Roof Design

Objective :

1. Main objective of the project is to create the Roof Sheet metal design in CATIA V5, and also designing Front roof rail, rear roof rail, bow roof, center roof rail and reinforcement.
2. Curvature study on Roof and performance calculations of Heat distortion and Snow load criteria to determine the position of the Bow roofs.
3. Finding the moment of inertia and section modulus for Bow-roofs and center roof rail.
4. Draft Analysis of each part.

Introduction :

1. A roof is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements.
2. Under the styling surface, there are many other structures under the roof which contribute to its strength and stability. Each structure/supports differ with respect to the size of the vehicle. They are primarily installed on flatter areas of the roof to enhance the strength. These are the Front Roof Rail, Rear Roof Rail, Centre Roof Rail and Bow Roofs and they are joined to the roof by the Ditch Area.

Reinforcements and Design:

Design Consideration :

• Visibility Criteria
• Head clearance
• Curvature study
• Head Distortion and snow load criteria
• Draft analysis

Ditch area:

The Ditch Area is the extended portion of the main Roof concealed by other BIW panels. The rail supports of the roof are attached to the Ditch Area using Spot welds, since it is not a styling surface and remains concealed. Headroom Clearance, visibility criteria, clearance for Wind shield and Back door are some of the factors to be considered while designing the Ditch Area. It is given a thickness of 0.75 mm.

Front Rail

It is the support which is joined with the Wind Shield Glass, Body Side Outer and the inner panels. It is usually made from Body Side Outer scrap and therefore the thickness of the rail is 0.75 mm. A draft angle of 7 degrees is provided keeping in mind the deep drawing operation. The Wind Shield end flange is the exact offset of the roof surface. The bottommost flange is decided after taking into consideration the visibility criteria and the Headroom Clearance of the passenger. Spot Welds are applied at Wind Shield end of the rail and the other end is joined with the roof using Mastic Sealants which also contributes to increased strength.

Bow roof:

Bow roof has been provided for additional strength especially against snow conditions and rollover situations. In this model, one bow roof has been provided between front roof rail and Centre roof rail and another bow roof has been provided between Centre roof rail and rear roof rail. But the number of bows on the roof is based on the size of the car like small cars do not require a bow because it increases the weight of the car as well as the other rails are enough to provide the support for the roof. But for large cars 3 or more bows are needed to support the roof. The thickness of Bow roof rails are 0.75 mm each.

Centre roof rail:

Similar to other Rails design, we have to take into consideration the Headroom Clearance and the visibility criteria while designing the Centre Roof Rail. But unlike the Front and Rear Roof Rails, Spot Welds are not applied for joining these rails to the roof. Instead Mastic sealants are introduced at both the ends since Centre Roof are fully positioned under the Styling surface of the Roof. To reduce the weight of the material used, a different styling has been followed with a thickness of 0.75 mm

Rear roof:

It is the support which is joined to the Back Door and the Body Side Outer and the inner panels. It is also usually made from Body Side Outer scrap and therefore the thickness of the rail is 0.75 mm. Similar to the Front Roof Rail design, we have to take into consideration the rear Headroom Clearance and the rear visibility criteria while designing the Rear Roof Rail. Input from the backdoor design team is also taken to accommodate the required clearance while work on rear roof. Spot Welds are applied at the Back door end of the rail and the other end is joined with the roof using Mastic Sealants.

Design Calculations and study:

Heat distortion :

Heat distortion temperature is the temperature at which the metal softens and deforms under a specific load. This study is done to check the ability of the roof to withstand high temperature with respect to the positioning of the reinforcements which contribute to increased strength.

Formula to be used:

W = (1.73 x 10^-3x L) + (1.85 x 10^-8 x R²t) + (1.10 x 10^-3 x l) - 2.68

Judgement Condition: OK < 2.7mm ≤ NG

Where,

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

R = Roof curvature= 2(Rx*Ry) / (Rx + Ry)

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness (mm)

l = Bow Roof Span (mm)

Snow Load Criteria :

Snow load criteria study is done to check the ability of the roof to withstand snowfall and bounce back to its original shape without any permanent deformation once the load is removed.

Formula:

Judgement Condition: Qr ≥ 3.1

Where,

α = My x Lx² x 10^-12, My = Y(Ly - Y)

t = Roof plate thickness (mm)

Ly = Distance between front and rear roof rails on vehicle 0-Y (mm) i.e. Length of Roof panel with the center point between Roof rail FR/RR as the reference point of the front/rear.

Lx = Distance between the left and right end of roof on the roof BOW (mm) i.e. Width of the Roof panel exposed on surface

Y = Distance from Front roof rail center to Bow roof (mm)

s = Distance for which Bow roof bears divided load (mm) = L1/L2 + L2/2

Iy = Geometrical moment of inertia of Bow roof (Y cross section) (mm⁴)

Rx = Lateral direction curvature radius of roof panel Y cross section on Bow roof (mm) i.e. Roof panel curvature radius of the length Lx in front view.

Ry = Longitudinal direction of curvature radius of roof panel X cross section on Bow roof (mm) i.e. Roof panel X curvature radius of the length s in side view.

Section Modulus:

Section modulus is the direct measure of the strength of the steel. Section modulus is a geometric property for a given cross-section used in the design of flexural members. The more section modulus, the more the member is resistant to bending of the beam.

Formula:

S = I/Y

Where,

S = Section Modulus (mm³)

I = Moment Of Inertia of the section about the bending axis. (mm⁴)

Y = Distance between the neutral axis and extreme end of the object (mm) .

Front roof rail:

I = 7927 mm⁴

Y = 109.99mm

S = 72.07019 mm³

Centre roof rail:

I = 6595 mm⁴

Y = 81.691 mm

S = 80.73105 mm³

Bow roof:

I = 2497 mm⁴

Y = 69.023 mm

S = 36.17635 mm³

Rear Roof Rail:

I = 70960 mm⁴

Y = 204.63 mm

S = 346.7722 mm³

Draft analysis:

Ditch area:

Front roof rail:

Bow roof:

Centre roof rail:

Rear roof rail:

Conclusion:

Thus the required reinforcements and Ditch Area for the Roof were designed and developed from the given styling. The curvature study was done after performing calculations for the Heat Distortion and Snow load conditions with the position criterion met for all reinforcements.

Further the Section Modulus study was conducted and Draft Analysis were done for each reinforcement successfully