Roof Design

ROOF DESIGN

AIM : For the given roof styling. Create an ASSEMBLY by developing the essential flanges for the outer roof and creating the reinforcements.

INTRODUCTION:

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. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure and to with stand the over head forces like snow

WHY REINFORCEMENTS :

• 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

• Different types of reinforcements that the car has are :
• FRONT ROOF RAIL :

REAR ROOF RAIL :

CENTRAL ROOF RAIL :

BOW ROOFS :

JOINING PROCESS :

Mastics are introduced between the roof outer panel and the reinforcements for joining . where ever the mastic are introduced there is remarkable improve of the strength

Spot welding is also introduced to join the reinforcements and the outer panel

• DESIGN CONSIDERATIONS FOR A ROOF :
• Visibility criteria
• Head clearance
• Curvature study
• Heat distortion and snow load criteria
• Draft analysis

• ROOF CRUSH TEST :
• Roof crush test is the design given by the IIHS ( insurance institute of highway safety ) . After analyzing the many accidents IIHS has then fixed some regulations , that how to perform the crush test .
• IIHS gives the rating based up on this test . For this they consider two points while giving the rating
• How fast the car can reforms to its original shape after the roll over ,How deep is the protrusion after the roll over

• GOAL OF ROOF CRUSH TEST :
• To minimize the protrusion into the occupant compartment , so that the seat belts and air bags can do there job in protecting the occupant

• CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST :

There are two conditions that should be followed for the roof crush test , they are

• INITIAL CONDITION :

Close and lock all the doors

Close all the windows

• SECOND CONDITION :

Fix the body sill or chassis frame on a hard lever surface horizontally

• METHOD OF APPLYING THE FORCE :
• The load is applied in such a way that 5⁰ is tilted from the front end to the rear end and the load will be in 1800 mm in length and 750 mm in width . The load is applied at the position of 25⁰ from the flatness of the roof . The load is applied on the roof for almost 120 sec and the crush speed at which the load applied is 13 mm/sec or less than that

• RESULTS :
• If the displacements goes above the requirements level then we have to increase the bow roof thickness or add more mastics points to increase the strength . the stronger the roof the better it can protect the occupants in the roll over crashes.

• HEAT DISTORTION STUDY ON THE ROOF :
• The heat distortion study plays a major role in sheet metal usage . heat distortion temperature is a temperature limit above which the material cannot be used for the structural applications . This study is used to predict the heat distortion temperature at where the material starts 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]
• Judgement Condition : OK< 2.7

Roof rail	Rx	Ry	Rx*Ry	Rx+Ry	R	L	t	l	W	result
fr - bow 1	5706	2547	14533182	8253	3521.91494	2030	0.75	448	1.630662493	pass
bow 1- center	4889	6411	31343379	11300	5547.500708	2030	0.75	428	2.061810848	pass
center - bow 2	3963	10712	42451656	14675	5785.574923	2030	0.75	471	2.175664304	pass
bow 2 - rear	3279	14561	47745519	17840	5352.636659	2030	0.75	369	1.94451774	pass

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

SNOW LOAD CRITERIA :

This test is done to know how is the roof 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 is the basic requirements for snow load criteria .

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

Where

α = My x Lx2 x 10-12 , My = Y(Ly-Y)

Judgement 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 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 rail	Rx	Ry	Lx	Ly	L1	L2	t	Y	Iy	My	s	a	((Rx+Ry)/2)^2)	qr	result
fr - bow 1	2806	4402	1134	2030	480	464	0.75	480	2823	744000	472	0.956751264	12988816	27.07215	pass
bow 1- center	6301	3776	1086	2030	464	512	0.75	937	5479	1024141	488	1.207867799	25386482.25	20.59596	pass
center - bow 2	11088	4276	1060	2030	512	470	0.75	1457	2819	834861	491	0.93804982	59013124	5.833935	pass

Section analysis: Section moment of inertia is used to predict the amount of deformation and deflection of a part. A higher moment of inertia is preferable as it reduces the amount of deflection for a given force. The thickness of the part and addition of flanges and embosses increase the section inertia of the part. Here the master sections of the roof and roof rails and their section analysis are given.

front roof rail

rear roof rail

bow roof 1

center roof rail

bow roof 2

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 7⁰ is considered for analysis . green colour infer on the parts that all face along the tooling direction has positive draft angle greater than 7⁰

front roof rail

bow roof 1

center roof rail

bow roof 2

rear roof rail

Conclusion:

1. Designed Roof reinforcements such as Front roof rail, Bow roofs, Centre Roof rail, Rear Roof Rail and Ditch area as per given master sections
2. Performed Draft analysis for all reinforcements
3. Performed Section Inertia Analysis
4. Peformed Curvature study and calculated Heat Distortion Criteria & Snow Load criteria