What Is a Mid-Drive Motor?

A mid-drive motor, also known as a central motor or crank motor, is an electric motor used in electric bicycles (e-bikes) that is positioned in the center of the bike’s frame, usually around the bottom bracket. Unlike hub motors, which are mounted in the wheel hub, mid-drive motors drive the wheel through a chain, belt, or cardan drive.

An often touted advantage of a mid-drive motor is the low center of gravity when compared to a hub motor. By engineers, this is considered mostly marketing talk as the physics do not really support claiming a big advantage for that reason.

Some mid-drives have gear shifting in the motor housing which when combined with a belt or cardan drive can drastically reduce maintenance. Compared to a bike with external gears, this is a great plus.

Mid-drive motors are usually more expensive and complex than hub motors. Another disadvantage of a mid-drive motor compared to a rear hub drive is that all the force (human + motor) needs to through the chain/belt causing more wear. With a rear hub motor, only human power goes through the chain/belt. For this reason a mid-drive motor is actually a really good solution from a technical point of view.

The cardan drive mentioned has been used by bikes before and may see a revival with e-bikes. A cardan drive, also known as a universal joint drive or propeller shaft, is a mechanical device that transmits power from a motor or engine to a rotating shaft. Vehicles with rear-wheel or four-wheel drive, such as cars, trucks, and SUVs often use cardan drives. The driveshaft of the cardan drive connects to the transmission at one end and the differential at the other end. The torque from the engine transmits through the transmission, then through the driveshaft, and finally to the wheels through the differential.

Cardan drives are commonly used because they are simple, durable, and efficient. However, they require periodic maintenance and may generate noise and vibrations at high speeds.