Uni Wheel: Hyundai Reimagines the Drivetrain

The drivetrain of the automobile has basically gone unchanged since its inception. A motor sends power to a transmission (in internal combustion engine applications) or reduction gears (in electric motor applications). That power is then sent through constant velocity (CV) joints and a drive shaft to power the wheels.

Hyundai wants to gain more interior space in EV applications by doing away with the CV joints, shortening the drive shafts, and being able to make the motors smaller and positioned directly at each corner of the vehicle. They plan to do this with the Uni Wheel.

Hyundai Uni Wheel

The Universal Wheel Drive System (Uni Wheel) just debuted at Uni Wheel Tech Day in Seoul, South Korea.

Uni Wheel moves an EV’s reduction gears inside the vacant space inside the wheel hub. This, combined with having small electric drive motors positioned just inside each wheel, allows for extremely reduced drive shaft lengths and no need for CV joints on the drive shaft.

This video explains it better than I ever could.

In Hyundai’s words: “Uni Wheel uses a special planetary gear configuration consisting of a sun gear in the center, four pinion gears on each side, and a ring gear surrounding this arrangement. Power generated by the motor is transmitted to the sun gear, which in turn engages the pinion gears to rotate the ring gear. This is connected to the wheel to drive the vehicle.”

All of this has been done specifically to free up space for people and cargo in EVs. Other benefits Hyundai that touts are a greater driving range, the ability to use smaller electric motors, and because of the Uni Wheel’s high reduction ratio, it can deliver a “significant torque output.”

Wheel Articulation Without CVs

The pinion gears in the Uni Wheel design are connected to form two linkages. This multi-link mechanism allows the wheel to articulate on its suspension, without the use of CV joints.

This setup should provide better durability and efficiency. In traditional drive systems, the CV joint angle changes as the tire goes over bumpy terrain. The more severe the CV angle, the less efficient it is at transmitting power and the more prone it is to failure. Hyundai says that the “Uni Wheel can transmit power with almost no change to efficiency regardless of wheel movement.”

Potential Issues

The only real red flags I see with this design are the limited wheel travel that it will allow and the increase in sprung mass. The limited wheel travel will exclude this tech from working on off-road vehicles. The heavier sprung mass means that the shocks/dampers will be required to do more work and will overheat faster.

Cooling of this mechanism inside the wheel hub could be an issue as well. And, the whole setup could require more maintenance than the tried and true drivetrain used across the board today.

Hyundai Uni Wheel in Production Applications

No timeline was given as to when we might see this new Uni Wheel tech on production products, but it has far-reaching applications. Hyundai and Kia have registered eight patents related to the Uni Wheel in South Korea, Europe, and the U.S.

The two companies, which are closely working together on all sorts of new tech, say they are “continuously verifying the stability, efficiency, and durability of Uni Wheel through various tests to perfect its development and will continue efforts to improve its efficiency by adjusting the reduction gear ratio, and upgrading the lubrication and cooling system.”

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