In the first phase of vehicle electrification, the focus was on acceleration and speed. Nowadays,…
In recent years, we’ve seen some vehicle developers moving towards powertrain configurations where the motor is mounted inside the wheel: an in-wheel motor system. We must admit, there is an impressive amount of new space available when the electric motors are integrated into the wheels. And sure, these so-called hub motors or in-wheel motors come with certain benefits, but they also create some challenges.
Motor size and cost
In-wheel motors aren’t new. At the beginning of the 20th century, Ferdinand Porsche’s first hybrid vehicle used hub-mounted electric motors in each wheel. These motors power the wheel directly. There’s no need for a gearbox or driveshaft. When using a reduction gearbox, the speed is reduced and the torque is multiplied. But with an in-wheel motor, there is no reduction. Wheel speed is equal to motor speed so the required torque and power need to be delivered in a direct-drive mode. What’s the impact of this? Since motor torque relates directly to motor size and there is no torque multiplication (as there is no gearbox), the motor needs to be sized bigger to reach the required performance. This also has a significant impact on cost since the level of torque production translates directly to the quantity of permanent magnet material needed. Especially with 4 wheels (and therefore 4 motors), the drivetrain cost increases significantly.
In-wheel motors are directly exposed to dust, salt, water, and other fluids but also to vibrations and shocks, which shortens their life expectancy. That’s one of the main reasons Ford eventually decided to ditch the in-wheel motor concepts they were working on for the new electric F-150.
Not much data is available about the practical long-term durability of in-wheel motors in vehicles, simply because there aren’t that many in operation. But for the bulk of use cases, we can expect that in-wheel motors are far from an ideal solution.
An important aspect in vehicle dynamics is “unsprung weight”. Adding mass to a wheel reduces driving comfort. Why is that? Unsprung weight is not supported by the suspension of the car. Reducing unsprung weight is, therefore, key to improving handling. The lower the unsprung weight, the easier it is for the shocks and springs to keep the tires in contact with the road over bumpy surfaces. A lot of problems, if not all of them, are caused by inertia. A larger mass means higher inertia. And higher inertia means more workload for shocks and springs. If unsprung components have a high mass they are harder to accelerate/decelerate, which makes it more difficult for the suspension to maintain a consistent tire load.
Consistent tire-to-road contact is essential for handling and safety. That’s why car manufacturers usually go all out to make the wheels as light as possible to ensure optimal performance.
In-wheel motors add directly to the unsprung mass of the vehicle since they’re located ‘before’ the suspension when starting from the road surface. This problem is severely compounded by the fact that in-wheel motors tend to be direct-drive, and therefore heavier than their onboard counterparts.
An in-wheel powertrain is typically implemented as a 4WD system. Having the four wheels driven has a positive impact on vehicle performance. With 4WD there’s better grip on the road. A torque vectoring system makes sure that each wheel has the optimum grip in every situation. This improves safety, acceleration, and handling. However, 4WD can just as well be implemented with inboard motors (like most electric ‘performance’ cars do nowadays), so this is by no metric an advantage for in-wheel motors. The same goes for the way energy is recuperated with regenerative braking: In-wheel motors don’t provide any advantage over on-board motors in this regard.
In-wheel motors will always suffer more from no-load losses and part-load losses because the motors can’t be decoupled from the wheels. There is a trend of mounting more motors in a vehicle and what we see, for example with the Porsche Taycan, is that the rear motors are decoupled from the wheels when cruising at highway speed to optimize the efficiency of the powertrain. This is very difficult to implement with in-wheel motors. They always rotate with the wheels, even when not used actively.
Some companies claim that the efficiency with in-wheel motors is higher because there is no gearbox. Although a gearbox does always introduce a bit of inefficiency, the 1 or 2-speed gearboxes that are typically used for EVs are much more efficient than the complex multi-stage transmissions seen in combustion engine powertrains. We’re looking at 1 or maybe 2 percent efficiency loss in the gearbox. This loss in efficiency is, however, overcompensated by the fact that, due to the gearbox, the electric motor can run on its most efficient operating area, resulting in an increase in vehicle range when compared to a direct-drive system, depending on the use case and driving cycle.
Skateboard platforms and cornering modules
In the recent trend of skateboard platforms we see 3 different setups:
- centralized inboard motors (e.g. Canoo),
- close-to-the-wheel (but not in-wheel) motors (e.g. REE), and
- in-wheel motors (e.g. Lightyear)
Putting the motors closer to the wheel introduces new engineering challenges. Take the example of braking. All EVs use some form of regenerative braking, which is very beneficial for efficiency. But not all braking can be done with regenerative braking alone, e.g. for an emergency stop. This means friction brakes are still necessary and in-wheel motors leave very little space to install them. As a result, cooling of the brakes becomes an issue.
When do in-wheel or close-to-the-wheel configurations make sense?
New configurations like in-wheel and close-to-the-wheel do have their advantages. Especially in cases when battery space is essential, modularity is key, and the vehicle will drive on smooth, flat surfaces, these new platforms can be a valid alternative.
An example are smaller commercial vehicles that drive at relatively low speeds and on smooth surfaces like urban areas. In combination with a skateboard-style EV platform, where all of the mechanical parts are embedded in the platform, this layout maximizes the usable space available.
At Traxial we believe in multiple motors per vehicle, especially when making use of the dimensional advantages of axial flux motors. The combination of an axial flux motor with a 1 or 2-speed gearbox is a very pragmatic solution, even when using 1, 2, 3, or 4 electric motors in one vehicle.
Many concepts and demonstrators have come along in the past 20 years but no single passenger car is series produced with in-wheel motors (yet). For now, we can say that Ford was right to nix in-wheel motors for their F-150 pickup truck. Looking at their use case, inboard motors make more sense as they offer better vehicle dynamics and durability because the motors are inside the car body and shielded from the environment. On top of that, it’s a more energy-efficient solution as one or multiple motors can be decoupled when relevant. The majority of ground e-mobility use cases benefit the most from inboard motors. For some specific use cases, it does make sense to use either in-wheel or corner modules. But for most applications, a powertrain combining a 1 or 2-speed gearbox with a Traxial yokeless axial flux motor will offer superior efficiency in the smallest possible package.
Talk to our sales team to explore the opportunities of integrating Traxial’s yokeless axial flux motors into your next vehicles.