Controllers for brushless motors: E-bikes require high initial torque and therefore models that use brushless motors typically have Hall sensor commutation for speed and angle measurement. An electronic controller provides assistance as a function of the sensor inputs, the vehicle speed and the required force. The controllers generally allow input by means of potentiometer or Hall Effect twist grip (or thumb-operated lever throttle), closed-loop speed control for precise speed regulation, protection logic for over-voltage, over-current and thermal protection. Bikes with a pedal assist function typically have a disc on the crank shaft featuring a ring of magnets coupled with a Hall sensor giving rise to a series of pulses, the frequency of which is proportional to pedaling speed. The controller uses pulse width modulation to regulate the power to the motor. Sometimes support is provided for regenerative braking but infrequent braking and the low mass of bicycles limits recovered energy. An implementation is described in an application note for a 200 W, 24 V Brushless DC (BLDC) motor.
Another type of electric assist motor, often referred to as the mid-drive system, is increasing in popularity. With this system, the electric motor is not built into the wheel but is usually mounted near (often under) the bottom bracket shell. In more typical configurations, a cog or wheel on the motor drives a belt or chain that engages with a pulley or sprocket fixed to one of the arms of the bicycle's crankset. Thus the propulsion is provided at the pedals rather than at the wheel, being eventually applied to the wheel via the bicycle's standard drive train.
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—and, as people often say, "the perfect is the enemy of the good." 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: This typical electric bicycle, a Sanyo Eneloop (now discontinued), had a range of about 30–55 km (17–35 miles) and a top speed of around 24 km/h (15 mph). Note the 250-watt hub motor on the front wheel and the 5.7Ah lithium-ion battery pack (black, marked "Sanyo," just in front of the back wheel). Picture by kind permission and courtesy of Richard Masoner, originally published on Flickr under a Creative Commons (CC BY-SA 2.0) licence.
At first glance, Wing’s e-bikes share some design features with Dutch e-bike company VanMoof’s flagship bikes, most noticeably the elongated top tube (23.3 inches) with embedded front and rear lights. There are differences — VanMoof’s battery is embedded in the frame, while Wing’s is external — but to look at them side-by-side, one could easily conclude that Wing is just a less-expensive version of the VanMoof.
Connect the battery to the speed controller and the throttle. Follow the instructions included with the kit to connect each part. Typically, you’ll only need to plug the connector on the speed controller into the connector on the battery, then repeat the process for the throttle. Be sure not to touch the battery wires together, as this could create a dangerous spark! 
(15.5) "Electric assisted bicycle" means a device with two or three wheels which has a saddle and fully operative pedals for human propulsion and also has an electric motor. For such a device to be considered an electric assisted bicycle, it shall meet the requirements of the Federal Motor Vehicle Safety Standards, as set forth in 49 C.F.R. Section 571, et seq., and shall operate in such a manner that the electric motor disengages or ceases to function when the brakes are applied. The electric motor in an electric assisted bicycle shall:
Electric Bicycles are defined by the California Vehicle Code . In summary, electric bicycles are to be operated like conventional bicycles in California. There are several exceptions to this. A person must be at least 16 years old, and anyone riding an electric bicycle must wear a bicycle helmet. The e-bikes must have an electric motor that has a power output less than 1,000 watts, is incapable of propelling the device at a speed of more than 20 miles per hour on level ground, is incapable of further increasing the speed of the device when human power is used to propel the motorized bicycle faster than 20 miles per hour, operates in a manner so that the electric motor is disengaged or ceases to function when the brakes are applied, or operates in a manner such that the motor is engaged through a switch or mechanism that, when released, will cause the electric motor to disengage or cease to function.
The federal Consumer Product Safety Act defines a "low speed electric bicycle" as a two or three wheeled vehicle with fully operable pedals, a top speed when powered solely by the motor under 20 mph (32 km/h) and an electric motor that produces less than 750 W (1.01 hp). The Act authorizes the Consumer Product Safety Commission to protect people who ride low-speed electric vehicles by issuing necessary safety regulations. The rules for e-bikes on public roads, sidewalks, and pathways are under state jurisdiction, and vary.
As electric bike options continue to expand, more brands are integrating the battery more seamlessly. That makes them look sleeker (and more like a real bike). Batteries are expensive, so make sure there’s a good way to lock the battery to your bike if you’ll be keeping it outside. Overall weight is important. Some battery and motors can add 15 pounds or more to the bike. With assist, you won’t feel that much when you’re riding, but you will if you have to carry your bike up stairs or lift it onto a bike rack.
Some of the Rad Wagon’s (small) flaws became apparent once I added more weight to the bike. The integrated rear rack can attach various panniers, platforms, or baskets to cary your cargo, but I was most concerned with hauling my two kiddos to school. Riding the bike with my three year old was a cinch; he held on to the bars in the caboose (available as an accessory add-on) and enjoyed the view. Adding my very tall, almost seven-year-old daughter, however, was a bit more complicated.
Perhaps the cleverest thing of all about the Electric Brompton is that despite all the extra stuff, it folds up exactly the same as the non-powered variety. It's so simple, and unlike certain folding bikes we could mention, what you're left with is a genuinely small thing, rather than something that's about the size of a bike with the front wheel taken off.