addChild doesn't work

Questions
#1

Hello I couldn’t add the tire to the sphere. line 72
Why doesn’t it show tire when I define addChild
how can i add 4 rubber 4 spheres to this

#2

addChild(mesh) just calls mesh.setParent(this) which keeps the mesh in place in the scene

You should use the parent-Property.
To add the wheels to the chassis:

wheel.parent = chassis;
#3

not working