There are different systems on the market, each using either a mid-drive pedal-assist or a wheel (hub) assist motor. Wheel assist systems usually give you the option of a throttle so you don’t necessarily have to pedal. The motor is either in the front or the rear wheel (hub).
Mid-drive systems like those from Bosch or Shimano's STEPS provide power through the cranks of the bike and enhance your pedal stroke – so you must be pedaling to engage the assist.
Mid-drive systems now make up approximately 90% of the e-bike market, and Bosch is a worldwide leader in the category. Their mid-drive system is light weight, centrally located on the bike, and the direct power is extremely efficient. With 4 different drive units, and various battery options, Bosch makes systems for electric bikes designed for recreational riding, commuting, off road riding and even cargo bikes.
The Bosch e-bike system offers a 4 tiered assist system and the batteries are made up of lithium ion cells, with a high quality information system.
Similar to Bosch, Shimano offers a mid-drive system for e-bike manufactures. Shimano's systems are very lightweight and smooth riding. With the drive unit placed around the crank, the bicycle's centre of gravity is quite low. The 3 tiered controller system gives you the option to run low, medium or high assist levels, or run the bike without assist.