The Pros and Cons of Using In-Wheel Motors in Electric Cars

By · May 08, 2013

The Toyota ME-WE concept, with in-wheel motors for better packaging

The Toyota ME-WE concept, with in-wheel motors for better packaging.

There are many differences between gas and electric cars. One of the most intriguing is the possibility for an EV to fit motors inside the wheels.

We've seen many concepts with in-wheel motors, including the Toyota ME-WE, which debuted in Paris a few weeks ago. However, no production car is available with this architecture yet, but it's certainly worth a closer look. Think of the great possibilities: more room for passengers and the elimination of all transmission losses. The motor turns and the car moves without any inertia. It sounds so simple, but it's not. There are several challenges with in-wheel motors (IWM). The first one, obviously, is size.

The Toyota ME-WE concept, with a perfect flat floor, no transmission bump

The Toyota ME-WE concept, with a perfect flat floor and no transmission bump.

There's not much space inside a wheel, and it's impossible to give it all to an electric motor. A reducer is also needed. An electric motor can easily go at more than 10,000 r.p.m., but no car on the road can have its wheel turn that fast. The limited space then implies that an IWM will make less power than a motor under a front hood. A second implication, which receives less thought, is that to be powerful despite the small size, an IWM will need the best available magnets. That means the very expensive neodymium ones (a rare earth material).

It's possible to buy an electric scooter with an IWM, but one that makes very little power. A car requires much more, and the special case of three-wheelers set aside, it also requires at least two in-wheel motors—say, for both rear wheels. That raises the cost issue again. Two small high-tech motors will never be cheaper than a single larger low-tech motor.

Another common expense for a car manufacturer is the development of a differential. There are many off-the-shelf parts for this in a conventional architecture, but none can be used with IWM. The device—which in a curve makes the inside wheel turn at a lower speed than the outside one—requires software. All modern cars have an electronic differential, stability control and traction control, but those systems would work differently with IWM, and few engineers have any experience in that field. It's especially tricky when you add regenerative braking into the equation.

Finally, there's the problem of unsprung weight. Racers have been using light alloy wheels for decades. Heavier wheels would be a return to the past. With today's technology, manufacturers can make a complete in-wheel motor that weighs less than 100 pounds—including the tire on a 16-inch rim. So that problem appears to be manageable.

A Mercedes hybrid bus which was used at the Davos Economic forum, it had in-wheel motors

A Mercedes hybrid bus which was used at the Davos economic forum, it had in-wheel motors.

Reinventing the Wheel

But the biggest issue, by far, is this one: building a car with in-wheel motors requires a clean sheet approach—a new start with a new way of thinking. For example, every car on the road has a hood, but there's no need for one in a car with IWM. Some engineers are forecasting that IWM could appear in hybrid buses before they show up in cars.

If you think way off into the long-term future, most car people believe that in-wheel motors have tremendous potential. The technology is not cheap yet, but it could be if it was mass-produced—with the idea that many different models could have the same motors. The car's body, its look, its battery and the software that controls the in-wheel motors would be individualized for each model, but the motor hardware itself would be the same. Well, luxury cars, of course, would still get cooler shinier wheels.

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