Advantages of packaging the motors directly in the wheels include eliminating such costly components as driveshaft, axles, transmission and differential. This would contribute to fuel economy by reducing drag and weight, and would enhance passenger and luggage space. There would also be reduced complexity, improved regenerative braking and simplified switching from two- to four-wheel drive.

In-wheel motors are not a new idea. In 1899 a young engineer named Ferdinand Porsche (later of Volkswagen fame), worked for the Austrian Lohner Electric Company. Lohner wanted to enter the automobile business, so Porsche created the Lohner-Porsche with battery-powered electric motors in each front wheel hub. He later modified it into his pioneering “Mixte,” a hybrid with an on-board internal combustion engine to generate electricity for the motors. This early vehicle won Porsche a gold medal at the 1900 Paris Exposition.

The in-wheel motor is not without challenges in size, packaging and un-sprung weight. The greater the power and torque desired, the heavier the motor will be. While it is challenging, it is now technically possible to design electric motors and their associated electronic control mechanisms that are thin and small enough to be packaged within the wheel. Several manufacturers, including Michelin and Protean Electric, have achieved it. 

The other problem is the change in vehicle dynamics associated with the increased unsprung weight, or mass, of motors located within the wheels. Unsprung mass includes wheels, tires, brake rotors, callipers and part of the suspension, all of which are not supported by the car springs. The traditional goal has been to decrease unsprung mass to achieve reduced wheel hop so that traction, vehicle ride and handling qualities are enhanced. A good example of lowering unsprung mass is moving the brakes from the wheels to a position inboard, making it part of the sprung mass. 

The unsprung mass associated with in-wheel motors has made vehicle manufacturers wary of them. California-based Tesla Motors, manufacturer of the all-electric Tesla roadster, considered in-wheel motors but decided the disadvantages outweighed the advantages.

So there is lots of scepticism about in-wheel motors, the conventional wisdom being that they add excessive wheel mass. But Protean Electric Ltd. of Farnham, Surrey, UK, designer and developer of the “Protean Drive” in-wheel electric drive system, argues that the negative impression is unfounded. As Protean’s principal applications engineer Tom Prucha succinctly puts it: “In reality, the unspsrung-mass penalty related to wheel motors is a myth.” 

Protean Electric (www.proteanelectric.com) plans to elaborate on this at the Society of Automotive Engineers 2011 World Congress with an oral presentation entitled: “Unsprung Mass with In-Wheel Motors; the Myths and Realities – Closing the Circle.” It has already demonstrated, and allowed the media to drive, a four-wheel drive Ford F-150 pickup with in-wheel motors at the 2011 North American International Auto Show in Detroit. 

To dispel doubts and validate its belief in in-wheel motors, Protean Electric used a 2007 Ford Focus as a test bed and engaged English engineering consultants Lotus Engineering (www.lotuscars.com/engineering) and Dunamus Ltd., to perform independent studies. The first step was mathematically modelling the impact of different suspension modifications such as spring rates and shock absorber compliance with each wheel carrying an extra 30 kg of rotating and non-rotating weight. This approximated the mass of Protean PD18 (18 in.) in-wheel motors, and their calculations suggested that Focus’s optional ST suspension would be suitable.

They then modified the vehicle with the sportier Focus ST suspension with its stiffer springs and bushings, firmer shock absorber compliance and stiffer anti-roll bar. The last step was testing the modified vehicle’s behavior on a two-post shaker rig and in actual driving. After subjective and instrumented testing the engineers concluded that while there were differences in vehicle dynamics using the ST suspension, they were not serious, and “should be overcome through the application of normal engineering processes within a product development cycle.” 

While Protean’s study sounds convincing, there are still sceptics about in-wheel motors. The advantages, however, would seem to make the concept well worth pursuing.