URDF: Continuous joints just slides down

Hi there. I'm using ROS Indigo Gazebo 2.2.6 I'm trying to set up a simple car simulation for gazebo which consists of a box as chassis and 4 cylinders as wheels. Also I would like to implement a node which transform the steering angle for a virtual bicycle model to the steering angles of the two front wheels. For that I tried to implement two perpendicular continuous-joints for the front wheels and one continuous-joint each for the back wheels. However, when I start the simulation the chassis seems not to be fixed to the wheels and the chassis just falls down in the front but stays in place in the back. Here's the code radius="${wheel_bearing_radius}"/>Gazebo/Black