The linear motion on the Y-axis is for damper, right? (Because damping is not really a need for me)

A convex shape fails, as some of the tires are just to low res, which results in a really un-smooth behavior.

However, my biggest problem is still the steering. Not locking the other two rotation dimensions results in uncontrollable behavior (even with LIMITED and limits set to 0). Using an additional entity and an additional 6D joints results in "lagging" of the wheel (behind the additional entity). The additional entity also behaves odd. I guess this could be related to the masses of the objects?