[ODE] constant wheel propulsion

[Tobias Zimmer]

Okay, I solved the problem with the linked wheels. I decided to just
skip the collision of the propulsion, which isn't of interest for the 
simulation,
and decided to use one wheel as engine for all linked parts. The system is
balanced now.

BUT: due to the weight of the connected bodies the force driving the wheel
is varying at each time step. If I understood it correctly, the 
timesteps work
as following:

1. setting of parameters
2. calculation of resulting forces on each object / joint

How is it now possible to make a wheel turn evenly? The speed is constant,
alright, but the calculated forces vary and such does the movement in each
timestep! I don't want to set the force on the wheel directly, so I'm 
looking
for an alternative to that.

If anybody got an idea, I would really appreciate that.

Regards,
Tobias

PS to Mr.Watte: I downloaded and examined the ray-car model precisely,
thanks for the hint!