Electric bicycles

by Chris Woodford. Last updated: August 4, 2011.

Simple, convenient, cheap, and economical—bicycles are one of the world's favorite forms of transportation. But they're not for everyone. They can be hard to pedal up and down hills or with heavy loads, and elderly or disabled people may find them impossible to manage. In the last few years, a new generation of electric bicycles has begun to revolutionize our idea of environmentally friendly transportation. These new cycles have all the convenience of cars with all the simple economy of ordinary cycles. Let's take a closer look at how they work.

Photo: A typical electric bicycle parked at a railroad station. Electric bikes are heavier and more cumbersome than ordinary cycles, so it's harder to carry them on trains, buses, and so on on.

The basic concept of the electric bike

Photo: This eZee Forte electric bicycle has a range of up to 48 km (30 miles) and a top speed of around 24 km/h (15 mph). Picture by kind permission and courtesy of 50 Cycles Electric Bikes.

If you have dynamo-powered bicycle lights, you already own an electric-powered bicycle! Consider: as you pump your legs up and down on the pedals, you make the wheels rotate. A small dynamo (generator) mounted on the rear wheel produces a tiny current of electricity that keeps your back safety lamp lit in the dark. Now suppose you could run this process backward. What if you removed the lamp and replaced it with a large battery. The battery would kick out a steady electric current, driving the dynamo in reverse so that it spun around like an electric motor. As the dynamo/motor turned, it would rotate the tire and make the bike go along without any help from your pedaling. Hey presto: an electric bike! It may sound a bit far-fetched, but this is more or less exactly how electric bikes work.

Key parts of an electric bike

There are four key parts to an electric bike: the batteries, the motor, the sturdy frame and spokes, and the brakes.

Batteries

The batteries are the most important parts of the bike, because (if you don't do any pedaling) they contain all the power that will drive you along. Typical electric bike batteries make about 350- 500 W of power (that's about 35-50 volts and 10 amps), which is about a quarter as much as you need to drive an electric toaster. In theory, you could use any kind of battery on a bicycle. In practice, however, you want to use something that stores lots of power without being too heavy—or you'll be using half your power just moving the battery along! Lightweight lithium-ion batteries, similar to those used in laptop computers, mobile (cellular) phones, and MP3 players, are now the most popular choice, though they're more expensive than older rechargeable battery technologies such as nickel-cadmium ("nicad"). Typical batteries will give your bicycle a range of 10-40 miles between charges (depending on the terrain) and a top speed of 10-20 mph (which is about the maximum most countries allow for these vehicles by law). You can extend the range by pedaling or free-wheeling some of the time.

Photo: Electric bicycles give themselves away with their large battery packs, usually mounted somewhere on the frame between the wheels.

Electric motor

In the theoretical electric bike we considered up above, we had the dynamo/motor driving the back wheel directly, simply by pressing on the tire. Most electric bikes work a different way. They have compact electric motors built into the hub of the back or front wheel (or mounted in the center of the bike and connected to the pedal sprocket). Take a look at the hub of an electric bike and probably you'll see it's much fatter and bulkier than on a normal bike. You can read more about how these motors work in our main article about hub motors.

Photo: Left: The hub motor of an electric bike. Note the thick copper coils of wire that convert electric power from the battery into the movement that pushes you along. Picture by courtesy of Fabian Rodriguez, published on Flickr under a Creative Commons Attribution 2.0 License. Right: Usually, the coils are not exposed and the hub motor looks much like an ordinary hub, only somewhat more bulky.

Frame

The frame of an electric bike also has to be slightly different. The main part of the frame (the bit that supports your weight) is usually made from lightweight aluminum alloy: the lighter the frame, the lighter the weight of the bike overall, and the further it can travel before you need to recharge the batteries. The spokes on the wheel also have to be stronger than the thin spokes on a traditional bicycle. That's because the electric motor in the hub spins the wheel with a lot of turning force (known as torque) and, if the spokes were ordinary lightweight ones, they could bend or buckle.

Brakes

Some electric bikes claim to use a neat trick called regenerative braking. If you start pedaling the bicycle or going downhill, the spinning wheels turn the electric motor in the hub in reverse and start charging up the batteries. In practice, regenerative braking is nowhere near as useful on an electric bicycle as it is on an electric train or car. You'd have to go down an awful lot of hills to charge up the batteries completely and that's usually not practical. And what's the point in pedaling the wheels simply to charge the battery? You might as well have bought an ordinary bicycle to start with!

How environmentally friendly are electric bikes?

There's no question that electric bikes are far better for the environment than petrol-powered car engines. But that doesn't mean they're completely perfect. Making and disposing of batteries can be very polluting. Not only that, but an electric bicycle is still using energy that has to come from somewhere. You may think you're using clean green power, but the electricity you use for getting about might have come from a filthy old, coal-fired power plant or one driven by nuclear energy. (If you're lucky, of course, it might have come from solar panels or a wind turbine!) Electric bikes are nowhere near as environmentally friendly as ordinary push bikes, but nothing is ever perfect. Electric bikes are certainly a step in the right direction. If everyone used them to get about instead of cars, global warming might be less of a problem, and the world would be a far cleaner and healthier place!

Photo: Could solar power be the future of electric bicycles? The large solar panel mounted over the back wheel of this experimental bike powers an electric motor connected by a chain drive to the back wheel, helping the rider when he doesn't feel like pedaling. Using clean, green solar power would remove the problem of having to charge electric bikes with electricity generated from fossil fuels—and help to extend their range significantly. Photo by Warren Gretz courtesy of US DOE/NREL

Further reading

On this website

• Bicycles
• Electric and hybrid cars
• Fuel cells
• Hub motors
• Lithium-ion batteries
• Regenerative brakes

Books

• Build Your Own Electric Bicycle by Matthew Slinn. TAB Green Guru Guides/McGraw-Hill, 2010. Introduces electric bikes and their benefits, discusses safety and legal issues, then goes on to explain how to build a bike with a hub motor kit. Also covers repair, maintenance, and more advanced projects.
• Electric Bicycles by David Henshaw and Richard Peace. Excellent Books, 2010. Introduces electric bikes, describes their history, and explains how to choose and use a bike of your own.
• Electric Bicycles: A Guide to Design and Use by William C. Morchin and Henry Oman. Electric Bicycle Manual, 2006. A basic introductory brochure.

Articles

• Town hosts electric bikes rally: BBC News, 7 May 2006. The Welsh town of Presteigne celebrates with the UK's first ever electric bike rally.
• China's march towards green revolution by Roger Harrabin, BBC News, 22 October 2009. Explains why the growth in battery-electric scooters is good for the environment, even if the electricity does come from coal.