Challenge

Roof Design

AAafiq Abdul Rahman29 Nov 2022

ROOF ASSEMBLY

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.

INTRODUCTION :

Design of any product based on the requirement.The design engineer sketches the product based on the the requirements. then the 2d sketches with the dimensional is converted into the 3d model with using advanced softwares . so the role of the design engineer is to design the product that can be easily manufactured and with less cost effective .This is the place where the Design for manufacturing [DFM] and Design of assembly [DFA] takes a role while developing any product .The process of development of design carries on until the design satisfies the customer needs .

Here in this project the product that we are going to develop is a car roof.

main parts of the roof

The roof has an outer panel which is the styling part.And also provide the strength to top roof the car.
Strengthening parts are present below an outer panel of a roof, which are front roof rail, Bow roof, Centre roof rail, and Rear roof rail.
The front roof rail provides provision for the windshield glasses. Visibility criteria and headroom clearance have to be followed while designing the front roof rail. Provision for windshield mounting and inside rearview mirror is provided on the front roof rail.
The Rear roof rail has hinge mounting for the back door. It is attached at the portion where the back door meets with the roof.
Bow roofs are provided in between the front roof rail and center roof rail as well as the Rear roof rail and center roof rail. They are provided to enhance the localized strength of the roof and are attached directly to the bottom of the roof with either spot welding or with the help of Mastics.
Reinforcement center or center roof rail is used as a cross member between Left hand and right-hand bodysides.
Both the front roof rail and rear roof rail provided with the Master section for the design.
The below image shows the location.

design and functional requirements for the roof

while design the roof following consideration have taken into account .

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

Roll over test on the 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.

design consideration

Clearance of the 5mm should be present between outer roof rail and front roof rail , rear roof rail , bow roof rail and reinforcement center where mastics are used for adhesion.
thickness of the sheet metal 0.75mm
all the edges should be filleted and there should not be any sharp edges present.
minimum draft angle 7 degree is required for all the faces .

Design the roof rail and bow roof rail :-

A. FRONT ROOF RAIL

By the given master section and dimension of front roof rail has been designed.
the image is given below is master section drawing.

The below image shows the dimensions used for the front roof rail.

bow roof 1

By following the given Master section Bow Roof has been designed.
The below image shows the given master section for Bow Roof.
the next given drawing is given thickess.

center roof

By following the given Master section center Roof has been designed.
The below image shows the given master section for Bow Roof.
the next given drawing is given thickess.

bow roof 2

By following the given Master section bow Roof 2 has been designed.
The below image shows the given master section for Bow Roof.

the next given drawing is given thickess.

rear roof

By following the given Master section rear Roof has been designed.
The below image shows the given master section for Bow Roof.
the next given drawing is given thickess.

section modulus 0f all roofs

front roof section modulus

center roof section modulus

bow roof 1

bow roof 2

rear roof rail section modulus

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> 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]

ANNOTATION

Calculation of "W":-

heres all heat distortion calculation passed .

SNOWLOAD CALCULATION:-

A snow load study is done to check the ability of the roof to withstand snowfall and bounce back to its original position without undergoing permanent deformation upon the removal of the load.

Procedure:

Create a center line at the 0-Y region of the roof and then offset the same to 100mm on each side.
Draw a vertical line at the midpoint of the line drawn from the center points of the FRR and BR 1 and followed the same till the rear roof rail.
Offset the vertical line to a distance of 100mm on each side.
Create intersection points and project them on the roof.
From these projected points, make a 3-point arc and the Rx and Ry values.
Project the extreme edges of the styling surface fro measuring the width at the respected sections.

FORMULA:-

Qr = [Iy x t²] / [α x s x [(Rx + Ry)/2]²x 10^-8]

α = My x Lx² x 10^-12 , My = Y(Ly-Y)

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]

The width of the roof panel is 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

ANNOTATION

CALCULATION:-