Roof challenge

Design of Roof

INTRODUCTION: The roof is an essential part of a vehicle which plays an indispensable role in providing effective protection for you and those who drive with you from heavy snow, harsh rain, strong wind, harmful UV rays, etc. As many other automotive body panels, the roof is subjected to environmental conditions. Hail and snow storm may burst, tree limbs and cause sharp dents, dings, and scratches. Your vehicle can also become ruined in an accident. Though a defect may seem insignificant at first, it can lead to serious consequences. Whether your roof was dented due to downfall or roughed up in a collision, it is urgently necessary to find a replacement for it. The roof panel is attached to the BIW of the car with the help of front and rear roof rail. This attachment is done with the help of sealants. Bow roofs 1 & 2 are attached depending upon the length of the roof along with the central roof rail. Longer the roof, more the number of roof rail. Other dimensions like strength and material is governed by the factors like head room, overall space, visibility, ergonomics and the rules and regulations of the states where it is sold.

OBJECTIVE:

1. To perform the section modulus study for the master sections and create for the Front roof rail, Rear roof rail, Bow roofs and center roof rail.
2. To create the Ditch area and to perform the draft analysis.

DESIGN CONSIDERATIONS:

1. Visibility criteria
2. Head clearance
3. Curvature study
4. Heat Distortion and Snow load criteria
5. Material thickness

MAIN COMPONENTS OF ROOF:

1. FRONT ROOF RAIL: It is the support that is joined with the windshield glass, body side outer and inner panels. It is usually made from bodyside outer scrap and therefore the thickness of the rail is 0.75mm. A draft angle of 3-5 degree is provided for deep drawing operation. The visibility criteria and head room clearance of the passengers are taken into consideration for the bottom most flange. Windshield end and the roof end are spot welded and the other end of the front roof rail is joined with the roof using mastic sealants for increased strength.

2. BOW ROOF 1: Bow roof center 1 is an additional stiffening member used in the roof assembly for the localised strength improvement and it is join directly to roof itself with several spot welding or even with mastic sealent.

3. CENTER ROOF RAIL: Center roof rail or reinforcement center is used as a cross member between bodyside left hand and right hand respectively. In some cases, the reinforcement center is not joined with the roof and in some other cases, it is made to join in both roof and bodyside. The reinforcement center and Bow roof are joined either spot weld or by mastic sealants.

4. BOW ROOF 2: It is similar to that of bow roof 1 which serves the same purpose as the previous but with a limited extend and hence it is used with a smaller cross.

5 REAR ROOF RAIL: It is the support that is joined to the backdoor, the bodyside outer and the inner panels. Inputs from the backdoor design is also taken to accommodate the required clearance along with headroom clearance and rear visibility criteria while designing the rear roof rail. Spot welds are applied at the backdoor end of the rail and other end is joined with the roof using mastic sealants.

DITCH AREA: It is the extended portion of the main roof concealed by other BIW panels and not a styling surface. Headroom clearance, visibility criteria, clearance for windshield and back door are the some of the factors to be considered while designing the ditch area.

HEAT DISTORTION CURVATURE STUDY:

The heat distortion study plays an important role in sheet metal usage. Heat distortion temperature is a temperature limit above which the material cannot be used for structural applications. This study is used to predict the heat distortion temperature at where the material starts to soften when exposed to a fixed load at elevated temperature. In order to avoid bending or damages on roof, based on heat distortion temperature, this study will predict the bow roof position on the roof to strengthen the roof.

Curvature study on the roof:

Bow – roof prediction Formula is given as

W = 1.73x10^-3 x L+1.85x10^-8 x (R^2)/t + 1.10x10^-3 x l - 2.68

Where

L = Roof Length in X – Direction (mm) (Roof dimension in o-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)

The condition to pass this test is given as,

2.7 ≤ W ≤ 3.5

S.No	Positions	Rx	Ry	R	l (mm)	L (mm)	t (mm)	R^2/t	W	OK/NG
1	FRR & BR1	3438.81	2543.17	2923.94	394.11	2049.05	0.75	11399233.5	1.51	OK
2	BW1 & CRR	3650.36	5933.76	4534.48	368.51	2049.05	0.75	27415345.16	1.77	OK
3	CRR & BR2	3072	2919.42	2993.76	484.69	2049.05	0.75	11950131.92	1.62	OK
4	BR2 & RRF	2638.3	16326.82	4542.55	304.49	2049.05	0.75	27513014	1.71	OK

For considering above calculations our design is safe according to heat distortion criteria for Bow Roofs.

SNOW LOAD CRITERIA:

Position	Iy	α	My	Lx	Ly	Y	S	Rx	Ry	t	Qr	Judgement
FRR & BR1	534.66	0.967	748848.86	1136.5	2025.39	486.67	460.72	5531.46	2371.22	0.75	4.323	Ok
BW1 & CRR	4228.3	0.815	691565.41	1085.94	2025.39	434.78	494.36	4825.20	5867.50	0.75	20.652	Ok
CRR & BR2	545.39	0.911	815095.01	1057,56	2025.39	553.94	498.93	4165.15	11777.10	0.75	1.06	NG
BR2 & RRF		0.758	702046.16	1039.73	2025.39	443.92		3477.50	16296.92	0.75

Moment of Inertia (Iy):

Bow Roof 1

Center Reinforcement:

Bow Roof 2

As we can see that all the values for Qr are not above 3.1, Hence it has not passed the snow test. So here we use embosses in between center reinforcement and Bow Roof 2 to increase the strength of the roof component.

Draft Analysis: - Draft analysis is done to check if the component does not have any negative draft can create a problem during the manufacturing process. Sheet metal parts must have a draft of at least 7 degrees for proper tooling.

Thus, our design is safe from draft analysis for 7-degree tool opening angle.