UR10 linked to MiR100 problem

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I want to attach an UR10 on top of a MiR100 in a Gazebo simulation. Each model works perfectly on it's own, the UR10 can receives trajectory and the MiR100 can be controlled by turtle_teleop_key. Then I made another urdf file, linking my MiR100 and UR10 with a fixed joint since I want the UR10 to be fixed relative to the MiR100. When I run the simulation the UR10 is still receiving the trajectories nicely but the MiR doesn't move properly, it' slow and It seems the UR10 is too heavy to be transported. I tried to separate them, linking to a "world" joint and everything works, but of course the UR10 is not linked the MiR. Only when I use this joint I see problems:

  <joint name="mir_ur_joint" type="fixed">
    <parent link="base_footprint" />
    <child link="UR_base_link" />
    <origin xyz="0 0 3.53696e-1" /> <!-- 3.4433e-01 + 0,018732/2 = 3.53696e-1 -->
  </joint>

I also tried to increase the z value because I thought the meshes were clipping but it doesn't seem to be the solution.