[ODE] Car stability with lowered centre of mass leads to problemswhen rolling

[Mattias Fagerlund]

As discussed previously, aside from lowering the center of gravity to a 
realistic position, simulating the hydrodynamic lift of the car body (or 
aerodynamic down force if you prefer) will also help stability.

Fdrag = DragCoefficient * LiquidDensity * Velocity ^ 2 * Area / 2
Flift = LiftCoefficient * LiquidDensity * Velocity ^ 2 * Area / 2

Drag opposes the direction of the relative velocity, lift acts 
perpendicular to it.

cheers,
mattias