Roof Design

Roof Design:-

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 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 reinforcements.

CAR 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. 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 over head forces like snow

WHY REINFORCEMENTS :

If the roof is flat it is very weak and cannot able to with stand the forces like snow , so in order to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process

Parts Of Roof:

1. Roof outer body
2. Front roof rail
3. Rear roof rail
4. Bow roof rail
5. Center roof rail
6. Roof outer body: This is the outer cover of the body on which different rails or reinforcement will be added

FRONT ROOF RAIL:-

:

This is the front roof rail and this is mounted on the front of the roof on which wind shield glass and front body will be joined. This front roof rail is created with given master section view.

Bow roof rail:

The bow roof are given to improve the torsional stiffness and load bearing capacity of the roof. The number bow roof will depend on the size of the roof, some times there will be no bow roof if the roof is too small for example in Alto, Mini Cooper etc., and it is not necessary that bow roof will always be in same place it will vary from design to design, in our design we take two bow roof one in front and another in back between center and rear roof rail.

bow roof rear

4. Center roof rail: Center roof rail plays crucial role because this will provide support to the roof in center. This also help during rool-over test. The center roof rail is connected to the B-Pillar.

Center bow roof rail

5. Rear roof rail: Rear roof rail is at end of the roof and rear glass and side body.

The next image shows the master section of the rear roof rail.

DESIGN CONSIDERATIONS FOR A ROOF :

• Visibility criteria
• Head clearance
• Curvature study
• Heat distortion and snow load criteria
• Draft analysis

ROOF CRUSH TEST :

Roof crush test is the design given by the IIHS ( insurance institute of highway safety ) . After analyzing 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 reforms to its original shape after the roll over and
• How deep is the protrusion after the roll over

GOAL OF ROOF CRUSH TEST :

To minimize the protrusion into the occupant compartment , so that the seat belts and air bags can do there 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 , they are

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 goes above the requirements 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 THE ROOF :

The heat distortion study 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 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< 2.7> 3.1

The heat is the one method to calculate whether the position of the roof is correct or not. Here is the image of the of the formula.

Similarly we will calculate,

From the above table it can be concluded that all values of w<2.7 and also not w>3.1 so thus infers that current positioning of bow row are good in state as per design and found ok

SNOW LOAD CRITERIA :

This test is done to know how is the roof behaving when there is a 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 removal the roof should go it its original position . This is the basic requirements for snow load criteria .

Qr = [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 10^-8]

Where

α = My x Lx2 x 10-12 , My = Y(Ly-Y)

Judgement condition = Qr ≥ 3.1

250 ≤ s ≤ 380

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 reference point of the front and the 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 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]

APPLY THE VALUES IN THE FORMULA :

In order to find the snow load criteria apply all the necessary values from the roof to the give formula

CALCULATIONS :

It is observed from the values that obtained values of QR > 3.1

Section analysis:

Section moment of inertia 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 2

Create an intersection curve with the help of the plane and select the bow roof 1

Moment of inertia maximum,( MAX) I _max =3.127×10³ ( mm⁴)

Moment of inertia maximum ,( MIN) I _min =195.706 ( mm⁴)

BOW ROOF 1

Moment of inertia maximum,( MAX) I _max =3.24× 10^3 ( mm⁴)

Moment of inertia maximum ,( MIN) I _min =167.156 ( mm⁴)

CENTRAL ROOF RAIL :

Moment of inertia maximum,( MAX) I _max =5.34× 10^3 ( mm⁴)

Moment of inertia maximum ,( MIN) I _min =419.32( mm⁴)

FRONT ROOF RAIL

Moment of inertia maximum,( MAX) I _max =1.64× 10⁴ ( mm⁴)

Moment of inertia maximum ,( MIN) I _min =954.29( mm⁴)

REAR ROOF RAIL

Moment of inertia maximum,( MAX) I _max =5.91x10⁴ ( mm⁴)

Moment of inertia maximum ,( MIN) I _min =4.45 × 10³( mm⁴)

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.

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

Conclusion

We had successfully created the parts of roof and also check the design, according to the result we can send part for drafting but before that we have to change the position of the bow roof rail to get under control heat distortion and snow load resistance.