Two motors?
Two identically geared hub motors with two controllers, could be an asset in slippery or extra steep conditions; 2WD will have better traction. I don't know how well two separate kinds of power systems would work together, but having an extra motor on the front hub can't hurt if it is geared low enough.
two speed hub motor
Apparently I
cannot over emphasize the need for gear enough. Hub motors that do not have a
gear reduction inside must use more amperage to overcome the
lack of gears. And that means that you will need a larger motor and battery,
just to handle the increased amperage. It also means that you will end up
turning your bike into an illegal motorcycle if you want to power 400lbs up
an 8% grade. Or at least have worse mileage, and you could run the risk of
burning out your controller if it is not big enough to handle the increased
wattage.
When you use gears it
really changes things. With gears you can move the "efficiency
peak" around to match your mph. On a fixed hub motor you are
permanently tied to your power-band and this presents a real problem at low
speeds. Low mph/rpm on a hub motor places you into the low
efficiency areas of the power-band. You can compensate a little if you have
an ammeter and are really careful about using even less current than the
current limit allows (so you are using almost no voltage down low in order to
be more efficient) but you are still fighting a less than desirable situation.
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Run a 40mph hub at less
than 10mph under load and you'll cook it. My best guess is that a hub would
have to be capable of better than 40mph to climb rough steep grades
efficiently, and in that case you would not be able to do so slowly or you'd
overheat the motor.
At 10 mph the efficiency
is running about 50%. The good efficiency areas don't even exist until you are
past 20 mph. So unlike with gears where you can "gas it" off the line
and be in the good efficiency areas all the time, gearless hub motors have
a real "issue" with quick starting. Quick starts on a gearless
hub motor are really bad news for battery usage... (much worse than
if you are geared right)
The only
reason to even think about hub motors is because the speed of operation is lower
(usually less than 500 rpm) so it makes finding sprockets much easier. An
excellent example is the Elf car.
That's why
they used one for the Stoke Monkey mid -drive.
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Geared Hub motors verses Un-geared hub motors  Hub motor verses gears debate How long does a geared vs direct drive last? |
| http://senseable.mit.edu/copenhagenwheel/ |
This is a self-contained, but wimpy
machine, made for people that live on flat land, and need to stay dry
cycling to work! But do not think it will carry you up steep hills with cargo!
While a single stage chain drive loses around 5-10%, a hub drive not operating near it's peak RPM is losing far more power than a properly geared motor would be.
San Francisco wheel?
If someone wants to make a dent in america's car
culture glut, they are going to have to engineer one that can drive 400lbs up a
10% grade at legal speeds. It has the potential to use thicker wires for lower
voltages when climbing hill, unlike normal hub motors, without planetary gears.
Only in america can we use a motor strong enough.
Geared hub motors are intrinsically complex. Most direct-drive hub motors only have a few moving parts, and a hub motor with a gear set has tens of moving parts.
This leads to the big 2
disadvantages of a hub motors: #1 they are more expensive, #2 it are more
fragile than a direct-drive hub motor. But they are much better hill climbers
than a non-geared hub motor of the same power
So not all hub motors are bad. Some are very well
made and they charge for that durability.
Hub motors in general have a problem running low
voltage in through the axle, because low voltage requires larger wire than will
fit. This may limit the slow hill climbing trust. But you can check the
performance ability on this Canadian motor simulator. http://www.ebikes.ca/tools/simulator.html
If you have steep hills, you are most likely going
to need a geared hub motor. I needed a 6:1 gear ratio for about 600lbs on our
steep hills. But I have not seen one that is more than 5:1 ratio.
There are other problems with hub motors, like
sensitivity to road shock and and water. Personally I do not think they are
worth the price. With steep hills and a lot of weight it would be better to use
a single stage gear reduction from a large motor set to the side.
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| The enabling feature of the GTF is its 3:1 reduction gearing. The setup is similar to this planetary gear system, also produced by Italian aviation company, Avio. |
Hub motors aren't quite right for the
purpose of hill climbing with cargo because they are happiest running at RPMs
too high for regular bicycle wheels.
We can get hub motors that weigh 10kg and more,
but we can't get them in a configuration that will efficiently drag a loaded
normal bike up a steep grade at less than 1000W input power.
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If done right for given terrain (ie: larger wheel
for flat land or a smaller wheel for steep hills, or at least an oversized motor
and controller that will give a wider range of efficiency), a large direct
drive hub motor is the most efficient motor system. But if you have steep
hills, and there is a legal limit to the size of motor you're allowed, driving
around in a single gear may not be the most efficient.
This is for a law that limits motor size.
If the law allows a bigger
motor go for a bigger motorThis book does not tell you how motors work,
only how to install a hub motor.
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| http://www.amazon.com/Build-Electric-Bicycle-Green-Guides-ebook/dp/B004G090CE |
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