Uh I found it already. Its stupid but check out the MMT plugin repository, I dont remember the file but it is i think inside the suspension class. What you have to do is whenever you change anything of a bodyinstance that forces it to update its mass properties let it do its thing. When it is done (you updated all properties and added all meshes) you take the mass space inertia tensor and multiply the axis you want by the factor you need. then it works without any problems.