Roof Design

Roof Design

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

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
• Bow roofs
• Central roof rail and
• Rear roof rail

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 and
• How deep is the protrusion after the roll over

The main purpose of the crash 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.

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.The system consists of an upright assembly and an attached loading head that can be fixed at varying heights from the ground as well as at pitch angles ranging from −5 to +5 degrees to accommodate testing on the driver or passenger side.The roll angle is permanently fixed at 25 degrees. Four hydraulic actuators control the movement of the platen along two linear guides. The entire system is mounted on a T-slot bed plate anchored to the floor of the test facility.

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.

Roof strength evaluations are conducted on a quasi-static test system manufactured by MGA Research Corporation.

Components of roof-

1.Front Roof Rail-

The front roof rail is located on the front of the roof where the wind shield joins with the roof. The visibility criteria and the head room clearance of the passenger are taken into consideration for the bottom flange. One end of the rail is joined using spot welding while the other end is joined with the roof using mastic sealants for increased strength.

2.Bow roof 1-

Bow roof is an additional siffening member for the roof to increase the strength and so that it can satisfy the norms. Both the ends are joined together using mastic sealants to increase strenght and also since this would not affect the styling of the roof.

3.Centre Roof Rail-

Centre Roof rail or the reinforcement centre is located at the centre of the roof . The cross section of this is more since it needs to provide more strength to the roof. Here both are ends are joined together using mastic sealants.

4.Bow roof 2-

This bow roof also have the same purpose as that of the first one. This one is located between the centre bow roof and rear roof rail.

5.Rear Roof Rail.

This roof rail joins with the back door and the roof. Inputs from the backdoor are taken into consideration for designing this roof rail.

Here one end is joined using spot welding while the other end is joined using mastic sealants.

CALCULATIONS:

For our study, we are considering the following conditions for testing the roof.

1. Heat Distortion Criteria:

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

Sample Calculation:

Rx=5718.244mm

Ry=2813.63mm

T=0.75mm

L1=427.3492mm

`R=frac{2RxxRy}{Rx+Ry}=frac{2xx5718.244xx2813.63}{5718.244+2813.63 } =3771.50997`

`W=[1.73xx10^-3xxL]+[1.85xx10^-8xx[frac{R^2}{2}]]+[1.10xx10^-3xxl]-2.68`

`W=[1.73xx10^-3xx2039.56]+[1.85xx10^-8xxfrac{3771.50997^2}{2}]+[1.10xx10^-3xx427.3492]-2.68=1.66939`

Heat Calculation
REGION	Rx	Ry	Rx*Ry	Rx+Ry	R	T	L	l	2.7<W<3.5	CONDITION
FRONT ROOF RAIL-BOW ROOF -1	5718.244	2813.63	16089024.9	8531.875	3771.50997	0.75	2039.5648	427.3492	1.66939698	PASS
BOW ROOF-1-CENTER ROOF	6746.353	4835.628	32622856.7	11581.98	5633.38086	0.75	2039.5648	378.2206	2.04728594	PASS
CENTER ROOF-BOW ROOF-2	11072.91	3882.179	42987009.8	14955.09	5748.81445	0.75	2039.5648	457.5376	2.16694386	PASS
BOW ROOF-1-REAR ROOF RAIL	14918.61	3230.43	48193526.4	18149.04	5310.86212	0.75	2039.5648	235.0778	1.80276234	PASS

we can conclude that the positions at which the bow roofs are placed satisfy the heat distortion condition and is good to go for manufacturing.

Section moment of inertia

The moment carrying capacity of an object is directly dependent on geometrical property (I) and material property (E) of an object,which is collectively termed as flexural rigidity (EI).Geometry of an object plays an important role in load bearing capacity of an object which is indicated by moment of inertia of a section.

Therefore, section modulus is the predominant factor which evidences the strength of an object and is defined as the ratio of the moment of inertia of the object about its centroidal axis to the distance of the extreme fibers of the object from the neutral axis.

There are two types of section modulus we have,

• Elastic section modulus
• Plastic section modulus

Elastic modulus is defined as,

S = I/Y

S – Section modulus,

I – Second moment of area

Y – distance from the neutral axis to outer most fiber of the object.

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

Bow Roof Raill-1

Center Roof Rail

Bow Roof Rail-2

Rear Roof Rail

2. 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 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]

SAMPLE CALCULATION:

Iy=1677.229mm^4

Y=427.35mm

`My=Y(Ly-Y)=427.35(1677.229-427.35)=534135.557``α=MyxxLx^2xx10^-12=534135.25xx1143.55^2xx10^-12=0.698492202`

L1=427.3492mm

L2=378.2206mm

`s=frac{L1}{2}+frac{L2}{2}=frac{427.342}{2}+frac{378.2206}{2}=402.785`

Rx=4372.95mm

Ry=12843.85mm

t= 0.75mm

`Qr=frac{Iyxxt^2}{αxxSxx[frac{Rx+Ry}{2}]^2xx10^8`

`Qr=frac{167.229xx0.75^22}{0.69849xx402.785xx[frac{4372.5+12843.85}{2}]^2xx10^8 }` =8.90806

so for the front roof rail and Bow1 the Qr is grater than 3.1 so for it passed the snow load condition and the position of Bow1 is correct.

REGION	Iy	Y	Lx	Ly	My	α	S	Rx	Ry	R	T	Qr	OK/NG
FRONT ROOF RAIL-BOW ROOF -1	1677.229	427.3492	1143.55	2031	534135.257	0.69849	402.785	4372.95	12843.85	37644102.8	0.75	8.90806	OK
BOW ROOF-1-CENTER ROOF	2096.14	378.2206	1093.85	2031	649752.13	0.77743	417.88	3800.33	6681.79	19917764.7	0.75	18.2217	OK
CENTER ROOF-BOW ROOF-2	1692.22	457.54	1067.85	2031	564911.98	0.644171	346.31	4312.92	11684.24	34497235.7	0.75	12.3689	OK

Judgemental criteria is Qr > 3.1

so the Qr value for the all the rail is grater than 3.1 so for it passed the snow load condition

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 color infer on the parts that all face along the tooling direction has positive draft angle greater than 7⁰ and passed in analysis

Outer Roof

Front Roof Rail

Bow Roof Rail-1

Center Roof Rail

Bow Roof Rail -2

Rear Roof Rail

ROOF IMAGES :

Conclusion :

Therefore,we design the roof,front roof rails,bow roof rails, center roof rails,and rear roof rails according to the master cross- sectional drawing,conduct moment of inertia and design analysis & verify the feasibility of manufacturing .