How In-wheel Motors Work

Michelin's Active Wheel system
Michelin is hoping that its Active Wheel system will catch on with automakers soon.
Michelin North America, Inc.

Most car owners don't give much thought to the wheels and tires on their cars -- and who can blame them, really? If your tires are working the way they're supposed to, there's usually not much more to think about. There's a significant amount of technology, design and research that goes into making a good tire, but only two things typically come to mind when someone is buying new tires: How much do they cost? And how long will they last?

However, for a little more than a decade, many car manufacturers and tire companies have been working hard to change how automobile owners think about and use their tires. More specifically, they want to take the vehicle's power supply, as well as many other integral parts of a functioning automobile, and place it all inside the wheel. Based on that description, it probably won't come as a surprise to you that these somewhat new components are called in-wheel motors. And companies like Michelin are hoping that this new technology will revolutionize the future of transportation.


The basic principle behind a vehicle equipped with in-wheel electric motors is simple. The internal combustion engine normally found under the hood is simply not necessary. It's replaced with at least two motors located in the hub of the wheels. These wheels contain not only the braking components, but also all of the functionality that was formerly performed by the engine, transmission, clutch, suspension and other related parts.

Although the concept is relatively simple in theory, in-wheel motors pose a number of questions about performance, function and efficiency. We'll take a look at all of these questions and more beginning on the next page.


In-wheel Motor Performance

The Protean F 150 EV has no engine; however it does have four in-wheel electric motors that could produce more than 100 horsepower per motor (over 400 hp in total).
Photo courtesy Protean Electric

Producing a vehicle that uses in-wheel electric motors is a process that's a lot more complex than just tearing out the engine and then cramming electric motors into the unused space inside the wheel. This type of electric motor is designed to work on hybrid vehicles, full battery-operated vehicles and even fuel-cell-powered electric vehicles.

The amount of power generated by these in-wheel motors can vary depending on the manufacturer and the size of the motor. For example, a company called Protean Electric unveiled a Ford F 150 truck at the Specialty Equipment Market Association (SEMA) show in 2008. Protean modified the Ford F-150 EV by removing the V8 engine and adding four in-wheel electric motors to the truck. Each of the four Protean Drive motors are capable of delivering over 100 hp each, a total of 400 hp from all four motors -- far more than produced by the standard V8 engine. Each motor weighed only 68 pounds (31 kilograms) and gained power from a 42 kWh lithium-ion battery that provided the truck with a range of 100 miles (161 kilometers) before recharging.


The number of in-wheel motors a vehicle actually uses can be adjusted to meet the vehicle requirements. For instance, in most cases, two motors will supply sufficient power; however, if you're talking about an all-wheel-drive (AWD) vehicle -- either an off-road truck or a performance car -- obviously that would require four in-wheel motors.

Up next, let's take a look at Michelin's Active Wheel system to better understand how this technology functions.


In-wheel Motor Efficiency

Michelin's Active Wheel system
Michelin's Active Wheel system contains not only the electric motor that actually drives the wheel, but also the braking system and an active suspension system, too.
Michelin North America, Inc.

Depending on the manufacturer, an in-wheel motor may contain a variety of components, but most have the same basic parts. We're using Michelin's Active Wheel system as our example.

The outside of an in-wheel motor has very little variation compared to a standard wheel. However, once the wheel is taken off the vehicle, the main elements of the in-wheel electric motor system are exposed. That relatively small area contains the braking system, an active suspension system and the electric motor that actually drives the wheel. The in-wheel active suspension system is an electrically operated system that can react in a mere 3/1,000ths of a second to automatically correct pitching and rolling motions.


Some in-wheel motor designs offer what's called regenerative braking as well. That means the system captures some of its own kinetic energy while braking and sends it back to charge the battery. Some hybrids, such as the Toyota Prius and Tesla Roadster, already incorporate this regenerative braking technology, which provides the automobiles with a longer driving range.

One of the greatest advantages of in-wheel electric motors is the fact that the power goes straight from the motor directly to the wheel. Reducing the distance the power travels increases the efficiency of the motor. For instance, in city driving conditions, an internal combustion engine may only run at 20 percent efficiency, meaning that most of its energy is lost or wasted via the mechanical methods employed to get the power to the wheels. An in-wheel electric motor in the same environment is said to operate at about 90 percent efficiency [source: Lepisto].

Sounds pretty good, right? Well, keep reading to find out how an in-wheel electric motor doesn't sacrifice power while increasing efficiency.


In-wheel Motor Power

The Mitsubishi CT concept
The Mitsubishi CT concept uses the Mitsubishi In-wheel Electric Vehicle (MIEV) hybrid powertrain, which employs an electric motor in each of the vehicles four wheels.
AP Photo/Paul Sancya

So far, we've learned that the combination of several in-wheel motors can put out more than 600 horsepower and that they can receive their own energy while braking, but what about the instantaneous power that's sometimes required at the wheels? In other words, do these in-wheel electric motors provide enough torque for every application? After all, torque plays an important role in any automobile's response time and performance, doesn't it?

In a vehicle equipped with in-wheel electric motors, there's plenty of torque available -- almost instantly, as a matter of fact. Electric motors produce a high amount of torque, and since that force is transmitted directly to the wheel, very little is lost in the transfer. Each wheel can be equipped with sensors to determine how much torque is required at any given time. Similar systems exist in cars on the road now, but the response times are slightly slower due to the number of components involved and the more complex electrical communication pathways.


On a vehicle equipped with in-wheel electric motors, several major systems are housed within the wheel itself. So, it only stands to reason that many of the core components of a traditional automobile can be removed. We mentioned at the beginning of this article that the engine, transmission, clutch, suspension and other related parts can be eliminated on vehicles equipped with in-wheel electric motors because the in-wheel components handle all of these functions. This replacement of mechanical functions with electrical functions is often referred to as by-wire technology -- such as drive-by-wire, or brake-by-wire, for example.

Eliminating the engine makes it possible to add design and structural enhancements to a vehicle. To date, testing of the in-wheel electric motor system has been conducted by many automakers and technology companies, including the Venturi Corporation of Monaco for use in its Volage concept vehicle, but questions of reliability, durability and safety are difficult to report without widespread usage of the system.

For more information about in-wheel electric motors and the companies developing and testing the technology, follow the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links

  • Doggett, Scott. "Michelin to Commercialize Active Wheel; Technology to Appear in 2010 Cars." Dec. 1, 2008. (March 10, 2009)
  • Eureka Magazine. "Making the case for brushless wheels." September 2008.(March 10, 2009)
  • Garrett, Jerry. "Exhibition for Red-Hot Performance Takes a Green Turn." The New York Times. Nov. 6, 2008. (March 10, 2009)
  • "Could In-Wheel Motors be the Next Big Thing?" Oct. 1, 2007. (March 11, 2009)
  • Lepisto, Christine. "Michelin Unveils Active Wheel in Affordable Electric Car." Nov. 30, 2008. (March 9, 2009)
  • Michelin. "Michelin Active Wheel." Oct. 2, 2008. (March 4, 2009)