Roof Design

ROOF DESIGN

AIM:For the Given Roof styling, we need to create an ASSEMBLY by developing the essential flanges for the outer roof and creating the reinforcements like Ditch area , Front Roof rail ,Bow roofs, Central Roof rail and Rear roof rail.

OBJECTIVE:

• Perform the calculations for the Heat Distortion and Snow Load criteria by using the formulaes to determine whether the bow roofs are at the correct positions are not.
• Finding Moment of Inertia and Section modulus for Bow roofs.
• Perform the Draft analysis for the reinforcements which we created.

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 weather conditions.
• When we are seeing a car from the outside, it is just top protion covering the inner cabin. But in design point of view,we need to create no of Roof rails for providing enough strength to withstand the load. The Bow roofs are created such a way that they are in a flat portion of the roof and need to be in a straight object for making the manufacturing simple.
• Front Roof rail & Rear Roof rails are fixed to the ditch area by using spot weld operation. But the BOr roofs and the center bow roof are fixed to the roof by using mastic sealants,to avoid damages on the aesthetic surface.

DESIGN CONSIDERATIONS FOR A ROOF:

Fro designing the roof we need to consider some considerations before proceeding with the design .They are

• Head room clearance.
• Strength of the roof.
• Visibility criteria.
• Draft analysis(must be greater than 7deg)

REINFORCEMENTS FOR THE ROOF:

If the surface of the roof is flat,it is very weak and cannot be able to withstand the forces.So in order to increase the strength of the roof we are introducing these reinforcements like front roof rail, centre roof rail, rear roof rail and the bow roofs.These acts as supporting members for the roof and are joined to the flat surface of the roof by using different joining process.

They are

• Front roof rail
• Rear roof rail
• Center roof rail
• Bow roofs.

FRONT ROOF RAIL:

The front roof rail is joined by using spot welding process,since it is covered by the wind shield.It is connected to the A pillar of the car.We can provide embosses to provide enough strength to the roof rail adn for designing we need to follow the master section.Once it is done we need to provide fillets to the sharp edges and then thicken it.

REAR ROOF RAIL:

The rear roof rail is connected to the back door and the C pillar of the car.We need to follow the master section.Once it is done we need to provide fillets to the sharp edges and then thicken it.

CENTRAL ROOF RAIL:

Generally centre roof rail is placed at the center of the roof which is connected to the B-pillar of the Car.For this joining the roof rail and the roof we are using mastic selants.

BOW ROOFS:

The bow roofs are provided to improve the torsional stiffness and load bearing capacity of the roof structure.To increase the strenth on the flatter region or surface we are providing the bow roofs. The number of bow roof present is depends on the overall size of the roof. In this project two bow roofs are sufficient before and after the centre roof rail.

CREATION OF DITCH AREA:

It is the extended portion of the roof outer panel styling surface.It is used to attach with the reinforcements by using spot welding process. It has a tool opening angle of 7 deg.It needs to beprovided with edge blends.

ROOF ASSEMBLY:

JOINING PROCESS:

• Spot welding is used for joining the reinforcements(front roof rail and rear roof rail) and the outer panel.
• Mastic selants are used in between the roof outer panel and the reinforcements like center roof rail,bow roof1,bow roof2 for the joining purpose.

ROOF CRUSH TEST:

A study was conducted at the Vehicle Research and Test Center to investigate roof crush resistance in passenger vehicles.Roof crush test is the design given by the IIHS (insurance institute of highway safety). Roof strength evaluations are conducted on a quasi-static test system manufactured by MGA Research Corporation.

IIHS gives the rating based up on this test. For this they consider two points while giving the rating

• How fast the car can reform to its original shape after the roll over and
• How deep is the protrusion after the roll over.

The main aim for th eroo fcrush test is 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 level surface horizontally.

CURVATURE STUDY ON ROOF:

• Heat Distortion study
• Snow Load study

HEAT DISTORTION STUDY:

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.

Heat Distortion Criteria :

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> 3.1

In order to find the heat distortion criteria apply all the necessary values from the roof and substitute them in the given formula and then evaluate in a table to test the judgement condition is satisfied or not.

BOWS	Rx	Ry	R	L	l	t	R^2/t	W	OK/NG
FRR-BR1	706.600	2400.141	1091.780	2029.1364	374.7866	0.75	1.5893*10^6	1.27	OK
BR1-CRR	4965.102	6219.295	5521.877	2029.1364	428.4607	0.75	40.6548*10^6	2.05	OK
CRR-BR2	3993.131	10496.388	5785.347	2029.1364	425.2588	0.75	44.6269*10^6	2.12	OK
BF2-RRR	3241.511	14837.813	5320.656	2029.1364	357.1055	0.75	37.7458*10^6	1.92	OK

From the above table it is concluded that all values of w are less than 2.7 and also not greater than 3.1(i.e OK< w > 3.1 ) so thus infers that current positioning of bow roofs are good in state for heat distortion as per design.

SNOW LOAD STUDY :

This test is done to know how is the roof behaving when there is a snow over the roof surface . Normally due to the snow weight the dent or roof bend happens. But the roof should be designed in such a way that when the snow is removed from the roof ,it should go to its original position . This is the basic requirements for snow load criteria .

SNOW LOAD CRITERIA:

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

Where

α = My x Lx^2 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].

Inorder to obtain the moment of inertia we need to apply section inertia analysis over the bor roofs so that we can get the value of Imin.

Bow Roof-1 Section Modulus:

Center Roof Rail Section Modulus:

Bow Roof-2 Section Modulus:

In order to find the snow load criteria apply all the necessary values from the roof and substitute them in the given formula and then evaluate in a table to test the judgement condition is satisfied or not.

BOWS	Iy(MM^4)	t	alpha	Lx	Ly	Y	s	Rx	Ry	Qr	RESULT(Qr>3.1)
FRR-BR1	1572.7292	0.75	0.9118	1141.0173	1964.6911	467.9052	489.1469	4623.354	2471.055	15.764	OK
BR1-CRR	1998.0434	0.75	1.13952	1086.6808	1964.6911	978.2938	508.7953	3764.912	6128.055	7.921	OK
CRR-BR2	1646.2761	0.75	0.7978	1058.6751	1964.6911	1485.4959	493.1986	3184.563	10981.320	4.691	OK
BF2-RRR	-	0.75	-	1043.3766	1964.6911	1964.6911	-	3942.159	16749.422	-	-

From the above table it is concluded that all values of Qr are greater than 3.1, so thus infers that current positioning of bow roofs are good in state for snow load as per design.

DRAFT ANALYSIS:

Draft analysis is used to predict the manufacturable feasibility of the part.The draft is provided in order open and closing the mold.The draft angle provided for design is around 7 degrees.

Roof Styling surface

Front Roof rail

Rear roof rail

Center roof reinforcement

Bow Roof-1

Bow Roof-2

CONCLUSION:All the required reinforcements are provided for the roof.Thus the roof suucessfully passed the judgement criteria for heat distortion and snow load and the draft analysis is also made.