Try selecting all constraints and then checking the box for disable collision(so collision is disabled between bodies connected by constraints).
If there are other bodies that overlap each other that are not connected by constraints you will need to disable their collision as well(to each other), by selecting them both and right clicking and to disable collision in the context menu.
I create a simple physics asset for my base mannequin character and when I press simulate it flies off into space.
I’ve read this from the docs: Troubleshooting Common Physics Asset Errors | Unreal Engine Documentation
I’ve tried to scale down some bodys, remove others which were overlapping etc such that I get no “explosion” issues. But I don’t think this is the case, since on a default mannequin simulation runs perfectly fine. However it still flies away. So I deleted that physics asset copied the default mannequin’s physics asset and assigned it to my skeletal mesh mannequin (which should be the same one? as all the bones are the same). While the simulation on the default mannequin works perfectly, the simulation on my mannequin forces the mesh to fly into space.
The mesh flies off erratically and continuously, meaning that forces are constantly applied to the bodys. This is a typical ue4 skeleton, running default ue4 animations. Anyone knows why this is happening? Perhaps there is some setting I’m missing, some setting that gets toggled?
I did exactly as you said and it didn’t help. In fact I’m trying to recreate the entire physics asset right now and the character still behaves very erratically with just 4 shapes in him, those of the left leg. I’m wondering if there’s something going on with the root; I don’t know.
it sounds similar to this issue:
https://answers.unrealengine.com/questions/825429/ue-4202-merging-bones-while-using-physics-body.html
I think the first thing to make sure is all your bones have scale of 1 when importing skeletal mesh. Make sure you are exporting in cm, and check if they have the same scale in engine.
Thank you for helping; I have checked the bones both in 3ds max and in UE4. In 3ds Max the scale of almost all bones is 100, except for a few of them mostly on the left hand fingers which are at 99.999 or 100.0001. Then checking the bones in engine the root and pelvis bones are at 1 and most of the other bones have a scale of 1 as well, some of them do have values like 1.0000005 or 9.9999996 or 9.9999997 you get the point. After checking the original mannequin’s bone scales admittedly they are all exactly 1. I don’t believe this could really be the problem, considering these extremely small differences; However, I’ll keep investigating and if I don’t find anything else then I’ll export the skeletal mesh reset everything to 100 and reimport it and check if there’s any difference. I always neglected the physics asset; and now it’s apparently the time to pay; I knew this day would come.
Not sure if it would help, but have you tried the “Skip Scale from Animation” Option on the bodies of the physics?
I could not find out what is causing the seemingly erratic behavior on the physics. Even if I add two bodys there is abrupt sudden movement that is very strange. On a normal mannequin everything works fine. Therefore I concluded that it must be the bone scale problem. So you were probably right. I went into Max, scaled the bones properly, reimported and then all physics worked normally right out of the box with the automatically generated physics asset. Then I deleted that physics asset, created another one, and again did tests and everything seems to work fine now. Apparently ue4 wants absolutely precise scaling on the bones… However, I don’t know how to proceed now as i have based my workflow with the old skeleton which has hundreds of animations and assets on it all working fine; Now I risk destroying everything with the new skeleton. Time for backups, and then more backups and then more wasted time, because well what else do I have to do with my life anyway. Thank you very much for the answer though.
Nope; it didn’t help.
Glad I could help~
I think, as long as you just edit bone transforms (and not hierarchy and names) you should be able to reuse your animations. Did you try to reimport your skeletal mesh with proper scales? If there still problems, you can check Animation Modifiers to edit animations in bulk from within the editor.
Don’t hesitate to ask further 
Ayan
I figured it out. The problem was that I was adding preview assets to my sockets that had collision enabled, which was constantly interferfering with my bodies collision. This is why the bodies never settled. So when I attach anything to a character (to a socket/bone) I must disable its collision.
I expected that collision for preview meshes would be disabled, but this was apparently not the case.
I had a similar problem, but my solution was very different. I am working with someone else’s level. For some reason, they added a skybox with a collision sphere on it. Any movable actor would rocket off when spawned. Simply deleting the collision sphere fixed this.
