Instead of using a hinge try using the spring action - i think it needs to be applied every frame. I cant really remember what values would be good, but just give it a try.
Every frame, d'oh! Thanks, David. I read about it, added one at the start of the layout and nothing happened. Now by using it always and adding a force between the wheels to sort-of hold them together the result is much better (also the cap is updated at the link above):
Thing is, there seems to be no way to point the forces at image points, thus the "base" just hangs there and spins around since it's only held by something right in the middle.
One idea I had was to disable rotations for it and just do a "Always - Base angle = Angle between the wheels", but that seems sort-of messy, is there a nicer, physics based way?
You might give the custom movement behavior a try.
I can tell you one thing. The triangles wont give proper collision detection at negative values.
Not sure if custom movement can supply all the wigglyness that the physics one can. But if this doesn't work out I'll give it a try. Speaking of triangles - had that problem, but placing the image point right in the middle solved it. Besides, these are just test graphics anyway.