Hub motor or mid-drive motor: advantages and limitations of each design

A light electric vehicle can have its motor in one of two places: either directly in the wheel itself (hub motor), or mounted to the frame with a chain or belt that drives the wheel (mid-drive motor). Neither solution is inherently better. Hub motors are simple, compact, and quiet; mid-drive motors benefit from gearing, put less stress on the suspension, and are easier to cool. That's why most scooters use hub motors, while light electric motorcycles tend to use mid-drive motors.

Here are two ways to make a wheel turn

The wheel motor

In a hub motor, the stator (the wound part) is fixed to the shaft, and the outer bell, which is lined with magnets, rotates and carries the rim. Therefore, the wheel rotates exactly at the speed of the motor. To produce a lot of torque without reduction, you need a large diameter, many poles, and wide magnets: this explains the weight of a powerful scooter motor. Our guide on hub motors details its construction.

Here's an example from our product descriptions: the XHS75 motor-wheel weighs approximately 8 kg and is rated at 6,000 W, with an unloaded speed of 2,340 RPM at 60 V. It's a low-speed, high-torque motor designed to directly drive a scooter wheel.

The mid-drive motor

A central motor is mounted to the frame. Its shaft carries a pinion or pulley, which drives the wheel via a chain, a belt, or both. The transmission divides the speed and multiplies the torque: a motor that spins quickly and produces modest torque becomes, at the wheel, a motor that turns slowly but delivers very high torque.

Example from our product descriptions: our RM30 motor for Sur-Ron Light Bee is a mid-drive motor advertised at 12,000 RPM, and it comes with a pulley for the machine's original belt. This RPM would be unusable when directly connected to a wheel; it is the transmission that brings it down to the wheel's speed.

Unsprung weight

Everything located below the suspension (wheel, tire, brake, and motor if it's integrated into the wheel) is called the unsprung mass. The heavier the unsprung mass, the more difficult it is for the wheel to follow bumps: it bounces, momentarily loses contact with the ground, and the suspension works less effectively.

On a scooter, this issue exists but remains acceptable: the speeds in turns are low, and the rider absorbs some of the impacts with their legs. However, on an off-road motorcycle, which handles jumps and rough terrain at high speeds, a heavy motor in the rear wheel significantly degrades the handling. This is one of the reasons why lightweight electric motorcycles often have a motor mounted on the frame.

The reduction, a key advantage of the mid-drive motor

For a given power output, a motor that spins faster can be smaller and lighter than a motor that spins slower: power is the product of torque and speed, and torque is what requires copper, iron, and magnets. Our guide on torque and power explains this relationship.

The reduction also provides flexibility:

• Choosing the gear ratio: A different sprocket or pulley changes the balance between acceleration and top speed, without affecting the motor itself
• The same motor is used in multiple machines: it's simply a matter of adapting the transmission;
• The motor is operating within its optimal range: it's spinning faster, which is where its efficiency is often better than at very low speeds.

The hub motor, on the other hand, has only one "gear ratio," which is determined by its winding (High Speed or High Torque) and the battery voltage. To make it go faster or provide more power, you either change the motor or adjust the battery voltage.

What the hub motor does better:

• Simplicity: No chain to grease or adjust, no belt to monitor, and no sprocket to wear out.
• The Silence: Without a transmission, the only sounds come from the bearings and the motor itself.
• Compactness: Everything is integrated into the wheel, which frees up space on the deck for the battery. On an electric scooter, there simply isn't any other place to put it.
• Effortless dual-motor drive: It simply requires a motor in each wheel, without any shaft or chain connecting to the front. This is the architecture of dual-motor scooters.
• Transmission efficiency: There is no transmission, so there are no losses in a chain or belt.

What the mid-drive motor does better

• Cooling: The motor is located in the airflow, away from the heat generated by the brakes. You can add fins, a fan, or a liquid cooling system to it. A hub motor is enclosed within its rim. See our guide on liquid cooling.
• Protection: A motor mounted within the frame is less exposed to impacts, water, and mud than a motor mounted directly to the ground.
• Tire replacement: The wheel can be removed without disconnecting any phase cables.
• Behavior: Less unsprung mass, and a mass positioned closer to the center of gravity.

How to choose the right components for a project

On electric scooters, the choice is quickly settled: hub motors are the standard due to space limitations. The next step is to select the appropriate size, winding, and cooling method.

When designing a machine, we start with the specifications: speed, weight, terrain, available space, acceptable maintenance, and the number of driven wheels. A robot that moves slowly is very well suited to hub motors; while off-road motorcycles or karts almost always require a central motor. This is the work of our electric two-wheeler design department, which offers hub motors or motors with a shaft output, sized for the specific machine.