretty much
all machine built wheels they may need the spokes tightened and adjusted
after the first 50 miles or so. A spoke wrench is around $10, and wheel
truing is a valuable and some what easy skill to learn. If
a retailer sells a e-bike kit and doesn't include or even mention a
torque arm, do not buy from them. The proper axle hardware and torque
arms are necessary to contain the power of the hub motor in bicycle
axles not designed for such power. While many people don't run torque
arms with little problems, it is like playing the lottery and if you win
your front wheel comes off and you go flying over the handle bars at
30mph. I don't mean to scare you, but the risks or real and it's
happened. Make sure you install the axle hardware correctly, and check
the axle nuts for tightness at regular intervals!
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| With out a torque arm the motor axle is free to wiggle, spin, break out of, or in this case spread the drop outs. He was lucky!
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