Roof Design

 AIM : Prepare a report which includes the curvature study on the roof based on the Bow roof prediction calculation as well as the manufacturing techniques.

           Provide appropriate dimensions and check for Draft analysis. Do a curvature study on the roof and perform calculations to determine the position of the Bow-roofs.

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.

The roof itself may be constructed of soft or rigid material. Soft-tops are made of vinyl, canvas or other textile material; hard-top convertibles have roofs made from steel, aluminium, plastic or carbon fibre composites.

Roof header materials must be high in tensile strength, low in weight to lower the vehicle center of gravity, high in toughness, and integration-friendly.

Design Consideration : 

• Visibility criteria
• Head Clearance
• Curvature Study
• Heat Distortion Criteria 
• Snow Load Criteria
• Clearance of 5 mm should be present between outer roof rail and Front roof rail, Rear roof rail, Bow roof, and Reinforcement center where mastics are used for adhesion.
• Thickness for all sheet metal parts - 0.75 mm.
• All edges should have fillets and there should not be any sharp edges present.
• A minimum draft of 7 degrees is required for all the faces.

Components of Roof : 

1. Roof Top : 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. 

   2. 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.

    3. First Bow Roof Rail : A Roof Bow is a sheet metal part, usually spot welded to the BIW inner frame, forming the inner structure of the vehicle body. Its primary function is to supporting the roof panel loads, such as Oil Canning and local stiffness.

    4. Center Roof Rail 

    5. Second Bow Roof Rail

    6. Rear Roof Rail 

Ditch area : The extended portion of the main roof concealed by other BIW panels.

Manufacturing Technique : 

The auto roof panel is one of the largest areas covering the driver and the passenger compartment. In the modern automobile manufacturing there are several different types of car roof panels such as, solid metal stationary hard tops, sun roofs, panoramic glass roofs, retractable hard tops, and few others, each of them are unique in design and function. Obviously, roof panel is the most important part that protects the automobile inner compartment from unfriendly environment. There are regulations regarding the amount of the force that the roof panel withstands in case of damage or collusion to sustain without causing injury to the occupants.

Other than the crashworthiness of the roof panel there are also manufacturing imperfections. As most of scholars described springback is a common manufacturing imperfections phenomenon in every sheet metal manufacturing process and it is a geometrical change of the part after the forming tool force has removed and occurs at the end of the forming process.

The first step, oil pressure, punch press: first iron plate is extended; Secondly to its trimming and punching; Then to its flanging and punching; Finally, after punching completes, it is carried out to the operation of side folding; Completed the making of outside plate.

Roll Over Test On Roof 

• The rollover test is given by the Insurance Institute of highway safety ( IIHS). After analyzing many accidents IIHS has fixed some regulations about how to perform the test.
• IIHS rating is based on the following things which are how fast a car can reform its original shape after rollover and how deep a protrusion is.
• The aim of this test is to minimize the protrusion of the roof.
• Two conditions need to be followed while performing this test. One is to lock all the doors of the car properly and the second is to close all the windows of a car.
• Also, it is mandatory to fix the chassis on the hard lever surface horizontally.

Bow roof prediction calculation : 

• Heat Distortion Criteria : 

This study is used to find out and predict the heat distortion temperature at where the material starts to soften when exposed to a fixed load. So in order to avoid bending or damage of the roof based on this calculation we will decide the position on the roof bows on the roof.

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]

Judgment Condition

OK< 2.7

• Snow Load Criteria : 

We predict the snow load in this of the roof bows and then place and change the design of the load accordingly to satisfy the condition.

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

Judgment condition

Qr ≥ 3.1

 Section Inertia Analysis : 

First BOW : 

Center Rail : 

Second BOW : 

Draft Analysis : 

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.

It is important that what-ever is designed should be easily manufactured. Draft analysis only is not a sure shot confirmation that the cad design is 100 % manufacturable but it checks that all the surfaces are greater than or equal to a particular angle (in sheet-metal it is 7 degrees). 

Drawing/Stamping direction: A sheet metal part is manufactured by the stamping process. In this process a punch and die are used to generate the desired shape. The stamping direction indicates the direction in which the punch moves.

Front Rail : 

First BOW : 

Center Rail : 

Second BOW : 

Rear Rail : 

Conclusion : In this report I have mentioned all the definitions, design considerations manufacturing            techniques involved in roof design and also mentioned about draft analysis. 

                    As all the criteria of bow roof predection calculations are satisfied.

 Therefore no need of doing emboss on roof outer panel and also the bow rails are designed at good positions according to the both criteria.

                              I have attached all the part files and igs file of roof assembly.

                                                              Thank You!