Many MMD models have extremely unrealistic breast physics that tend to vibrate and collide poorly with your model's body. This guide tells you how to delete the breast physics entirely and remove this problem.
Important preface that I have no experience with this and only managed to do it through following guides and trial and error. If this doesn't work or your model is different I unfortunately can't help you. The guides I used are:
- Removing Physics by Kysen on Amino
- PMX error checker quick reference on Github
You will need PMX Editor to do this. The English translated version can be found here.
Opening PMXeditor, you should see this box. Follow File → Import and click on your model. (Your model must be in .pmx format, or the editor can't open it. If your model is a .pmd, you SHOULD be able to follow these exact same steps in PMDeditor- to my knowledge the only difference is the file format they reference, but I can't guarantee it will work.)
When your model opens in