Ferdinand Porsche, it seems, was always ahead of the curve. He conceived one of the earliest electric cars back in 1900, which later added a gas engine to compensate for its primitive lead batteries. Today, we’d call that a range-extended EV, and those may be the next big thing in electrification. 

But that car had another bit of technology that’s coming around again: in-wheel hub motors, which eventually gave rise to the first all-wheel-drive vehicle. In-wheel motors differ from the ubiquitous inboard motors because they are essentially part of the wheel, eliminating the need for drive shafts or differentials.

They allow for more direct power transmission to the road and have several advantages that you’d think would have made them more popular by now.

Photo by: Hyundai

Hub motors also seem like a great way to turn a conventional combustion car into a hybrid with minimal cost. Yet, while a few automakers and tech suppliers have toyed with these ideas, nothing has come to fruition, so I couldn’t help but wonder: Why haven’t we seen them on production EVs or as a third-party solution to electrify vehicles?

What Makes Hub Motors Great?

1901 Porsche in-wheel electric motor

Photo by: Porsche

First, a primer on electric motors may be necessary here. On a conventional EV, the traction battery (i.e. the main battery) supplies power to electric motors that drive the wheels. Most modern EVs have one electric motor for front- or rear-wheel-drive, or two motors for all-wheel-drive. Some automakers are adding three or even four motors to their cars for even more power.

But those motors are always located inboard between the wheels, integrated into the transaxle itself. Considering their increasingly compact size, couldn’t electric motors go into the wheels themselves? 

Having the motors in the wheels seems like a great way to make more room in the vehicle. This can either allow for a bigger battery to be used, or it can simply be a way to increase the level of cargo or passenger room in the cabin.

Hub motors would also simplify EV platforms and make subframe designs considerably simpler, too. This lower complexity would make them cheaper to manufacture and also facilitate platform modularity by making it easier to vary the wheelbase and track width without requiring modifications to many other components. 

In other words, if you don’t have to make room for motors somewhere in the middle or body of the car, you get a lot more freedom in terms of design. 

Photo by: Renault

It would also make having front-, rear- and all-wheel-drive vehicles on the same platform with minimal modifications. Being able to build multiple different vehicles on the same platform would certainly help startups reduce the upfront cost of engineering and manufacturing, while also reducing the time it takes to actually build cars, giving fledgling companies a higher chance of success.

Another big advantage of hub motors is the significant reduction of drivetrain losses. These are highest in a combustion vehicle, which sends power from the engine to the wheels through a gearbox, differential and drive shafts, all of which incur losses. Even current EVs still have reduction gears, differentials and drive shafts, and some of the power is still lost when transmitted from the motor to the wheels.

Having the motor inside the wheel powering it directly results in less friction and less wasted energy. It not only ensures that most of the motor’s power is put to the ground, but it should also improve efficiency and range. You can never have enough range in an EV, so having more of it by having a simpler, cheaper drivetrain seems like a good idea.

It would also allow for more precise torque control and torque vectoring. Having individual motors can not only simulate what a limited-slip differential does, but it can go beyond that and offer traction levels you simply couldn’t achieve otherwise through exact control of each motor.

What Are Hub Motors’ Disadvantages?

Photo by: Porsche

Hub motors have many advantages over having them inboard, but they also bring new problems to the table. Increased unsprung mass is the first issue that springs to mind. Unsprung mass refers to the weight of the suspension components (and the wheel) not supported by the suspension, and keeping it low improves comfort, grip and handling precision.

This could be offset by having the heavy brakes inboard, but this would also increase the complexity of the suspension and subframe design, likely eliminating much of the advantage of having hub motors in the first place. Mercedes wants to integrate and fully enclose the brake disk in the drive unit, which has a number of benefits, including eliminating emissions of brake pad dust, which is a source of air pollution that has mostly been ignored but is starting to gain attention.

The rotor part of the electric motor would probably be directly connected to the wheel, while the stator would be fixed to the suspension. With the stator essentially being part of the wheel, this would increase its rotational inertia, meaning it will resist speed and direction changes more, reducing its ability to accelerate and change direction, thus potentially lowering a vehicle’s agility.

Another potential issue with hub motors has to do with them taking a considerably more brutal beating than if they were inboard—whatever abuse your tires and suspension take could potentially transfer to the motor in some way.

Every pothole or road imperfection would send a jolt through the motor, affecting the longevity of moving parts such as bearings or the rotor itself. The motors would also be exposed to water, snow and salt, which could cause accelerated wear on seals and potentially seep into the motor, causing all sorts of problems.

Cooling could be another potential hurdle to overcome, although engineers could come up with clever wheel designs that dissipate heat, as well as ways of channeling cool air toward the motors to prevent overheating. Many performance cars employ this to keep their brakes cool, and it should also work for drive units, albeit not quite as efficiently, since you wouldn’t be cooling the actual internal components that get hot.

There would also be high-voltage cables running through the suspension from the battery to the motors. These would be needed to power the motors, but they would also be subjected to a lot of movement, and, in the event of a crash that causes the vehicle to lose a wheel, they would potentially expose the high-voltage cable, posing a significant danger of electrocution for occupants and emergency services around the vehicle.

There are ways that this risk can be mitigated, but it would also add complexity to a system whose main attraction was supposed to be its simplicity. Hub traction motors’ rarity also drives up their price, and the economic factor surely plays a big part in why OEMs haven’t embraced them.

Hub Motors Make Sense In Niche Uses

Photo by: Renault

But some are trying. 

Renault has announced its ambition to equip the upcoming 5 Turbo 3e with two hub motors driving each of its rear wheels individually. The big advantage that Renault touts with in-wheel motors is responsiveness, “for an effect not unlike the turbos of yesteryear, but without the lag time.” Allowing careful control of how much power goes to each wheel will allow for fun tail-out handling and an obligatory drift mode.

Using this type of drive unit, which also houses the disk brakes, brings “significant weight and space savings on the rear axle.” The car needs 20-inch wheels to be able to accommodate the drive units, each of which produces 268 horsepower and a combined peak of 536 hp. Renault expects it to accelerate to 62 mph (100 km/h) in around 3.5 seconds with a top speed of 167 mph (270 km/h).

Hyundai is one of the few other OEMs working on hub motors. Its project, called Uni Wheel, envisions a broad range of drive units for uses in EVs and other products, touting the same benefits of improved packaging and freeing up more interior room. The manufacturer says it plans to use in-wheel motors in all types of vehicles, from small cars to performance EVs, in the future.

A view of a Hyundai EV platform without in-hub motors (left) and with them (right.) Notice the difference in space savings that could be used for new designs and packaging choices.

Photo by: Hyundai

Hub motors would also make sense in an urban delivery vehicle, where they could allow for even more cargo room within a set track width and wheelbase. Since you can get rid of a conventional axle, there’s no reason why you couldn’t use the space between the wheels for extra storage capacity, allowing the van to carry even more boxes.

Automotive driveline supplier Neapco teamed up with in-wheel motor specialist Elaphe and designed a hub motor called SuperBear specifically for this purpose, touting the above advantages. Its drive units even feature a built-in two-speed gearbox, and they are designed to fit on a regular commercial-grade rim for both front- and rear-wheel drive applications.

Having heavy motors that power the front wheels can’t be good for steering, though. Having hub motors on the front axle may require additional reengineering of the front suspension and steering system to make it controllable and reduce what could potentially be very severe torque steer.

They are designed to power a vehicle on their own or together with a combustion engine as part of a hybrid system. Electrifying older vehicles is also a viable use for these motors, although the idea isn’t new and other companies have also tried developing similar solutions but ultimately failed to deliver them to market as promised.

We also can’t forget the Lordstown Endurance, which had four Elaphe-sourced in-wheel motors, and it almost made it into production. Maybe it was too ahead of its time with its unique motors and extended-range electric powertrain that featured a small battery. Range extenders are seen as a possible next big thing in the electrified vehicle space, but none of them will have motors inside their wheels.

Like Neapco, UK-based Protean Electric is also working on in-wheel motors for both passenger and commercial vehicles, as well as more heavy-duty applications. Its most recent Gen 5 Proteandrive motors make 138 hp, so up to 552 hp in a quad-motor configuration. It fits inside an 18-inch wheel, and they say it’s compatible with vehicles weighing up to 11,400 pounds (5.2 tonnes).

Many companies say their vehicle is built on a “skateboard platform,” but, in reality, it’s just a structural battery pack and two subframes bolted to it—it’s not a true skateboard. Having four in-wheel motors would allow for a true skateboard-like platform, freeing up the space between the wheels to be used in other ways (like for additional battery modules to extend the range).

Hub motors have found widespread success in two-wheeled vehicles of all shapes and sizes. But in four-wheeled vehicles, they were only sporadically used over the decades, typically just for testing purposes (or in Moon rovers), but they never became a staple of electrified automotive engineering. That could all change once a company brings them to market, taking advantage of all their benefits and seeing success with such a project.