differential drive caster wheel problem

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Hello, I am trying to build a sdf model of a differential drive, but I am having some issues. The problems are caused by the caster wheel joint.

Here is the sdf code:

<link name="caster_wheel">
  <pose>0.045 0 -0.0528 0 -0 0</pose>
  <gravity>1</gravity>
  <self_collide>0</self_collide>
  <kinematic>0</kinematic>
  <inertial>
<mass>0.01</mass>
<inertia>
  <ixx>0</ixx>
  <ixy>0</ixy>
  <ixy>0</ixy>
  <iyy>0</iyy>
  <iyz>0</iyz>
  <izz>0</izz>
</inertia>
  </inertial>

  <collision name="caster_wheel_geom">
<laser_retro>0</laser_retro>
<geometry>
  <sphere>
    <radius>0.01</radius>
  </sphere>
</geometry>
<surface>
  <friction>
    <ode>
      <mu>0.01</mu>
      <mu2>0.01</mu2>
      <fdir1> 0 0 0</fdir1>
      <slip1>0</slip1>
      <slip2>0</slip2>
    </ode>
  </friction>
  <bounce>
    <restitution_coefficient>0</restitution_coefficient>
    <threshold>100000</threshold>
  </bounce>
  <contact>
    <ode>
      <soft_cfm>0</soft_cfm>
      <soft_erp>0.2</soft_erp>
      <kp>1000000000000</kp>
      <kd>1</kd>
      <max_vel>100</max_vel>
      <min_depth>0.001</min_depth>
    </ode>
  </contact>
</surface>
  </collision>

  <visual name="caster_wheel_geom_visual">
<cast_shadows>1</cast_shadows>
<laser_retro>0</laser_retro>
<transparency>0</transparency>
<pose> 0 0 0 0 -0 0</pose>
<geometry>
  <sphere>
    <radius>0.01</radius>
  </sphere>
</geometry>
<material>
  <script>
    <uri>file://media/materials/scripts/gazebo.material</uri>
    <name>Gazebo/Yellow</name>
  </script>
</material>
  </visual>
</link>



<joint name="caster_joint" type="revolute">
      <pose>0 0 0 0 -0 0</pose>
      <parent>chassis_link</parent>
      <child>caster_wheel</child>
      <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0</damping>
      <friction>0</friction>
    </dynamics>
    <limit>
      <lower>-10000000000000000</lower>
      <upper>10000000000000000</upper>
      <effort>0</effort>
      <velocity>0</velocity>
    </limit>
      </axis>
    </joint>

before starting the simulation, everything looks normal:

the caster wheel is the front yellow sphere. Right after clicking on the play button of gazebo this happens:

I tried to understand what is wrong, but I can't see any problem.

Thank you

Andrea