Roof Design

ROOF DESIGN

ROOF

INTRODUCTION

The roof of a car or other vehicle is the top part of it, which protects passengers or goods from the weather. The roof also

protects the passenger from any injury when the car gets crashed or is rolled over or something heavy falls on the

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 otherexternal elements. Because the earliest automobiles were designed in an

era of horse-drawn carriages, early automobile roofs used similar materials and designs.

OBJECTIVE

The main objective of this project is to understand the design process to design the roof with all the roof rails and to

understand the calculations behind it. We also get to learn about the different tools used in NX cad for designing.

METHODOLOGY

We get the model of the roof of the from the styling team. Then we will have to do all the engineering work. We create the

ditch area for the roof and also the roof bows and roof rails for the roof which would we designed by us considering all the safety norms and regulations.

DESCRIPTION

• Components of Roof
• Calculations
• Hear distortion and snow load criteria
• Draft analysis

COMPONENTS OF ROOF

1. Front Roof Rail
2. Rear Roof Rail
3. Bow Roof 1
4. Bow Roof 2
5. Center Roof.

FRONT ROOF RAIL

The front roof rail is located on the front side of the roof where the wind shield joins with the roof. The visibility and the head room clearance of the passenger are taken into consideration for the bottom

flange. One end of the rail is joined using spot welding while the other end is joined with the roof using mastics to increase strength.

BOW ROOFS 1 AND 2

Bow roofs are additional members for the roof to increase the strength and so that it can satisfy the norms. Both the ends

are joined together using mastics to increase strength and also this would not affect the styling of the roof.

Center Roof

Centre Roof or the reinforcement centre is located at the centre of the roof. The cross section of this is more since it needs to

provide more strength to the roof. Here both are ends are joined together using mastics.

Rear roof

This roof joins the back door and the upper roof with the help of mastic seals.

CALCULATIONS

Heat Distortion

This study is used to predict the heat distortion temperature at where the material starts to soften when exposed to a fixed

load. So in order to avoid bending or damage of the roof based on this calculation we will decide the position on the roof bows

on the roof.

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:

Bow position	Rx	Ry	R	l	L	t	R2/2	W	OK/NG
FR and 1st	5629	3068	3970	468.56	2033.7	1.5	21034825.94	1.54	OK
1st and CTR	4856.7	6623.7	5612	375.81	2033.7	1.5	43857067.77	1.62	OK
CTR and 2nd	4017	10598.9	5275.67	493.2	2033.7	1.5	46131333.73	1.76	OK
2nd and RR	3425	14472.6	5638.68	392.33	2033.7	1.5	40912692.46	1.65	OK

Snow Load Criteria

We predict the snow load in this of the roof bows and then place and change the design of the load accordingly to satisfy the condition.

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 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 exposd 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]

Moment of inertia-

Bow roof 1-

Centre bow roof-

Bow roof 2-

Result-

Bow position	Iy	α	Lx	Ly	Y	S	Rx	Ry	t	Qr	Determination
FR and 1st	162.1	1.07	1129.69	2033.7	510.65	447.74	4005.6	15113.26	1.5	0.9	NG
1st and CTR	268	1.23	1011.72	2033.7	919	464.47	3654.03	10543.14	1.5	2.2	NG
CTR and 2nd	165.3	0.95	1069.39	2033.7	1418.08	447.71	4231.18	6602.02	1.5	2.99	NG

Now, these values for the snow load does not satisfy the condition.So to make the roof stronger we have to create embossses on the roof.

Draft analysis

Before we give the part for manufacturing we have to some analysis ourself to check all the constraints and to check if we have any negative draft on a part .

We have a tool called draft analysis in NX Cad to check the following.

Front roof rail

Bow roof 1

Centre roof rail

Bow roof 2

Rear roof rail

Roof

I gave a angle limit of 3 degree.When the colour is green it means it has a positive draft it means area which are more than 3 degree is shown as green. If it is shown in yellow that means it

has less then 3 degrees and if it is in blue or red then it means it has negative draft.

Section modulus-

Original

S_min=149 mm³

Offsetting the bottom curve by 0.5mm downwards

S_min=171 mm³

Offsetting the bottom curve by 0.5mm upwards

S_min=175.5 mm³

CASE	SECTION	Imin(mm4)	Smin(mm3)
1	Original	1.5677x 105	149
2	Offset 0.5mm down	1.73089x 105	171
3	Offset 0.5mm up	1.7362 x 105	175.5

From the above observations we can say that when we offset the curve 0.5mm downwards, we get a better section modulus.

CONCLUSION

• Design of the roof with all the roof rails for better support and strength is done sucessfully.
• We have designed it to satisfy the rules and regulations by calculating the heat distortion criteria and the snow load critera.
• So for further processing we can send this roof for strength test and manufacturing processes.