I animated the ROOT bone, and added a 'Limit Location' on the BottomRR set to be fixed on Z=0. But somehow the ROOT bone forces the mesh to dip under Z=0 during the rotation (while the bone BottomRR stays perfectly in place). How can I solve this?
I would say its due to the default f-curve created on the ROOT bone, change its type (channel) to 'linear Extrapolation' that could fix it, but you may have to add one of the 'modifiers' to it. Also see this Rotate a Cube on a Plane Using Drivers