Monday, October 24, 2016

Hub Motors for Cargo bikes?

Hub motors are inherently heavier and bulkier than gear driven wheels. And they are worthless for hill climbing unless they are mounted like these are.

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

cheaper to build your own.

You could use one of these YubaMundo mid-mounts
to build a 3 to one gear reduction from a 5:1
geared hub motor (or a 15:1 reduction). Then
put a direct-drive hubmotor on your drive
wheel, for cruising flat land. And switch between
the two motors for a legal power system.
To run only the axle there is a complex operation

The faster the motor output RPM; 
the more torque thrust you will have.
So a high voltage low amperage controller 
setting will give you more hill climbing power, 
even with smaller motors and
 much more gear reduction.

There is a new Stoke-Monkey G02 with internal gear reduction, but I don't know if it is a 5 to 1 ratio or what. In this chart you can see that if you make a 1.6 to 1 gear reduction to a 26” wheel it may do the job.

A 12” wheel is a gear reduction of 2.16 to 1, of a 26” wheel (tire). So this gear reduction would equal about 10½ mph on the 26” wheel. With this reduction ratio it is possible to drive a little faster if you're willing to gamble the motor over heating. Or you can build a smaller ratio reduction like 1.7 to 1 for about 12-13mph (always count on less output)


higher voltage is better

E. Hub motors should be rated by how much torque they can produce rather than how many watts they can produce, It is not the motor power that causes a motor to overheat, but the motor torque. To avoid this problem, use a larger motor or a higher voltage for a higher RPM. At a higher RPM a given motor can produce more power.

High voltage with a low amperage will keep the motor from over heating. But of course you need a larger gear reduction. To build it in a single stage you will need a large pulley on the drive wheel. Possibly a 12inch rim pulley  custom made with epoxy teeth molded to fit the timing belt that will be long and wide enough. Although a two stage reduction maybe better it would be more expensive to build.

This chart shows that a 2.705 to 1 ratio of my 23 inch drive wheel at 72volts and 25 amps will make high enough rpms to keep the motor from over heating.

Read this for more info: MOTOR POWER RATINGS


A 16” wheel is a ratio of 1.6¼ to 1 of a 26” wheel. (20” is 1.3 : 1)

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 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 with out using way too much power. (as direct drive hub motors do).

Higher voltage with a custom controller

Using higher voltage will be like using a lower gear because it allows more wattage without increasing the amperage (higher amperage requires thicker wires), but then you have to increase the gear ratio even more because the motor will be moving faster at that higher voltage.

If you have a 26 inch drive wheel and steep hills to climb, trade your bike for a long tail cargo bike with a 20 inch drive wheel.

Start with a 'geared' hub-motor with a 5 to 1 ratio and if that is not enough, you will need to build an external gear reduction.

Very few hub motor can use 88volts with out a special operation. And so I say forget the hub motors and buy a mid drive then drive the rear wheel directly, not through the bikes pedal powered chain, to save the chain and sprockets from wearing out too fast.

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.

Gear reduction from a hubmotor:

do not put a hub motor on the front wheel!
Vector Control does not add horse power, it can only move it to when and where you need it. Such as when you need extra thrust, like climbing steep hills with cargo or starting off from a dead stop at a traffic light.

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