Roof challenge

ROOF DESIGN

Aim:-

To design the roof of an car along with its reinforcement following the master section & design procedure.

Objective:-

• To design the roof and its reinforcement
• Perform draft anaylsis
• To do curvature study on the roof and perform calculations to determine the position of the Bow-roofs.
• Curvature study - Heat disortion criteria and snow load criteria

Introduction

Roof:-

An automobile roof or car top is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.

 

Design and Functional Requirements for the Roof

While designing of roof following considerations have to be taken into account.

• Visibility Criteria
• Head clearance
• Curvature Study
• Head Distortion and snow load criteria

Roll Over Test On Roof 

• The rollover test is given by the Insurance Institute of highway safety ( IIHS). After analyzing many accidents IIHS has fixed some regulations about how to perform the test.
• IIHS rating is based on the following things which are how fast a car can reform its original shape after rollover and how deep a protrusion is.
• The aim of this test is to minimize the protrusion of the roof.
• Two conditions need to be followed while performing this test. One is to lock all the doors of the car properly and the second is to close all the windows of a car.
• Also, it is mandatory to fix the chassis on the hard lever surface horizontally.

• Load is applied at 25 degrees tilt when view from the front over the length of 762 mm.

• Load is applied at 5 degrees tilt when view from the side, over the length of 1829 mm.

• Start with designing the roof outer panel ditch area with the help of given styling surface following the master section.

• By following the given Master section and dimensions front roof rail has been designed

        

• By following the given Master section Bow Roof-1 ,center bow roof, bow roof-2 & bow roof-3 has been designed. Bow roof & center bow roof are added as additional reinforcement in the roof assembly. These are joint with the roof outer panel by means of spot welding.

Bow roof-1

 Centre bow roof

Bow roof-2

Bow roof-3

• By following the given Master section Rear Roof Rail has been designed.

• Necessary flanges have been created for the roof outer panel to join a side, front, and rear of the car roof with the main body.
• Embosses are added to increase the strength of outer panel

Draft Analysis of front roof rail

Draft Analysis of Bow roof-1

Draft Analysis of center bow roof

Draft Analysis of bow roof-2

Draft Analysis of bow roof-3

Draft Analysis of rear bow roof

Heat Distortion Analysis

• Heat distortion analysis plays a pivotal role in the strength of the roof panel. It is the temperature above which material cannot be used for structural applications.
• This study is used to predict the heat distortion temperature where the material starts to soften when exposed to a fixed load at elevated temperature.
• In order to avoid any bending or damages on the roof, heat distortion temperature can be recalculated using the trial and error method by changing the position of bow roofs. In this way, it can come in the range which is shown below.

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 using w : OK< 2.7

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]

Positions of Rx and Ry

Position of bow roof are ok for heat disortion anaylsis (All 'W' values are lesser than 2.7)

Snowload Analysis

• The purpose behind this analysis is to check the strength of the roof when snow is present on it acting as a load.
• A roof should be designed in such a way that it should rebound to its original shape after the removal of snow.

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 center point between Roof rail Front /Rear as the front and rear reference point.

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]

 Bow roof positions satisifying the snow load criteria.

 Assembly view:-

Conclusion:-

         All the parts have successfully cleared the draft analysis. The proposed Roof design has successfully cleared the heat distortion and snow loaded analysis.Embosses have been provided onto the roof outer panel portion to increase the strength.

The roof has been successfully designed following master section and design procedure.