Link without inertia for ball joint

[muttistefano]

Environment

• OS Version: Ubuntu 24.04 , binary gz harmonic

Hi all,
i am trying to create a ball joint by having two consecutive rotated revolute joints in the same origin.

To do this, I need to define links with no inertia to tie together the joints, in this manner :

<joint name="${prefix}_ankle_1" type="revolute">
  <origin xyz="0 0.17 0" rpy="${-pi/2} 0 0"/>
  <parent link="${prefix}_KFE"/>
  <child link="${prefix}_fake_ankle_1"/>
  <axis xyz="1 0 0"/>
  <limit lower="${-16*pi/180}" upper="${16*pi/180}" effort="0" velocity="0"/>
</joint>

<link name="${prefix}_fake_ankle_1"/>

<joint name="${prefix}_ankle_2" type="revolute">
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <parent link="${prefix}_fake_ankle_1"/>
  <child link="${prefix}_fake_ankle_2"/>
  <axis xyz="0 1 0"/>
  <limit lower="${-16*pi/180}" upper="${16*pi/180}" effort="0" velocity="0"/>
</joint>

<link name="${prefix}_fake_ankle_2"/>

When I load the model in gz sim, it complains about links with no inertial tag:

  Error Code 19: Msg: urdf2sdf: link[LF_fake_ankle_1] has no <inertial> block defined. Please ensure this link has a valid mass to prevent any missing links or joints in the resulting SDF model.
  [gazebo-2] Error Code 19: Msg: urdf2sdf: parent joint[LF_ankle_1] ignored.
  [gazebo-2] Error Code 19: Msg: urdf2sdf: [1] child joints ignored.
  [gazebo-2] Error Code 19: Msg: urdf2sdf: [1] child links ignored.

I tried adding the tag

<gazebo reference='${prefix}_ankle_1'>
  <disableFixedJointLumping>true</disableFixedJointLumping>
</gazebo>

in the urdf, but it does nothing.
Is there a way to implement this ?
Thanks

[pran-d]

Hi, facing this issue too. Has there been any fix for it yet?