I just did a deep dive on this. I had a Biped rig made in 3ds max, which is in itself a modular system thats impossible to customize, so I had to jump through a few hoops to make it work.
the main takeaways:
- You need a root bone at 0,0,0. So, the first bone in the hierarchy. If your rig doesn´t have one (like the mixamo rig afaik), you need to create it manually and don´t forget to add it to your skin modifier (or equivalent when you´re not using 3ds max).
- You might need to reparent some bones. In my setup the hand was linked to the last arm twist bone, in the modular setup you must have the hand linked to the lowerarm joint.
If your rig has toes, they need to be linked to the “ball” joint, instead of directly to the “foot”. - You need to make sure your naming convention is correct.
I just exported the unreal mannequin and renamed my joints accordingly.