An electric bicycle can easily be built by taking an existing bicycle and adding five parts: 1) a motor, 2) a linkage from the motor to a wheel or pedals, 3) batteries, 4) a throttle and 5) a motor controller (the brain that regulates how much battery power to send to the motor based on the position of the throttle). Buying parts that work together is the hardest part. Here’s the cheapest solution, which is a bottom bracket driven, non-freewheeling electric bike.