How to make seperate parts for vehicles?

If for example you just want to make your own separate part (off an existing one) for a vehicle, copy that whole part section
Example from 'hatch_bumper_F' (line 275)

Spoiler

"hatch_lip_F": {
"information":{
"authors":"BeamNG",
"name":"Front Lip",
"value":30,
},
"slotType" : "hatch_lip_F",
"flexbodies": [
["mesh", "[group]:", "nonFlexMaterials"],
["hatch_lip_F", ["hatch_bumper_F"]],
],
"nodes": [
["id", "posX", "posY", "posZ"],
//--redefine nodes to move them
{"collision":true},
{"nodeMaterial":"|NM_PLASTIC"},
{"frictionCoef":0.8},
{"nodeWeight":0.68},
{"group":"hatch_bumper_F"},
{"selfCollision":true},
["fb2rr", -0.71, -1.87, 0.167],
{"selfCollision":false},
["fb2r", -0.38, -1.94, 0.167],
["fb2", 0, -1.95, 0.167],
["fb2l", 0.38, -1.94, 0.167],
{"selfCollision":true},
["fb2ll", 0.71, -1.87, 0.167],
{"selfCollision":true},
["fb4rr", -0.78, -1.58, 0.158],
["fb4ll", 0.78, -1.58, 0.158],
{"group":""},
],
},

And paste it somewhere under.
Change the name on the first line ( hatch_lip_F > hatch_lip_F_yourmod )

That part will now appear as a new part in the part selector.

 

you can name it whatever you want, as far it's unique.