Everything you need to know about hub motors

A hub motor is an electric motor which is built into the hub of the wheel to provide propulsion. They have risen in popularity greatly with ebikes due to their ease of use and installation.

How a hub motor works

The hub motor can be split into two parts; the rotating and the stationary part called the Stator.

The stator contains a number electromagnets, windings that create electromagnetic fields when current is applied. This is the heaviest part so it kept stationary.

The rotating part contains a number of Rare earth magnets which create a constant electromagnetic field. The magnets are attached to the motor housing which provides the mounting points for the spokes.

An electronic speed controller (ESC) will control the current flowing to each of the motor phases (windings). Each of the phases will be supplied with electrical current successively to create a rotating electromagnetic field. The rare earth magnets will interact (attract and repel) with the rotating field created by the electromagets resulting in a rotational motion around the axle.

Phases

The most common number of phases is in a hub motor is 3 but there could be more or less. With an increased number of phases you are able to control the position of the motor to a higher degree of accuracy. Additional phases come with at a cost, both in the motor and the speed controller would be slightly more expensive to handle the increased number of phases.

Increased motor accuracy is useful in industrial applications but not so much with ebikes hence use of standard 3 phase motors.

Rare earth magnets

The most common rare earth magnet used in hub motors is the Neodymium magnet due it's affordability and high magnetism.

Despite their name Rare earth magnets aren't partircularly rare in the Earth's crust. Neodymium magnets are made from the elements Iron, Boron and Neodymium which are produced at 1,150,000,000, 9,400,000 and 7,000 tonnes per year respectively although demand for Neodymium is rising. The "rare" part of the term is due to the elements required being congregated rather than evenly distributed through the earth's curst like iron and aluminium.

how long do rare earth magnets they last?

Neodymium magnets are "permenent", if left undisturbed they will last forever. Heat and corrosion will however cause the magnet to degrade but if used within their intended limits will loose a small fraction of its magnatism over time but it is negligable. Other magnetic fields may also have a small impact on your magnet.

The magnets will most likely outlive other components in your hub motor.

Sources: [1] [2] [3] [4]

Pros and cons of hub motors

The alternative to hub motors are mid-drive motors which are mounted to the frame. Power is then transfered to the drive train via a chain or a belt.

Here are the pros and cons of hub motors compared to mid drive motors:

Pros

Cons

Things to take into account when choosing a hub motor

Axle flats

If you've been shopping around for a hub motor for your ebike you may have noticed manufactuers specifying axle flats size.

Hub motors are different to normal bike hubs in that the axle must be fixed and should not rotate. This is achieved by the flat part on the axle slotting into the fork ends on your frame.

When buying your hub motor you should ensure the axle flats match the size of your fork ends. Most hubs have 10mm axle flats but some more powerfull motors (usually 4000w+) with 12mm axle flats.

Fork ends are often referred to as dropouts