I tried being clever and pretty much wasted 6 hours….

I spent a bunch of time rigging up this model, a large portion of the time was spent undoing the dumb modifiers I put on the legs. Specifically I used array modifiers and mirror modifiers to generate the legs after making the first one.

Using this technique ensures that when you finally go to export you need to manually re-assign origins somewhere on the model which is something you really need to keep track of when rigging properly.

This all went good but I decided to put the bones 90 degrees offset because I thought it would make it nicer to manipulate. Which I would say is true if I was going to manually manipulate the bones. However, I tried getting smart and moving into unreal engine control rigs: https://dev.epicgames.com/documentation/en-us/unreal-engine/control-rig-in-unreal-engine

Which in theory should let me animate in unreal, which would save me an export/debug step which I normally have to do from blender. However I kept hitting weird issues with setting up controls and having unreal auto-recognize things… Then I noticed the issue once I tried testing the physics asset:

So that isn’t good. Essentially unreal makes a set of simple geometric shapes that it glues onto your model when you import to try to get a good collision box setup. In this case me twisting the bones make the system give up and just surrounded the body with a capsule and legs with one big box:

So I attempted to fix this (which looked good)

…But then I hit the next problem…

What’s going on here is that there’s a parenting loop going on. Looking back at the original model I put the bones 90 degrees offset from the mesh (again because I think I’m smarter than how everyone does stuff).

With the bones offset from the mesh I had to apply the physics geometries to the mesh itself rather than the bone. So then unreal was trying to modify the position of the mesh, but the bone had no constraints to the physics so it tried putting the mesh back, then the physics manipulated the mesh, then the bone manipulated the mesh, then the physics manipulated the mesh, then the bone manipulated the mesh….So that comes out with the result you see above which is everything kind of giving up. The fix here is to re-orient the bones so I can attach the physics geometries to the bone….I basically wasted my morning setting myself up for failure. However this is the first complex mechanical mesh I’ve ever done so it’s still a learning experience in my head.