In-Wheel Hub Motors vs. Integrated E-Axles: The Trade-Offs Dictating EV Drivetrain Packaging

The binding constraint of automotive suspension is the requirement to keep the tire contact patch pressed firmly against the pavement. For a damper to effectively control a wheel over broken ground, the wheel assembly must remain relatively light. If the mass of the wheel exceeds the suspension's ability to force it back down after an impact, the tire loses contact with the road, destroying both grip and ride quality.

For a century, this physical constraint dictated a single drivetrain architecture. The heavy propulsion unit—whether an internal combustion engine or a central electric motor—had to be mounted directly to the chassis. By keeping this mass "sprung," engineers isolated it from the violent vertical movements of the wheels, sending power outward through a network of differentials, half-shafts, and constant velocity joints.

The electric vehicle transition challenged this centralized model. Because electric motors can be scaled down to the diameter of a brake rotor, engineers realized they could move the propulsion system entirely out of the chassis. By placing a motor directly inside each wheel hub, the mechanical middlemen could be eliminated.

This in-wheel motor architecture offers immediate packaging advantages. Removing the central e-axle frees up the space between the wheels, allowing manufacturers to install larger battery packs, lower the cabin floor, or design novel aerodynamic shapes. The 2022 Lightyear 0, a highly efficient solar-assisted vehicle, utilized in-wheel motors specifically to eliminate the mechanical drag of transmission joints and maximize its limited solar energy.

Furthermore, placing the motor at the wheel hub fundamentally changes how a vehicle handles traction. Because the magnetic flux acts directly on the wheel, torque delivery is nearly instantaneous. Gorazd Gotovac, Chief Technology Officer at Elaphe, notes that their in-wheel motors can deliver full torque in just 4 milliseconds.[2]

That response time is roughly 20 times faster than a conventional inboard e-axle system. This millisecond-level control enables true torque vectoring, allowing the vehicle's software to independently accelerate or brake individual wheels to pivot the chassis. In theory, this precision could replace mechanical anti-roll bars and stability control systems entirely.[2]

Yet, moving the motor into the wheel violates the suspension's binding constraint. It adds significant unsprung mass to each corner. A modern in-wheel motor, such as the DeepDrive IW 2000, weighs 34 kilograms. Protean Electric's PD18 Gen 5 unit adds a similar 36 kilograms per wheel.[1]

When a 34-kilogram heavier wheel strikes a pothole, it generates substantially more upward kinetic energy. The passive dampers on a standard passenger car struggle to arrest that violent motion. The result is a harsh, bouncy ride and a momentary loss of traction over uneven surfaces.[1]

To compensate for the heavier wheels, manufacturers must stiffen the suspension components and increase the damping rates. While this keeps the tire on the road, it transfers more of the impact force directly into the cabin, compromising passenger comfort.

The severity of this trade-off depends entirely on the vehicle's baseline mass. In a heavy commercial vehicle, the sprung-to-unsprung mass ratio is already high enough to absorb the penalty. The 2023 Lordstown Endurance, a 2,948-kilogram electric pickup truck, utilized four Elaphe hub motors producing a combined 440 horsepower.[3]

Because the Endurance carried a massive 109-kilowatt-hour battery pack on its chassis, the heavy sprung mass effectively masked the heavy wheels. Reviewing the truck, automotive journalist Paul Eisenstein observed, "The hub motors clearly add some unsprung mass. But the truck was nowhere as rough-riding as I anticipated."[3]

For lighter passenger cars, however, the math remains unforgiving. The unsprung mass penalty is too severe for passive suspension systems to hide without ruining the ride quality. This physical reality explains why the automotive industry has largely bifurcated its approach to EV drivetrains.

The dominant paradigm for mass-market passenger vehicles remains the integrated e-axle. In this architecture, the electric motor, inverter, and reduction gear are packaged together in a single housing mounted on the vehicle centerline.

E-axles keep the heavy components sprung, preserving the delicate balance of passenger car suspension dynamics. They also shield the sensitive high-voltage electronics from the harsh environment inside the wheel well, protecting them from direct road impacts, water submersion, and abrasive brake dust.

Manufacturers are continuously refining the central e-axle to reduce its size and cost. DeepDrive, for instance, has developed a dual-rotor, radial-flux motor that can be used centrally. "We've minimised the use of sheet metal, copper, steel and magnets, which not only reduces weight but also lowers cost," explains Jannik Stammler, an engineering specialist at the company.[1]

While e-axles introduce a slight mechanical delay and consume central packaging space, they represent the safest, most comfortable compromise for everyday driving. The mechanical linkages are a known quantity, and the suspension tuning requires no radical reinvention.

In-wheel motors, meanwhile, are finding their niche in specialized applications. Beyond heavy trucks, they are being adopted for high-performance vehicles where active suspension systems can manage the unsprung mass, and where millisecond torque vectoring provides a competitive edge on the track.

Renault has announced that its upcoming 5 Turbo 3E, slated for 2026, will utilize Protean's in-wheel motors to deliver 550 horsepower to the rear wheels. The vehicle relies on stiffer suspension settings to offset the hub weight, prioritizing aggressive handling over daily comfort.

The architecture also offers unique advantages for retrofitting older platforms. "If you have a car which has been engineered and crash-tested and you want to hybridize it, you have to redo all the crash-testing," Gotovac explains. Because in-wheel motors do not require a central powertrain block, they can be added without altering the vehicle's primary crash structures.[2]

The drivetrain architecture of the next decade will not be defined by a single winner, but by a strict adherence to physics. Until active, predictive suspension systems become cheap enough for mass-market cars, or motor topologies shed another 15 kilograms per corner, the heavy lifting will remain on the chassis.