Roof Design

ROOF DESIGN

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.

OBJECTIVE

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

DESIGN CONSIDERATION-

1. Visibility criteria
2. Head clearance
3. Curvature study
4. Hear distortion and snow load criteria
5. Draft analysis

Components of roof-

 

1. Front Roof Rail-

The front roof rail is located on the front of the roof where the wind shield joins with the roof. The visibility criteria and the head room clearance of the passenger are taken into consideration for the bottom flange. One and of the rail is joined using spot welding while the other end is joined with the roof using mastic sealants for increased strength.

 

2. Bow Roof 1-

Bow roof is an additional stiffening member for the roof to increase the strength and so that it can satisfy the norms. Both the ends are joined together using mastic sealants to increase strength and also sine this would not affect the styling of the roof.

 

3. Centre Roof Rail-

Centre roof rail or the reinforcement center is located at the center 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 mastic sealants.

 

4. Bow Roof 2-

This bow roof also has the same purpose as that of the first one. This one is located between the center bow roof and rear roof rail.

 

5. Rear Roof Rail

This roof rail joins with the back door and roof. Inputs from the backdoor are taken into consideration for designing this roof rail.

Here one end is joined using spot welding while the other end is joined using mastic sealants.

 

We then also make the ditch area around the roof.

Ditch area the extended portion of the main roof concealed by other BIW panels.

CALCULATIONS

Heat Distortion curvature Study-

This study is used to find out and 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] + [1.10 x 10^ (-3) x 1] – 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:

  

Sr.No.	Bow	Rx(mm)	Ry(mm)	R(mm)	I(mm)	W	Judgment
1.	FRR and 1st Bow	5715.22	2840.97	3795.32	624.49	2.0265	OK
2.	1st Bow &CRR	4821.04	6805	5643.7	329.73	2.2096	OK
3.	CRR/2nd Bow	3965.38	10642.91	5777.9	415.75	2.3576	OK
4.	2rd Bow & RRR	3236.02	14881.3	5316.03	347.05	2.2998	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 [9Rx + Ry)/2] 2 x 10-8]

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

Where

α = My x Lx 2 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 the roof panel with the centre 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 exposed on the surface.

Y = distance front roof rail to roof BOW [mm]

s = distance for which roof bow bears divided load [mm]

s = L2/2 + 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]

Result-

 

sr.no.	BOWs	ly (mm)	alpha	Lx (mm)	Ly (mm)	y (mm)	s	Rx (mm)	Ry (mm)	t (mm)	Qr	determination
1	FR & BOW 1	195.7	0.993	1076.35	2032.25	1434.48	432.4	4357.86	2869.36	0.75	20.379	ok
2	BOW 1 & CRR	465.91	0.891	1068.96	2032.25	514.16	369.69	3794.60	6608.31	0.75	12.81	ok
3	CRR & BOW 2	200.69	0.746	1055.38	2032.25	414.1	309.875	4300.10	11449.92	0.75	7.96	ok

Now since we these values for the snow load values it dose not satisfy the condition.

So, to make the roof stronger we create embosses on the roof to strength the roof.

Draft analysis-

Before we give the part for manufacturing, we have to some analysis out self 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-

 

We have a limit angle of 3 degree.

When the colour is green it means it has a positive draft it means area which are more then 3 degree is shown as green. If it is shown in yellow that means it has less then 3 degrees and if in blue or red then it means it has negative draft.

CONCLUSION

We were successfully able to design the roof with all the roof rails for better support and strength. We designed it so as to satisfy the norms and regulations by calculating the heat distortion criteria and the snow behind designing the roof of the car and what all we take into consideration while designing so.