[PhAsset] Movement lock overriden by parent bone, unwanted spring force

I’ve made this gif that illustrate the problem I’m facing: [http://gfycat.com/GaseousCarefreeAstrangiacoral][1]

The chasis collision body is in the root bone. The wheels are other bones that, while being under the root bone in the hierarchy, are unconnected to it. These are the settings of the constraint of one of the wheels.

If the XYZMotions of the wheels are completely locked, why do they act as a “spring” when being forced by the parent bone? How can I force the chasis movement to be limited by the wheels collisions as well? I mean, like a car without suspension.

Advanced thanks