Introduction: Engineering a Power System for Heavy Cycle-trucks and Velomobiles
Heavy utility cycles will become a very large part of transportation as people wake up to the reality of climate change. It is not easy to understand how to provide enough low-speed torque thrust that is needed to climb steep hills at legal speeds. Bigger motors are not the best solution. Proper gear to power matching is much more efficient and economical, especially in terms of mileage.
Many people confuse torque with power. They are different terms. Motors can have high torque-thrust at slow hill climbing speed, or low torque-thrust at a high top speed and yet have similar power output.
Step 1: Determine Your Hill's Steepness
Measure the steepness of the worst hill that you cannot avoid climbing. Use a level and a ruler (metric or decimal), then divide rise into the span to get the grade percentage.
Step 2: Calculate Your Total Combined Weight
Total combined weight, plan for at least 400-500lbs (181-226 kg). If you have a cycle-truck or a velomobile that can hold two people, think about at least 600-800lbs (272-362 kg).
a.) Find a power calculator or find a graph for your motor, like the bicycle speed calculator at kreuzotter.de that has a setting for Velomobiles. http://www.kreuzotter.de/english/espeed.htm
b.) Then a gear ratio calculator, like the motor and gear ratio calculator at Electric Scooter Parts Support Center.
c.) Take the RPM speed that your motor will produce at the wattage you will need (see a performance graph of the motor). Calculate a gear reduction ratio you will need to reduce the speed of the motor to get the maximum thrust. Subtract a little speed (1-2 mph or 1.6-3.2 kph) for the lack of accuracy.
Find the peak efficiency R.P.M.s and wattage on the label. When a motor is pushed past the peak efficiency limits too long, it can over heat. Brushless motors can produce about twice the power on their label for a few minutes without over heating too much; to get past the steepest part of the hill.
Something else to consider is that if your motor nameplate says 1200W, that's the electric power it draws. Its actual output will be less, possibly considerably less depending on the motor design. Yet brushless motors in general can use twice the nominal wattage.
In normal mode, or the constant torque region, the motor exerts constant torque (rated torque) over the entire speed range until the rated speed is reached. Once past the rated RPM speed of the motor, the torque will decrease proportionally with speed, resulting in a constant power (rated power) output. The constant power region eventually degrades at high speeds, in which the torque decreases proportionally with the square of the speed.
Step 3: Choose the Best Way to Power-assist Your Human Powered Hybrid Vehicle:
Mid-Drives are very good for low wattage motors. But over 500 watts can put too much stress on bicycle drive chains, especially if it is a narrow chain for more than 7 speeds. BMX single speed chains are the best if you don't want to use a motorcycle chain on the other side of the drive wheel. And slow acceleration is easier on the chain.
Direct drive hub-motors are set at a very high gear, therefore are not good for hard acceleration like in stop-n-go city driving. Even if they are Brushless motors and can be pushed past the peak efficiency wattage for a few minutes, they need more ventilation.
Geared hub-motors are not available in more than a 1:5 gear reduction. Hub-motors can obtain full thrust at a slower speed than most other motors. Most hub-motors do not ventilate well, so they have heat problems when using high amperage while climbing hills in a gear that is too high.
High speed high voltage motors are better than slower motors because they do not build up so much heat. And are more efficient than hub motors, even with the large amount of gear reduction needed to produce the amount of thrust needed.
Two speed transmissions are all the gear ranges needed for electric motors, unless you want a very wide range of speed, or if your using a high speed motor that does not have enough low speed thrust. ie: your government may not allow powerful enough motors for your needs. If you are allowed all the power you will need, but just need to cut it off at the legal speed, that can be done with the controller. If you live in very steep hill country and you're going to be moving a lott more weight, you may want three gears.
Two motors in tandem can pump-up the output thrust without increasing the overall speed that is set by the fixed gear ratio. A controller for each motor is needed but then you can program them to power-up the extra motors when needed. And both controllers can be wired into one accelerator.
What are the advantages? Electric motors loose thrust ability when climbing hills at a slow speed; if you want your high speed gear-ratio to be set at about 20mph and you need a 10mph gear ratio to get enough thrust to climb your hill with only one motor, a second or third should provide enough thrust at 20mph without having to use a larger motor. The bigger motor would take more amperage to keep running at a high speed when cruising on flatter land, where the extra power is not needed. Also motors made for high speed are more efficient than slow speed motors; less energy is turned into heat.
It would be possible to use a gearless hub motor for a cruising gear with a mid-mounted motor with a large gear reduction for hill climbing.
Two Separate Reduction Ratios
I like the idea of using two motors separately. One with a large gear ratio reduction and the other with a higher speed reduction. Then the controllers set to turn on the slower gear reduction when needed and the other to turn off. Making it a two speed machine with the same legal wattage.
Vector Control (field oriented control) is a better way to make motors produce more thrust at lower and higher speeds than using gears. With controllers having more computing power now, they can control a motor much better than ever before. It requires a three-phase AC motor and a more expensive controller. These would be very good for a motorcycle that needs a wider range of torque-thrust than a simple electric bicycle.
Increasing the number of phases above three allows the stator MMF's to be shaped so that the motor produces significantly greater torque. http://commutercycling.blogspot.com/2016/05/vector-control.html
E-bike laws should be changed to reflect the gear speed so that you can still have enough power to climb your worst hills. i.e. Washington state allows only about 1000 watts for a power assisted bicycle and 1500 watts for a moped. Well if you want to move 600lbs (272 kg) up an 8% grade you are going to need a gear ratio that will drive at about 12mph (19 kph) at the r.p.m. that your motor will produce at that 1500 watts.
One reason the laws don't let you have more than “a given power output”, is because even if you have a controller set to turn off at a given speed, the sensor can be by-passed. And it is very unlikely that a law restricting the bike to the use of controllers that do not work without speed sensors, will be functional in the USA.
Some states require that the crank be powered as well as the drive wheel. And others require a torque sensor on the cranks. This is probably the only way to restrict the speed that cannot be bypassed easily. But this would also need to be examined by an “engineer”.
Step 4: Brakes Are the Most Important Part.
If they are not strong enough to over power the motor, you can get into worse trouble than you want. Double cylinder hydraulic disc brakes should be used if you are going to be moving 600-800lbs (272-362 kg) faster than walking speeds, or down a steep hill.
Step 5: Leverage
There are a lot of people that think gears waste enegy. Boy are they wrong! Direct drive hub motors waste way too much energy just getting up to speed. So they are no good for stop and start city driving and for hill climbing.
Vector control: “field oriented control of permanent magnet motors” on youtube:
This cannot add horse power, it can only put it where you need it, like at the slow end so that you have more thrust when starting off from a stop light or when hill climbing with cargo.
A 2 speed gear box: https://www.youtube.com/watch?v=ZTIpMyUn8Qk
Choosing a battery: https://endless-sphere.com/w/index.php/Choosing_a...
Read more on my non-profit blog: Utility Cycling Technology
My facebook page about utility cycling technology: https://www.facebook.com/utilitybikeproject/
How gears work:
Hub Motors for Cargo bikes? http://commutercycling.blogspot.com/2016/10/blog-post_24.html
Step 6: Excellent Examples
Step 7: State E-bike Laws Must Change!
Washington state e-bike laws are not adequate for heavy duty cargo bicycles.
The maximum power allowed for a motorized bicycle is 1500 watts at the drive axle. The motor will have to draw more than that.
1500 watts will move 600lbs (total combined weight) up an 8% grade at about 13mph if using the right gear. Unfortunately most people think they need as much speed as they can get. So if a moped bike had a gear box with at least two gears, there would not be a problem.
But if you want to use a fixed gear drive the 13mph is the maximum speed the vehicle would drive at. So the laws should reflect the amount of gear reduction your vehicle has; ie: for 20mph you need about 2200 watts to move that 600lbs up an 8% grade.
The wide torque band of electric motors, particularly the torque available at low RPM, eliminates the need for more than one gear.
Set the single fixed reduction gear at the highest speed that the available power will still be capable of driving your fully loaded vehicle up your steepest hill. Then if you still need a faster gear for less strenuous terrain, you can think about using a two speed gear box.
However it would be easier to just use a gear box, not to exceed 20 or 30mph. And have your vehicle examined for stop-ability with a total combined weight of 800lbs.
The real problem is people's insane lust for speed. They want to be able to turn their bicycles into illegal motorcycles. Well there are better ways to kill your self.