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The electric bicycle is a new class of vehicle, combining the best qualities of bicycles with the mobility of motorized vehicles. On an electric bicycle, an electric motor spins a bicycle wheel in addition to human pedal power. The human controls how fast the motor spins by twisting a throttle or pressing a button. Some electric bicycles are powerful enough to propel the bicycle on their own power, without human pedaling. The energy used by the motor is stored in batteries which are charged by plugging into a wall outlet through a charger.

Electric bicycles use an electric motor to spin a bicycle wheel. The power for the electric motor comes from batteries and is regulated by a motor controller (otherwise, the motor would be stuck full on all the time!). The rider tells the controller how much power they want to deliver using a throttle or “pedelec” sensor (which senses how hard the rider is pedaling).

There are five major parts in the electric drive train:

1. electric motor
2. transmission between the motor and the wheel
3. motor controller
4. throttle
5. batteries

Electric Motors

Electric motors come in many types but for electric bikes only a few are common. There are two methods of “commution” (basically, how do you get it to spin when you send electricity through it):

• brushed DC: cheap and rock solid but inefficient (50%-80%) and heavy, also low end power is limited because of electrical characteristics
• brushless DC: these have 3 “phases” that need to be activated in the correct sequence for the motor to spin with the benefit of higher efficiency (60-95%) but also slightly less reliability because some motors tend to have problems with the phase sensors

The efficiency numbers are vague because efficiency varies with speed and other factors, but generally you will get more range out of a brushless powered vehicle than a brushed one with all else equal.

Transmission

With that out of the way, one of the most important things to consider is how to link the motor to the wheel(s). This is very much up to personal preference and your requirements.

	Hub motors have the motor built into the hub (the center part) of a bicycle wheel Gearless hub motors are very heavy (15lbs+) but usually built to be reliable and sturdy Geared hub motors get more power out of a smaller package (7lbs+) but are susceptible to having the gears wear out; they are also more expensive
	Chain and belt drive motors link to the wheel via a chain and possibly some gears in between, much like your legs connect to the wheel on a regular bike; the upside is you can choose your gearing for speed or torque and change your mind later, and the parts tend to be of higher quality; the downside is added complexity (most common types are motor connect straight to the wheel or a bottom bracket drive where the motor is connected to the pedals)
	Bottom bracket drives connect to the bike’s pedals. Freewheeling cranks and a freewheel on the motor are necessary to keep the motor from turning the pedals when the rider is not pedaling and to keep the rider from using energy to turn a motor that is off
	Outrunner motors are the latest innovation in ebikes: massive amounts of power (think ~4hp) in a light weight package with the major drawbacks of high complexity and cost
	Friction drives are old school: ineffecient and not very good for your tires, anyone selling these at this point is probably trying to clear out old stock

Battery

There are five characteristics to watch for with batteries:

• capacity (measured in volts and amp-hours or, when multipled together, Watt-hours): how much juice the batteries store. This is usually measured at the “C20″ rate, or how much capacity is available if the battery is discharged over 20 hours, which gives optimistic numbers for electric vehicles
• calendar life is how fast the batteries degrade over time just sitting on the shelf, to 80% of their original capacity
• cycle life is how many times a battery can be discharged and then charged again before its capacity drops to 80% of its original capacity
• discharge rate is how fast a battery can be discharged. This is usually written as a multiplier of ‘C’, the battery’s capacity. So if a 7 Amp hour battery is being discharged at 2C, then 14 Amps is being drawn from the battery
• price: since they are so expensive, you have to be smart about accounting for the above factors. Often times marketing people will spout one of the metrics and price without telling you that the battery has a short cycle life or abysmal discharge rate (so you have buy a lot of them to get good enough performance)

Batteries come in several different chemistries (the chemicals inside that allow them to store energy). If you want the kitchen sink, click here.

• Lithium ion batteries come in many types; uninformed news media like to talk about battery fires but only certain types have this problem. These are the state of the art, have high discharge rates, low weight, but fairly high price and a 3-5 year calendar life
• Nickel Metal Hydride batteries are reliable but don’t perform as well as lithium ion, although they have a better calendar life. They cost about the same as lithium ion, so unless you get a deal on them they’re usually not worth it.
• Nickel Cadmium batteries were used in older power tools. They have similar performance to Nickel Metal Hydride.  They can last a very long time if properly maintained.
• Lead Acid batteries are very cheap but also extremely heavy. They have limited cycle life and poor performance.

One often overlooked characteristic is packaging. The Tesla Roadster’s 6800 cylindrical batteries are more of a headache than several large prismatic (box shaped) cells.

• Cylindrical: AA, Sub-C, 18650, 28650, etc. These are round batteries that are often packaged together inside another container. Power tool batteries usually contain 5+ cylindrical batteries put together in a plastic box.
• Prismatic: Box shaped batteries. Lead acid batteries often come in this form (just think of a car battery or a 9V battery) as do many mass produced NiMH batteries (like the ones in hybrid cars)

Batteries capacity is often over stated.  Actually, let’s be honest.  Battery capacity is usually over stated.  The cheaper the batteries, the greater the overstatement.

Remember, capacity is measured in Watt-hours and is calculated using Volts x Amp-hours.

A typical bike ridden hard uses 30 watt-hours per mile, light to medium use is 15 watt-hours per mile. Since a typical pack has about 250 watt-hours of capacity (and you never want to go all the way down to zero), it’s going to last about 7 miles with a lead foot. The more aerodynamic your vehicle the better, the more you pedal the better, the slower you accelerate the better. Most places publish numbers assuming a bicycle riding at 5 mph on flat ground. Let’s be honest, no one is going to use a bicycle like that.

Controller

So you have a motor attached to a wheel, you have some batteries, how do you connect the two together? You need a controller so that the batteries don’t just run the motor at 100% all the time.

Controllers need to be matched to the motor type and batteries. They are usually rated to work at a given voltage and have a maximum power rating (in Watts or in amps; if in amps, just multiply the voltage by the amps to get Watts).  To get around legal restrictions on power, you’ll often find that the volts x amps written on the side of the controller don’t match the labeled power.

• Brushed DC motors and controllers are cheap and usually reliable due to their simplicity
• Brushless DC motors and controllers come in a few types:
  • sensorless: these allow you to control a brushless DC motor with or without “hall” sensors for the 3 phases (whether the motor never came with any or they broke) with the trade off that low speed performance is worse. They are more expensive than sensored, and, as of 2010, they have really awful reliability.
  • sensored: these require that hall sensor wires for the phases are present, reducing reliability and presenting a huge headache if the connectors don’t match. Many of the cheaper controllers don’t work with high speed motors (only low voltage, non-geared hub motors)
  • high quality sensored: these will usually have some fancy options for regenerative braking and will work with all motors with hall sensors

Throttle

There are several different throttle options.

Full twist throttles are like motorcycle throttles; the whole grip twists, and as you twist further, more power is applied
Half twist throttles only allow half the grip to twist; this is quite handy in case you should bump your handlebars against something
Thumb throttles are handy when you don’t want to replace your hand grip; the throttle has a little lever you operate with your thumb
Pedelecs assist your pedaling by sensing how hard you are pedaling

Additionally there are two technologies that make throttles work and most controllers only work with one type of throttle.

• Hall effect throttles are common and cheap; unfortunately many are really cheesy and unreliable
• Resistive type throttles (basically, throttles made by Magura or Curtis) turn a little potentiometer and are very reliable but expensive; sometimes “pot boxes” are used, where the potentiometer is in a box and you can use a standard motorcycle throttle

A well designed electric bicycle travels faster and farther than a human power bicycle but costs a fraction of the price of an automobile to buy and operate. Typical charging cost is less than a few pennies and a new, high quality electric bike costs less than an 10 year old used car.

• Costs less than 1/10th the price of an automobile to buy and operate
• Only reason to visit the gas station: tire pump
• No license, registration, or insurance required
• Don’t need to pay for parking
• Take it on the bus or train (where available)
• Less physical demand on the rider (that means you), so it’s easier to go out for a ride than with a conventional bicycle
• Finally a bicycle commute where you don’t have to change clothes
• Extra power means it’s easy to carry stuff (like groceries)
• Two wheels are more fun (ok, that’s more a matter of opinion)

One great thing about electric assist is that humans get tired very quickly when they have to work hard, so even a small boost will make it much easier to ride a bike.

That said, most electric bikes sold today have weak motors and heavy batteries. The overall benefit to you is negligible or negative.

Unless you are an athlete, your legs are good for about 300 watts of output in short, sweaty bursts. Pedal at half that power and you will be able to ride comfortably for hours but will not get anywhere very quickly. A 1 horsepower electric assist not only makes you as powerful as Lance Armstrong, but you’ll be able to keep riding as long as your batteries have juice and you’ll be much more comfortable. Which brings us to the next point.

If you ride often you will discover that most bicycles aren’t up to the job of daily usage. Most of the United States bicycle industry is focused on recreation, and the expectation is that you won’t ride more than a couple of miles. Cheap bicycles tend to be heavy and uncomfortable. Many of the more expensive ones are tuned for racing, not riding to work or getting groceries. Look for the bicycle that offers useful features such as:

• Layout – make sure the bicycle fits your body shape; for example, you should be able to do full leg extensions when pedaling
• Comfort – seats and handlebars that fit your body
• Cargo – riding with a backpack is not much fun (especially in warm weather), so get a bicycle that can have racks and baskets attached
• Accessories – head lights and tail lights, mirrors, and locks will make your experience safer and more fun

Bicycles are the most efficient form of personal transport available and have the lowest green house gas emissions (Wilson, 2004). Their small size and low weight are crucial and the circular motion of pedaling also uses energy very efficiently. A simple comparison proves this point: compare your top speed when running versus bicycling versus pushing a car down the road. You get the best results on the bicycle for the same amount of energy.

What about CO2 emissions? We all know gasoline powered cars aren’t efficient, but maybe the CO2 emissions from growing and transporting food and then feeding it to a bicyclist are greater than the emissions from refining and transporting petroleum and then burning it in an engine.

Well, the numbers just don’t work out that way. Compare the bicycle versus the car. Automobiles output 30 or more times the CO2 of a bicycle– electric or human powered.  Trucking food and gasoline around are probably comparable in terms of emissions.  Growing food is unlikely to be more energy intensive than refining petroleum– it’s definitely not going to make up the 30x difference!

Carbon Emissions Comparison
Transportation Mode	CO2 Output (lbs/mile)
Bicycle	0.04
Electric Bicycle	0.04*
Segway	0.08
Motorcycle	0.30
Train/Subway	0.60
Jet Airplane	0.97
Car	1.10
SUV	1.57

*estimated

Actually, according to my crude calculations, an electric bicycle is slightl less polluting than a human powered bicycle. I calculated this number based on bicycle power usage and the CO₂ output of a coal fired powerplant. In reality, utility power will probably also come from much greener sources than coal, so the situation for human powered bicycles could be worse. But, I have not factored in transmission losses, etc, so it might also balance out. In any case, the difference is very small– probably about the same CO2 emissions difference as rolling down the window of a car on the freeway.

Details of Estimates

For those looking to run the numbers, the CO₂ output of a coal power plant is 850kg of CO₂ per MWh, which comes out to 1.87 lbs per KWh. A bicycle riding at 20mph needs 300 watts of power, so at 66% efficiency that makes for about 400 watts of power needed at 20mph. It will take 3 minutes, or 1/20th of an hour, to travel one mile, requiring 20 Watt-hours, or 1/50th of a Kilowatt-hour. That gives 0.0374 lbs of CO₂ per mile.

References

Wilson, David Gordon (2004). Bicycling Science (3rd Ed.), MIT Press

DOE (200), Carbon Dioxide Emissions from the Generation of Electric Power in the United States
http://www.eia.doe.gov/cneaf/electricity/page/co2_report/co2report.html

Sightline Institute (2004), Air Travel Heats Up the Planet
http://www.sightline.org/research/energy/res_pubs/rel_air_travel_aug04

Heinzmann, John David, and Taylor, B. Michael (2001), The Role of the Segway PT in Emissions Reduction and Energy Efficiency

http://www.segway.com/downloads/pdfs/emissions_reduction_whitepaper.pdf

Bicycle have been around for two centuries. The first bicycles were wooden “Drasiennes” which lacked pedals and brakes and today there are many different types of bicycles, from folding bicycles to rugged downhill mountain bikes.

Two wheeled wooden “Drasiennes” first appeared in 1817. On a Drasienne, the rider propelled the bicycle by pushing off the ground with their feet. Later, the Drasienne was improved with the addition of pedals fitted to the front wheel (chains were not practical yet) to make the “Bone Shaker,” so named thanks to their solid wooden wheels. Frames started to be made entirely of metal and to achieve higher speeds, the front wheel grew in size, leading to the famous high wheel bicycles. Enlarging the wheel came with a steep price because the bicycles were difficult to mount and turned into human catapults when the wheel fell into a rut or hit a curb.
Better manufacturing techniques allowed chains to appear on bicycles and led to the standard diamond frame layout we are all familiar with. This was called the “safety” bicycle at the time in contrast to the high wheels. Pneumatic tires and brakes made bicycles even more practical and they enjoyed their heydey in the 1890s until they were eclipsed by the automobile in the early 1900s. In the 1930s, recumbents began appearing– and even with 2nd rate riders, winning– bicycle races in Europe so the safety bicycle industry successfully lobbied the bicycle racing governing body, the UCI, to ban them. Bicycle development was stagnant until the 70s and 80s when small manufacturers began introducing new types of bicycles like the mountain bike.
In the late 90s, electric bicycles began to appear but it wasn’t until the 2000s that they started to take off in a big way, especially in Asia.