By Florian Güldner.    

One of the greatest benefits of electric motors is their high rpm and the high initial torque, which eliminates the need for a gear box.  Now, to reduce the number of mechanical parts even further the idea is to have more than one motor, which also would allow you to control each wheel independently.  The latter clearly would create a “better car”.  As an example, look at Audi and its success with the Quattro, the electric version, is already on the streets for testing and is the Audi e-tron.  

The possibilities for multiple motors in a car are:  Two motors at one “axis” that control the two front/rear wheels directly, or four motors that control each wheel individually.  Another question would be, are these motors part of the wheel, or are they, like conventional motors, part of the car.  One of the first e-cars, the Lohner-Porsche, already had in wheel motors.  So one might think the technology is already developed, but even though engineers are working on creating a reliable car with in-wheel motors, the technology is not ready for mass production, yet. 

To look at the technology of in wheel motors, please look at this video from Siemens (turn of the sound first, its terrible). 

The main hurdle is the unsprung weight that is added to a wheel.  For the motorcycle brand buell, low unsprung mass is the central part of its philosphy.  For example, a motor with 25kW adds roughly 40kg to one wheel.  Unsprung weights has negative impact on driving dynamics, which is already heavy by the high weight of the battery, and also impacts many components of the car, especially the dampers and tires.  Also electronics need to be placed directly in the wheel.  Next to the benefit of directly controlling each wheel individually, the in-wheel motor also offers the benefit of more room in the engine bay, which can become a trunk, and it reduces the mechanics to an absolute minimum.

The Fraunhofer Institute recently announced the age of the in-wheel motor. Fraunhofer scientists are testing it on the demonstration vehicle »Frecc0«.  The main issue was to increase power and torque densities to reduce the unsprung mass.  

My personal opion is that the 4-motor alternative will controbute to the success of e-cars, especially pure electric vehicles, PEVs, significantly, since they are the chance to actually build better cars.  However, I am very sceptical about the progress with in whell motors.  The problem is fundamental physics, and there is no way around it, so the only solution would be:  Add elecotrinics, power, etc. to make the driver not feel the shortcomings.