Roof challenge

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.

OBJECTIVE :

            *The objective of this project is to design car roof with essential flanges and reinforcements like ditch area , front roof rail ,bow roofs, central roof rail and rear roof rail for the the given styling surface

            *Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct position

            *Finding moment of inertia and section modulus for Bow roofs ,central roof rail . front roof rail and rear roof rail .

            *Perform the draft analysis for each reinforcements

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.

                                                            

             * The roof is a styling surface part which is fixed with the BIW body through the rails and panels inside the upper roof to provide it strength and stiffness.

Types of car roofs:

1. Convertible roofs
2. Hardtops.
3. Sunroofs.
4. T-tops.
5. Targa tops.
6. Vinyl roofs.

Here are the images of different car roofs.

1. Convertible Roofs:

2. Hardtops: 

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

                                                 

3. 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.

                                       

    

4. T-tops:

          A T-top 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.

                          

5. Targa Tops:

               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 rear window is normally fixed, but on some targa models, it is a removable plastic foldable window, making it a convertible-type vehicle. Any piece of normally fixed metal or trim which rises up from one side, over the roof and down the other side is sometimes called a targa band, targa bar, or a wrapover band.

              

 

6. Vinyl Roofs:

                    This covering was originally designed to give the appearance of a convertible to models with a fixed roof, but eventually it evolved into a styling statement in its own right. When leather, canvas and vinyl were sometimes used along with landau bars, to give a fairly accurate reproduction of a horse-drawn carriage's movable top.

        

Master Section and its Parameters:

                                                                

                                                                

                                                                

                                                                

                                                                

Joining Process:

       Mastics are introduced between the roof outer panel and the reinforcements for joining. Where ever the mastics are introduced there is remarkable strength. Spot welding is also introduced to join the reinforcement and the outer panel.

Design consideration for a roof :

• Visibility criteria
• Head Clearance
• Curvature study
• Heat distortion and snow load criteria
• Draft Analysis

Roof crush test :

              The roof crush test is the design given by the IIHS (Insurance Institute of Highway Safety). After analyzing many accidents IIHS has then fixed some regulations in performing crash tests.

              IIHS gives the rating based on this test. For this, they consider two points while giving the rating.

             *How fast the car can reform to its original shape after the rollover and

             * How deep is the protrusion after the rollover?

The goal of the roof crush test :

               To minimize the protrusion into the occupant compartment, so that seat belts and airbags can do their job protecting the occupant.

Conditions to follow for roof crush test:

               Two conditions should be followed for the roof crush test, they are :

Initial conditi0n:

       * Close and lock the all doors

       *  Close all the windows

Second condition:

        Fix the body sill or chassis frame on a hard lever surface horizontally.

Method of applying the fore :

                 The load is applied in such a way that 5 degrees is tilted from the front end to the rear end and the load will be 1800 mm in length and 750 mm in width. The load is applied at the position of 25 degrees from the flatness of the roof. The load is applied on the roof for almost 120 sec and the crushing speed at which the load is applied is 13 mm/sec or less than that.

Result :

                   If the displacement goes above the required level then we have to increase the bow roof thickness or add more mastic points to increase the strength. The stronger the roof the better it can protect the occupants in roll-over crashes.

 

Reinforcement and its types:

           If the roof is flat, it is weak and cannot able to with stand the external load like snow load, roll over and etc. so in order to increase the strength of the roof, reinforcement like front roof rail, central roof rail, rear roof rail and the bow roof are introduced and joined in the flat surface of the roof with different joining process. In general, there are four types of reinforcement are used in the hard roof top they are,

        * Roof outer panel

        *Front roof rail

        *Central Roof rail

        *Rear roof rail

        *Bow roof rail                                                                                                                              

Roof Outer Panel:

                                                       

Front roof rail:

           *Front roof rail is the one which joins the wind shield glass, body side outer and the inner panel also. Here the front roof rail is designed based up on the master section given.

                                                        

Rear roof rail:

           *Just as the front rail roof, the rear rail roof provides reinforcement to the rear part of the car roof. Similar to the front rail roof, the rear roof rail is to be designed considering the rear visibility criteria and the rear passenger’s headroom clearance.

                                                          

                                                          

Center roof rail :

          * Central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load. Generally central roof rail is placed at the center of the roof which is connected to the B-pillar support structure. 

Bow roof:

 

          *The bow roof are given to improve the torsional stiffness and load bearing capacity of the roof structure. The number of roof presented is depends on the overall size of the roof. Presently in this project two bow roofs are calculated.

Screen shot of Different Views of Complete Roof Design:

THEORY:

Section modulus is to find the geometric property for a given cross-section used in the design.

Calculate the section modulus using the formula S=I/Y

S= Section modulus

I= Moment of Inertia

Y= Distance between neutral axis and the extreme end of the object

Inertia:

Front roof rail:

Center roof rail :

Bow roof 1 :

Bow roof 2 :

Rear roof rail:

Heat distortion study on the roof :

                  The heat distortion study plays a major role in sheet metal urges. Heat distortion temperature is a temperature limit above which the material cannot be used for structural applications. This study is used to predict the heat distortions temperature at where the material starts to soften when exposed to a fixed load at an elevated temperature. To avoid bending or damage to the roof, based on the heat distortion temperature, this study will predict the bow roof position which helps to strengthen the roof.

Bow–Roof Prediction Formula

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]

Apply the values  in the formula :

           To find the heat distortion apply all the necessary values from the roof to the given formula.

Calculation :

Snow load criteria :

              This test is done to know-how is the roof is behaving when there is snow over. Normally due to the snow weight, the dent will happen. But the roof should be designed in such a way that when the snow is removed the roof should go to its original position. These are basic requirements for the snow load criteria.

 

Snow Load Prediction 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)

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

Apply the values in the  formula :

        To find the snow load criteria apply all the necessary values from the roof to the given formula.

Calculation :

Draft analysis :

           The Draft analysis command enables you to detect if the part you drafted will be easily removed. This type of analysis is performed based on color ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.

Roof Outer Panel :

Rear roof rail:

Bow roof 1 :

Bow roof  2 :

 

Center roof rail :

Front roof rail:

Roof image :

Top view :

                                              

Side view :

 

Isometric view :

Tree Structure :

               

            

       

      

Conclusion :

        Hence the roof, front roof rail, bow roof rails, central roof rails, and rear roof rails are designed by following the master sections, and the moment of inertia, section modulus, and draft analysis is carried out to check manufacturability.