Why do constraints not work on bones with no mesh in Physics Asset

I have run into what I think is a bug involving skeletal mesh physics assets:

  • I import a skeletal mesh which has the lower body and upper body connected through a spine bone.
  • This spine bone does not have any mesh associated with it.
  • When creating the physics asset it creates all the constraints I need.
  • Reviewing the constraints the chest is connected to the spine, which is connected to the pelvis.
  • However when I simulate this physics asset the mesh separates at the connection between the spine and chest.
  • If I go and manually add a collider to the spine body the chest and spine will no longer separate, however the constraint still ignores angular limits.
  • If I go back to the skeletal mesh and add some geometry to the spine bone and create a new physics asset the constraints now work as expected.
  • So in summary there appears to be something wrong with constraints without geometry. Is this intended?

You didn’t happen to find an answer to this did you?

We ended up just putting a small cube in the hierarchy to work around the problem.

If there are no rigid bodies, such as when you’re creating a new Physics Asset.

You skeleton is broke or you didn’t use the correct settings when generating the physics asset TelltheBell.

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