Roof Design

Roof design

Objectives

1. To create an Roof Assembly by developing the essential flanges for the outer roof and creating the reinforcements.

2. To perform draft analysis in every parts.

3. Perform a curvature study on the roof and perform calculations to determine the position of the Bow-roofs.

Theory

Car 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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.

Reinforcements used in Roof design

If the roof is flat it is very weak and cannot able to with stand the forces like snow , so in order to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process

Types of reinforcements used-->

• Front roof rail
• Bow roofs
• Central roof rail
• Rear roof rail

Front Roof Rail

Front roof rail is the one which joins the wind shield glass , body side outer and the inner panel also. The design is made following the given master section.

Rear Roof Rail

Rear roof rail is the one which joins the back door and the body side outer and it is made using the design criterias followed by the given master section.

Central Roof Rail

Central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load. Generally central roof rail is placed at the center of the roof which is connected to the B-pillar support structure. This central roof rail helps in adding the strength to the roof during the rool- over test. The thickness of the central roof rail is 0.75 mm.

Bow Roof Rail

The bow roof are given to improve the torsional stiffness and load bearing capacity of the roof structure. The number of roof presented is depends on the overall size of the roof. Presently in this project two bow roofs are calculated.

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 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

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]

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.

The draft analysis is performed on each reinforcements.

Calculation for Heat Distortion

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.

position	Rx	Ry	R	t	l	L	R^2/t	W	Result
front to Bow1	3068.2	3269.4	3165.6	0.75	286.7	1921.9	14251968.5	0.986	ok
Bow 1 to center	3738.4	26519.1	6552.5	0.75	333.6	1921.9	57247008.3	2.067	ok
Center to Bow 2	4187.9	22008.8	7036.8	0.75	488.7	1921.9	66022072.3	2.397	ok
Bow 2 to rear	3326.9	209416.7	6549.7	0.75	400.54	1921.9	57198093.5	2.138	ok

From the above table it can be concluded that all values of W < 2.7 and also not W > 3.1. So thus infers that current positioning of bow row are good in state as per design and found ok.

Calculation for snow load criteria

Section modulus Bow roof 1

Section modulus Bow roof 2

Section modulus Center roof

Roof position	Rx	Ry	Lx	Ly	t
rear to Bow 1	3493.8	1910.5	1158.24	1921.9	0.75
Bow 1 center roof	2933.27	3754.5	1109.37	1921.9	0.75
Center to Bow 2	581.4	20422.2	1076.5	1921.9	0.75
Bow2 to rear	4076.2	83372.2	1057	1921.9	0.75

Y	S	My	α	Iy	Qr	result
286.7	414.9	468811.8	0.63	525	15.47	ok
333.6	1532.2	529856.88	0.65	2008	10.35	ok
488.6	555.9	700310.38	0.81	2210	2.50	Not ok
400.5		609320.7	0.68

Conclusion-

1. Using all input data we have successfully designed Ditch area flanges, Front roof rail, Bow roof rails, Rear roof rail as per the requirement.
2. As per heat distortion criteria we come to know all bow roof positions are OK.
3. As per Snow load criteria we come to know there is improvement required.
4. All roof rails draft analysis are found OK.