Incorrect opening a urdf file in Gazebo

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One of the issues you have, the one that make the model appear collapsed is due to errors in the inertial values. For some reason, when the inertias are not correct , as soon as the model touches the ground and you exert some kind of force in the continuous joints of the wheels, they break and appear in the origin of the model.

You can test this by spawning the model in a zero gravity environment and it won't break any joint. I've made a full video tutorial talking about the issue and explaining how to recalculate your inertal values, at least for basic geometric shapes. Here is the VIDEO ]

Here you have some images of the differences in the inertias for a simple base_link and a wheel:

Colapsed due to wrong Inertias

Same urdf except that it has recalculated inertias

Here I leave a geometric version of your URDF that works and the launch files and python code I used to do this. If you can upload the stl I could update this answer with the complete model as well as maybe add the control.

Inertia Calculator:

#!/usr/bin/env python
"""
Made by TheConstructSim
You can use this script as you please
"""

import math


class InertialCalculator(object):

    def __init__(self):
        print "InertialCaluclator Initialised..."

    def start_ask_loop(self):

        selection = "START"

        while selection != "Q":
            print "#############################"
            print "Select Geometry to Calculate:"
            print "[1]Box width(w)*depth(d)*height(h)"
            print "[2]Sphere radius(r)"
            print "[3]Cylinder radius(r)*height(h)"
            print "[Q]END program"
            selection = raw_input(">>")
            self.select_action(selection)

        print "InertialCaluclator Quit...Thank you"

    def select_action(self, selection):
        if selection == "1":
            mass = float(raw_input("mass>>"))
            width = float(raw_input("width>>"))
            depth = float(raw_input("depth>>"))
            height = float(raw_input("height>>"))
            self.calculate_box_inertia(m=mass, w=width, d=depth, h=height)
        elif selection == "2":
            mass = float(raw_input("mass>>"))
            radius = float(raw_input("radius>>"))
            self.calculate_sphere_inertia(m=mass, r=radius)
        elif selection == "3":
            mass = float(raw_input("mass>>"))
            radius = float(raw_input("radius>>"))
            height = float(raw_input("height>>"))
            self.calculate_cylinder_inertia(m=mass, r=radius, h=height)
        elif selection == "Q":
            print "Selected Quit"
        else:
            print "Usage: Select one of the give options"


    def calculate_box_inertia(self, m, w, d, h):
        Iw = (m/12.0)*(pow(d,2)+pow(h,2))
        Id = (m / 12.0) * (pow(w, 2) + pow(h, 2))
        Ih = (m / 12.0) * (pow(w, 2) + pow(d, 2))
        print "BOX w*d*h, Iw = "+str(Iw)+",Id = "+str(Id)+",Ih = "+str(Ih)

    def calculate_sphere_inertia(self, m, r):
        I = (2*m*pow(r,2))/5.0
        print "SPHERE Ix,y,z = "+str(I)

    def calculate_cylinder_inertia(self, m, r, h):
        Ix = (m/12.0)*(3*pow(r,2)+pow(h,2))
        Iy = Ix
        Iz = (m*pow(r,2))/2.0
        print "Cylinder Ix,y = "+str(Ix)+",Iz = "+str(Iz)

if __name__ == "__main__":
    inertial_object = InertialCalculator()
    inertial_object.start_ask_loop()

car_design_full_geometric_fixed_physics.urdf

<?xml version="1.0"?>
<robot name="car_design_full">  
<material name="material_base_link">
    <color rgba="0.929411765 0.619607843 0.149019608 1"/>
</material>    
<material name="material_sensors">
    <color rgba="0.792156862745098 0.819607843137255 0.933333333333333 1 ...