how to make stronger joints

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I add some code (inertial mass) to the robot example shown on the tutorial pages of Gazebo. But when I start simulation, the robot collapses. Any idea how tho make 'stronger' joints?

<sdf version="1.6"> <model name="my_robot"> <static>false</static>

    <link name='chassis'>
      <pose>0 0 .1 0 0 0</pose>

      <inertial>
        <mass>40.000000</mass>
      </inertial>

      <collision name='collision'>
        <geometry>
          <box>
            <size>.4 .2 .1</size>
          </box>
        </geometry>
      </collision>

      <visual name='visual'>
        <geometry>
          <box>
            <size>.4 .2 .1</size>
          </box>
        </geometry>
      </visual>

      <collision name='caster_collision'>
        <pose>-0.15 0 -0.05 0 0 0</pose>
        <geometry>
          <sphere>
          <radius>.05</radius>
        </sphere>
      </geometry>

      <surface>
        <friction>
          <ode>
            <mu>0</mu>
            <mu2>0</mu2>
            <slip1>1.0</slip1>
            <slip2>1.0</slip2>
          </ode>
        </friction>
      </surface>
    </collision>

    <visual name='caster_visual'>
      <pose>-0.15 0 -0.05 0 0 0</pose>
      <geometry>
        <sphere>
          <radius>.05</radius>
        </sphere>
      </geometry>
    </visual>
  </link>
  <link name="left_wheel">
    <pose>0.1 0.13 0.1 0 1.5707 1.5707</pose>
    <collision name="collision">
      <geometry>
        <cylinder>
          <radius>.1</radius>
          <length>.05</length>
        </cylinder>
      </geometry>
    </collision>
    <visual name="visual">
      <geometry>
        <cylinder>
          <radius>.1</radius>
          <length>.05</length>
        </cylinder>
      </geometry>
    </visual>
  </link>

  <link name="right_wheel">
    <pose>0.1 -0.13 0.1 0 1.5707 1.5707</pose>
    <collision name="collision">
      <geometry>
        <cylinder>
          <radius>.1</radius>
          <length>.05</length>
        </cylinder>
      </geometry>
    </collision>
    <visual name="visual">
      <geometry>
        <cylinder>
          <radius>.1</radius>
          <length>.05</length>
        </cylinder>
      </geometry>
    </visual>
  </link>

  <joint type="revolute" name="left_wheel_hinge">
    <pose>0 0 -0.03 0 0 0</pose>
    <child>left_wheel</child>
    <parent>chassis</parent>
    <axis>
      <xyz>0 1 0</xyz>
    </axis>
  </joint>

  <joint type="revolute" name="right_wheel_hinge">
    <pose>0 0 0.03 0 0 0</pose>
    <child>right_wheel</child>
    <parent>chassis</parent>
    <axis>
      <xyz>0 1 0</xyz>
    </axis>
  </joint>

  </model>
</sdf>