How does an electrically-assisted bicycle work?

energy-efficient, bicycle, battery, electric engine, electrical assistance Created nearly two decades ago in Japan, electrically-assisted bicycles allow almost effortless riding. Electric bikes were rapidly successful in countries such as China, where 120 million units were reportedly sold (compared with 300,000 in Japan, 200,000 in the United States and 100,000 in the Netherlands). In France, too, the electric bike market has been growing steadily, from 15,000 sales in 2009 to 40,000 in 2011. How do electrically-assisted bikes actually work? Here is an exhaustive insight into this mature and reliable technology which is too little-known in France.

The engine and battery set makes paddling more energy-efficient

An electrically assisted bicycle is nothing but a regular bike combined with an electric engine and a battery to power it. It includes a technical control device along with a magnetic sensor and sometimes a handlebar-command unit. When the rider paddles, the magnetic sensor informs the control device which instantly calculates the power the engine will need to supply to provide adequate electrical assistance. The magnetic sensor consists of a round magnet located on the bottom bracket spindle and a sensor cell located on the frame. The engine starts to assist paddling as soon as the sensor cell recognizes the magnet, that is for each spin of the wheel.

European law has narrowly defined the energy capacity of an electrical bike: the engine alone must not set the vehicle in movement. That is why engines get started only when wheels are in motion and brakes are not being used. The engine’s maximum power is 250 watts, which corresponds to the energy a person weighing 80kg must spend to reach a speed of 32 km/h on flat ground. Concretely, experts say that users « truly experience a much easier ride », especially uphill or when starting up.

Technical aspects

Electrically- assisted bikes can rely on a number of technologies:

  • Bracket-based assistance: it is the cheapest and thus the most common system. As soon as the wheel spins, the magnetic sensor perceives the movement and triggers the engine.
  • Pressure -based assistance: as soon as pressure is exerted on the paddle, the engine starts. The more intense the effort, the more the engine assists the rider.
  • Effort-based assistance: engine assistance depends on the pace of the paddling: the faster the paddling, the harder the engine works.
  • Chain-based assistance: thanks to this system, the engine is very responsive as it starts as soon as the chain is tensed, that is as soon as the rider sets foot on the paddle.

It is generally easier to install the engine in the front, which also provides for better balance (engine in the front, battery in the back). The engine can be installed in the hub of either wheel, thus replacing the wheel spindle. It can also be located elsewhere: in this case, transmission will occur through a chain or a belt connecting a chain ring, located on either wheel spindle, with the engine. Another system is a “paddle-engine”, i.e. a single unit located on the bottom bracket spindle. This technique makes the whole system lighter as the engine, the sensor and the control device are all located on the same spot.

Whatever the technology, electrically-assisted bikes require a battery able to provide the electrical power needed to assist paddling. There are several sorts of batteries, relying on metals and technologies similar to those used in some electric cars. Lead batteries are the most common but are quite heavy (4 to 5 kg). They are rather energy-efficient, but their limited power is the reason why they are less and less used in Europe, along with the fact that their efficiency decreases at sub-zero temperatures.

Nickel batteries are lighter and offer a higher maximum power. Contrary to lead batteries whose lifetime is equal to 350 charge-discharge cycles, nickel batteries can reach up to 500. The more advanced lithium-ion technology is expensive (300 to 500€) but offers a higher maximum power (150W/h/kg, compared with 90W/h/kg for a nickel battery), which is why a lot of recent electric bikes are fitted with such batteries. The cost of the battery, which is the most expensive element of the bicycle, directly determines its price; today this is the main obstacle to its development, although the technology is mature (price from 1000 to 2500€ on the European market).

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