Challenge

Roof challenge

GG. Saffwan Syed Asan27 Aug 2021

Automobile 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. It plays an important role in aerodynamics and internal strength of vehicle. A roof panel assembly includes a roof outer panel/ditch area and inverted hat like reinforcements rails which underlie the outer side edges of the roof outer panel. Roof rails are provided to give strength to the outer panel and also provide support to other commodities present in a car. It also acts as a cross member between body sides of the car body and provides provisions for front windshields, backdoor (in case of hatchback vehicles). Roof assembly also one among the parts which provide safety for rollover accidents.

1. Hardtops

2. Sunroof:

a. Pop-up:

b. Spoiler:

c. Panoramic sunroof:

3. Convertibles:

a. Retractable hardtops:

b. Detachable hardtops:

c. Soft tops:

Apart from aerodynamic shape of roof it has to withstand the induced drag force on surfaces. To make the roof more structurally rigid and to improve its local strength the BIW parts designed. The internal components of roof are

Front roof rail
Rear roof rail
Reinforcement roof rail
Bow roofs

Roof designing in software with sunroof:

The roof outline surface is given which is then developed and added flanges.

Roof front flange and front rail:

Roof rear flange and rear rail:

Design in software:

Front and side flange:

Rear flange:

Sketch and design for sunroof portion:

Final design of the roof with provition for sunroof:

Front roof rail:

This is the place where the roof connects with front windshield and adds more support.

Design of Front roof rail in software:

Rear roof rail:

Design of Rear roof rail in software:

Rear Bow roof:

Bow roof design in software:

Front Bow roof:

Due to prescence of sunroof, this bow row is created in a way where it takes the part of front bow roof and centre bow roof, and so it gives support on the sides of the sunroof.

Full Roof assembled:

Heat Distortion study :

In heat distortion study we have a mathematical formula which has to be satisfied in order to justify the bow roof positions. The material softens under increased temperature conditions. While applying load at this state makes the material much vulnerable to static loads. So to formulate this scenario the heat distortion criteria is introduced.

The formula for heat distortion criteria is

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]

S.no	BOW	Rx(mm)	Ry(mm)	R(mm)	l(mm^4)	L(mm)	t(mm)	W	OK/NG
1	FR & FBR(start)	5418.888	1538.816	2396.961	132.871	2110.65	0.75	1.12	OK
2	FBR start & end	4756.367	6549.771	5510.832	1063.42	2110.65	0.75	2.65	OK
3	FBR & RBR	3493.167	12329.198	5443.933	261.857	2110.65	0.75	1.99	OK
4	RBR & RR	3051.609	14927.489	5067.313	253.109	2110.65	0.75	1.88	OK

Snow Load criteria

Snow load criteria study is done to check the durability of the roof to withstand snowfall and reflex back to its original shape without any permanent deformation once the load is removed.

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

Roof panel X curvature radius of length s in Side view

S. no.	Bow	I(y) (mm^4)	α	Lx	Ly	Y	s	Rx	Ry	t	Qr	OK/NG
1	FR & FBR(start)	933.192	0.597	1210.23	2110.65	214.91	662.71	6030.35	133.21	0.75	13.97	OK
2	FBR start & end	75.26	1.414	1128.512	2110.65	1110.51	706.991	4711.43	1063.96	0.75	5.079	OK
3	FBR & RBR	5.782	0.657	1094.753	2110.65	303.47	325.246	3717.93	261.91	0.75	3.84	OK
4	RBR & RR	1.184	0.723	1086.655	2110.65	347.02	173.51	3280.33	252.71	0.75	17.13	OK