Talk the torque

More thoughts on in-wheel motors and their effects on twisting force

- MARK ORTIZ

Q Regarding the question about in-wheel motors in Racecar Engineering issue V31N11, I was surprised not to see any mention of torque vectoring with hub motors. Or was this in the catch all about making vehicles go directions other than where they are pointed?

When you mix performance vehicle dynamics and hub motors, independent control of the motors is the first thing that crosses my mind. This may be as simple as using software to link the speed of two wheels together, rather than a set of clutch discs. Once you do it in software, it can be cognisant of the yaw rate of the vehicle. Obviously, this is more of a challenge with a conventional limited slip.

THE CONSULTANT

A That was a significant oversight on my part. Yes, having a separate motor for each wheel does open up interesting possibilities for generating yaw moments by applying unequal power to the right and left wheels, in addition to steering them individually. Of course, it is not necessary for this to occur that the motors be hub motors. They could be sprung. There just has to be one motor for each wheel, or at least one for each side of the car.

The control strategy for such a feature, or combination of features, really poses some puzzles. It's more complex than thrust vectoring for an aircraft, for example. Near the limit of adhesion, the relationship between torque applied and ground plane force produced is complex and non-linear, and longitudinal force capability plays off against lateral force capability.

If you are directing the exhaust gases of a jet or rocket engine, you can be pretty sure that more fuel to the engine will result in more reactive propulsion opposite the direction of the discharge. The relationship may be somewhat non-linear, but it doesn't reverse.