Roof challenge

Roof Challenge

Objective: To develop the essential flanges and reinforcements with appropriate dimensions and check the draft by using draft analysis. Calculating the Heat distortion, Snow load, and section inertia on the roof

Introduction

An automobile roof is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements

The roof is a styling surface part that is fixed with BIW body through the rails and panels inside the upper roof to provide its strength and stiffness

Types of car roofs:

1. Convertible roofs

2. Hardtops

3. Sunroof

4. T-tops

5. Targa tops

6. Vinyl roofs

Design of Roof Includes

• Ditch Area
• Front roof rail
• Rear roof rail
• Bow roofs 1 & 2
• Centre Roof rail

The following factors should be considered for the roof design.

• Visibility Criteria
• Head clearance for comfort
• Safety Parameters
• Draft analysis
• Heat distortion and snow load

To design a roof with reinforcements we have a master sketch and some reference images

Rear Roof Rail.

Front Roof Rail

Bow Roofs 1& 2

Centre Bow Roof with reinforcement

Procedure

1. Front Roof Rail:

We constructed a front roof with suitable measurements using the master sketch, created embosses to strengthen the part, and provided mounting brackets.

2. Bow Roof 1&2

We created the bow roof by offsetting the outer surface and combining those surfaces with the studio surface command by providing a certain draught angle. Mastics are added to the sides of the portion to make it stiffer.

3. Centre Bow Roof with reinforcements

Follow the same technique as the bow roof creation but with a larger size for the center reinforcing.

4. Rear Roof Rail

We designed a rear roof with suitable measurements and created embosses to strengthen the component using the master sketch.

5. Roof

Assembly images:

After the Completing of the design part, we will be calculating the strength of the roof using two methods

• Heat Distortion Calculation Method
• Snow Load Calculation Method

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

Judgment Condition

OK< 2.7

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]

To find the value of Rx and Ry values.

• Draw a horizontal line in the center of the top roof sketch.
• Then, below 300mm, offset this line and label it the -3Y line.
• Convert all these lines into reference lines using the -3Y line offset 100mm above and below.
• Draw vertical lines across the middle of each reinforcement, offset by 100mm on both sides.
• Where the lines meet, an intersection point is generated.
• These points should be projected onto the outer roof panel.

After Projection of points

Rx Value

Ry Value

S.no	Distance	Rx	Ry	R	t	l	L	R^2/t	W	Result
1	FRONT RAIL TO BOW 1	6300.3719	3254.264	4291.754008	0.75	462.8	2033	24558869.96	1.800509094	ok
2	BOW1 TO CENTER BOW	5363.6068	7010.5355	6077.472679	0.75	510.6	2033	49247565.56	2.309829963	ok
3	CENTER BOW TO BOW 2	4086.3066	11507.862	6031.05605	0.75	510.7	2033	48498182.78	2.296076381	ok
4	BOW 2 TO REAR RAIL	3348.2366	15495.9937	5506.646059	0.75	468.6	2033	40430867.76	2.100521054	ok

According to the judgment criteria ok <2.7>3.1, All values are satisfied

Snow Load calculations

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)

Judgment 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 front and rear reference point.

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

The width of the roof panel is 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]

Roof panel X curvature radius of length s in Side view

To find the value of Rx and Ry values.

• Draw a horizontal line in the center of the top roof sketch.
• On both sides, offset this line by 100mm. All of these lines should be converted to reference lines.
• Draw vertical lines across the middle of each reinforcement, offset by 100mm on both sides.
• Where the lines meet, an intersection point is generated.
• These points should be projected onto the outer roof panel.

After Projection

Rx value

Ry Value

To find out the geometrical moment of inertia, we need to create an intersection curve and use the section inertia command

BOW Roof 1

Centre BOW Roof with reinforcements

BOW Roof 2

S.no	Distance	Rx	Ry	Lx	Ly	L1	L2	t	Y	S	My	alfa	ly	Qr	Result
1	FRONT RAIL TO BOW 1	6937.123	3234.9789	1045.99	2033	464	511	0.75	464	487.27	728016	7.96519E+23	89.45	5.01159E-41	ok
2	BOW1 TO CENTER BOW	5875.714	6959.1466	1060.277	2033	510.5	511	0.75	510.5	510.585	777236.3	8.73759E+23	155	4.74539E-41	ok
3	CENTER BOW TO BOW 2	4203.539	12195.409	1089.35	2033	510.7	469	0.75	510.7	489.605	777438.6	9.22574E+23	88.9	1.64667E-41	not good

As per the Calculation report, the Qr value supports for 2 Criteria (According to the judgment criteria Qr>3.1) but fails on the third criteria

In order to meet the 3^rd criteria, embossing has been given on the roof section in order to give better stiffness and strength to the roof

Draft Analysis:

Draft analysis was performed with 7 degrees value.

Front Roof Rail:

BOW roof 1

Centre Bow roof

BOW roof 2

Rear Roof Rail

Outer Panel

Conclusion

The roof was built with all of the required specifications, including rails, heat distortion, snow load, and draught studies for safety issues.