A electric motor system for supplying assistive propelling power to a bicycle, comprising: one or more drivetrains, each comprising a motor-drive shaft having a pinion with fixed drive system driving a crown and bearing assembly connected to a spindle; and a controller for executing a motor control sequence comprising: an initial state activated by a rider's start command where the motor speed is being detected for an initial period, and if the motor speed is higher than a first speed threshold continuously for a first period, then the motor is turned on to output a rider-selected torque level; a continuous motor speed detection state where the motor speed is being continuously detected, and if the motor speed falls below a second speed threshold continuously for a second period, then the motor is turned off; and the motor is turned off if a stop command from the rider is received.