Roof Design

                                                        ROOF DESIGN AND ANALYSIS

OBJECTIVE : In this project we are going to learn how to design roof and what all design consideration we need to follow and at the end we will analysis the heat detortion and snow load condition of the roof.

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.It also offers aerodynamic support to the car when it is in motion. A roof panel assembly includes a roof outer panel/ditch area and inverted hat like reinforcements rails which underlie the outer side edges of the roof outer panel. An automobile roof is considered as an integral part of BIW (body in white) design.

In roof also there is diffirent type which is mention below:

• Convertible roof
• Hardtop roof
• Sunroofs
• T-tops
• Targa-tops
• Vinyl roofs

Convertible roofs :

A convertible or cabriolet  is a passanger car that can be driven with or without a roof in place. The methods of retracting and storing the roof vary among eras and manufacturers.

A convertible car's design allows an open-air driving experience, with the ability to provide a roof when required. A potential drawback of convertibles is their reduced structurely rigidity(requiring significant engineering and modification to counteract the effects of removing a car's roof).

The majority of convertible roofs are of a folding construction framework with the actual top made from cloth or other fabric. Other types of convertible roofs include retractable hardtops (often constructed from metal or plastic) and detachable hardtops (where a metal or plastic roof is manually removed and often stored in the trunk).

Hardtops roof :

A hardtop is a rigid form of automobile roof, which for modern cars is typically constructed from metal. A hardtop roof can be either fixed (i.e. not removable), detachable for separate storing or retractable within the vehicle itself.

Sunroof :

A sunroof is a movable panel that opens to uncover a window in an automobile roof, allowing light and 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.

T - tops :

A T-top (UK: T-bar) is an automobile roof with a removable panel on each side of a rigid bar running from the center of one structural bar between pillars to the center of the next structural bar. The panels of a traditional T-top are usually made of auto grade safety glass.

Targa top, or targa for short, is a semi-convertible car body style with a removable roof section and a full width roll bar behind the seats. The term was first used on the 1966 Porche 911Targa, and it remains a registered trademark of Porsche AG.

Vinyl roof :

Vinyl roof refers to a vinyl covering for an automobile's top.This covering was originally designed to give the appearance of a convertible to models with a fixed roof and eventually evolved into a styling statement in its own right. Vinyl roofs were most popular in the American market, and they are considered one of the period hallmarks of the 1970s domestic cars. Vinyl roofs were also popular on European- (especially UK-) and Japanese-built cars during the 1970s, and tended to be applied to sporting or luxury trim versions of standard saloon (sedan) models.

WORK FLOW CHART :

CLASS A SURFACE :

Class A surface is an aesthetic surface which is directly visible to the person or customer. By using class A surface we will develope the roof outer panel by giving the thickness of 0.75mm. Class A surface we will use for outer panel where other component will attached for complete assembly.

OUTER PANEL :

Outer panel of the roof which is going to developed from the class A surface with the thickness of 0.75mm inward direction. The length of the outer panel depends on the requirement of the project. normaly for small car the outer panel of roof are curvy in nature and for the length of the car is more then the outer panel of the roof will be more flat in nature. The curvature outer panel required less inner support and for length outer panel we nned to provide more support in different position to give more strength to the roof.

FRONT ROOF RAIL :

Front roof rail is used to provide strength to the roof panel front and it also acts as link between windshield glass, roof panel and headliner(interior trim). Headroom clearance and visibility are two important criteria for the driver and both of them depends upon the front roof rail dimensions, hence it is important to select appropriate dimensions for the roof rail.

the master section we are going to follow for front roof rail are shown below:

CENTER ROOF RAIL :

Center roof rail is the supportive component for the roof which is fixed with the outer panel with the help of spot welding at the ditch area and with the mastic in the middle surface of the area. 

The master section for center roof rail are mention below :

REAR ROOF RAIL :

Rear roof rail or we can say rear roof flange is another supportive component for the roof which is going to give support the end or back side of the car. The rear roof rail will attached with the c-pillar and the back door compartment. It's is very importent to consider the backdoor design when we are design the rear roof rail , because the back door function will also depend on the rear roof rail position.

The master section using for rear roof rail are mention below :

BOW ROOF RAIL :

Bow roof rail is an addition rail when our roof length is more and need extra strength we will go for bow roof rail. The section of the bow roof rail is same as center roof rail or we can use different section as well as per requirment. The width of the bow roof rail is also very as per the requirment and its also vary the number of bow roof rail as per the requirment

The master section of the bow roof rail is mention below :

DRAFT ANALYSIS :

Draft analysis is very import for any model which we are going to design, because we need to design the model as per the manufacturing consideration. For manufacture the component we need tooling direction for the movement of dies wheich are givng the required shape for the model. If the angle of the surface and die is not maintain then its very difficult to eject the model or tools from the die. For sheet metal design we should maintain minimum 7 degree draft angle for all the surfaces along with tooling axis.

Below shows the draft analysis for the room model :

DIFFERENT VIEW OF ROOF ASSEMBLY :

CALCULATION FOR THE ROOF DESIGN IN DIFFIRENT CONDITION :

HEAT DETORTION CALCULATION AND ANALYSIS :

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>

SNOW LOAD CALCULATION AND ANALYSIS :

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)

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

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

CONCLUSION :

As per the output result for the heat detortion and the snow load calculation we can see the design consideration is satisfying the condition. In this model we got the "s" result is greater then 380 . So we nned to adjust the position of our bow roof rail and center roof rail as well if required to achieved the " s " value as per standered.