Roof challenge

Design of Car Roof

Abstract

This report presents the design and assembly of the car’s Roof along with the justifications being carried out.

Objective

To Design a car’s Roof NX CAD using the given master section and design parameters.

Introduction

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.

Types of Roofs

1. Hardtops
2. Sunroof
3. Convertibles

Hardtops

sunroof

Panoramic roof

Pop-up sunroof

Spoiler sunroof

Convertible

Components

1. Roof outer panel
2. Front roof rail
3. Bow roof 1
4. Bow roof center
5. Bow roof 2
6. Rear roof rail

Roof outer panel

Front roof rail

Bow roof 1

Bow roof center

Bow roof 2

Rear roof rail

Section modulus

Section modulus is the geometric property for a given cross-section used in the design beam other geometric properties used in design include area for tension, radius of gyration for compression and moment of inertia for stiffness. Any equation for the section moduli, The elastic section modulus (S), plastic section modulus(Z)

Section inertia analysis of Bow roof 1

Section inertia analysis of center bow roof

Section inertia analysis of Bow roof 2

Draft analysis

The design has been analyzed using the draft analysis tool in NX CAD with having 7-degree allowance

Bow roof 1

Bow roof center

Bow roof 2

To justify the bow span and bow position we have to perform certain studies. In this project we have performed two studies on bow roof positioning and roof curvature.

1. Heat distortion study
2. Snow load criteria

Heat distortion study

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

Were,

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]

Judgment condition

OK< 2.7 <3.1

s.no	Bow position	Rx	Ry	L	I	t	R^2/t	W	Ok / NG
1	Fr-Br1	5699.97	2692.60	2030	515.90	0.75	87001137	3.0	ok
2	Br1-Cbr	4751.67	7084.64	2030	378.66	0.75	43137792	2.05	ok
3	Cbr-br2	3876.70	11024	2030	379.09	0.75	43853633	2.06	ok
4	Br2-Rr	3270.15	14071.6	2030	299.6	0.75	37538181	1.85	ok

Snow load criteria

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

Judgement condition

Qr ≥ 3.1

s.no	Bow position	α	Lx	Ly	Rx	Ry	s	Iy	t	Qr	Ok/NG
1	Fr-Br1	0.99	1130.5691	2030	4290.97	2988.6	533.86	1726	0.75	13.4	OK
2	Br1-Cbr	0.72	1251.56	2030	3832.32	7042.29	457.41	2281	0.75	15.9	OK
3	Cbr- Br2	0.57	1055.94	2030	4315.53	11738.53	443	1738	0.75	6.01	OK

Here we got a Qr values is greater than 3.1 but as you can as the S value for the three is greater than 380mm hence we introduce beads on the roof panel. The longer beads cover the front and rear portion of the roof to provide more strength.

Result

Final assembly of the car roof