Car roof design

Objective-

1. For the given Car Roof styling, design and develop the essential flanges and reinforcements, namely the Ditch Area, Front Roof Rail, Rear Roof Rail, Centre Roof Rail and Bow-Roofs.

2. Do a curvature study on the roof and perform calculations for Heat Distortion and Snow load criterion to determine and justify the position of the Bow-roofs.

3. Do a Section Modulus study for the Master sections created for the Front Roof Rail, Rear Roof Rail, Bow Roof Rails and Centre Roof Rail

4. Perform Draft Analysis of each reinforcement.

TOOL USED- NX

Introduction-

An automobile 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.

Under the styling surface that is seen on the outside, there are many other structures under the roof which contribute to its strength and stability. The number of such structures/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-

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 Roof 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 3-5 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.

Rear Roof Rail-

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. Inputs from the backdoor design is also taken to accommodate the required clearance. Spot Welds are applied at the Back doorend of the rail and the other end is joined with the roof using Mastic Sealants.

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 and Bow Roofs. 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 Rail and Bow Roofs are fully positioned under the Styling surface of the Roof. For increasing the strength of the rails, embossing is done. To reduce the weight of the material used, a different styling has been followed with a thickness of 0.75 mm.

Bow roof 1-

Bow roof 2-

All roofs-

Cross sectional view-

Final assembly-

Analysis-

1. Curvature Study

It is done to justify the positioning of the Bow Roofs. There are mainly three criterion to fulfill

1. Roof Crush test

2. Heat Distortion Criteria

3. Snow Load Criteria

Roof Crush Testing is done only after the part or prototype is manufactured in physical conditions.

a. Heat Distortion Criteria

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.

Prediction Formula : W=1.73×10-3×L+1.85×10-8×R2t+1.10×10-3×l-2.68\">W=1.73×10−3×L+1.85×10−8×R2t+1.10×10−3×l−2.68

W=1.73×10-3×L+1.85×10-8×R2t+1.10×10-3×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)

When applying the corresponding measurements into the formula, the criterion is met for all the reinforcements.

b. 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.

Prediction Formula: Qr=Iy×t2α×s×(Rx+Ry2)2×10-8\">Qr=Iy×t2α×s×(Rx+Ry2)2×10−8

Judgment condition: Qr ≥ 3.1

Where,

α=My×Lx2×10−12,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 centre 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 centre 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

When applying the corresponding measurements into the formula, the criterion is met for all the reinforcements.

CONCLUSION-

The roof and ditch area is successfully designed, along with the heat distortion test and snow load test. All the requirements are calculated and gets passed through them, so it could be sent to further steps of production.