An e-bike is a two-wheeler that helps power your pedaling via a motor. The battery-operated bikes have become more widespread over the years. But they still conjure up some questions about how exactly the motor works and what it means for the riding experience.
How E-Bike Motors Work
Electrical energy is converted into mechanical energy by electric motors. These motors are brushless, meaning that they do not use brushes to change the direction in which current flows. Older electric motors did this with brushes. For more than a decade, brushless motors have been the standard because the brushes made the motors inefficient and prone to wearing out over time.
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 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.
Knowing how e-bike motors work requires understanding the interaction between the rotor and the stator. The rotor spins when current flows through the stator's electromagnets in a circular pattern, repelling and attracting the permanent magnets. The stator is attached to a shaft. Through a small chainring attached to the shaft of a mid-drive motor, torque is generated, and that torque provides pedaling assistance. Because hub motors have shafts as axles, they don't spin. It is actually the rotor itself that spins, causing the entire motor (hub) to spin, causing the front or rear wheel to spin.
The Different Types of Motors
Modern e-bike motors come in three basic varieties, despite sharing much of the same technology. Motors for mid-drive bikes are mounted at the center of the frame, where the bottom bracket is typically located. There are two types of hub motors for hub-driven e-bikes. The motors are located inside the front or rear hub.
Apart from their bearings, direct drive hub motors do not have moving parts: The motor simply revolves around the axle, which is fixed to the frame's dropouts. In gearless hub motors, planetary gears are used to increase torque output and decrease motor RPM.
There are also aftermarket e-bike kits available that allow you to attach an e-bike motor to a standard bike. These kits include friction drives, which use spinning wheels that contact the rear tires to produce propulsion.
Mid-Drives
The mid-drive motors are located between the cranks of an e-bike. A chainring is connected to a shaft that is spun by an electric motor. By adding the motor, you are enhancing your pedaling power rather than adding a separate power source. The motor pack also includes a gear-reduction system. Despite spinning hundreds of times per minute, Bosch mid-drive electric bike motors reduce the shaft's RPMs at the shaft, which optimizes performance to a rider-friendly cadence of 50 to 80 RPM, according to Bosch's Weinert. Except for the lowest-end mid-drive systems, all feature gear sensors that shut off power to the motor while you're changing gears to prevent the chain from breaking.
Direct-Drive Hub Motors
The simplest electric bike motor is the direct-drive hub motor. The motor shaft serves as the rear axle. The hub motor spins around the shaft because the shaft is fixed. Direct-drive motors are typically larger than geared hub motors because larger hubs provide more leverage and higher torque outputs, which are essential in delivering adequate power at lower RPMs. During braking, direct-drive e-bikes can generate electrical energy, which is known as regenerative braking.
The motors are perfectly bidirectional. The brakes can be slammed forward and backward equally. When the brakes are squeezed, the motor controller becomes a generator, and the resistance generates electricity. Regenerative braking only gains a little energy. Someone found a 3.5 percent improvement in range with his regenerative system, although energy gains increase on hilly routes - but it provides the benefit of brake-savings on long descents since the braking energy is absorbed electronically rather than through friction.
Geared Hub Motors
Geared hub motors operate similarly to direct-drive hub motors, except that an electric motor is located within the hub and spins at a much faster speed. This motor's shaft is connected to a series of planetary gears, which spin the hub at a slower speed. This results in more torque, but less top-end speed.
A gear hub motor typically has a smaller diameter than a direct drive motor because it doesn't require as large a motor to generate the same amount of torque on the wheel, but it's also wider because of the planetary gears. In addition to the motors, the geared hub motors also come with a freewheel: This means you can't use them for regenerative braking, but when they're not in use, they're free to coast instead of creating drag, so they're more like a traditional bicycle.
Friction Motors
Friction-driven e-bikes might seem archaic compared to modern e-bikes with hub motors and mid-drive systems, but they're an affordable option for cyclists who want to transform a traditional bicycle with minimal effort. The bolt-on motor drives wheels that contact the tires, usually below the chainstays or above the seat stays, though some kits attach to the fork brake mounts. By spinning the tire, the motor propels you forward.
As a general rule of thumb, mid-hub motors tend to be more expensive than front or rear-hub motors, but most people find they offer a smoother ride.
There are different kind of motors that you can find fitted on electric bikes. Except the types of above motors, electric bike motors are classified as its different position on ebike. The majority of modern ebike motors are placed at the bottom bracket (in between the cranks), while they can also be positioned within the hub of the rear wheel. This latter positioning is generally found on cheaper electric bikes. The details are as below.
Front-hub motors Positioned on the front wheel hub, they’re common on folding bikes and cheaper touring e-bikes (typically under £1,500).
Pros:
The motor doesn't wear down the chain - the motor is directly powering the wheel, rather than the drive chain.
You can customise gears easily.
Cons:
The bike can be unbalanced - the front is heavier than the back.
Tricky to remove the front wheel for transportation.
Front wheel can slip on steep climbs if you don’t distribute your weight properly.
The motor pulls you forward, rather than pushing you, which can feel strange.
Rear-hub motors These are situated on the rear wheel hub. They also tend to cost less than £1,500.
Pros:
The motor pushes you forward, which feels more natural to most people than being pulled forward by a front-hub motor.
The motor should wear down the chain less quickly than a mid-hub motor.
Easy to customise the gears.
Cons:
You need a specially designed rear wheel.
Tricky to remove the rear wheel for transportation.
The bike can be unbalanced - the back is heavier than the front. This issue is exacerbated if the battery is also located at the back and the bike doesn’t have front suspension.
Mid-hub motors
E-bikes with a mid-hub motor (sometimes called a mid-engine motor) are typically more expensive. They do have some significant advantages though.
Pros:
More balanced weight distribution, making it easier to lift and carry.
Best for off-road riding because the centre of gravity is lower compared with front or rear-hub motors.
You can easily customise, switch or replace wheels.
Wheels are easy to remove for transportation.