Much like batteries, there are many different motor and controller systems being used on electric bicycles today. Early and some low-price models use a chain or belt drive and an externally mounted motor. This type of design allows the installation of the motor without having to design a special frame. Most current models use a brushless hub-centric motor mounted into the front or rear wheel. These motors are very efficient, require no maintenance, and often have a reduction gear integrated in them.
Like horsepower in cars, motors are rated in watts—and there are many ways to measure (and manipulate) that rating. Look for bikes that have a continuous wattage rating, it may be lower but it is a more accurate representation of the performance of the motor. Additionally, some motors will use a reduction gear to help boost the performance of the motor. A 250w motor with a reduction gear can perform as well as a 450w non-geared motor.
Electric bikes are available in both front-wheel-drive (FWD) and rear-wheel-drive (RWD) configurations. RWD bikes tend to be easier and safer, to ride as most of your body weight is carried on top of the drive wheel. Though, the RWD motor location limits the number and style of gearing the bike can use. With FWD bikes, the motor can cause the front tire to skid in slick conditions causing difficulty with steering.
Electric motors have the ability to deliver 100% of their power instantaneously. To make the bike safe to ride, a controller is installed to act as the brains of the bike, and to control the acceleration of the motor. Controllers can have additional features installed in them, such as: multiple assist speeds, light controls, battery diagnostics, or a trip computer.