Friday, October 29, 2010

Motor-pedal hybrid design

More about the laws

velomobile cgi

Mid-drives / Zero Trail Steering / Variable Speed Crank / Tilting trike technology

The most important parts

Hubs with cartridge bearings, the idiot bicycle industry has virtually stopped making bearings with loose balls. And the last one I bought no longer has parts available; like the cones! (never buy Shimano hubs)

Wheels built with rigid spoke configuration, when spokes move due to looseness they are guaranteed to break. Heavy duty wheels

Hydraulic disc brakes, it will take you much longer to stop in the rain with so much weight that you need a motor. A double hydraulic disc brake is the only way to go for 600 to 800lbs.

Proper gear reduction for enough torque thrust to keep the motor form over heating. Using a 20 inch drive tire will make this much easier to achieve. And cheaper to find a heavy duty tire (16” moped is 20”O.D.)

How to power a heavy Velomobile or Cycle-truck.

Velomobiles can get a get an efficiency of 40 miles to 85 miles per kilo watt hour of electricity. While it is easy to build a Vehicle light enough to get the really high mileage number at a slow speed on flat land, try thinking about how to power one that will hold two adults and 80lbs of cargo. Or a cycle-truck that will not be heavier than 600lbs (total combined weight) that will be driven up a 9% grade. We have worse hills but they are avoidable. If you think that is bad do the numbers on the new electric cars! 3.7 per kWh is high mileage.

I think the main reason that there is not much knowledge about this kind of vehicle is no one knows how to build one.

Most people build only for speed, unfortunately if you’re in a wreck and you get blamed for it the cops have to send your vehicle to be checked out by a technician, to make sure it cannot be driven faster than the speed limit for this kind of vehicle. So your best bet is to have it registered as a moped, or a motor-cycle. Using only one motor you could gear the motor down to the 30 mph limit, if it is still powerful enough to climb your steepest hills.

An option you might consider is using a multiple Kw capable motor and using a programmable controller to keep the total power running through the motor within legal limits but still allow multiplying up the phase amps for torque when hill climbing in the motors lower end.

For example let’s say you used a 4.5-Kw rated 72V high power scooter motor. Normally you would use a controller with such a motor that would allow for motor phase amps to reach nearly 200-Amps with a battery side Amp limit of 65-Amps or so. You would end up with an electric motorcycle that could really move.

For a lower total power build, within e-bike legal limits, you still use the same motor and still use a large powerful controller that can handle phase amp multiplication. One that goes all the way up to the that 200-Amp level when the motor gets bogged down when climbing a hill, to get the full torque. But if you program in a much lower Amp limit on the battery side, thus ensuring you stay within legal total power limits, you still get the bottom end torque. Also you don't need a great big battery that can handle sustained 65-Amps pull on the battery end.

My state the legal power limit, for it to still be a Moped bicycle by the letter of the law is "2-brake-horsepower maximum" which is equivalent to 1.49-Kw physical output power. "Brake-horsepower" defines a motors physical output power not the electric power input. Thus in my situation I would examine the motors performance graphs and figure out what battery-side amp limit I should program in, to ensure that I stayed within that limit (knowing the general performance curves of such motors on a 72V system that would probably end up being about a 25-to-30 Amp limit on the battery side).

Basically use a much bigger motor and controller and just dial down the Amp limit on the battery side and thus tapper off the middle to the top band of the motors performance but still keep all, or nearly all, of the high torque capability of the bigger motor on its bottom end.

You do not want not to add speed to a 600lb vehicle. One of these motors can pop a bicycle drive chain easily. You will need a “Cycle Annalist” to help control the vehicle. And a controller that will read the Cycle Annalist. And at 600lbs with a speed of 30 mph, you will probably need hydraulic disc brakes. And I do not mean the small kind made for simple bicycles.

I calculated the power it would take to climb some our steep hills. There are other calculators on the internet, but they use metric.

Efficiency pretty much doesn't change down to 80% max speed, may fall off some at 60% max, and may for certain motors fall off up to 10% at 30% max speed. That means if the bike can go 25 mph it may at most take a 10% efficiency hit if ridden at 7.5 mph (without pedaling). It also means that if the bike can go 40mph, it will take about a 5% efficiency hit if ridden at 20 mph.  36V 20A at 10 kph. Efficiency about 42%. Switch to 11V custom battery. Now at 10 kph, efficiency is about 72%.

Lower voltage systems are safer, have easier BMS requirements (less channels), less complexity, more compatibility (with DC converters and other components), and easier on electrical components

Bigger motors are not always better. the most important parameter to optimize in a motion system is torque. If you have an application that requires high torque at slow speed, a gear reduction of some sort can sometimes dramatically reduce the motor size or increase the motor's efficiency. If you need high speed at low torque, a large motor can have excessive iron loss. This will manifest itself as a high no-load current. If you notice that the no-load current goes up dramatically with speed, then the motor probably has a lot of eddy current loss. If the no-load current remains the same over its RPM range, the iron loss is mostly attributable to hysteresis drag torque. Knowing what components make up the iron loss is important because it can point you in two different directions: 1. reduce the frequency (RPM) of the motor to reduce the eddy current loss or, 2. reduce the size of the motor to reduce the hysteresis drag. 

A well designed motor and proper gearing would eliminate heat as a problem... my current Unite motor never gets hot because I downshift when it's in that "bad heat" zone and so it never needs to do "bad things" in "bad situations". People who use fixed gears deal with heat because.... well... because you have no gears to prevent heat.

The majority of the heat that can damage the motor and controller comes from the amps of current. By choosing a system that has higher volts, you can achieve the same amount of power with fewer amps.

use this simulator to figure it out:

converting a hub motor to mid drive motor:

something like this could be built with pedals.
Links to Enineering tool box. com:

Gear calculations

Bicycle gears

Pulley diameters and speed

 First stage gear reduction:

Motor-pedal hybrid design

If you live with steep hills and want to drive a more than the usual bicycle-weight up them, you really need to put some thought into how to motorize the machine. Some people think that all e bikes do not last long, because most of them are designed for light weight duty on virtually flat land.

Hub motors are the most efficient, but not for this scenario. Climbing hills at legal speeds up steep hills with only one gear, is hard on a motor and controller. Even if electric motors do have a much wider range of ability than gasoline motors.

So most people build for momentum, that takes calculation. Or just a reasonably low gear on the drive wheel, and possibly a larger than legal motor.

The federal government says that one horsepower should be the legal limit, but in the state of Washington 1.2 hp is OK. Most people I know, chant the phrase “the cops don't care”. But when I asked the cops in Seattle they told me how they do care, and how they deal with it.

In Europe the cops are allowed to have road blocks and do any thing they want to keep the people from using a motor bigger than one-third horse power. That is almost impossible to drive even one person, with out cargo, up a steep hill.

It is possible to use more than one motor, see the white trike car above with two or three hub motors.

Geared hub motors are available in 5-1 gear reductions or less. When I calculated for a 30 mile range velomobile, I had to use a 6-1 gear reduction because lead acid batteries are very heavy. Better to use lithium. The price works out to be a little less than lead acid over the life of the battery.

geared hub motor

If it is true that electric motors must spin near peak efficiency RPM to get full horse power and that you need a very large gear reduction to allow the motor to spin that fast and still keep the vehicle down to a legal speed. Then most e-bikes are illegal.

For high torque at low speeds you'll need a motor and controller that can allow high amperage at slow hill climbing speed. If you get a programmable controller you can limit the power to legal levels.

Or just use a vary large gear reduction for the high speed motors that run cooler than slow speed motors. like the Astroflight motors

DaVinci drive unit

This is the best motor you can get for a human/ power hybrid:

Most electric motor systems will have a power/torque curve which looks similar to this:

You then ask two questions:
Is acceleration in constant torque region enough for every use of the vehicle?
Is constant power region and max rpm enough to give an adequate top speed?

If the answer to both questions is yes, then a 1 gear transmission is enough. Otherwise, the vehicle can benefit from gears. Then you get to choose whether to use a gearbox or to change the electrical system. 


“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.”

The ‘Cycle Analyst’ maybe able to control the speed with out eliminating the slow speed amps. But there may still be a chance you could burn out your power system if you run too many amps through it at too low of a speed. A heat sensor could be good.                          

If 3 to 7 years life span is long for a big hub motor, then forget them.

(For Life expectancy, hub motors could work 5000 hrs without damage in Lab.
In real life it depends on the motor's usage (e.g. hitting bumps and water getting in).

You can make a good heavy-duty motor like the brushed Mars 909 last twenty years if you do not over power them with a cheap controller.

  One other thing 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. 

How to calculate a fixed gear 

 First measure the average grade of your worst hill. Use a carpenter level and a metric or 'tenths of inch' ruler.

And divide the span by the rise to get the percent of grade.

Decide what the total combined weight of your machine will be. Include every thing!

Then use this calculator to find how much energy it takes to climb your worst hill, as a 'streamlined trike'.

Find a gear ratio that will bring the drive wheel RPM down close to the legal speed limit (or what ever you can live with) when at the wattage you need to climb the nasty hills. See graphs provided by manufacturer. (this motor should have enough torque for any hill)

Decide what you think maybe a good sized pulley for the motor, you may have to rethink the size later. Divide the diameter into the size of a larger pulley on the drive wheel.

use this simulator to figure it out:

When using a rim sheave the belt may slip! Especially in wet weather! Unless you add something inside the rim like matte textured electrical tape or ridges some how.

To use a Gates power grip belt 8mm pitch GT3 down load the manual and rack your own brains to find the parts to fit together to get a close ratio to slow your drive-wheel down far enough! It will be easier to find a large belt pulley for a 20” wheel than any larger wheel. 

Look for a 

"One-Way Locking  Bearing" 

to make a ratcheting pulley.

"One-Way Locking  Bearing" 

Trapezoidal or square drive Timing Belts??

A thin section belt may be the best, whereas the pitch change at the periphery would be quite small

This should be ratcheting so it will eliminate the drag of the motor when just pedaling 

Use gates power grip GT3 8mm pitch
get the pulley at the same time your finding the length you need.

Trike stability

Delta trikes and quads need a differential to use a single stage reduction; unless you can live with a lopsided traction drive (but that can push the nose of a delta trike to one side on wet hills).

Two-wheel positive traction can be had with a jack shaft, but will need a two-stage reduction. Delta trikes also have a tendency to roll over on a fast turn. The only thing that can be done for that is to weight down the rear wheels and lower the seat, then slow down.

A tadpole trike can spin out of control if the rear wheel is too close to the front and may be forced into a spin by too much weight on the rear; don’t use slick tires. But if the rear wheel is farther back it will need more weight to keep a grip on the road in an emergency stop-turn. In line tandem tadpole trikes probably will not have this problem. But the front wheels of heavy tadpole trikes should be built with the spokes all the same length to handle the lateral forces.

Low seats help keep trikes from flipping over, but they could still use a crash cage even if just to climb out of the seat.

Wheels should have at least 36 14-gauge spokes, 20" wheels with box wall rims are stronger than big wheels, although there have been good results from using the new tubular rims with only 32 spokes for downhill racers (jumping off cliffs).

Tires are a problem for this much weight. High pressure tires are essential but will give you a rough ride. I have a lot of expensive tires split down the middle. Maybe Moped tires?

Brakes are the most important thing

Only hydraulic disc brakes can be synchronized perfectly, but a double lever is not available, except for the Magura BIG brakes:

But you will still need a third brake on the rear wheel with extra-large rotors.

Drum brakes modulate better and give you more control over stopping than cable pull disc brakes, but are available only in cable pull and are no good in the rain.

Most hydraulic systems that are on the market today are dual piston (more powerful systems can have up to eight pistons), though a few entry level brakes operate similar to mechanical brakes with one piston that moves and one that is stationary.

Systems that utilize two or more pistons, provide more braking force, better modulation, and little or no drag and both pads that will retract from your rotor after you apply the brakes. By using hydraulic fluid instead of a cable, there is less drag on the brake system.

Moped laws of Washington state says that you can't have more than
 2 Brake Horsepower

Actually I don't know if it is possible to build one of these that is perfectly legal unless you can keep the weight (450lbs?) and speed down to about so the brakes work like they should.

'Build your own electric motorcycle' by Carl Vogal. I think people who lust for speed should be building electric motorcycles.

When the center of weight is so far forward, it is possible to drop your feet, and have them dragged under the front axial. It is much safer to have the crank behind the front wheels.

Note that the crank should be behind the front wheels. But the front wheels should also be wider apart.

This one could use three hub motors

Dynamic Stability

When a vehicle is said to be dynamically stable it is meant that it reacts safely and predictably under various driving conditions.

When designing a chassis, we can choose how the car will react when turning too fast. One of two things will always happen: either the car wheels will slip relative to the ground, or the vehicle will tip over. Obviously, slipping is the desired outcome. Keep this in mind for the moment.

When the car does slip out of control on a fast turn, we can design it in such a way that we know whether the front or rear wheels will slip first. This is important because if the rear wheels slip first, the vehicle runs the risk of spinning out of control (oversteer). If the front wheels slip first (understeer), you won’t spin out and it is easier to regain control. Understeer is considered a safe dynamic response to slipping in a turn and is designed into all commercial cars. Which wheels will slip first is a simple matter of weight distribution and weight transfer.

The problem for delta vehicles is how to distribute their weight and control their weight transfer during a turn to avoid undesirable outcomes. If you design the weight distribution for a heavy front bias to achieve understeer, you increase the risk of tipping over. If you increase the weight distribution on the rear tires, the vehicle will oversteer in hard turns.

We also need to consider nose diving, which is exactly what it sounds like. When you slam on the brakes as hard as possible, the vehicle will either skid to a halt or the rear wheels will lift off the ground. This is also a function of weight distribution and weight transfer. It would seem that the delta design has an advantage here because it naturally lends itself to having a rear biased weight distribution. But in the real world, a hard stop doesn't always occur when traveling in a straight line. If you stop hard enough while turning with a delta vehicle, the weight will transfer to the front wheel enough (despite suspension designs to prevent this) to cause the vehicle to flip over at an angle.

Keep a good 30% of the vehicle weight on the drive wheel to maintain good traction. (Ideally 33/67 rear weight distribution)

No comments: