Roof Design

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. Though it just looks like a styling surface that protects from sun, wind and other elements, various environmental conditions are taken into account for designing of the same. Roof should be provided with proper reinforcements for strengthening. Vehicle roof can be subjected to high heat conditions or load during snowy conditions. To make the roof withstand these conditions, various calculations are done. This ensures the safety of the passengers in the vehicle.

Objective:

The main objective of the project is to design a roof and roof reinforcements from the styling surface received. After the design is done, heat load and snow load calculations will be done on the 0Y section of the roof.

Design and calculations:

Roof and roof reinforcements are designed in NX CAD from the given styling surface. Sheet thickness is considered as 0.75 mm for all components. Two bow roofs are provided for the strength of the roof.

Front Roof rail:

This is the reinforcement provided at front. This part is joined to BIW body and windshield. Visibility criteria is taken in account for joining it with windshield. It is fixed to BIW with spot welds and roof surface is attached with the help of mastic sealants. A draft angle is provided for deep drawing operation. Draft analysis is also done on the section to check its manufacturing ability.

Bow roof 1:

It is an additional reinforcement provided to increase the strength of the roof. It is fixed with roof body with mastic sealants.

Centre rail roof:

Centre rail roof can be considered as reinforcement joining the left and right part of the roof. It is fixed to roof by mastic sealants and to BIW by spot welds.

Bow roof 2:

It is similar to the bow roof 1 provided for better strength.

Rear Roof rail:

It is the reinforcement provided at rear side of the roof. Backdoor and bodyside panels are attached with this part. It is also attached with mastic sealants and spot welds.

Heat distortion study:

This study is done to check how the sheet metal components perform in the extreme heat conditions. Sheet metals have tendency to change its shape in high temperatures. To avoid any failure of sheet metal components this study is done to determine its capacity.

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

Judgement Condition: OK< 2.7> 3.1

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]

Result table:

This shows the design is safe in heat distortion criteria.

Snow load Prediction formula

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]

        Roof panel X curvature radius of length s in Side view

Result Table:

This shows that at the third section, design does not fulfil snow load criteria.

Hence, to increase the strength of the roof, embosses are provided on the roof surface.

Draft analysis of roof was done to check for proper draft angle.

Conclusion:

Design of roof was done and checked for heat load and snow load criteria. Designed passed the heat load criteria but failed at some sections in snow load criteria. To overcome this, embosses are provided on the roof surface.