The
only reason to use two freewheels on the freehub
is to let you use a smaller
sprocket
than is possible with a freewheel, on
the motor.
Cutting
splines [see the video] into a single speed freewheel needs a special
machine. Two freewheels are needed on a cassette holding freehub for
a motor and still be able to pedal together or alternately. Other
ways to do this is mount a freewheel on the motor spindle and use a
freewheeling crank [both available with a mid-drive kit].
The reason to build a single speed drive is to separate the motor drive
from the human powered drive. Having the motor's power not going
through the crank will not only save the sprockets wear but also make
it possible to use a single low-slow gear reduction ideal for saving
your motor for steep hill climbing and hard starts at stop lights
etc.
With
three speeds on the crank, matching the motor's speed with your legs
will become much easier. But you will probably need to use
your legs on the steep hills; not good for people with debilitated legs. However if you don't haul cargo up these hills, you don't need
such low gears so you really need a motor with less power or a programmable controller to keep you out of trouble.
I
can tell you the approximate gear reductions I use but every one is
different: the motor has a 6 to 1 reduction in the planetary gears;
this is a slow motor output. Then there is a 2.2 to 1 ratio to my
drive wheel which is only 23” OD. This is god for most of my hills
[but then 2.74 to 1 to climb a really steep hill.] If you are patient
enough you can figure out the rpms and speed. Thereby making a low
enough gear ratio that will keep you under 20 mph on flat land and
give you more than enough power to climb the hills with cargo.
Unfortunately most people are insane, so they build a drive to go as fast as
possible. This gives ebikes a bad reputation and the cops will harass you just because they can't believe that anyone wants to drive their
bike under 20mph.
There
are hubs made for two sprockets, one on each side, but they do not
both “freewheel”. Pedaling the extra freewheel on the motor does
not take more effort. And if you use a larger voltage you will get
more RPMs but may need to make your phase wires thicker to handle
more heat if you do not keep the amps down. I want to use 72 volts with a lower gear ratio just to get more kinetic energy for steep
hill climbing. Not to increase my speed.
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| Double drivetrain |
 |
| Cool Knight BMX Chain |
 |
| with motor adapter |
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| http://sickbikeparts.com/front-freewheel-heavy-duty/ |
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| adapter Sickbike parts.com |
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