Porblem with URDF model and tf_tree

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Hello all,

I am developing a robot from scratch. I created a urdf model (see here) and when I run $ urdf_to_graphiz Last_Mile_Robot.urdf I receive a closed kinematic chain as depicted in the following picture:

But strangely when I visualize the robot description model in rviz the kinematic chain is broken, I verified this also using $ rosrun rqt_tf_tree rqt_tf_tree resulting in the following output:

The "front_right" and "front_left" links are connected to "chassis" using a continuous joint which looks like this (see code lines 165-172 & 190-197):

<joint name="front_right_hinge" type="continuous">   
<parent link="chassis"/>   
<child link="front_right"/>   
<axis xyz="0 0 1"/>   
<origin rpy="0.0 0.0 0.0" xyz="0.325 -0.325 -0.30"/>   
<limit effort="0.1" velocity="0.1"/>   
</joint>

But when I change the type of said joints to fixed the kinematic chain is closed.

Any idea why this is happening?

Thanks in advance!

All the best, Georg