Roof Design

AIM: To design a roof with all the components like front roof rail, rear roof rail, and bow roofs by following the given master sections and the high-quality surface, considering all the design requirements as per the OEM standards.

ROOF

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, rain, and other external elements. The roof is not a single panel as seen from the exterior it consists of front roof rail, bow roofs, center roof rail, and rear roof rail with different thicknesses which act as a reinforcement for the top.

Roof Crush test for passenger car:

Safety point of view vehicle roof plays a very crucial role especially roll over kind of accidents, In which vehicle tips over onto its side of the roof Vehicle rollover crashes are the cause of many fatalities and head, neck and spine trauma around the world.

Strength of the roof structure to occupant protection in real world roll over crashes, requiring the maximum moving distance of the roof structures should be less than 127 mm, (5 in), where the induced load is 1.5 times the vehicle weight.

For the test, the passenger ca shall be rigidly placed or positioned on a horizontal surface with the doors locked and the window closed. Furthermore, a flat, rigid block with a lower surface a rectangle measuring 30 inches wide by 72 inches long (762mm x 1829 mm) shall not move more than 5 inches (127mm), measured as the distances between the original location of the lower surface of the test device and its location as the specified force level is reached, when it is used to apply a forces of 1.5 times the unload vehicle weight (UVW) or 5000 lbs. (22224N), whichever is less. To either side or forward edge of the vehicle.

This test can be conducted on either side of the roof structure, the front left or the front right side but not on the both sides in one single test and still meet the requirement of the test. The rigid plate shall not move faster than 0.5 inches/second and the force applied on the plate shall not exceed the lesser of 1.5 times the unloaded vehicle weight (UVW) expressed in kilograms multiplied by 9.8 or 5000 lbs (22240N).

Also, the direction of the force shall be downward and perpendicular to the lower surface of the rigid plate and such that it moves in a straight line without rotation. The duration of the test shall not exceed 120 seconds. The longitudinal axis of the plate is pitched forward at a 5’ angle as viewed from the side of the passenger car as shown,

The lateral axis or the roll of the plate is such that it forms a 25’ outboard angle with a horizontal surface as viewed from the front of the passenger car as shown in below, fig

Design Of Roof Includes

• FRONT ROOF RAIL
• BOW ROOF 1
• CENTER ROOF RAIL
• BOW ROOF 2

The Styling Surface of the roof.

Master Section of the Front Roof Rail.

Front roof rail is the one which joins the wind shield glass, body side outer and the inner panel also. Here the front roof rail is designed based up on the master section given.

Rear roof rail:

Rear roof rail is the one which joins the back door and the body side outer.

Center Roof Rail

Here the minimum thing we need to take care is the width of the Center Roof Rail should be between 80-110.By following the master section by styling team the physical shape shown below.

BOW ROOFS

• Mastics are applied between roof and the roof rail and bow roof.When the mastic gooses out it joins the roof rail and bow roof with the Roof.
• As we have curvature in front and more flat in the back side of the roof,it is sufficient to have reinforcement in the center of the roof.
• If the roof was small with better curvature we need to use only one reinforcement (center) that wiil act as bow roof.
• No.of bow roof increases if the car size becomes longer and taller.If flat region increases bow roofs also increases.

Center Roof along with reinforcement rail:

• These roof rails are positioned mostly on the center portion of the roof or at the B Pillar of the vehicle i.e, the most flatter region of the car which is susceptible to failure under the action of load.

Joining Processes

The Flanges of the front and rear roof rail and flanges of the bow roofs and center roof rail are joined using the spot welding, whereas mastic sealants are provided along the roof rails to improve the strength and also to avoid bad marks on the aesthetic surfaces.

FINAL ASSEMBLY

Design Calculation of Roof:

To make sure the roof design passes the Analysis test some of the criteria to be considered before the design of bow rails under the roof. They are,

• Heated Distortion study on the roof
• Snow Load Condition

Heated Distortion study on the roof:

The heat distortion study plays a major role in sheet metal usage. Heat distortion temperature is the temperature limit above which the material cannot be used for the structural applications. This study is used to predict the heat distortions temperature at the material where it start to soften when exposed to a fixed load at elevated temperature. In order to avoid bending or damages on the roof, based on the heat distortion temperature, this study will predict the bow roof position which helps to strengthen the roof.

BOW – roof prediction study

W= [1.73x10 ^ (-3) x L] + [1.85 x 10^ (-8) x (R^2)/t] + [1.10 x 10^ (-3) x1] – 2.68

Where’

L = Roof strength 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 < 2.7 > 3.1

Here we have to find out Rx and Ry values. To find out

• Create a sketch on Top roof and draw a horizontal line at a center.
• Then offset this line below 300mm and called it as _3y line.
• From the _3y line offset 100 mm above and below and convert all these line into reference line.
• Draw the vertical lines at the middle of each reinforcements and offset it at 100 mm both sides.
• Intersection point is created where the lines are met.
• Project there points the outer roof panel.

S.NO	CASE	Rx	Ry	R	l	L	R^2/t	t	W	REMARK
1	Case-1	5732.2010	2660.7198	3634.4393	443.9787	2034.3985	17612198.70	0.75	1.383	ok
2	Case-2	41975.7708	6424.6832	5076.3363	372.2704	2034.3985	34358920.30	0.75	1.614	ok
3	Case-3	4028.8934	10320.8459	5795.4485	362.8524	2034.3985	44782964.42	0.75	1.797	ok
4	case-3	3330.7407	14257.6142	5399.9838	396.5022	2034.3985	38879766.72	0.75	1.725	ok

Snow Load Condition:

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

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 < 380

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 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 = L ½ + 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]

Here we have to find out Rx and Ry values. To find that

• Create a sketch on Top roof and draw a horizontal line at a center.
• Offset this line to 100 mm on both sides. Convert all these line into reference line.
• Draw the vertical lines at the middle of each reinforcements and offset it at 100 mm both sides.
• Intersection point is created where the lines are met.
• Project there points the outer roof panel.

Section Modulus:

Section Modulus is a geometric property for a given cross section used in the design of beams or flexural members. Other geometric properties used in design include area for tension and shear, radius of gyration for compression, and moment of inertia and polar moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape in question. Equation for the section moduli of common shapes are given below.

There are two types of section moduli:

The elastic section modulus and

The plastic section modulus.

The elastic section modulus is defined as

S = I / Y

Where:

S = Section modulus

I = second moment of inertia (or area moment of inertia, not to be confused with moment of inertia)

Y = distance from the neutral axis to any given fiber. It is often reported using Y = C, where c is the distance from the neutral axis to the most extreme fibre.

Section Modulus Calculation for Bow Roof 1:

Section Modulus Calculation for Bow Roof 2:

Section Modulus Calculation for Center Roof:

S.NO	CASE	RX	RY	LX	LY	Y	S	MY	ALPHA	ly	t	Q	REMARK
1	1	4465.0970	2696.9085	1147.0749	2034.7985	521.7476	486.4949	789430.6757	1.038	2800	0.75	2.4	N-OK
2	2	3778.5691	6375.4729	1096.3813	2034.7985	974.4695	449.6370	103258.2704	1.242	3305	0.75	12.9	OK
3	3	4247.1722	10723.0839	1071.5115	2034.7985	1417.1574	458.2080	875294.6554	1.004	2798	0.75	6.1	OK
4	4	4082.4397	15932.8252	1056.8347	2034.7985	1926.644		208374.4911	0.232		0.75	###

Draft Analysis:

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 zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.

Minimum draft angle of 7 degree is considered for analysis, green color infers on the parts that all face along the tooling direction has positive draft angle greater than 5 degree and passed in analysis.

Outer Roof

Front Roof Rail

Rear Roof Rail

Center Roof

Center Roof Reinforcement Rail

BOW Roof Rail

Conclusion:

Thus the required reinforcement and the ditch area for the roof were designed and developed from the given styling. The curvature study was done after performing calculation for the Heat Distortion and Snow load conditions with the position criteria met for all reinforcements.

Further the section modulus study was conducted and Draft analysis were done for each reinforcement successfully.