Tractor Simulation: Conflicting Collision / Link / Joint values in SDF

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Hello All,

I am new to ROS and Gazebo, so I'm sure I'm dealing with some very basic problem. I will give a basic description of the problem in the hopes that someone might point me in the right direction. You can visually understand the problem better, by looking at the picture at the end of the post.Thanks!

I have a tractor model, which I extended with a front loader. The entire frontal-shovel is made up of 2 links registered in my sdf file. I also registered 2 joints: the shovel-joint has base as parent and the front-arm-joint has the shovel-joint as parent. When I position the front loader, my entire tractor gets pushed back and upwards in a way that the front wheel lose contact to the ground. This of course makes the entire tractor uncontrollable.

I did also make other changes to my simulation by creating a publisher and subscriber,as well as messages to catch keyboard press in order to move the frontloader up and down. Despite of this, I believe that the main problem is in the SDF file where I declare the positioning of my two frontloader links and collisions.

Somehow my configurations are conflicting with the tractor. I have tweaked values several times, but nothing seems to get me what I need, which is the tractor with all four wheels on the ground and the frontloader positioned in front of it as it should.

Any help with this would be highly appreciated.

Thanks you all in advance

 <link name="base">
  <pose>0.0 0.0 0.1 0.0 0.0 0.0</pose>
  <inertial>
    <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
    <mass>150.0</mass>
    <inertia>
      <ixx>0.01</ixx>
      <ixy>0.0</ixy>
      <ixz>0.0</ixz>
      <iyy>0.01</iyy>
      <iyz>0.0</iyz>
      <izz>0.01</izz>
    </inertia>
  </inertial>
  <collision name="base_geom">
    <pose>0 0 1.7 0.05 0.0 1.570796326794897</pose>
    <geometry>
      <box>
        <size>3.2 4.25 2.5</size>
      </box>
    </geometry>
    <surface>
      <contact>
        <ode/>
      </contact>
      <friction>
        <ode/>
      </friction>
    </surface>
  </collision>
  <visual name="base_visual">
    <pose>0.3 0 0.0  0.05 0.0 1.570796326794897</pose>
    <geometry>
      <mesh>
        <uri>model://Fendt/base.dae</uri>
      </mesh>
    </geometry>
  </visual>
  <gravity>1</gravity>
  <self_collide>0</self_collide>
</link>
<link name="back_left_wheel_link">
  <pose>-1.5 0.9 1.1 0.0 0.0 0.0</pose>
  <inertial>
    <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
    <mass>5</mass>
    <inertia>
      <ixx>0.01</ixx>
      <ixy>0.0</ixy>
      <ixz>0.0</ixz>
      <iyy>0.01</iyy>
      <iyz>0.0</iyz>
      <izz>0.01</izz>
    </inertia>
  </inertial>
  <collision name="back_left_wheel_link_geom">
    <pose>0.0 0.375 0.0 -1.570796326794897 0.0 0</pose>
    <geometry>
      <cylinder>
        <length>0.75</length>
        <radius>1.1</radius>
      </cylinder>
    </geometry>
    <surface>
      <contact>
        <ode/>
      </contact>
      <friction>
        <ode>
          <mu>1</mu>
          <mu2>1</mu2>
        </ode>
      </friction>
    </surface>
  </collision>
  <visual name="back_left_wheel_link_visual">
    <pose>0 ...