MMD Tutorial: Leg Physics

Going to go over how to do leg physics since I don’t see a tutorial for it anywhere. I figured out at least a method to do it that works decent enough.
I am assuming if you want to add this to your model, that you already have at least a intermediate level of knowledge of how to use pmxe. Like how to create bones and stuff. If something is not mentioned in detail there is probably tons of tutorials for it already out there.

This is the bone structure for legs that you need to do leg physics.
mmd_leg_physics_structure

It might look normal at first but this is a closer look.
mmd_leg_physics_structure_close
extra Ik bones are added with the child of the bones being the lower bone portion of the leg bones.

there is the structure with physics and joints.
mmd_leg_physics_structure_w_physics_and_joints

ok you should have a good idea how this works. basically the inner physics colliders will oscillate in various directions based on the joint constraints to create the jiggle effect.

steps:
I’ll start with a nude cirno model. I ran the through semi standard bones plugin. I did to auto correct any possible issues with physics or anything else that might be a problem later. Basically I have simply found that when editing a model it is good to run that first.
mmd_leg_physics_tutorial_pic1

Add two new bones and place them under where the thigh bones begin. I have them right under so they are easy to click on again later. Make their parent bones match the Ik thigh bones parent bones.
mmd_leg_physics_tutorial_pic2

make the child bones of the new bones you created also match the thigh bones.
mmd_leg_physics_tutorial_pic3

now you need to make the bones IK and part of the leg system. you need to click the ankle bone in the IK leg chain. make on the bone section window you have the dark orange bone selected. you will see two bones there. 0 and 1. you need to copy the index of the new bone and add into the list in between the other two bones.
mmd_leg_physics_tutorial_pic4

mmd_leg_physics_tutorial_pic5

mmd_leg_physics_tutorial_pic6

mmd_leg_physics_tutorial_pic7

mmd_leg_physics_tutorial_pic8

now do the same for the other leg.
next add joints and physics to the regular leg joints
mmd_leg_physics_tutorial_pic9

mmd_leg_physics_tutorial_pic10

make the leg physics smaller
mmd_leg_physics_tutorial_pic11

note the physics for the legs. make them not collide with other physics according to your model. other then that, all 4 leg joints have the same stats. and both physic colliders have these stats below.
mmd_leg_physics_tutorial_pic12

the joints need to be linked between the new colliders and the existing leg physic colliders. I don’t think a specific direction is necessary but I haven’t checked. the pic illustrates the joint connections.
mmd_leg_physics_tutorial_pic13

make sure the physics are attached to the new bones.
mmd_leg_physics_tutorial_pic14

you can test in transform view if you want
mmd_leg_physics_tutorial_pic15

now it is time to weight paint to the new bones. Gradient from 80 to 0 both legs.
mmd_leg_physics_tutorial_pic16

and that is it. watch the legs jiggle in your motions.

mmd_leg_physics_tutorial_pic17

when I tired it with this particular model the legs were jiggling too much. way to tone it down are making the weight beginning at less than 80. like from 60 to 0 instead of 80 to 0. you can reduce the X and Z axes movement on joints (not rotation) to soften it up a bit. You need the joint rotation almost at full 360 degrees of movement so it won’t vibrate the leg.