Roof Design

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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 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 positions.

· 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 reinforcement.

 

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. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure and to with stand the overhead forces like snow.

                           

 

 DESIGN CONSIDERATION

• Visibility Criteria
• Head Room Clearance
• Curvature Study
• Heat Distortion & Snow Load Criteria
• Draft Analysis

 

REINFORCEMENTS & 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:

1.     Front Roof Rail

2.     Bow Roof

3.     Centre Roof Rail

4.     Rear Roof Rail

 

1. Front Roof Rail:

Front roof rail is the one which joins the wind shield glass, body side outer and the inner panel. It is used for supporting the roof when externally applied forces come into play in case of accidents.

                          

2. Bow Roof:

The bow roofs are given to improve the torsional stiffness and load bearing capacity of the roof structure. Longer the roof area, a greater number of bow roofs are given to the structure. Positioning of the bow roofs are decided by the curvature study. The Heat distortion and Snow load criteria are checked to analyse the bow roof position by various condition. Here 2 bow roofs are taken.

                          

 

                         

 

3.Center Roof Rail:

Center roof rail helps in providing effective 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. This central roof rail helps in adding the strength to the roof during the roll- over test.

                         

 

4. Rear Roof Rail:

Rear roof rail is the one which joins the back door and the body side outer.

                   

ROOF CRUSH TEST

Roof crush test is the design given by the IIHS (insurance institute of highway safety). After analysing the many accidents IIHS has then fixed some regulations, that how to perform the crush test.

IIHS gives the rating based up 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 roll over and
• How deep is the protrusion after the roll over

                      

 

GOAL OF ROOF TEST:

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

 

CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST:

There are two conditions that should be followed for the roof crush test

 INITIAL CONDITION:

• Close and lock all the doors
• Close all the windows

SECOND CONDITION:

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

 

METHOD OF APPLYING THE FORCE:

The load is applied in such a way that 5⁰ is tilted from the front end to the rear end and the load will be in 1800 mm in length and 750 mm in width. The load is applied at the position of 25⁰ from the flatness of the roof. The load is applied on the roof for almost 120 sec and the crush speed at which the load applied is 13 mm/sec or less than that

                         

 

RESULTS:

If the displacements go above the required level, then we have to increase the bow roof thickness or add more mastics points to increase the strength. The stronger the roof the better it can protect the occupants in the roll over crashes.

 

HEAT DISTORTION STUDY ON ROOF

The heat distortion study plays a major role in sheet metal usage. Heat distortion temperature is the temperature limit above which the material cannot be used for the structural applications. 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 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]

 Judgement Condition: OK< 2> 3.1

                            

After applying all the values in the above formula, the following table is created:

                  

From the above table it is concluded that all the values are of W< 2> 3.1. So, that all bow roofs areas are placed at the right place.

 

 

SNOW LOAD CRITERIA

Snow load criteria study is used to check the durability of the roof to withstand the snow fall on the roof and reflex it in original shape without any permanent deformation after load being removed. 

Q_r = [I_Y x t²] / [α x s x [(R_X + R_Y)/2]² x 10^-8]

Where,

α = M_y x L_X² x10^-12, M_{y =} Y(L_y–Y)

Judgement condition = Q_r > 3.1

 t = Roof plate thickness[mm]

L_Y = 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 rails front/ rear as the front and rear             reference point.

L_X= Distance between the left and right end of the roof on the roof bow[mm],

Y = Distance between Front roof rails to roof bow [mm],

s = Distance for which roof bow bears divided load [mm],

          s = L₁/2 + L₂/2

I_y = Geometrical moment of inertia of roof bow (Y cross – section) [mm4],

R_X = Lateral direction curvature radius of roof panel Y cross section on roof bow [mm4],           

          Roof panel curvature radius of the length L_X in front view.

R_Y = Longitudinal direction curvature radius of the roof panel X cross section on roof bow[mm4],

                       

 

MOMENT OF INERTIA

It is used to predict the amount of deformation and deflection of a part. A higher moment of inertia is preferable as it reduces the amount of deflection for a given force. The thickness of the part and addition of flanges and embosses increase the section inertia of the part. Here the master sections of the roof and roof rails and their section analysis are given.

BOW ROOF 1:

                 

 

CENTER ROOF RAIL:

               

 

BOW ROOF 2:

               

After applying all the values in the above formula, the following table is created:

                

From the above table it is concluded that all the values are of Qr> 3.1. So, that all bow roofs areas are placed at the right place.

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 colour ranges identifying zones of analysed element where the deviation from the draft direction at any point, corresponds to specified values.

Minimum draft angle of 7⁰ is considered for analysis. Green colour infers on the parts that all face along the tooling direction has positive draft angle greater than 7⁰ and passed in analysis.

 

FRONT ROOF RAIL:

 

BOW ROOF 1:

                                      

 

CENTER ROOF RAIL:

                               

 

 

BOW ROOF 2:

 

REAR ROOF RAIL:

ROOF IMAGES

 

Front View:

                     

SIDE VIEW:

                    

 

ISOMETRIC VIEW:

                     

 

CONCLUSION

Hence the roof, front roof rail, bow roof rails, central roof rail and the rear roof rail is designed by following the master sections and they are found to be at correct positions and the moment of inertia and draft analysis are carried out to check feasibility manufacturing.