Aim:

To design roof with necessary reinforcements. 

Objective:

The design contains, roof outer panel, front roof rail, rear roof rail, center roof rail, bow roof 1 and bow roof 2. Heat distortion and snow load prediction calculations are performed after the complete design of the roof. The design is followed by given master sections. The given skin data is used to design complete roof. 

Introduction:

Roof is an automotive component, which lies above the passenger compartment. Roof protects passengers from the sun, rain, wind and other natural phenomenon. During accidents in case the car rolls on the roof, a good roof should be able to withstand certain deformation, thus providing safety to its passengers. In countries like Canada and Russia, snow is very common during winter. The weight of the snow acts much on the roof. For such harsh weather conditions, the roof should be able to withstand the snow load. The outer panel of the roof serves for styling of the vehicle, where the strength of the roof lies beneath the outer panel. Parts under the outer panel are front roof rail, rear roof rail, center roof rail, bow roof 1 and bow roof 2. These parts provide strength and stability to the roof. These parts come with necessary reinforcements and attaches to the outer roof with mastic sealants and spot welding. These structures vary with the size of the vehicle.

Types of car roofs:

• Convertible roofs
• Hard Tops
• Sun roofs
• T- Tops
• Target tops
• Vinyl roofs

Design Consideration of roof:

• Visibility Criteria
• Head Clearance
• Curvature Study
• Heat Distortion and snow load criteria
• Draft Analysis
• Safety Parameters

Master Section and its Parameters:

Master section is a set of data with necessary dimensions provided to design the components. In other words, master section is pre design data.

Reinforcement and its types:

• Front roof rail
• Central Roof rail
• Rear roof rail
• Bow roof rail

Roof Outer Panel:

Roof outer panel is taken from the given skin data only, the given sheet is extracted and untrimmed. Flanges are created using law extension command. The image given below is the final output of the roof outer panel.

The image provided below shows the incomplete design of the outer roof panel. The design is followed by the master sections provided, but the rear roof flange data is incomplete. 

Front roof rail:

Front roof rail joins roof outer panel, wind shield glass and inner panel. The image shown below is, front roof rail designed by the master section given for front roof rail. The design is followed by several embosses/reinforcements to increase its strength and stability.  Visibility criteria and head room clearance must be considered in the design of front roof rail.

Rear roof rail:

Rear roof rail is also designed according to the given master section, rear roof rail design needs to be followed by the visibility criteria. The image given below is the final output of the rear roof rail. The rear roof is designed my law extension command. Darts are also provided to the rear roof rail to give strength and stability.

Central Roof rail:

Center roof usually connects with the B pillar of the car. Center roof rail is more vulnerable in case of a car roll over. This should withstand during accidents thus providing safety to the passengers.  

 Bow roof:

Depending on the car size, number of bow roofs in a car changes. In the current design only two bow roofs are created.

Design Calculation of Roof:

Heated Distortion study on the roof:

To calculate heat distortion, points need to be projected on the outer roof panel as shown in the figure. To create the intersection points on the outer roof panel, we need to follow the given procedure. A center line Y-0 need to be drawn in the sketch workbench. The Y-0 line is offset to 300mm below, two symmetric lines are drawn with 100mm offsets. These lines create horizontal lines in the sketch. To create vertical lines the edges of roof rails are projected, as per the given instructions. Vertical lines are drawn at the center of the line drawn between each rail. The lines created are further offset with 100mm both sides of the vertical line. Intersection points are created and then projected on the roof outer panel. Using arc/circle tool, an arc is drawn using three points. Then the curvature is measured using local radii command. Using the formula below ‘w’ is calculated.

BOW – roof prediction study:

         W= [1.73x10 ^ (-3) x L] + [1.85 x 10^ (-8) x (R^2)/t] + [1.10 x 10^ (-3) xl] – 2.68

Where’

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

R = Roof curvature

R = 2 (Rx*Ry) / (Rx + Ry)

Rx = X Curvature

Ry = Y curvature

t = Roof plate thickness [mm]

I = Bow Roof span [mm]

Judgement Condition: OK < 2> 3.1

S.no	BOW ROOF	Rx	Ry	R	T	I	L	R^2/t	W
1	FRR-BR1	2773	5722	3735.63	0.75	453.05	2033.70	18606575.32	1.68	Pass
2	BR1-CRR	6868	4805	5654.20	0.75	465.72	2033.70	42626636.85	2.13	Pass
3	CRR-BR2	11193	3859	5739.27	0.75	383.38	2033.70	43918960.17	2.07	Pass
4	BR2-RRR	14750	3255	5333.10	0.75	317.56	2033.70	37922607.48	1.88	Pass

Conclusion:

From the above table, it is concluded that the design has good heat distortion values.

Snow Load Condition:

Snow load calculations are very important in countries where winter is severe with snow fall. The roof design should pass the calculations to witstand snow and to have good structural stability. The formula given below calculates the load bearing capacity of a car roof. A center line is created on the roof in sketch workbench, 100mm offset is created on the both sides of the line created horizontally. Edges of the roof rails are projected to create intersection points. To create vertical lines, first draw lines between the projected points of the roof rails. from the midpoint of the drawn lines, draw vertical lines as shown in the figure below. The intersection points are created and then projected on the outer panel. Using arc/circle tool create arcs using three points. measure radius of the arcs using local radii. To obtain moment of inertia of the roof rails, a datum plane is created and curve is created by using intersection between the datum plane and the roof rails. 

Qr = [Iy x t2]/ [a x s x [(Rx + Ry) / 2] 2 x 10-8]

Where,

A = My x L x 2 x 10-12, My = Y (Ly- Y)

Judgment Condition = Qr > 3.1

250 < s>

T = Roof plate thickness [mm]

Ly= Distance between the front and rear roof rails on the vehicle along with 0-Y [mm]

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

Lx = Distance between the left and right end of the roof on the Roof Bow [mm]

Width of the roof panel exposed on the surface.

Y = Distance front Roof Rail to Roof BOW [mm]

S = Distance for which roof Bow bears divided load [mm]

S = L1/2 + L2/2

Iy = Geometrical moment of inertia of Roof Bow (Y Cross section) [mm4]

Rx = Lateral direction curvature radius of roof panel Y cross section on Roof BOW [mm]

Roof panel curvature radius of the length Lx in front view

Ry = Longitudinal Direction curvature radius of the roof panel X cross section on Roof Bow [mm]

Case 1: Moment of inertia (Min)

S.no	Bow roof	Rx(mm)	Ry(mm)	Lx(mm)	Ly(mm)	L1(mm)	L2(mm)	t(mm)	Y(mm)	S(mm)	My	α	Iy(mm^4)	Qr
1	FRR-BR1	2718.83	4450.77	1086.48	1962.27	538.91	534.62	0.75	538.91	536.76	767062.93	0.90	269.43	3.25	Pass
2	BR1-CRR	6861.74	3815.53	1140.54	1962.27	534.62	452.90	0.75	1073.53	493.76	954089.05	1.24	252.75	1.08	Fail
3	CRR-BR2	11787.51	4317.84	1061.39	1962.27	452.90	435.82	0.75	1526.44	444.36	665268.34	0.74	268.24	0.94	Fail
4	BR2-RRR	16990.77	3897.90	1048.54	1962.27	435.82	217.91	0.75	1962.27	217.91	0	0

Case 2: Moment of inertia (Max)

S.no	Bow roof	Rx(mm)	Ry(mm)	Lx(mm)	Ly(mm)	L1(mm)	L2(mm)	t(mm)	Y(mm)	S	My	α	Iy(mm^4)	Qr
1	FRR-BR1	2718.83	4450.77	1086.48	1962.27	538.91	534.62	0.75	538.91	536.76	767062.93	0.90	23156.90	373.00	Pass
2	BR1-CRR	6861.74	3815.53	1140.54	1962.27	534.62	452.90	0.75	1073.53	493.76	954089.05	1.24	51079.21	292.71	Pass
3	CRR-BR2	11787.51	4317.84	1061.39	1962.27	452.90	435.82	0.75	1526.44	444.36	665268.34	0.74	22058.73	103.45	Pass
4	BR2-RRR	16990.77	3897.90	1048.54	1962.27	435.82	217.91	0.75	1962.27	217.91	0	0

Conclusion: In table 1 moment of inertia is consider at minumim value, at MOI(min) the design has failed to clear the snow load bearing capacity. Where as in the table 2 (MOI max) it has cleared jundement condition for Qr. On the other hand in both the cases the design has not cleared 's' value. In order to make the design to clear this condition, one more bow roof is required. 

Draft analysis:

Draft analysis has done for all the roof rails, all the roof rails are given draft angle above 7degrees. By draft analysis it is concluded that, all the parts are manufacturable. 

Conclusion: 

The objective of this project is clearly implemented. The design is provided with necessary support structures, in other words reinforcements. All the roof rails, i.e., front roof rail, rear roof rail, center roof rail and bow roofs are created by the given skin data. All the design is followed by the master-sections provided for the project. Heat distortion calculations, snow load calculations and draft analysis is performed to check and validate the design. The design has cleared heat distortion values where as the design showed poor performance in snow load calculations. To perform better in snow load calculations, one more support structure, that is one more bow roof would increase the structural rigidity in the design. The design has also cleared draft analysis, hence the parts are manufacturable.