Fundamentally speaking, electric motors translate electrical energy into mechanical energy. E-bikes use brushless DC motors, or BLDC motors, meaning they don’t use brushes to alternate the direction of current flowing to the motor, as older electric motors did. Those brushes made the motors less efficient and tended to wear out over time, so brushless motors have been the standard for more than a decade.
Open up a BLDC motor and you’ll see a bunch of wires wound around a circular series of poles. That’s the stator; it becomes an electromagnet when the electric motor controller draws current from the battery into the wires. You’ll also see a circular series of permanent magnets, either directly inside or outside the stator. The orientation of the magnets relative to the stator depends on the type of BLDC motor, but either way, that’s the rotor.
Grasping the interaction between the rotor and the stator is crucial to understanding how e-bike motors work. When current runs through the stator’s electromagnets in a circular sequence, those electromagnets repel and attract the permanent magnets on the rotor, causing it to spin. The stator is attached to a shaft. On a mid-drive motor, the shaft spins to generate torque, and that torque gives you pedaling assistance via a small chainring connected to the shaft. On hub motors, the shaft becomes the axle and therefore doesn’t spin. Instead, the rotor itself spins, causing the entire ebike motor (hub) to spin, thus creating torque to spin the front or rear wheel.
How Motors Work With the Rest of the E-Bike.
In addition to the motor, all e-bikes have motor controllers and batteries. The controllers modulate the amount of power flowing to the motor, which uses your input to transfer the desired amount of current from the battery into the ebike motor. What makes an e-bike an e-bike is the experience of how power is being doled out . Pedal-assisted e-bikes might use a speed (a.k.a. cadence) sensor, which regulates e-assist by detecting the rider’s pedaling cadence, or torque sensors, which sense how much torque the rider is putting into the pedals. Some e-bikes have throttles that allow you to use the electric motor independent of your pedaling, although regional laws define where you can and cannot use throttle-equipped e-bikes.
The Different Types of Motors
Despite sharing the same basic tech, the electric motors you’ll see on today’s e-bikes come in three basic variants. Mid-drive motors are positioned at the center of the bike’s frame, where you’d normally find the bottom bracket. Hub-driven e-bikes have motors within the front or rear hub, and there are two types of hub motors. Direct-drive hub motors, apart from their bearings, have no moving parts: The motor just spins around the axle, which is secured to the frame’s dropout. Geared hub motors use a series of planetary gears to lower the motor’s RPM and increase its torque output. You’ll also find aftermarket e-bike kits that allow you to equip a standard bike with a mid-drive or hub motor, and among aftermarket kits, there are friction drives, which use a spinning wheel that contacts the rear tire to create propulsion.
Mid-drive motors are located between an e-bike’s cranks. An electric motor generates torque that spins a shaft that’s connected to a chainring. The motor is therefore supplementing your pedaling power within the bike’s chain-drive, rather than adding an additional power source. There’s also a gear-reduction system within the motor pack. Bosch mid-drive motors spin hundreds of times per minute—much faster than you could pedal—so the motor’s internal gearing reduces the RPMs at the shaft, therefore optimizing the system’s performance to a rider-friendly cadence of 50 to 80 RPM. All but the lowest-end mid-drive systems include gear sensors that cut the power to the motor while you’re changing gears to avoid breaking the chain while the bike isn’t in gear.
Direct-Drive Hub Motors.
Direct-drive hub motors are the simplest ebike motors. The motor’s shaft becomes the rear axle. Because the shaft is fixed in place, the motor (a.k.a. the hub) spins around the shaft, propelling you forward. Direct-drive motors tend to be larger in diameter than geared hub motors, because bigger hubs mean increased leverage and higher torque outputs, which is needed to supply adequate power at lower RPMs. Direct-drive e-bikes can also generate electrical energy during braking in a process called regenerative braking. Motors are perfectly bidirectional.They can go forward and backward with equal efficiency. When you squeeze the brakes, a cutoff switch tells the motor controller to become a generator, and the resistance generates electrical energy. The energy regained from regenerative braking is minimal, although energy gains increase on hilly routes—but the primary benefit is brake-saving stopping power on long descents, as the braking energy is absorbed electronically rather than through friction.
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