Roof Design

                                                                      ROOF DESIGN

Aim

To design a roof with all the components like front roof rail, rear roof rail, and bow roofs by following the given master sections and the high-quality surface, considering all the design requirements as per the OEM standards.

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.
• Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure.
• The roof is not a single panel as seen from the exterior it consists of front roof rail, bow roofs, center roof rail, and rear roof rail with different thicknesses which act as a reinforcement for the top.

Objective

• To design a roof of the car with all the reinforcement from the given styling surface.
• To perform the curvature study on the roof by considering Heat Distortion and Snow load criteria.
• To calculate the moment of inertia for the reinforcements.
• To Perform the draft analysis for analyzing the components' manufacturing feasibility.

1)Ditch Area of Roof

• The ditch area is the extended portion of the main roof which is concealed by the other BIW components followed by the other aesthetic surface.
• It also serves the joining purpose for the reinforcement which is joined using the spot welding.
• Visibility, Clearance for windshield and back door mountings are some of the factors needed to be considered while designing the ditch area. 

 

 2)Front Roof Rail 

• Front Roof Rail is a reinforcement that features to back of the bracket lamp and the room lamp and it also serves the purpose of attaching the windshield.
• The development of the FRR is based on the given master section and also references from the given styling and the front ditch area are taken into consideration.
• Embosses and darts are incorporated for strengthening the roof.

 

3)Bow Roofs

• These are the reinforcements provided on the most flatter regions of the roof which are more susceptible to failure.
• The number of bow roofs depends upon the length of the roof i.e, longer roofs have more bow roofs like vans. 
• For the development of the bow roofs, the styling surface and side ditch area are taken as references. 

Bow Roof 1

Bow Roof 2

Bow Roof 3

Center Roof  

• These roof rails are positioned mostly on the center portion of the roof or at the B Pillar of the vehicle i.e, the most flatter region of the car which is susceptible to failure under the action of load.

Rear Roof Rail

• It is a reinforcement provided to the rear portion of the roof and it also features the tailgate mountings.
• The development of RRR is dependent upon the rear headroom clearance and rear visibility criteria.
• The minimum clearance from the headliner to the roof is should be 80mm which will vary from variant and region to region.

 

 Bow - Roof Prediction Calculation

 1)Heat Distortion Criteria

Heat distortion is the temperature at which the metal softens and deforms under a specific load. This study is done to check the ability of the roof to withstand high temperatures concerning the positioning of the reinforcements which contribute to increased strength.

Procedure

• Create a center line at the 0-Y region of the roof and then offset the same to -3Y.
• Create a symmetric line from the -3Y region at a distance of 100mm.
• Draw a vertical line at the midpoint of the line drawn from the center points of the FRR and BR 1 and followed the same till the rear roof rail.
• Offset the vertical line to a distance of 100mm on each side.
• Create intersection points and project them on the roof.
• From these projected points, make a 3-point arc and the Rx and Ry values.

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            

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]

Judgment Condition

OK< 2.7

 

Sl.No	Bows	Rx	Ry	R	l	L	t	W	Remark
1	FR & BR 1	5625.922	2493.69	4984.2	304	1923.1	0.75	1.594	OK
2	BR 1 & CTR	4939.14	5763.24	5319.46	364	1923.1	0.75	1.745	OK
3	CR & BR 2	4203.4	8783.51	5685.82	196	1923.1	0.75	1.66	OK
4	BR 2 & BR 3	3618.53	11628.1	5519.46	349	1923.1	0.75	1.782	OK
5	BR 3 & RR	3059.12	14968.3	5080.1	288	1923.1	0.75	1.6	OK

2)Snow Load Criteria

A snow load study is done to check the ability of the roof to withstand snowfall and bounce back to its original position without undergoing permanent deformation upon the removal of the load.

Procedure

• Create a center line at the 0-Y region of the roof and then offset the same to 100mm on each side.
• Draw a vertical line at the midpoint of the line drawn from the center points of the FRR and BR 1 and followed the same till the rear roof rail.
• Offset the vertical line to a distance of 100mm on each side.
• Create intersection points and project them on the roof.
• From these projected points, make a 3-point arc and the Rx and Ry values.
• Project the extreme edges of the styling surface fro measuring the width at the respected sections.

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)

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof Rails on the Vehicle along with 0Y[mm]

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]

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]

Ry = Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW [mm]

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

 

Sl.No	Bows	Iy	α	Lx	Ly	Y	s	Rx	Ry	t	Qr	Remark
1	FR & BR 1	2499	0.84	1160	1962	402.1	425.5	4777.6	2280.9	0.75	31.57	OK
2	BR 1 & CTR	2342.6	1.14	1098	1962	850.5	357.2	3758.7	5290.6	0.75	15.806	OK
3	CR & BR 2	2534.3	1.11	1082	1962	1116.4	350.18	3999.4	8484.8	0.75	9.41	OK
4	BR 2 & BR 3	2538.3	0.726	1067	1962	1550.7	422.78	4333.6	12119.4	0.75	6.874	OK
5	BR 3 & RR			1052	1962	1962		3877.3	17098.2	0.75

Moment of Inertia 

 

DRAFT ANALYSIS 

Draft Analysis is carried out to check the manufacturing feasibility of the roof and all the reinforcements rails. All the components should be designed by considering at least the minimum draft value of 5 Deg.

Roof 

 

Front Roof Rail

Bow Roof 1

Bow Roof 2

Bow Roof 3

Center Roof Rail

Rear Roof Rail

Final Roof Assembly

Conclusion

Thus the roof is designed with all the necessary reinforcements and roof rails according to the given master section and the styling surface, by considering all the design parameters. Also, the curvature study is done by taking the heat distortion and snow load criteria for determining the strength of the roof.