Why Not One Motor Per Wheel

One electric motor per wheel may sound like a nice idea but it is rarely used. The SLS AMG Electric Drive is the most memorable one in my mind. There are some trade off for this configuration that I haven’t seen mentioned very often. Mercedes advertises the vehicle with 552 kW (751 hp) total power (source), each motor outputting a quarter of that amount. However if we lose traction at one or more wheels, the total amount of power available to us is now a fraction of the total. There is no fancy LSD to transfer power left to right or forward to aft.

This situation gets worse for serious off road vehicles. While climbing a steep grade the front axle may have zero traction. With independent motors at each wheel we are now relying on half of the available power to get us up the hill. In a traditional 4×4 we would lock all of the diffs and the total engine power would get down to the ground to the tires that have traction. This is an unfair comparison of course. But if I were designing an electric 4×4 with independent motors at each wheel, each motor would need to be sized much larger than the equivalent vehicle with a single motor and locking diffs.

There are many advantages to having one motor per wheel. Improved torque vectoring, traction control, and packaging (arguable) are some of them. However the down side is that in some cases the total combined power needs to be much higher to match the equivalent conventional drivetrain with LSD or locking diffs.