Roof Design

ROOF DESIGN:

AIM:

        For the given roof styling, create an assembly by developing the essential flanges for the outer roof and creating the reinforcements.

OBJECTIVES:

• The main objective of this project is design a car roof with reinforments like front roof rail, bow roof rails, central roof rail, rear roof rail and ditch area and essential flanges using given styling surface.
• Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct positions.
• Finding moment of inertia and section modulus for Bow roofs, central roof rail, Front roof rail and rear roof rail.
• Perform the draft analysis of all parts individually.

INTRODUCTION:

        An automobile roof or car top is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rail and other external elements. Roof is the structure to the overall structure and to with stand the over head forces like snow.

        When is seeing from outside of the car, it is just closing the top portion. But in design point of view, it is hard to create the roof. because of you have to create no of support rails for enough strength to withstand the load. Bow roofs are primarily fixed in flatter region to enhance strength.

        Front roof and  rear roof rails are fixed to the ditch area using spot weld operation. But front, rear bow roof and center roof rails are fixed to roof using mastic sealants. We don't want so much gap between roof rails and bow roof area. Because we reducing the NVH issues.

ROOF OUTER PANEL:

FRONT DITCH AREA:

        Front ditch area is creating with help of the dump solid of class A surface developed by considering the parameters to design the front ditch area. We create the extension sheet with minimum 15mm, and given the positive draft angle 3 degree considering the following drawing operation.

REAR DITCH AREA:

        It is same to the front roof ditch area but we have to fix the rear door in this area, so we considering the paramenters to design the rear roof ditch area.

DIFFERENT TYPE OF REINFORCEMENTS:

•  Front roof rails
• Bow roof rails
• Central roof rail
• Rear roof rail

Front Roof Rail:

        Front roof rail is used for supporting the roof for externally applied forces occuring at accidents. When we increasing the front roof rail is good for strength but also it is not for drivers because of it hide visibility of road.

        When considering the parameters of roof rail, give them 80mm length as per the master section, proper draft angle of 7^o or more degrees and also using the embosses for increasing the strength of the rail. Front section of front roof rail had a spot welded to the front ditch area and other end is fixed with help of mastic sealants.

Rear Roof Rail:

        Rear roof rail is creating to length the of 80-100mm with 7^o or more draft angle. Also create the embosses on the sheet to ensure the enough strength for holding the roof structure. When considering the all paramenters like length, width, sealants, rubber materials and wind shield to create the design properly.

Bow Roof Rails:

        We want to create minimum two bow roof for the roof structure. One is placed after front roof rail and another one is placed between center roof and real roof rail. The bow roofs are used to improve the torsional stiffness and load bearing capacity of the roof structure. When number of bow roofs are given by the roof dimension. Longer roof area need more bow roof to the structure.

        Positioning of the bow roofs are decided by the curvature study. The heat distortion and snow load criteria are checked to analyze the bow roof position by various condition. We also give the mastic area for fixing the bow roof to the outer. Thickness os the bow roof is 0.75mm.

Bow Roof Rail 1:

Bow Roof Rail 2:

Reinforcement Center Roof Rail:

        Reinforcement center roof rail is effective support to the flat area structure roof. Because of center area of roof is more susceptible to failure under action of load. Usually, reinforcement center roof rails are fixed at the center of the roof for connecting to the B-pillar support structure, it will used roll over test.

        Center bow roof and reinforcements are placed at the center of the roof structure. They two are connecting to help of spot welding operaation. Thickness of the reinforcement center roof rail is 0.75mm.

Joining process:

Mastic Sealants of Roof:

        The mastic sealant or the mastic point is on elastic compound that behaves like a rubber. The mastic sealants are applied at the many places like roof outer to front roof rail, rear roof rail, bow roof and reinforcement center roofs. It helps roof outer panel to come back to its original shape after it gets deformed. the mastic has an influence area of 80mm.

        It gives the three benefits in one operation. It fills the gap between the two or more substrates, it forms a barrier through the physical properties of the sealant itself, and by adhesiion to substrates.

Spot Welding:

        It is also help to join the two rails like reinforcement roof rail and the inner roof rail.

DESIGN CONSIDERATION OF ROOF:

• Visibility criteria
• Head room clearance
• Bow roof position
• Heat distortion and snow load criteria
• Draft analysis

ROOF CRUSH TEST:

        Roof crush test is the design given by the IIHS (Insurance institute of Highway Safety). After analyzing the many accidents IIHS has fixed some regulations, that how to perform the crush test. IIHS gives the rating based on this test. For this test they are considering two points while giving the rating.

• How fast the car can reforms to its original shape after the rool over 
• How deep is the protrusion after the rool over

Purpose of Crush Test:

        To minimize the protrusion into the occupant compartment, so that the seat belts and air bags can do there job in protecting the occupant.

Conditions to follow for roof crush test:

        There are two conditions that should be followed for the roof crush test, they are 

INITIAL CONDITION:

• Close and lock all the doors
• Close all the windows

SECOND CONDITION:

        Fix the body sill or chassis frame on a hard lever surface horizantally

Method of applying the force:

        The load is applied in such a way that 5^o is tilted from the front end to the rear end and the loaf will be in 1800mm in length and 750mm in width. The load is applied at the position of 25^o from the flatness of the roof. The load is applied on the roof for almost 120 sec and the crush speed at which the load applied is 13mm/sec or less than that.

        If the displacement goes above the requirements level then we have to increase the bow roof thickness or add more mastic points to increase the strength. If we stronger the roof, we able to protect better in the roll over crashes.

CURVATURE STUDY ON THE ROOF:

•  Heat Distortion 
• Snow Load Condition

Heat Distortion:

        Heat distortion is temperature limit above which material cannot be used for structural application. This study is useful to find the where the material starts to soften when exposed to a fixed load at elevated temperature. In order to avoid the bending damage to the roof based on the heat distortion temperature. This study will provide which position of the bow roof give enough strengthen to the roof design.

Bow Roof Prediction Formula

     W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [1.10 x 10^(-3) x I] - 2.68

Where,

        L = Roof Length in X-Direction (mm) [Roof dimension in 0 - Y]

        R = Roof curvature, R = 2(R_x * R_y)/(R_x + R_y)

        R_x = X curvature

        R_y = Y curvature

        t = Roof plate thickness (mm)

        I = Bow roof span (mm)

Judgement Condition:

        OK < 2> 3.1

        Now from the above equation, for our roof design the parameter is as per below the 2.7 is ok condition. Here, we have taken the thickness for the sheet for all compartment is 0.75mm. So, t = 0.75mm

        In order to find the heat distortion apply all the necessary values from the roof to the given formula.

HEAT DISTORTION CRITERIA CALCULATIONS

ROOF - RAIL	Rx (mm)	Ry (mm)	R (mm)	L (mm)	t (mm)	l (mm)	W	RESULT
Front Roof -  Bow Roof 1	3259.26	5653.34	4134.75	2029.24	0.75	573.77	1.883	OK
Bow Roof 1 -  Center Roof	7181.85	4727.69	5701.91	2029.24	0.75	265.74	1.925	OK
Center Roof -  Bow Roof 2	10106.53	4071.81	5804.89	2029.24	0.75	259.89	1.948	OK
Bow Roof 2 -  Rear Roof	13984.86	3373.17	5435.33	2029.24	0.75	499.07	2.108	OK

        From the above table it can be concluded that all values of W  is < 2>

Snow Load:

        This test is done to know how is the roof behaving when there is a snow over. Normally due to the snow weight the dent will happen on the roof. But the roof should be designed in such a way that when the snow is removed the roof should go its original position. This is the basic requirements for snow load criteria.

Formula of Snow Load Criteria:

     Q_r = [I_y x t²] / [α x s x [(R_x + R_y) / 2]² x 10^-8]

        Where,

        α = M_y x L_x² x 10^-12, [M_y = Y (L_y - Y)]

         t = Roof plate thickness (mm)

       L_y = Distance between the front and rear roof rails on the vehicle along with 0Y (mm)

       Length of the roof panel with the center point between Front roof rail / Rear roof rail as the front and rear reference point.

       L_x = Distance between the left and right end of the roof on the roof bow (mm)

        The width of the roof panel is exposed on the surface.

        Y = Distance of front roof rail to roof bow (mm)

        s = Distance for which roof bow bears divided load (mm)

            s = L₁ / 2 + L₂ / 2

        I_y = Geometrical moment of inertia of roof bow (Y cross section) (mm⁴)

       R_x = Lateral direction curvature radius of roof panel Y cross section on roof BOW (mm)

        Roof panel curvature radius of the length L_x in front view

       R_y = Longitudinal direction curvature radius of the roof panel * cross section on roof bow (mm)

        Roof panel * curvature radius of length s in side view

Judgement Condition:

        Q_r ≥ 3.1

        250 ≤ s ≤ 380

        If roof structure pass to the snow load condition.  They must have a Q_r value above 3.1.

X & Y Curvature:

Rx values:

Ry values:

L_x values:

L_y value:

MOMENT OF INERTIA VALUES:

Bow Roof 1:

Center Roof Rail:

Bow Roof Rail 2:

Snow Load Criteria Calculation:

ROOF - RAIL	Iy (mm4)	∝ (mm4)	t (mm)	Rx (mm)	Ry (mm)	s (mm)	Y (mm)	L1 (mm)	L2 (mm)	Lx (mm)	Ly (mm)	My (mm2)	Qr	RESULT
FRONT ROOF - BOW ROOF 1	1.6118x103	1.10	0.75	3251.30	4162.52	504.17	668.58	668.58	339.76	1128.576	1960.596	863816.06	11.897	OK
BOW ROOF 1 - CENTER ROOF	1.0304x104	1.128	1.5	7150.24	3843.12	336.25	1008.34	339.76	332.74	1083.87	1960.596	960197.82	202.31	OK
CENTER ROOF - BOW ROOF 2	2.4882x104	0.944	0.75	10503.39	4227.69	476.125	1341.08	332.74	619.51	1065.723	1960.596	830820.52	57.40	OK

        Qr ≥ 3.1. So, we done the curvature study on the roof. Heat distortion and snow load condition the roof is pass the two criteria. So position of the bow roofs and center reinforcement roof placed at right place.

DRAFT ANALYSIS REPORT:

        The draft analysis command enables you to detect if the part you drafted will be easily removed. This type of analysis is performed based on color ranges identifying zone on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.

        Minimum draft angle is 7 degree is considered for analysis. Green color shows positive draft angle.

Roof:

Front Roof Rail:

Bow Roof 1:

Center Roof Rail:

Bow Roof Rail 2:

Rear Roof Rail:

VARIOUS VIEWS OF ROOF:

Isometric View:

Top View:

Front View:

Back View:

CONCLUSION:

        I created the roof with ditch area, front roof rail, rear roof rail, bow roof rails and center roof rail and did the curvature study of heat distortion and snow load criteria. Finally we got a roof panel with reinforcements.