I've noticed something odd about my vehicle sim, and I was wondering if
anybody else had come across the problem.
I have a vehicle which has 4 tyres. I drive forwards until I get
considerable velocity, then hit hard left on my controller (and hold it
down). This causes my front tyres to rotate some angle theta to the car's
axis. I also keep the accelerator held.
As a result of this slip angle, I generate forces in a lateral direction to
the front tyres (using Pacejka, or whatever method any other method used to
calculate cornering forces). This causes a torque to my car which will
rotate the whole vehicle to the left (-ve torque around the y axis in my
coordinate system).
As the car rotates, the rear wheels also begin to generate forces in their
lateral direction to oppose their motion (+ve torque around the y axis).
OK, now here's the weird bit. The lateral forces at the rear can never be
greater than at the front (due to part of the traction budget being used to
accelerate the car). After initially doing a skid round to the left (as I'd
expect) after a while it begins to spin back partly to the right, before
going back to the left again etc etc.
This seemed odd to me, until I sat down and figured out what was happening.
Currently my tyres are symmetric about the car's COM, and I'm ignoring any
weight transfer due to cornering/braking etc. (ie no suspension). So you
would expect that applying equal forces to all four tyres would have a
resultant torque of zero. However, if the front wheels are turned, then the
magnitude of the torque due to these wheels decreases (since there's a
smaller angle between the tyre's lateral direction and the line from the
tyre to the vehicle's COM). So in the situation above, equal forces to all
four tyres would result in the back end swinging around.
This is what's happening in my sim. As I'm turning, the torque generated by
the rear wheels is gradually increasing until eventually it is greater than
for the front wheels, causing the direction of the overall torque to
reverse. This is behaviour that seems to make sense mathematically, but
which I'm sure doesn't happen in real life (I've never noticed it anyway) or
in other sims I've seen.
Possible reasons I've thought of are different mass distribution, weight
transfer etc, but I can't really see how this would make a difference in all
situations.
I've tried applying the force at the front wheels in the car's lateral
direction rather than the wheel's lateral direction, and this overcomes the
problem (obviously) but it just seems like a bit of a hack to me.
Has anybody else noticed this problem? And if so, what did you do?
Cheers,
Mike.