Roof Design

Roof Design

• Objective: -

1. 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.
2. 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
3. Finding moment of inertia and section modulus for Bow roofs , central roof rail . front roof rail and rear roof rail .
4. Perform the draft analysis for each reinforcement.

• Introduction: -

Product design starts with requirements. The designer will sketch the product based on your requirements. He then uses advanced CAD software to transform the 2D sketch into his 3D model with proper dimensions. Another important aspect of design is that any designed product is manufacturable.

Therefore, the designer's role is to design products that are easy and inexpensive to manufacture.

This is where Design for Manufacturing [DFM] and Design for Assembly [DFA] play an important role in product development.

The design development process continues until the design meets the customer's requirements.

The product to be developed this time is car roof.

CAR ROOF:

A car roof or top is the part of the car that sits above the interior of the car and protects the car's occupants from the sun, wind, rain, and other external elements.

The roof is the structure above the windshield glass that adds strength to the overall structure and has stiffeners to withstand overhead forces such as snow.

Reasons to reinforce:

A flat roof is very weak and cannot withstand forces like snow. To increase the strength of the roof, stiffeners such as front roof rails, middle roof rails, rear roof rails and vaulted roofs are introduced and connected to the flat roof surface by various joining methods.

Here are the different types of reinforcements that cars have:

• Front roof rail
• Bow roof
• Center roof rail
• Rear roof rail

FRONT ROOF RAIL :

Front roof rail is the one which joins the wind shield glass,

body side outer and the inner panels. Here the front roof rail is designed based up on the master section given below: -

REAR ROOF RAIL:

The rear roof rail connects the rear door and the outside of the body.

CENTRAL ROOF RAIL:

The central roof rail helps effectively support flat areas of the roof that are prone to damage under load. The central roof rail is normally placed in the middle of the roof and connected to the B-pillar support structure.

These central roof rails help stiffen the roof during rollover testing.

Middle roof rail thickness is 1.25mm.

BOW ROOF:

Bow roofs help improve the torsional stiffness and load-bearing capacity of roof structures. The number of vaulted roofs present depends on the overall size of the roof.

Two bow roofs have now been added to the project.

CONNECTION PROCESS:

A filler is placed between the outer roof skin and the stiffener to connect them. Wherever mastic is introduced, strength is noticeably improved.

Spot welding is also introduced to join the reinforcing material and the outer panel.

DESIGN CONSIDERATIONS FOR A ROOF :

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

1. Roof:-
2. We do not reinforce the center roof rail because we are building a hardtop roof from the given main
3. Add flanges to all sides of the roof for spot welding the front roof rail, center roof rail, rear roof railand

• Flange length and angle are determined based on visibility criteria and drawing angles required for sheet metal

1. Roof thickness is 75mm

2. Front Roof Rail:-
3. The front roof rails are used as roof face stiffeners to add You can also install brackets for some functions such as interior lamps, map lamps, and rear view mirrors.
4. The depth of the front roof rail is determined based on established line-of-sight criteria for the driver's field of

• Thewidthofthefrontroofrailisdesigned to provide sufficient headroom between the driver's head and the end face of the roof rail to prevent

1. On the front we do spot welds and on the back of the front roof rails we use mastic dots to connect totheroof
2. Theminimumradiusforedge rounding is 3

3. Rear Roof Rail:-
4. The rear roof rails serve to stiffen the rear roof
5. Theconnection method ofthisrailisthesameasthefrontroof

• Theminimumradiusoffilletis

4. Bow Roof Rails:-
5. Make one arch between the front and middle roof rails and two arches between the middleandrearroof
6. Both ends of the bow are spot welded to the outer flanges of the roof and the inner side of the bow is attached to the inner face of the roof usingmastic

• The minimum fillet radius is 3

1. In other words, the cross-section of the arch is made to obtain the maximum value of the minimum moment of inertia at which the arch

5. Centre Roof Rail:-
6. A canter roof rail is created to provide additional strength to the area where the roof surface is flat.
7. Using the Command extract & then offset to generate the necessary surfaces at different dimns. to select the extracted roof and create the dich area part.

• Use a datum plane on the curve at 50% or middle point.

1. Use Offset command for datum plane to create the different datum planes to obtain flange surfaces.
2. Use Trims command to Intersect offset surfaces as per desired dimns. to obtain desired surfaces.
3. Use Blend command and join the sheets at 7deg.

• Join all these surfaces & use edge fillet command.
• Now extract & then offset the dich are to 1.5mm in downward direction.

1. Now create boundaries of connecting surfaces & create splines which can be used as a guide.
2. Now use Multisection Surfaces command to join these surfaces using already created guide curves.
3. Use Thick surfaces command to it for 0.75 mm.

• And the overall length should be 80 to 90mm.

ROOF CRUSH TEST:

The roof crush test is a design specified by the IIHS (Insurance Institute for Highway Safety). After analysing several accidents, the IIHS has established some rules on how crash tests should be conducted.

The IIHS bases its rating on this test; for this they consider two points in the evaluation

• Time it takes for a car to return to its original shape after a rollover &
• How deep is the protrusion after the roll over.

Purpose of the roof crush test:

Protrusion into the passenger compartment should be minimized, allowing seat belts and airbags to do their part to protect the occupants.

Conditions to be observed for roof pressure test:

The roof crush test must comply with two conditions:

First state: Close and lock all doors, close all windows.

Second condition: Lock the rocker or chassis his frame horizontally to the rigid surface of the lever.

How to apply force:

The load is applied so that 50 is tilted from the front end to the rear end, and the load is 1800 mm in length and 750 mm in width. The load is applied at 250 degrees from the flatness of the roof. A load is applied to the roof for about 120 seconds, and the crushing speed when the load is applied is 13mm/s or less.

Result:

If the displacement exceeds the required level, the thickness of the vault should be increased, or mastic points added to increase strength. The stronger the roof, the better it protects the occupants in a rollover accident.

Rooftop heat distortion studies:

Thermal distortion analysis plays an important role in the use of sheet metal. Heat distortion temperature is the temperature limit at which a material cannot be used for structural purposes. This study is used to predict the heat distortion temperature at which a material will begin to soften under constant load at elevated temperatures. To prevent roof deflection and damage, predict the position of the arch roof from the heat distortion temperature and use it to reinforce 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> 3.1Top of Form

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

SNOW LOAD CRITERIA:

This test is done to find out how the roof behaves when it snows. Dents usually appear under the weight of the snow.

However, the roof should be designed to return to its original position when snow is removed. These are the basic requirements of the snow cover standard.Bottom of Form

Snow Load Formula:

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

Where

α = My x Lx^2 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 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

Judgement condition = “ Qr ≥ 3.1 “ and “ 250 ≤ s ≤ 380 “

cross-section on Roof BOW [mm]

For Bow 1:

For Bow2:

For Center:

Applies a value to an equation:

To find the snow cover standard, apply all the values ​​you want from the roof to the given formula.

Calculation:

SECTION MODULUS:

Section modulus is a geometric property of a specific section used in beam or flexure design. Other geometric properties used in the design include areas for stress and shear, radius of gyration for compression, and rigid and polar moments of inertia. The relationship between these properties is highly dependent on the particular geometry. The section modulus formula for common geometries is shown below.

There are two types of section moduli :

1. The elastic section modulus and
2. The plastic section modulus.

The elastic section modulus is defined as

S = I / y

where :

S= Section modulus,

I = second moment of inertia (or area moment of inertia, not to be confused with moment of inertia),

y = the distance from the neutral axis to any given fibre.

It is often reported using y = c, where c is the distance from the neutral axis to the most extreme fibre.

Bow roof 1 –

Create a section curve using a plane and select Bow roof 1.

Moment of inertia maximum,( MAX) I max = 2.726 × 10⁴ (mm⁴)

Moment of inertia maximum ,( MIN) I min = 2.218 × 10⁴ ( mm⁴)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = Ly/2=34 mm

FOR MAXIMUM MOI :

S= 2.726 × 10⁴ ( mm⁴) / 34 mm

S max = 801.76 mm³

FOR MINIMUM MOI :

S= 2.218 × 10⁴ (mm⁴)/ 34 mm

S min = 625.35 mm³

Bow roof 2 –

Create a section curve using a plane and select Bow roof 2.

Moment of inertia maximum,( MAX) I max = 2.7093 × 10⁴ (mm⁴)

Moment of inertia maximum ,( MIN) I min = 2197.74 (mm⁴)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = Ly/2=34 mm

FOR MAXIMUM MOI :

S= 2.7093 × 10⁴ ( mm⁴) / 34 mm

S max = 796.85 mm³

FOR MINIMUM MOI :

S= 2197.74 ( mm⁴)/ 34 mm

S min = 64.64 mm³

CENTRAL ROOF RAIL :

Create a section curve using a plane and select Central roof rail.

Moment of inertia maximum,( MAX) I max =4.86115 × 10⁴ ( mm4)

Moment of inertia maximum ,( MIN) I min =2810.23 ( mm4)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = Ly/2= 42.5 mm

FOR MAXIMUM MOI :

S= 4.86115 × 10⁴ ( mm⁴) / 42.25 mm

S max = 1143.8 mm³

FOR MINIMUM MOI :

S= 2810.23 ( mm4)/ 42.25 mm

S min = 66.51 mm³

FRONT ROOF RAIL:

Create a section curve using a plane and select Central roof rail.

Moment of inertia,( MAX) Imax =5.51946× 10⁵ ( mm⁴)

Moment of inertia,( MIN) Imin = 41174 ( mm⁴)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = Ly/2= 165.133/2=82.5665 mm

FOR MAXIMUM MOI :

S= 5.51946× 10⁵ ( mm⁴) / 82.5665 mm

S max = 6,684.86 mm³

FOR MINIMUM MOI :

S= 41174 ( mm⁴)/ 82.5665 mm

S min = 498.677 mm³

REAR ROOF RAIL:

Create a section curve using a plane and select Rear roof rail.

Moment of inertia,( MAX) I max =9.31932× 10⁵ ( mm⁴)

Moment of inertia,( MIN) I min =6.0484× 10⁴( mm⁴)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = Ly/2= 231.484/2=115.742 mm

FOR MAXIMUM MOI :

S= 9.31932× 10⁵ ( mm⁴)/ 115.742 mm

S max = 8,051.80 mm³

FOR MINIMUM MOI :

S= 6.0484× 10⁴ ( mm⁴)/ 115.742 mm

S min = 522.57 mm³

Snaps of different parts:-

Roof Assembly:-

Rear Roof Rail & Front Roof Rail:-

Front, Center & Rear Roof Rail:-

Roof Rail construction:-

Draft analysis:

You can use the draft analysis command to see if the drawn parts can be easily removed. This type of analysis is based on colour ranges that identify zones on the analysed element whose deviation from the design direction at any point corresponds to a specified value. A minimum draft angle of 70 is considered in the analysis. Green indicates parts that are all along the tool direction, have more than 70 positive draft angles, and pass in the analysis.

Front Roof Rail:

Rear Roof Rail:

• Conclusion: -

The results obtained after investigating the curvature of the roof were reasonable.

Bow roofs positioned (above flat areas) and help to increase the strength of the roof.

Embossing can be added to improve roof strength and aerodynamics but increases manufacturing costs and can be avoided if not required.