If you open the hood of a Tesla Model S Plaid, you’ll find a marvel of engineering. But it still looks like a motor—a heavy, cylindrical barrel of copper and steel. This is a Radial Flux motor, and it has powered the entire EV revolution up to this point.
But if you look at the Mercedes-Benz Vision One-Eleven concept, the motors are gone. Or rather, they have disappeared into the wheels.
This is the promise of Axial Flux technology. It isn’t just a better motor; it’s a form factor change that could delete driveshafts, differentials, and axles from the car forever.
The Physics: Radial vs. Axial
To understand why this matters, we have to go back to Physics 101. Torque = Force × Radius
Radial Flux (The Standard)
In a standard motor (Radial Flux), the magnets spin inside a stationary coil (stator). The magnetic force acts perpendicular to the axle, pushing it around.
- The Limitation: To get more torque, you need a longer motor (more magnets) or a wider motor (more leverage). But the copper windings on the ends of the cylinder (end windings) do nothing but generate heat. They are “dead weight.”
- The Heat Trap: The copper coils are buried deep inside the iron core. Getting heat out requires it to travel through layers of steel and insulation before it hits the water jacket on the outside capabilities.
Axial Flux (The Pancake)
In an Axial Flux motor, the rotor is a disc, and the stator is a disc next to it. The magnetic flux flows parallel to the axle, sandwiching the coils.
- The Leverage Hack: Because the motor is a flat disc (like a pancake), the active magnets are pushed further out from the center. Remember
Torque = Force × Radius? By moving the force to the outer edge of the disc, you get massive mechanical leverage without adding weight. - No Dead Copper: The coils are simple flat loops. Almost 100% of the copper contributes to torque.
The Manufacturing Nightmare
If Axial Flux is better, why isn’t everyone using it? Because building one is a nightmare.
In a standard radial motor, if the gap between the rotor and stator is off by a millimeter, it runs slightly inefficiently. In an axial flux motor, the attractive force between the two magnetic discs is immense—tonnes of pressure trying to slam them together.
- The Air Gap: Manufacturers must maintain a sub-millimeter air gap between two spinning discs that are flexing under massive magnetic loads.
- Bearing Load: The axial forces wreck standard bearings. You need massive, heavy thrust bearings to keep the faces from touching, which eats up your weight savings.
The YASA Breakthrough: “Yokeless” Cooling
Axial flux motors aren’t new—they’ve existed in theory for a century. But they had a fatal flaw: Heat. Sandwiching two magnetic discs together creates a pressure cooker. There was no way to cool the center of the “pancake” without destroying the motor.
Enter YASA (Yokeless And Segmented Armature), an Oxford University spin-out acquired by Mercedes-Benz.
Their breakthrough was a “Yokeless” stator.
- 3D Oil Channels: Instead of just spraying oil on the outside, Yasa pumps cooling oil through the center of the stator coils.
- The Dielectric Difference: They use a dielectric (non-conductive) oil that flows in direct contact with the copper. It doesn’t just cool the case; it cools the wire itself.
- The Result: A motor that weighs 50 lbs (23 kg) but produces 480 HP.
- Comparison: A standard EV motor with that power would weigh nearly 200 lbs.
Material Science: The Secret Powder
Standard motors use laminated steel sheets (silicon steel) to guide magnetism. Yasa uses Soft Magnetic Composites (SMC).
- What is it?: Instead of sheets, they use iron powder grains coated in an insulating ceramic layer, pressed into 3D shapes.
- The Physics:
- Eddy Currents: In solid iron, changing magnetic fields create rogue electrical currents (eddy currents) that generate heat. In SMC, the insulation around every single grain stops these currents dead.
- 3D Flux: Laminated steel only guides magnetism in 2D (along the sheet). SMC allows magnetism to flow in 3D, enabling complex “claw pole” designs that squeeze even more torque out of the magnets.
Why It Matters: The “Unsprung Weight” War
The holy grail of car design is the In-Wheel Motor. If you can put the motor inside the wheel, you can delete the transmission, the driveshaft, the axles, and the differential. You get a car that is entirely flat—a true “skateboard” with infinite interior space.
The Problem: “Unsprung Weight.”
- Defined: Weight that is not supported by the suspension (wheels, tires, brakes).
- The Physics:
F = ma. When a heavy wheel hits a bump, it gains massive upward momentum. The suspension spring has to fight that momentum to keep the tire on the road.- Heavy Wheel: Hits a bump -> flies into the air -> tire loses contact -> you crash.
- Light Wheel: Hits a bump -> moves up quickly -> suspension pushes it back down -> tire stays griped.
The Axial Flux Solution: Because Yasa’s motors are so impossibly light (less than a brake rotor), they are the only technology that makes high-performance in-wheel drive possible.
- Vectoring: With four independent motors, you can torque vector instantly. The computer can spin the left wheels backward and right wheels forward to turn the car on a dime (Tank Turn).
The Future: Mercedes AMG.EA
Mercedes didn’t buy Yasa for a science project. They bought it for the upcoming AMG.EA platform. We are about to see a bifurcation in the EV market.
- Economy Cars: Will keep using cheap, heavy Radial Flux motors.
- Performance/Luxury: Will switch to Axial Flux.
When you see the next-generation AMG electric supercars doing tank turns with four independent wheels, or the Vision One-Eleven cruising with zero hood height, remember: it’s not magic. It’s just a pancake motor that finally figured out how to stay cool.
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