Axial Flux Hub Motors and EV Motors: Why the Disc Fits the Wheel
By the Turncircles engineering team · Published
A wheel is round and shallow. So is an axial flux motor. That coincidence makes the axial flux hub motor one of the most natural applications of the topology, from e-bikes and wheelchairs to wheeled robots and light electric vehicles.
This page does the direct-drive arithmetic, explains where "axial flux EV motor" does and does not describe what we make, and lists the trade-offs of putting a motor in a wheel.
Configure a hub motor Read about direct drive
Why a Disc Suits a Wheel
A wheel is a big circle with not much depth. An axial flux motor is a big circle with not much depth. That is the whole pitch, and it happens to be true. A conventional radial motor in a wheel needs a tall cylinder of stator and rotor, which pushes the design towards long, heavy hubs or an outboard motor with a gear train. A disc fits inside the rim, uses the large diameter where torque is made, and leaves the wheel space for brakes, bearings and your sense of proportion.
The Direct-Drive Arithmetic
A hub motor is usually direct drive: no gearbox, so the motor turns at wheel speed. That makes speed low and the required torque high. Wheel speed in RPM is v / (π × d) × 60, with v in m/s and d the wheel diameter in metres.
| Application | Wheel | Speed | Wheel RPM | Shaft torque at stated power |
|---|---|---|---|---|
| E-bike | 0.66 m | 25 km/h | 201 | 11.9 Nm at 250 W |
| Wheelchair | 0.60 m | 6 km/h | 53 | 18.0 Nm at 100 W |
These are illustrative numbers for steady cruising; acceleration and hills need more. The pattern is what matters: low RPM, so power turns into a lot of torque, and torque is exactly what an axial flux disc delivers per gram. If you would like the reasoning behind going gearless, see our direct drive motor definition and the starting torque note.
Axial Flux EV Motors: Where We Fit and Where We Do Not
"Axial flux EV motor" covers everything from a mobility scooter to a performance car, and these are not the same business. Automotive traction motors at hundreds of kilowatts, with their qualification programmes, are made by specialised companies, and we list some of them honestly in the axial flux manufacturers guide. Our platform tops out at 56 kW. Within that, light electric vehicles and wheeled robots are a natural fit:
- Electric bikes, cargo bikes, scooters and wheelchairs.
- Wheeled robots and autonomous platforms, see the robot joint actuator guide.
- Vehicles with in-wheel drive, such as the Mars mobile concept.
- Regenerative drivetrains, as in the four-quadrant system for hybrid electric vehicles.
A Load Case Worked Through: A Wheelchair on a Slope
Take a wheelchair and rider of 150 kg moving at 6 km/h (1.67 m/s) up a 6% slope, with a rolling resistance coefficient of 0.015. The force along the slope is about m × g × (0.015 + 0.06) = 150 × 9.81 × 0.075 ≈ 110 N, which at 1.67 m/s is 184 W in total. With two driven wheels of 0.30 m radius, each wheel carries 55 N, or about 16.6 Nm, at 53 RPM and about 92 W.
Notice how the numbers fall out: modest power, but torque that a typical high-speed motor would only reach through a gearbox. A disc-shaped direct-drive motor delivers 16.6 Nm at 53 RPM without one, which is the point of the whole exercise. Add margin for starts, kerbs and a heavier rider, and these are the numbers you type into the configurator.
Illustrative load case. Your mass, slope and rolling resistance decide the real numbers.
The Trade-Offs of Putting the Motor in the Wheel
- Unsprung mass. The motor is now part of the wheel and moves with it over bumps. Light motors reduce the penalty, which is another argument for a lightweight design.
- Environment. Water, dust and shock go straight to the motor. Protection up to IP67 and sealed or wet designs matter here.
- Heat. The motor sits in a closed hub. A coreless stator has no iron losses, so there is less heat to remove, but it still has to leave somewhere.
- Position feedback and drive. Smooth low-speed torque needs rotor position sensing and a suitable controller.
Designing the Hub Motor Around the Wheel
Diameter, stack count, winding and voltage are all parameters, so the motor can be shaped to a given rim, a given battery voltage and a given top speed instead of the other way round. Set the wheel RPM from the table above, the torque from your load case, and let the configurator return the motor. For choosing starting values, see the electric motor selector guide.
Frequently Asked Questions
What is an axial flux hub motor?
A motor with an axial flux (disc) design built into the hub of a wheel, usually as a direct drive without a gearbox. The flat shape fits the limited depth of a wheel.
Are axial flux motors good for EVs?
For light electric vehicles and wheeled robots, yes. They offer high torque for their weight and a short axial length. For automotive traction at hundreds of kilowatts, other specialised manufacturers serve that market; our configurable range tops out at 56 kW.
Do hub motors need a gearbox?
Usually not. Hub motors run at wheel speed, which is low, so they are designed for high torque at low RPM. This is the direct-drive approach, and it removes gearbox losses and noise.
What speed does a hub motor run at?
Wheel speed. For a 0.66 m wheel at 25 km/h that is about 200 RPM; for a 0.6 m wheelchair wheel at 6 km/h about 53 RPM. The formula is v / (pi times d) times 60.
Can I get a custom axial flux hub motor?
Yes, that is what we do. Diameter, stacks, winding and voltage are set from your requirements in the configurator. Housing and protection can be matched to a wheel or tyre.
Design a motor for your wheel
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