AUTOMOTIVE ROOF DESIGN

Objective:

To design the Automotive Roof based on the given styling surface and to analyze the prepared model to check the manufacturability and the feasibility of the design.

Introduction:

The Roof of a car is the component which encloses the passenger compartment from the top. It protects the Passengers and the interior of the car from external environments thus providing a safe and isolated cabin. The roof plays an important role in the safety, especially in the case of roll-over accidents. To achieve better safety results the roof is designed with reinforcement structures such as rails.

The major components of the Roof are

1. Outer panel/ Shell
2. Front roof rail
3. Rear roof rail
4. Bow roofs
5. Central roof reinforcement rail.

All these components are designed to satisfy the basic functions of the roof, to meet the safety requirements and to improve the performance of the vehicle by weight optimization.

1) Outer Panel / Shell

The outer panel is the A-class surface of the roof which encloses the top of the passenger compartment. It is designed to match the input styling surface, thus getting better aerodynamic performance as intended. It also has a significant role in the aesthetics of the vehicle. The outer panel is mounted to the roof rails with the help of mastics and the ditch areas are mounted with spot welds. The commonly used materials for the outer panel are Steel and Aluminum alloys.

Design Considerations

Geometric requirements:

1. Thickness: 0.75 mm
2. Ditch area-minimum flange width: 15 mm

Manufacturing requirements:

1. Small emboss depth should be less than 6 mm
2. Mounting and welding areas should be flat
3. Minimum fillet radius should be 3mm+thickness
4. Tool opening angle is 3 degrees

Functional Requirements:

1. Provide adequate space for windshield glass supporting elements
2. Space for the hinges of backdoor and allow the swinging of backdoor

Design methodology

1: Creating the Outer panel surface from the given input skin.

2: Developing the ditch area

Flanges are created on four sides of the roof as per the above-mentioned requirements. The front end flange is given with a width of 21.50 mm to provide adequate space for the rubber sealing, mounting of windshield glass, etc. The rear ditch area is created to allocate the hinges of the backdoor and to allow the swinging of the backdoor to the required opening angle. Some embosses are also provided on the rear to increase the strength locally.

The thickness of 0.75 mm is given as per the geometric requirements.

2) Front Roof Rail

The front rail supports the roof outer shell at the front end. It is mounted with brace roof rail reinforcement on both ends. The rails are provided with embosses to improve the strength and the rail is connected to the outer panel using mastic sealants on one side and the other joinery is spot welded.

Design Considerations:

Geometric requirements:

1. Thickness – .75 to 1.5 mm

Manufacturing requirements:

1. Small emboss depth should be less than 6 mm
2. Mounting and welding areas should be flat
3. Minimum fillet radius should be 3mm+thickness

Functional requirements:

1. Headroom clearance: 80 mm minimum
2. Front visibility criteria
3. Enough flange length to support the windshield glass & its components
4. 2 mm clearance at the flange edge for trims.

Design methodology

1: Creating the swept surface that meets the design requirements with the outer surface geometry as reference.

The section is created by considering the front visibility criteria and minimum headroom clearance of 80mm. A flange of 15 mm is provided to support the windshield glass & its components,

And 2 mm clearance is provided at the flange edge for allocating the trims.

2: Embosses are given to improve the strength. They have created considering the manufacturing requirements.

The rail surface is thickened by 1.2 mm

Section Modulus Analysis of Front section:

Section Modulus S = I/Y

S = (1.988* 10^4)/17.2 = 1155.8 [mm ^3]

3) Rear Roof Rail

The rear roof rail is similar to the front rail. It is designed to support the back end of the roof and the hinge of the backdoor is also mounted on it.

Design Considerations

Geometric requirements:

1. Thickness – .75 to 1.5 mm

Manufacturing requirements:

1. Small emboss depth should be less than 6 mm
2. Mounting and welding areas should be flat
3. Minimum fillet radius should be 3mm+thickness

Functional requirements:

1. Rear headroom clearance: 80mm minimum
2. Rear visibility criteria
3. Space for mounting the backdoor hinge

Design methodology

1: Creating the swept surface that meets the design requirements and the outer surface geometry as reference.

The section is developed by considering Rear headroom clearance of 80mm, Rear visibility criteria, and space for mounting the backdoor hinge is also given.

2: Embosses are given to improve the strength. They are created according to the manufacturing requirements.

One row of embosses which exactly matches the embosses on the outer surface is provided to achieve the perfect locking between panels.

The rail surface is thickened by 1.2 mm

Section Modulus Analysis of rear section:

Section Modulus S = I/Y

S = (6.219* 10^4)/30.9 = 2012.62 [mm ^3]

4) Reinforcement roof CTR

The reinforcement roof at the center is the supporting member at the middle which is mounted to the B pillars on either side. They are attached to the outer panel using mastic sealants. Sometimes it acts as bow roof and sometimes bow roofs are also provided with the reinforcement roof center. In some vehicles a room lamp mounting provision is also given on this part.

Design Considerations:

Geometric requirements:

1. Thickness: 0.75 to 1.5 mm

Manufacturing requirements:

1. Small emboss depth should be less than 6 mm
2. Mounting and welding areas should be flat
3. Minimum fillet radius should be 3mm+thickness

The strength can be further improved by giving separate mastic points that extend outwards.

Design methodology

1: Creating the swept surface that meets the design requirements, with the outer surface geometry as reference.

2: The rail surface is thickened by 1.5 Mm

Section Modulus Analysis:

Section Modulus S = I/Y

S = (1.931* 10^4)/10.17 = 1898.72 [mm ^3]

5) BOW roofs

The Bow roofs are given to improve the torsional stiffness and load-bearing capacity of the roof structure. The number of bow roofs provided will depend upon the overall dimensions of the roof. They are attached to the outer panel using mastic sealants. Each mastic point will have up to 80 mm circular effective area. The positioning of bow roofs is done based on curvature study. The heat distortion criteria and snow load criteria are checked to analyze the position of the bows in various conditions.

Design Considerations

Geometric requirements:

1. Thickness – 1.5 mm minimum

Manufacturing requirements:

1. a) Mounting and welding areas should be flat
2. b) Minimum fillet radius should be 3mm+thickness

Design methodology

1: Creating the swept surface that meets the design requirements with the outer surface geometry as reference.

2: Providing separate mastic points

The mastic sealant holds the bows and outer panel together. Each point will have an effective circular area of 80 mm in diameter. The mastic sealant helps to improve the elasticity of the outer panel. They also improve the NVH characteristics of the roof.

The rail surface is thickened by 1.5 mm

One BOW roof is also provided within the reinforcement roof CTR

Section Modulus Analysis of Bow roof1 section:

Section Modulus S = I/Y

S = (2.487* 10^4)/13.1 = 1898.5 [mm ^3]

Curvature Study

To satisfy heat distortion and snow load performance of the roof, we have to determine the desired optimum location of bow roofs and roof plate thickness. If the roof is exposed to high temperatures for a long time the roof structure will tend to deform. This study will help to check whether our design is valid and if not will help to determine the optimum position of the rails to avoid the deformation of the roof. Similarly if the roof is subjected to the heavy load of snow, it will tend to deform or fail. To improve the strength under the snowed condition we have to position the rails at desired specific locations.

Heat distortion criteria:

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

Judgment Condition: OK<2.7mm<NG

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]

However if 2.7 W< 3.5, then provide the outer panel reinforcements to improve the strength.

Results:

Snow load criteria:

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)

Judgement condition = Qr ≥ 3.1

250 ≤ s ≤ 380

where;

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof Rails on the Vehicle along 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/L2 + 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]

Results:

In the heat distortion study, the 3ed section (between RR CTR & Rear Rail) came out to be (NG) not good. Also, In the snow load study the value of 'S' (the distance of bearing divided load) for RR CTR appeared as more than the desired range. So to improve the heat distortion and snow load performance of the roof at the rear section I gave emboss to the outer panel.

Draft analysis:

We use Draft analysis to see that the part can be drawn or not. It helps to find the negative draft present in the component which will cause difficulty in manufacturing.

Draft analysis done on roof rails:

The limiting angle is set to 5 degrees.

All these rails don't have any negative drafts at all, so they can be drawn successfully & have good manufacturability.

Final Model :

CAD Model: https://drive.google.com/open?id=1CJzziTLHCIn8l5Khhx_oQEG-fYPU_9Mt

Result: The Roof assembly is designed based on the given styling surface, considering all the geometric, manufacturing, and functional requirements.