@Ventilator: I have it working now, but I still need to figure out, why it did not work. Before I get back to you on this, some general tips:
- use hinges for the back wheels
- don't use addtorque on the wheels- set a motor value, otherwise the wheels will get kicked away in different directions by the environment. Motor values try to maintain a certain speed/direction no matter what.
- don't set the spring-damper values unless you have to, just pass a nullvector in order to use the defaults

Now how do I get that boulder off of the roof of my car?