Challenge

Roof Design

MMohammed Ashiq7 Dec 2022

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 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 roof rails.
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.
Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure.

Types of Car Roofs:

Convertible roofs.
Hard Tops
Sun roofs.
T- Tops.
Target tops.
Vinyl roofs.

Design Consideration of roof:

Visibility Criteria
Head Clearance
Curvature Study
Heat Distortion and snow load criteria
Draft Analysis
Safety Parameters

Roof Crush Test:

Roof Crush test for passenger car:

Safety point of view vehicle roof plays a very crucial role especially roll over kind of accidents, In which vehicle tips over onto its side of the roof Vehicle rollover crashes are the cause of many fatalities and head, neck and spine trauma around the world.
Strength of the roof structure to occupant protection in real world roll over crashes, requiring the maximum moving distance of the roof structures should be less than 127 mm, (5 in), where the induced load is 1.5 times the vehicle weight.
For the test, the passenger ca shall be rigidly placed or positioned on a horizontal surface with the doors locked and the window closed. Furthermore, a flat, rigid block with a lower surface a rectangle measuring 30 inches wide by 72 inches long (762mm x 1829 mm) shall not move more than 5 inches (127mm), measured as the distances between the original location of the lower surface of the test device and its location as the specified force level is reached, when it is used to apply a forces of 1.5 times the unload vehicle weight (UVW) or 5000 lbs. (22224N), whichever is less. To either side or forward edge of the vehicle.

Method of Applying the Force :

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.

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 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.

Results :

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 :

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

Types of Reinforcement :

Front Roof Rail
Bow Roofs
Central roof rail
Rear Roof Rail

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.

Master Section and its Parameters:

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:

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

bow roof rail :

bow roof rail 2 :

center roof rail :

Roof Outer Panel:

Screen shot of Different Views of Complete Roof Design:

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.

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.

Qr = [ Iy x t2] / [ α x s x [ ( Rx + Ry) / 2] 2 x 10^-8]
Where
α= My x L x 2 x 10-12,
My = Y (Ly - Y )
Judgement Condition = Qr ≥ 3.1
250 ≤ s ≤ 380
t = roof thickness [mm]
Ly = Distance between the front and rear roof rails on the vehicle along the OY [mm]
Length of Roof panel with the center point between roof rails 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]
The width of the roof panel is exposed on the surface.
Y = Distance of 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 :

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

Calculations :

Section Modulus :

Section modulus is a geometric property for a given cross-section used in the design of beams or flexural members. Other geometric properties used in the design include an area for tension and shear, the radius of gyration for compression, and the moment of inertia and polar moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape in question. Equations for the section moduli of common shapes are given below.
The elastic section modulus is defined as S = I/y
Where,
S = Section modulus
I = Moment of inertia
y = distance between the neutral axis and the end of the object
It is also often used to determine the yield moment (M y) such that
M y = S X σ _y
Where
σ _y= yield strength of the material
M _y= yield moment
Ʃ _yfor stainless steel is 520MPA

Calculation of the section modulus :

Moment of Inertia (Iy):

FRONT ROOF RAIL :

BOW ROOF 1 :

BOW ROOF 2 :

CENTER ROOF RAIL :

REAR ROOF RAIL :

DRAFT ANALYSIS:

Draft analysis is a method of checking the draft angle on various faces of a part with a particular direction.
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.

Draft Analysis for Front Roof Rail :

Draft Analysis for Bow Roof Rail

Draft Analysis for Central Roof Rail

Draft Analysis for 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 ] + [ 1.10 X 10^(-3) X1] -2.68
Where
L = Roof length in X -Direction (mm) (Roof dimension in 0-Y)
R = Roof Curvature
Where R = 2 ( Rx*Ry)/ (Rx+Ry)
Rx = X curvature
Ry = Y curvature
t = roof plate thickness (mm)
I = 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.

calculations :

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

Roof Images :

Top View :

Side View :

Isometric View :

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.