Monday, October 24, 2016

Hub Motors for Cargo bikes?

This is the right place for a hub motor
 but a second drive chain is needed

You could use one of these YubaMundo mid-mounts
without the jack shaft and sprockets that are there
just to drive from the left side over to the right side.
But you will still need a sprocket adapter that
fits onto the free-hub body on the drive wheel,
to use a stronger #35 chain. And mount it behind a
5 or 6 speed cassette. But if you’re looking for
maximum efficiency, use a LightningRod's kit
To run only the axle there is a complex operation
If you are thinking about a Stoke Monkey mid drive
consider this. This graph is done in a low gear
 and an even lower gear will work better but bicycle drive chains 
were made for only human power abilitys of about 1/3rd HP, so they wear 
out more often when you use an electric motor strong enough 
to climb steep hills with cargo. That is why I say mount your
 motor under the seat and drive the wheel through a gear reduction.

The only reason to use a hub-motor is the width, they fit between the pedals.
And they are slower than long motors, making it easier to gear down to the drive wheel.
A 16” wheel is a ratio of 1.6 to 1 of a 26” wheel.

Smaller drive wheels are like using a lower gear. But it is hard to find a cargo bike with a 16” wheel. Mounting the hub motor in the center will let you build a lower gear. You may need an even lower gear for a steeper hill.

You can see from these graphs that it is hard to find a hub motor that will drive you up a 9% grade with 440lbs (200kg; total combined weight) at any speed with a one-to-one ratio as direct drive hub motors are.

Using higher voltage will be like using a lower gear because it allows more wattage with out increasing the amperage (higher amperage requires thicker wires), but then you have to reduce the gear even more because the motor will be moving faster at that higher voltage.

I didn't see any 36 volt hub motors that could climb any hills steeper than a 8.6% grade with the weight that I haul, with a 26” wheel. Although there must be some. Lower gears use much less power than a one-to-one ratio to climb a given hill, so you will need a larger battery with a hub motor. (more money)

If your hub motor seller does not have a graph like these, or cannot understand these graphs, do not buy from them!

Motor simulator graphs:

Converting a hub motor to a middrive motor

eBike school:

velomobile gear reduction video

At 36 volts the motor is slower but not as powerful

Pay close attention to the Motor Power Curve (Red Line) and the Black Curve (Load Line). Typically the Motor power curve will rise up in an arc, and then abruptly fall off on a straight line down to "0" The highest point (Apex) of the motor power curve is when the motor is demanding the full current output of the controller (and where it is least efficient).

If the Load line intersects to the right of the apex of the motor power curve, then the controller is powerful enough for the system. If the load line intersect the motor power curve to the left of the apex of the motor power curve, then the controller is too small, and we should look for a more powerful (higher current) controller. 

    The black Load Line shows the power required to propel the configured bike at any speed. This has nothing to do with the simulator - it reflects the same results you get from any standard bike speed calculator - basic physics - nothing to do with motors. This is all about aero drag, rolling coefficient, grade, etc.

    The red line is the mechanical power the motor can deliver at any speed where speed varies according to load not throttle. This is based on dyno data and Justin's modeling - this is the 'simulation' part and has nothing to do with drag, grade, etc.

    The intersection of these two lines is the point where the power to propel the bike is exactly equal to the power produced by the motor. This indicates the terminal speed for that bike configuration and load. This is simply a graphic solution to two separate sets of power equations (i.e. it could be done algebraically - the graphic solution is equally valid...)
This means that if you want know how much power it takes to push your bike to 35mph, you just turn off all the lines except the Load Line, configure the bike, then look at the power at 35mph... This is the amount of power required regardless of the motor or drive system. It's only when you play this curve against a specific motor that you get into the terminal speed, etc where the specifics of that motor/battery/controller can be balanced against the requirements of the bike/grade/etc.

For example, in the plot above, we see that it takes about 3200W (motor power) to push that bike to 45mph. Using the same efficiency as shown (86.9%) we see that although it takes about 2680W (battery power) to achieve 40mph, it would require 3200/.869 = 3680W (battery power) to get to 45mph - a fairly staggering increase for 5mph - but perfectly understandable when you look at the steepness of the Load Line at that speed.

do not put a hub motor on the front wheel!

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