Roof Design

Report on roof design

OBJECTIVE:

• To create an assembly of a car roof by developing the essential flanges for the outer roof and creating the reinforcements by given styling surface.
• Provide appropriate dimensions and check for Draft analysis.
• Do a curvature study on the roof and perform calculations to determine the position of the Bow-roofs for snow load by given formula.

Software Used : NX-cad

DESIGN CONSIDERATION:

• Visibility criteria
• Head clearance
• Curvature study
• Hear distortion and snow load criteria
• Draft analysis.

Introduction: An automobile roof or car top portion is the portion of an automobile that fix above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements.design point of view, it is hard to design. Because it is consists of different parts such as at least two or three bow roof rail,center roof rail ,front roof rail,top portion of roof and rear roof rail.

STYLING SURFACE:

Master Section :Here are some Reference images for the Master sections - These images are just to give you a brief idea about the design.

OUTER ROOF :

Reinforcements :The reinforcements of a roof are:

• FRONT ROOF RAIL: The front roof rail is located on the front of the roof where the wind shield joins with the roof. The visibility criteria and the head room clearance of the passenger are taken into consideration for the bottom flange.One end of the rail is joined using spot welding while the other end is joined with the roof using mastic sealants for increased strength.
• 

• REAR ROOF RAIL: This roof rail joins with the back door and the roof. Inputs from the backdoor are taken into consideration for designing this roof rail.
• 

• BOW ROOF RAIL1:Bow roof is an additional siffening member for the roof to increase the strength and so that it can satisfy the norms. Both the ends are joined together using mastic sealants and spot weilding to increase strenght and also since this would not affect the styling of the roof.
• 

• CENTER BOW ROOF RAIL:Centre Roof rail or the reinforcement centre is located at the centre of the roof . The cross section of this is more since it needs to provide more strength to the roof. Here both are ends are joined together using mastic sealants.
• 

• BOW ROOF RAIL2:This bow roof also have the same purpose as that of the first one. This one is located between the centre bow roof and rear roof rail.
• 

FORMULA USED:

• Bow–Roof Prediction Formula

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

l = Bow Roof Span [mm]

Judgment Condition

OK< 2.7

Required values for the Bow–Roof Prediction Formula and Hear distortion for different section
Section	Rx(mm)	Ry(mm)	R(mm)	L(mm)	t(mm)	l(mm)	w	Judgment Condition(w<2.7)
Front roof rail-Bow roof rail1	3073.60	3074.92	3074.26	2023.71	0.75	390.1	1.09	OK
Bow roof rail1-CENTER BOW ROOF RAIL	11336.12	4990.70	6930.34	2023.71	0.75	399.4	2.04	OK
CENTER BOW ROOF RAIL-Bow roof rail2	3991.88	2543.78	3107.40	2023.71	0.75	338.82	1.43	OK
Bow roof rail2-Rear roof rail	13996.44	2693.21	4517.23	2023.71	0.75	463.5	1.83	OK

for heat distortion all four section of roof are in ok condition.

• Snow Load Prediction Formula
• Snow load criteria study is used to check the strength of the roof to withstand the snow fall on the roof and reflex it is original shape without any permanent deformation after load being removed.

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

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

Where,

α = My x Lx² x 10^-12 , My = Y(Ly-Y)

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof Rails on the Vehicle along with 0Y[mm]

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

Lx = 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 front 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]

Roof panel X curvature radius of length s in Side view

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

Values for Snow Load Prediction
Section	Rx(mm)	Ry(mm)	Iy(mm^4)	t(mm)	L1(mm)	L2(mm)	Lx(mm)	Ly(mm)	Y(mm)	s(mm)	a	Qr	Judgment condition Qr ≥ 3.1
Front roof rail-Bow roof rail1	4539.46	2587.81	1505.586	0.75	480.9	479.2	1103.9	1963.2	480.9	480.05	0.8686	15.993	OK
Bow roof rail1-CENTER BOW ROOF RAIL	3760.56	6003.06	2140.851	0.75	479.2	418.7	1067.5	1963.2	960.1	448.95	1.097	10.255	OK
CENTER BOW ROOF RAIL-Bow roof rail2	6152.39	10308.60	1521.375	0.75	418.7	584.3	1049.2	1963.2	1378.8	501.5	0.887	2.842	Fail

Note: for snow load two value are passes the given condition and one value fail so we have to do some modification to increase the strength .we can do some modification to get the desired value of Qr .By creating embosses in our bow roof rail2 or by increasing the depth of bow roof rail or by creating embosses in our outer roof .so we have created embosses

Draft Analysis:The draft angle of 7° is considered for analysis. The green color infers the parts that all face along the tooling direction has a positive draft angle greater than 7° and passed in analysis.

Conclusion: The assembly design of roof is created and look like as shown in the image-