Design of Car Roof Outer Panel, Front/Rear/Central Roof Rails

Design and development of a car roof assembly

Introduction:

An automobile roof is considered as an integral part of BIW (body in white) design. A car roof is responsible for protecting the passenger from rain, wind, sunlight, accidents etc. It also offers aerodynamic support to the car when it is in motion. A roof panel assembly includes a roof outer panel/ditch area and inverted hat like reinforcements rails which underlie the outer side edges of the roof outer panel. Roof rails are provided to give strength to the outer panel and also provide support to other commodities present in a car.

There are different rail reinforcements in a roof panel like:

1. Front roof rail
2. Rear roof rail
3. Bow roofs

Reinforcement roof centre


Fig 1 Car Roof Components

Fig 2 Roof panel with rail reinforcements

Software used : CATIA V5

Given : Roof SkinStyling surface Design Procedure

1. Styling surface cleanup :
It is necessary to remove any discontinuity in the styling surface which may result in faulty surfaces.Also it helps to generate smooth surfaces while performing complex operations. Surfaces were extracted from the given styling data and untrimmed to get the original surface back.





2. Origin point and axis system:
It is very important to ensure that all the surfaces, sketches etc created while developing the roof ditch area have no link with the styling surface, also it is very important to link everything with one point and one axis system. The default planes and part-body in CATIA V5 should never be used for creating the the cad model. This point is set to the co-ordinates (0,0,0) and the axis system is defined using this point.

3. Parameters:
It is important to create pre-defined parameters and use these while making the model which makes the model editable at every point of the cad model development.
Here the opening angle is taken as 7 deg because the model will not pass the draft analysis as for sheet metal the draft angle is 7 deg.

The following parameters have been defined

4. Roof ditch area:

In CATIA it is important to design a cad model with a particular method, the whole point is to make the model editable and have everything organised. This helps other engineers to understand how the cad model is prepared. Some key features while designing this panel were

• Specification tree maintenance
• Fully constrained sketches (Iso-constrained)
• The sketches should have the origin point and axis system as their parents.

Keeping all these points in mind the cad model for the roof panel was developed.

5. Front roof rail:

Front roof rail is used to provide strength to the roof panel front and it also acts as link between windshield glass, roof panel and headliner(interior trim). Headroom clearance and visibility are two important criteria for the driver and both of them depends upon the front roof rail dimensions, hence it is important to select appropriate dimensions for the roof rail.

6 Rear Roof Rail :




7. Roof Central





8. Bow roof 2

9 Bow roof 3 & 4

Map Lamp Mounting Bracket :




10 DRAFT ANALYSIS :

It is important that what-ever is designed should be easily manufactured. Draft analysis only is not a sure shot confirmation that the cad design is 100 % manufacturable but it checks that all the surfaces are greater than or equal to a particular angle (in sheet-metal it is 7 degrees).

Drawing/Stamping direction: A sheet metal part is manufactured by the stamping process. In this process a punch and die are used to generate the desired shape. The stamping direction indicates the direction in which the punch moves.

11. SECTION MODULUS STUDY :

Section modulus study is done to determine the strength of the roof panel. The more is the inertia of the panel more is its ability to handle the impact and provide safety to the passenger.

Formula used : S=I/y, where I = principal moment about the axis and y = half of the total distance of the section.

Here we take the least of the Principal moments

M1 = 1.356e-06 m^4
(As the distance measured is in mm we have to convert the principal moment into mm^4)

M1 = 1.35e06 mm^4

Y= 2124.497 mm

y=Y/2 = 1062.2485mm

S = (1356 / 1062.2485) x 1000 mm^3 = 1.277e03 mm^3

Front roof rail



Here

Y = 109.537 mm

y = 109.537/2 = 54.7685 mm

M1 = 1.119e-8 m^4 = 1.119e4 mm^4

S = 1190/54.7685 mm^3 = 21.728 mm^3


Rear Roof Rail

Y = 133.891 mm

y = 66.945 mm

M1 = 7.726 E-09 m4 = 7.726 E+3 mm4

S = 7726/66.945 mm^3

Roof Central :

M1 = 23280 mm^4

Y = 160 mm , y = 80 mm

S = M1/y = 23280 / 80 = 291 mm^3

Bow roof 3

Y= 160 mm , y = 80 mm

M1= 5835 mm^4

S = M1/y = 5835/80 = 72.8125 mm^3

Bow roof 4

We take

M1= 3280 mm^4

Y = 70, y = 35 mm

S = 3280/35 = 93.714 mm^3

Bow 1&2



Y = 60.437 mm, y = 30.2185

M1 = 2621 mm^4

S = M1/y = 2621/30.2185 = 86.735 mm^3

12 Roof curvature study and bow roof position justification

Before sending the CAD models to the simulation team it is important to justify the position of the bow roof and their bow span. It actually leads to the prevention of most of the problems that may occur in the design stage. There are two ways in which we can perform this study Before performing the study we ourselves analyse the roof curvature.

Above is an image of the cross section of the roof panel, as we can see that the front portion(from the left) has more curvature as compared to the rear half(to the right) we can place the bow roofs in such a way that more strength is given to the rear half. For this I have proposed the following design

To justify the bow span and bow position we have to perform certain studies. In this project we have performed two studies on bow roof positioning and roof curvature.

A) Heat Distortion study :

In heat distortion study we have a mathematical formula which has to be satisfied in order to justify the bow roof positions. The following formula has been used

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

Judgement Condition : OK<2>

Where

L = Roof Length in X-Direction[mm](Roof dimension in 0-Y)

R = Roof curvature

where, R = 2(Rx*Ry)/(Rx+Ry)

Rx = X curvature[mm]

Ry = Y curvature [mm]

t = Roof plate thickness [mm]

l = Bow Roof Span [mm]

Calculation for front roof rail & bow roof 1 :

Divide the roof assembly into propotional parts




We take the horizontal line as OY and make another line 3Y at a distance of 300 mm from the OY axis now we make 2 lines at 100 mm distance on either side of the 3Y line. The intersection of these lines will give us the points which will be projected on the Styling surface to determine Rx



Rx = 5754.393 mm

Similarly at the OY we take the Ry by taking the center between the bow roof edge and front roof rail edge.



here L =2039.05 mm
t= 0.75 mm
I = 244.194mm
from the formula we get W = 1.352 which is < 2.7 hence OK

S.no	BOW	Rx(mm)	Ry(mm)	R(mm)	I(mm^4)	L(mm)	t(mm)	R^2/t	W	OK/NG
1	FR & 1ST	5754.393	2106.777	3085.05	244.194	2039.05	0.75	12689633.333	1.352	OK
2	1ST & 2ND	5412.839	4281.777	4781.33	287.552	2039.05	0.75	30481463.442	1.729	OK
3	2ND & CTR	4633.607	7240.37	5650.85	299.951	2039.05	0.75	42576140.964	1.966	OK
4	CTR & 4th	3829.088	12183.347	5826.86	356.404	2039.05	0.75	45269729.946	1.690	OK
5	4th and RR	3204.403	16938.373	5389.26	312.19	2039.05	0.75	38725497.797	1.908	OK

B) Snow Load Study :

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 panel X curvature radius of length s in Side view

Calculation :

Here we take all the values along OY axis . Rx and Ry are taken along the OY.






Lx is the width of the roof at the center point

S. no.	Bow	I(y) (mm^4)	α	Lx	Ly	Y	s	Rx	Ry	t	Qr	Determination
1	Fr & 1st	2621	0.88	1169.045	2309.05	324.687	336.5645	4962.675	2060.314	0.75	40.37	OK
2	1st & 2nd	2654	0.87	1121.719	2309.05	348.449	377.5535	3854.644	4301.116	0.75	27.02	OK
3	CTR & 2nd	23280	0.92	1091.605	2309.05	406.658	440.767	3878.618	7482.885	0.75	100	OK
4	CTR & 3rd	3280	0.99	1070.393	2309.05	474.876	438.304	4324.888	11953.235	0.75	6.42	OK

Here we got the Qr values greater than 3.1 but as you can see the s value for the last two is greater than 380mm hence we introduce beads on the roof panel. The longer beads cover the rear portion of the roof to provide more strength.

Conclusion :

Roof panel with all the necessary reinforcements have been created and roof curvature and bow roof position justification has been provided.