actually, remove all wheels and it seams to sit fine. So the wheels (both non steerable and steerable) seam to somehow cause what looks like collision between the chair the beams connected to it

Hmm... doesnt seam to have anything to do with the wheels. It seams like the beams are colliding with the chair, even though they are jointed to it. So they are constantly being pushed away while being connected, and it causes the vehicle to flip around