BIW Roof Component Design and Curvature Study.

Aim -

• To develop necessary flanges for the roof styling surface and create the necessary rail reinforcements for the part.
• To perform the Curvature study on the roof bow to check if they pass the snow load and heat distortion criteria.
• perform the draft analysis to check the feasibility of roof rail for manufacturing purpose.

Theory -

The roof is the topmost structural member in an Automobile. The main purpose of the roof is to protect the passenger and Compartment from all types of environmental factors such as dust, wind, Rain, Sunlight etc. As the component is exposed to such rigorous conditions, careful material selection is really Important. Generally, alloys of aluminium are preferred as they are lightweight and results in lower CG and more stability. Steel is also used to develop a panel for the roof.

The outer panel of the roof is supported by the Roof rails. Roof Rail and Roof part are joined by Mastic glue preferably Butyl mastic as it retains flexibility even after adhesion and helps in maintaining flexibility and reducing NVH. Generally, the roof rails are developed from scrap material; from the blanks used to manufacture the rest of the body panel. Front and Rear roof rails are present in every car body. But, the number of bow roof and Reinforcement centre depends upon the curvature and length of the roof outer panel.

The better the curvature of the roof the better is the torsional stiffness of the component. If the curvature is not adequate at certain locations we provide Bow Rails to provide strength at the area. The positioning of the roof rails is governed by certain conditions. In this project, we are going to perform a Curvature study by using the Conditions of Heat Distortion and Snow load criteria to check if our component is feasible or not.

Formulas -

• Heat Distortion - it is an important factor to determine the position of the roof. Heat Distortion temperature is the temperature at which the material is no longer suitable for structural applications. When exposed to high temperature and load the material begin to soften and cannot be used.

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

Were,

L - Length in X-direction(mm)

Rx - Value of X curvature

Ry - Value of y Curvature

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

t - Thickness of the component

l - Bow roof Span

• Snow Load Condition - The roof of an automobile must be able to withstand load especially snow which results in heavy load acting on the roof. If the roof is not able to withstand it, there will be deformation of the roof. To prevent that, Snowed criteria calculation helps us to determine whether the roof design will withstand the snow load.

Formula - 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 centre 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

Design -

• Front Roof Rail -

• Rear Roof Rail -

• Reinforcement Center -

• Bow Roof Rail 1 -

• Bow Roof Rail 2 -

• Assembled Part -

If the curvature is not sufficient we can provide embosses to the roof panel to increase strength.

Draft Analysis - Draft analysis is done to check if the component does not have any negative draft. a negative draft can create a problem during the manufacturing process. Sheet metal parts must have a draft of at least 7 degrees for proper tooling.

• Front Roof Rail -

• Rear Roof Rail -

• Reinforcement centre -

• Bow Roof 1 -

• Bow Roof 2 -

Section inertia Calculation -

• Front Roof Rail -

• Rear Roof rail -

• Reinforcement Center -

• Bow Roof 1 -

• Bow Roof 2 -

Calculations -

• Heat Distortion -

The part satisfies the Heat distortion Criteria.

• Snow Load Criteria -

The part satisfies the snow load criteria.

• If any of the above conditions are not fulfilled the position of roof rails must be adjusted. Still, If the conditions were not met the styling team must be notified with required curvature value.

Part link - https://drive.google.com/drive/folders/11lkiI5bH3wpI2-FT5U4EzTCu9FsDN0GI?usp=sharing