Robot jumps up in the air, why?

edit

Hi, its my first post. So I am trying to write a urdf for my real robot and test/program it in a gazebo world.

Problem: my launch file spawns my robot with a paused world. When I un-pause it, it does this weird flip/jump/fly thing.

Solutions tried: Spawn robot just of the ground > same thing''''' used gazebo "view" of collisions and so on > looks fine to a newbeee''''''''' played with inertia values > no help''''''' ..... and bunch of other stuff

what does make a difference is when I comment out my caster link, the robot behaves normal? I am sure... no weirdness in gazebo.

Might have some thing to do with the small inertia's of the links?

A side node: I know about xacros > lets get it working first, then go there''''''

Cant attach URDF file so here is the caster link snip:

<!--.............................................................caster_link...................................................................-->

<link name="sphere_link">
    <visual>
    <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
    <geometry>
        <sphere radius="0.026"/>
    </geometry>   
    </visual>
    <collision>
    <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
    <geometry>
        <sphere radius="0.026"/>
    </geometry> 
    </collision>
    <inertial>
        <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
        <mass value="0.002"/>
        <inertia ixx="0.0000015417" ixy="0.0" ixz="0.0" iyy="0.0000015417" iyz="0.0" izz="0.0000004"/>
        <!--I added 1x zero to all inertia calculations to get gazebo view of enertia looking write? all sceuars?-->
    </inertial>
    <!--gazebo reference="sphere_link">  gazebo tags outside links, posibly in one gazebo tag
      <material>Gazebo/Blue</material>
    </gazebo-->

</link>

 <joint name="sphere_joint" type="fixed">
    <parent link="caster_link"/>
    <child link="sphere_link"/>
    <origin xyz="0.0 -0.008 -0.02" rpy="0.0 0.0 0.0"/>
</joint>   

<link name="caster_link">
    <visual>
        <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
        <geometry>
            <cylinder radius="0.0095" length="0.05"/>
        </geometry>
    </visual>
    <collision>
         <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
        <geometry>
            <cylinder radius="0.001" length="0.5"/>
        </geometry>    
    </collision>
    <inertial>
        <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
        <mass value="0.002"/>
        <inertia ixx="0.0000015417" ixy="0.0" ixz="0.0" iyy="0.0000015417" iyz="0.0" izz="0.0000004"/>
    </inertial>

    <gazebo reference="caster_link">
      <material>Gazebo/Blue</material>
    </gazebo>

</link>

<joint name="caster_joint" type="continuous">
    <parent link="base_link"/>
    <child link="caster_link"/>
    <axis xyz="0 0 1"/>
    <origin xyz="0.0 -0.089 0.045" rpy="0 0 0"/>
</joint>

How does one share the complete code?