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.

With tyres on a road surface, however, applying more power only gets you more thrust up to a point, beyond which you break traction and lose force in all directions.

Same applies to steer or slip angle. Up to a point, steering a tyre more, or running it at a greater slip angle, adds lateral force, but beyond that point lateral force diminishes.

With reactive propulsion, there is a trade off between thrust in the direction of travel and thrust perpendicular to that, but there is no point at which more nozzle angularity gets you less perpendicular force.

Performance envelope

With a road vehicle, accelerations change tyre loading, and the entire performance envelope of the tyre grows and shrinks with that. It also varies with temperature, inflation pressure, nature of the road surface and ageing of the tyre, which can occur very rapidly in racing rubber.