Linked test project contains already set up actor as described in steps to reproduce. Using a kinematic body on a root bone, setting all other bodies sleep family to Custom, and their custom sleep threshold to 1000000. You will notice when simulating that the kinematic root body will prevent the skeletal mesh from sleeping. There is an actor using exactly the same physics asset except it has no constraint. This actor falls asleep as intended.

This issue was reported and tested in 4.21.2 (CL-4753647). It was reproduced in 4.20.3 (CL-4369336), 4.22 (CL-5275346), and Main 4.23 (CL-5295869)

Steps to Reproduce
  1. Download and open the attached project
  2. Press Simulate on the ThirdPersonExampleMap


  1. Create a kinematic body on a root bone of character's mesh.
  2. Create an all-free constraint from pelvis to root.
    ( similar setup was shown in Epic's Live Stream [Link Removed] )
  3. Set sleep family to Custom and custom sleep threshold to 1000000, so that sleep effect will be noticeable.
  4. Create a test actor using this skeletal mesh.
  5. On BeginPlay set all bodies below and including pelvis to simulate physics.
  6. Place this actor in the world.
  7. Play.

Results: Skeletal mesh fails to sleep.

Expected: Constraint does not prevent skeletal mesh from sleeping.

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Won't Fix
ComponentUE - Simulation - Physics
Affects Versions4.
Target Fix4.23
CreatedMar 5, 2019
ResolvedJun 13, 2019
UpdatedJul 8, 2019
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