Roof Design

Aim- To develop the essential flanges and reinforcements with appropriate dimensions and checking draft by performing Draft analysis, Calculating Section inertia, Heat distortion and Snow load on the roof.

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.

In later years, many variations on the automobile roof developed. These include:

• Convertible roofs - A convertible or cabriolet is a passenger car that can be driven with or without a roof in place. The methods of retracting and storing the roof vary between models. A convertible allows an open-air driving experience, with the ability to provide a roof when required. Potential drawbacks of convertibles are reduced structural rigidity (requiring significant engineering and modification to counteract the effects of removing a car's roof) and cargo space.

• Hardtops - A hardtop is a rigid form of automobile roof, which for modern cars is typically constructed from metal.

• Sunroofs - A sunroof is a movable panel that opens to uncover a window in an automobile roof, allowing light and/or fresh air to enter the passenger compartment. Sunroofs can be manually operated or motor driven, and are available in many shapes, sizes and styles. While the term sunroof is now used generically to describe any glass panel in the roof, the term "moonroof" was historically used to describe stationary glass panes rigidly mounted in the roof panel over the passenger compartment. A moonroof has a glass panel that is transparent and usually tinted. Previous terms include Sunshine Roof, Sliding Head and Sliding Roof.

Roof is designed by following some criteria-

1. Visibility criteria as safety and functionality
2. Head clearance for ease and comfort
3. Draft analysis fro easy manufacturing and tooling
4. Safety parameters for users
5. Heat distortion 
6. snow load criteria

To create the Roof Given data is-

Roof Skin

Front Roof Rail Mastersection

Center and Reinforcement Roof Rail Mastersection

Bow Roof Mastersection

Rear Roof Rail Mastersection

Front Roof Rail View :--

Embosses are created to give strength to the roof rail and mounting for brackets are given in which lights and switches will position.

Center Roof Rail View- 

Mounting holes for brackets are created for cabin/reading light positioning.

Side Flanges on Bow Roof designed/created is  for Mastic Sealant application after that

Rear Roof Rail View- 

Roof Rails Position=

Embosses are created to provide strength to the material and holes are created to somehow reduce the vehicle weight along force distribution.

Complete Assembly View--

Roof Bow Calculations

Heat Distortion Calculation

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]

Except Rx and Ry all values can be measured on the model.

To get the Rx and Ry curvature a sketch was required-

• A sketch was created on a plane on top of the Roof 
• 0-Y line was created in the X-Direction in center of roof
• Then a Horizontal line below 300mm was created named as -3Y Line
• Two lines on both sides at 100mm were created from -3Y Line
• Intersection point on line was created and projected on outer Panel of Roof

Result:

Bow position	Rx	Ry	R	l	L	t	R2/2	W	OK/NG
FR and 1st	5626.39	3067.66	3970.5	448.56	2033.7	1.5	21019825.94	1.53	OK
1st and CTR	4854.3	6624.76	5602.99	365.81	2033.7	1.5	41858066.77	1.63	OK
CTR and 2nd	4065.16	10593.91	5875.67	463.2	2033.7	1.5	46031333.73	1.77	OK
2nd and RR	3424.34	14478.23	5538.68	352.33	2033.7	1.5	40902692.46	1.6	OK

Then Three point Arc was created by using these points

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]

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 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 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]

Result-

Bow position	Iy	α	Lx	Ly	Y	S	Rx	Ry	t	Qr	Determination
FR and 1st	161.19	1.05	1119.69	2033.7	569.65	447.74	4105.6	15113.26	1.5	0.84	NG
1st and CTR	267.05	1.21	1081.72	2033.7	979.65	464.47	3794.03	10543.14	1.5	2.09	NG
CTR and 2nd	161.3	0.91	1059.39	2033.7	1488.08	447.71	4231.18	6602.02	1.5	3.03	NG

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

So to make the roof stronger we create embossses on the roof to strengthen the roof.

We gave a limit angle of 3 degree.

Section modulus-

Original-

S_min=147.9884mm³

Offsetting the bottom curve by 0.5mm downwards-

S_min=165.1002mm³

Offsetting the bottom curve by 0.5mm up-

S_min=164.9576mm³

CASE	SECTION	Imin(mm4)	Smin(mm3)
1	Original	1.5667x 105	147.99
2	Offset 0.5mm down	1.73099x 105	165.1
3	Offset 0.5mm up	1.7352 x 105	164.96

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

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 load critera. While doing this we also understood about the methodology behind designing the roof of the car and what all we take into consideration while designing so.