Roof Design

Objective

<!--[if !supportLists]-->·       <!--[endif]-->To Design an assembly of Vehicle Roof by developing the essential flanges and creating the reinforcements for it by providing appropriate dimensions.

<!--[if !supportLists]-->·       <!--[endif]-->Check the individual part is good to draft by Draft Analysis.

<!--[if !supportLists]-->·       <!--[endif]-->Perform Curvature study on the Roof and Cheek for the Heat distortion Criteria and Snow load Criteria to determine the feasible number and position of the Bow Roof Rails and other reinforcements.  

Introduction

The Roof of a car is often overlooked. Yet it plays a crucial role in a vehicle’s aesthetic appeal and functionality. Car roof comes in various designs and materials. Each serves a specific purpose and caters to different needs. There are some types of roofs explained below,

<!--[if !supportLists]-->1.    <!--[endif]-->Solid Roof

It is the most common type of roof found in vehicles. It is typically made of steel, aluminium, or a combination of both. Solid Roof provides excellent protection against the elements. It provides insulation from rain, wind, and noise. They are known for their strength and durability which contributes to the safety of the vehicle. Also, it enhances the structural integrity of the car which improves the handling and stability of vehicle.

Figure 1

<!--[if !supportLists]-->2.    <!--[endif]-->Panoramic Glass Roof

This type of roofs has gained popularity in recent years. It is due to their ability to create an open and

Spacious feeling inside the vehicle. This type of roofs is typically made of tempered glass panels. It provides a wide, unobstructed sky view for both front and rear passengers. It can be fixed, tilted or slide type. It allows fresh air and natural light to enter in the cabin. They add a touch of luxury and elegance to a vehicle's interior. But it's important to consider the potential increase in heat and glare during sunny days.

Figure 2

<!--[if !supportLists]-->3.    <!--[endif]-->Convertible Soft top

They are also known as Rag Tops or Soft Roofs. They provide the exhilarating experience of open-air driving. It can be folded and stowed, allowing the driver and passengers to enjoy the freedom of the open roof. These roofs are made of fabric materials which are lightweight and flexible. It contributes to better fuel efficiency and improved handling. But this roof on a car may requires more maintenance.

Figure 3

<!--[if !supportLists]-->4.    <!--[endif]-->Retractable Hard Top

Retractable Hard Tops combine the best of both worlds. It offers the versatility of a convertible with security and insulation of a solid roof. These roofs are typically made of lightweight materials like Aluminum or its composites. They can be electronically folded and stored in the trunk with a button. They provide noise isolation and weather protection when closed.

Figure 4

<!--[if !supportLists]-->5.   <!--[endif]-->Targa Top

The Targa top is a unique roof design that originated from Porsche. This type of car roof features a removable center section/ mid-section. It leaves a solid structure behind the front seats and a large opening above. They provide the convenience of removing the center panel to enjoy open-air driving. While they are maintaining a rigid frame structure for added safety and structural integrity which provides the security with the freedom of a convertible.

Figure 5

<!--[if !supportLists]-->6.   <!--[endif]-->Sunroof/Moonroof

Sunroofs and Moonroofs are popular features found in many modern vehicles. They consist of a panel made of glass or transparent material which allows sunlight and fresh air to enter the cabin. Sunroofs are typically tilt or slide open while moonroofs are usually fixed. They provide a panoramic view of the sky. These roofs on cars are great for enjoying a breezy drive or stargazing at night. But consider the potential increase in cabin heat and the impact on headroom.

Figure 6

Design Parameters

To design a complete Roof, it may consider the Outer Roof with its ditch area. Along with the Outer Roof the Front Roof Rail, Bow Roof Rails and Centre Roof Rail are also designed to provide strength to the Outer Roof. The Bow Roof Rails and the Centre Roof Rail are connected to upper roof rail using the mastics. During designing these parts the following criteria is considered,

<!--[if !supportLists]-->·     <!--[endif]-->Visibility criteria

<!--[if !supportLists]-->·     <!--[endif]-->Head room clearance

<!--[if !supportLists]-->·     <!--[endif]-->Draft analysis

<!--[if !supportLists]-->·     <!--[endif]-->NVH (Noise, Vibration and Hardness)

<!--[if !supportLists]-->·     <!--[endif]-->Curvature study

<!--[if !supportLists]-->·     <!--[endif]-->Heat distortion criteria

<!--[if !supportLists]-->·     <!--[endif]-->Snow load criteria

Master Sections

Master sections are the cross-sectional views along with measurement details which guides us to use the parameters to Design the part and gives a general idea about the part design. Master sections are widely used in OEMs and automobile industries to convey the functional requirements of the products.

Front Roof Rail

Figure 7 Master section of Front Roof Rail

The Front Roof Rail is used to support the Outer Roof and increase the strength of the Roof. It also used to mount some features in the cabin such as room lamp, sunglass holders and rear-view mirror etc. The Front Roof Rail is also attached to A-pillar of the vehicle. There are some criteria like visibility of road to driver and head room area in cabin are kept in mind during providing width and depth of front roof rail. Also, give a draft angle of minimum 7 degree so we can easily produce the parts. Figure 8 shows the sectional view and the full view of the Front Roof Rail.

Figure 8 Front Roof Rail

Bow Roof Rails

The Bow Roof Rails are used as reinforcement for the Outer Roof. These Roof Rails enhance the strength of Outer Roof and reduces the NVHs values which improves the stability of the vehicle. The number and position of Bow Roof Rail is depending on the length and thickness of the Outer Roof and curvature study of the Roof which is held at the end of the design.

Figure 9 Master section of Bow Roof Rail 1 and 2

The figure 9 shows the Master Section of the Bow Roof Rail which is considered during designing the Bow Roof Rail. It consists all the necessary data required to design the Bow Roof Rail. The figure 10 shows the Sectional view and Full view of the Bow Roof Rail.

Figure 10 Bow Roof Rail 1 and 2

Centre Roof Rail

The Centre Roof Rail is used to support the Outer Roof at the mid/center. It is attached at B pillar of the vehicle and Outer Roof. The positioning of the center roof rail is also decided by the curvature study of the Roof to withstand the Heat distortion at high temperature environment and Snow load at ice places.

Figure 11 Master section of Centre Roof Rail

The above figure 11 shows the Master section which consist all the data necessary for designing the Centre Roof Rail. The Below figure 12 shows the Sectional and Full view of the Centre Roof Rail designed using the above Master section

Figure 12 Centre Roof Rail and sectional view

Rear Roof Rail

The Rear Roof Rail is Designed to support the Rear part of the Outer Roof and it is also used to mount the Tail gate of the vehicle. One end of the Rear Roof Rail is attached to Outer Roof by mastic and other one is with the weld.

Figure 13 Master section of Rear Roof Rail

As shown in the figure 13 the master section Rear Roof Rail is shown with all necessary data by which we can design the Rear Roof Rail. The figure 14 shows the Sectional and Full view of the Rear Roof Rail.

Figure 14 Rear Roof Rail and sectional view

Draft Analysis

 Draft analysis is the medium in the software by which we can analysis that the part can be made properly by using the process of punching and stamping along the tooling axis. For which we do draft analysis of the part along the Tooling axis. So, we generally give a 7 degree of draft value by which the part can easily be formed.  All the ranges are shown in the colors,

Figure 15 Range of Draft angles with Color Code

From the figure 15 The Green color represents the range of 7 degree or above, yellow color represents the range between 0-to-7-degree, Blue is for 0 to -7 and Red is used for -7 and below of it.

We prefer the range of 7 degree or its above i.e. green color. Below are the all reinforcements we used for the Roof Designing,

<!--[if !supportLists]-->·       <!--[endif]-->Front Roof Rail

Figure 16 Draft analysis of Front Roof Rail

<!--[if !supportLists]-->·       <!--[endif]-->Rear Roof Rail

Figure 17 Draft analysis of Rear Roof Rail

<!--[if !supportLists]-->·       <!--[endif]-->Bow Roof Rail 1 & 2

Figure 18 Draft analysis of Bow Roof Rail 1 and 2

<!--[if !supportLists]-->·       <!--[endif]-->Centre Roof Rail

Figure 19 Draft analysis of Centre Roof Rail

Final Assembly

<!--[if !supportLists]-->·       <!--[endif]-->Front view

Figure 20 Front View

<!--[if !supportLists]-->·       <!--[endif]-->Side view

Figure 21 Side view

<!--[if !supportLists]-->·       <!--[endif]-->Top view

Figure 22 Top view

<!--[if !supportLists]-->·       <!--[endif]-->Isometric view

Figure 23 Isometric view

Curvature Study

It is the study of the curvature of the Roof which shows the strength of the vehicle and Aerodynamic stability of the vehicle. This study may include Roof Crash Test, Heat Distortion Criteria, Snow Load Criteria etc.

<!--[if !supportLists]-->·       <!--[endif]-->Roof crash test

This test is designed by IIHS (Insurance Institute of Highway Safety) after analysing many accidents. It decides to test vehicle on some parameters to improve the vehicle and passenger safety. On the basis of those parameters IIHS gives rating to the vehicle. For rating the vehicle, they consider two things First one is how fast the car regain the original shape after roll over and second is how deep is the protrusion after the roll over.

Figure 24 Crush Test at OEMs

Figure 25 Side views of Crush Testing

The system used to test consists an upright assembly and an attached loading head that can be fixed at varying heights from the ground as well as at pitch angles ranging from -5 to +5 degrees to accommodate testing on the driver or passenger side.

The roll angle is permanently fixed at 25 degrees. Four hydraulic actuators control the movement of the platen along two linear guides. The entire system is mounted on a T-slot bed plate anchored to the floor of the test facility.

If the result is not appropriate then do changes in the Roof rails to improve strength of the roof.

<!--[if !supportLists]-->·       <!--[endif]-->Heat Distortion Calculation

The heat distortion calculation plays a major role in sheet metal usage. Heat distortion temperature is a temperature limit above which the material cannot be used for the structural application. 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 the roof, based on the heat distortion temperature, this study will help in positioning the bow roof at which roof got more strengthen

Figure 26 Rx and Ry values for the Heat distortion Criteria

Figure 27 Projection and intersections for the measurement

Bow-Roof prediction Formula

W=[1.73*10^ (-3) *L] + [1.85*10^(-8)*(R^2)/t] + [1.10*10^(-3)*I] – 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]

I = Bow Roof Span [mm]

Judgement Condition: Ok<2>

Result of the Heat Distortion Calculation

The value of w is less than the judgement condition i.e. less than 2.7 in all the four section that shows that the positions of all the Reinforcements are feasible and good according to the Heat distortion criteria.

<!--[if !supportLists]-->·       <!--[endif]-->Snow Load Criteria

This test is done to know the Roof Behavior when there is a snow over the Roof. Normally due to the snow load on the Roof the Rood get dented. But the Roof should be designed in such a way that when the snow is removed from the roof then the roof should go it its original position. This is the basic need for snow load criteria.

Figure 28 Rx and Ry values for Snow Load Criteria

Figure 29 Projection and intersections for the measurement

Judgement Formula for the Snow Load Criteria

Qr = [Iy * t^2] / [α*s*[{(Rx + Ry) / 2} ^2] * 10^ (-8)]

where,

α = My * Lx^2 * 10^ (-12)

My = Y (Ly - Y)

Judgement Condition, Qr >3.1

250< s>

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof rails on the vehicle along with 0-Y [mm] length of roof panel with the centre point between roof rail front / rear as the reference point of the front and 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 from front roof rail to bow roof rail [mm]

S = (L1/2) + (L2/2)

Iy = Geometrical moment of inertia of bow roof rail (Y cross-section) [mm^4]

Rx = Lateral direction curvature radius of roof panel Y cross-section on Bow roof rail [mm]

Ry = Longitudinal Direction curvature radius of the roof panel X cross-section on Bow roof rail [mm]

Section Analysis:

<!--[if !supportLists]-->1.    <!--[endif]-->Bow roof rail 1

Figure 30 Iy for Bow Roof Rail 1

<!--[if !supportLists]-->2.    <!--[endif]-->Bow roof rail 2

Figure 31 Iy for Bow Roof Rail 2

<!--[if !supportLists]-->3.    <!--[endif]-->Centre roof rail

Figure 32 Iy for Centre Roof Rail

Result of Snow Load Calculation

As per the above Data collected from the Design, the value of Qr is above 3.1 means the Snow Load Criteria is Passed but the value of s is not lying between the range. It means that the number of Bow Roof Rails are have to increase or we can provide Embosses to the Outer Roof to increase the strength of the roof.

Embossed Outer Roof

<!--[if !supportLists]-->·       <!--[endif]-->Outer Roof Upper side

Figure 33 Embossed Outer surface

<!--[if !supportLists]-->·       <!--[endif]-->Lower side view

Figure 34 Lower side of Improved Roof

Conclusion

Design of Roof was successfully completed using the Siemens NX as per the given Master sections. Checked the manufacturability of the part by doing the Draft analysis and section analysis also completed the curvature study to check the position and number of Bow Roof Rails are good to provide proper strength and stability to the vehicle.

Link: https://drive.google.com/file/d/1YMfvxhMdwcbD1UZ3YvHXDAMmaJAM_Xcv/view?usp=sharing