Roof Design

Roof Design:

• A car roof or car top is the portion of a car that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements.The external sheet metal part is called roof panel which is mounted on and supported by roof bows.

Design Consideration:

While designing a roof following parameters have to be followed.

• Visibility criteria
• Head room clearance
• Bow roof position
• Curvature study i.e., Heat distortion and Snow load criteria
• Draft analysis

Components in a roof:

Roof consists of structural members such as:

• Front roof rail
• Rear roof rail
• Centre roof reinforcement
• Bow roof rails

Front Roof Rail:

Bow Roof Rail 1:

Centre Roof Rail Reinforcement:

Centre Bow Roof:

Bow Roof Rail 2:

Rear Roof Rail:

Positioning of Roof Rails:

Assembly of Roof Rails:

Draft Analysis:

The draft analysis results of all the roof rails are as follows:

Section Modulus Study:

Section modulus may be defined as the ratio of total moment resisted by the section to the stress in the extreme fibre which is equal to yield stress.

Section Modulus is given by,

S = I/Y

Where,

S = Section Modulus

I = Moment of Inertia

Y = distance between the neutral axis and the extreme end of the object

Front Roof Rail: 

where, I = 1.67555e+03 mm4

Y = 70 mm

S = 23.935 mm3

Bow Roof Rail 1:

where, I = 2.06005e+03 mm4

Y = 100 mm

S = 20.600 mm3

Center Roof Rail:

where, I = 2.77166e+03 mm4

Y = 120 mm

S = 23.096 mm3

Bow Roof Rail 2:

where, I = 2.152365e+03 mm4

Y = 100 mm

S = 25.126 mm3

Rear Roof Rail:

where, I = 4.107269e+03 mm4

Y = 80 mm

S = 51.340 mm3

Roof Bow Calculation (Curvature Study):

This study is done based on the following two criteria :

1. Heat Distortion Criteria
2. Snowed Criteria

1. Heat Distortion Criteria: 

Bow–roof prediction Formula : W = 1.73x10^-3 x L+1.85x10^-8  x (R^2)/t + 1.10x10^-3 x l-2.68            

Judgement Condition : OK<2>

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]

Heat Distortion Criteria
S.NO	BOWs	Rx (mm)	Ry (mm)	R (mm)	I (mm)	L (mm)	t (mm)	R^2/t	W	Determination
1	FR & BOW 1	5634.65	2076.44	3034.59	355.61	2033.22	0.75	12278315	1.45	OK
2	BOW 1 & CTR	4712.81	5801.71	5200.86	292.1	2033.22	0.75	36065259	1.82	OK
3	CTR & BOW 2	3533.47	10975.88	5345.92	309	2033.22	0.75	32105147	1.88	OK
4	BOW 2 & RR	2873.26	16191.32	4880.45	200.7	2033.22	0.75	31758389	1.64	OK

2. Snowed Criteria:

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

Judgment condition:  Qr ≥ 3.1 and 250 ≤ s ≤ 380

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

Snowed Criteria
S.NO	BOWs	Iy (mm)	α	Lx (mm)	Ly (mm)	Y (mm)	s (mm)	Rx (mm)	Ry (mm)	t (mm)	Qr	Determination
1	FR & BOW 1	2060	0.8091	1100.6	1797.5	524.9	257.52	2815.08	1653.31	0.75	111.41	OK
2	BOW 1 & CTR	2771	0.7468	1064.6	1797.5	513	2.65.27	3200.36	5411.14	0.75	42.43	OK
3	CTR & BOW 2	2152	0.7321	1044.8	1797.5	528.6	191.53	4112.78	10402.38	0.75	16.38	OK

Manufacturinf Techniques:

Joining Process

• At centre portion, the outer roof panel and reinforcement roofs are joined by introducing mastic between them.
• At end, they are joined by spot welding

Stamping Process

• The roof are made by a process called stamping (also known as pressing).
• Stamping is the process of placing flat sheet metal in either blank or coil form into a stamping press where a tool and die surface forms the metal into a net shape.