Roof challenge

Roof challenge

Introduction

The roof part starts from A-pillar and goes up to D-pillar. The roof's purpose is to provide cover to the passenger inside the car and protect the passenger from elements like sun, rain, wind, etc. the roof also prevents the unwanted object from falling inside the car. Roofs are generally made up of aluminum. An aluminum-based roof panel may reduce the overall weight of the assembly.

Types of automobile roof

• Convertible: A convertible, cabriolet, or spyder is a passenger car that can be driven with or without a roof in place.
• Hardtop: A hardtop is a rigid form of automobile roof, which uses for modern cars typically constructed from metal. A hardtop roof can be either fixed, detachable for separate storing, or retractable within the vehicle itself.
• Sunroof: An automotive sunroof is a movable panel that is operable to uncover a window in an automobile roof. which allows light and fresh air into the passenger compartment.
• A-Top: A-Top is an automobile roof with a removable panel on each side of a rigid bar running from the center of one structural bar running from the center of the next structural bar. The panels of a traditional T-top are usually made of auto-grade safety glass.
• Targa top: it is a semi-convertible car body style with a removable roof section and a full-width roll bar behind the seats.
• Vinyl roof: Vinyl roof refers to a vinyl conversion for an automobile’s top. This converting was originally designed to give the appearance of a convertible to models with a fixed roof but eventually, it evolved into a styling statement in its own right.

Design consideration

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

Components of roof:-

• Front roof rail
• Rear roof rail
• Bow roof rail
• Ditch area

1. 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. Spot welds are applied at wind shield end of the rail and the other ends is joined with roof using mastic sealant which also increases the strength.

2. Rear roof rail

Rear roof rail is again used to strengthen the roof and it acts as a link between back glass and roof which connected to the backdoor of the car. Spot welds are applied at back door end of the rail and the other end is joined with roof using mastic sealant.

3. Centre bow roof rail

This is a roof rail that will be at the center of the roof to provide strength and stability to the roof. Here the center roof rail is joint with the roof through mastic sealant.

4. Bow roof rail

Bow roof is used for strengthening purpose

5. Ditch area

Ditch area is a extended portion of the main roof concelled by other BIW panels. The rails support of the roof are attached to the ditch area using spot weld.

Calculations to determine bow position

1. Heat distortion criteria

Heat distortion temperature is the temperature at which the metal softens and deforms under a specific load. This study is done to check the ability of the roof to withstand high temperature with respect to the positioning of the reinforcements which contribute to increased strength.

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

Judgement condition : OK<2.7<NG

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]

From the above table, it is concluded that the designed roof satisfies the heat distortion criteria.

2.Snow load criteria

Snow load criteria study is done to check the ability of the roof to withstand snowfall and bounce back to its original shape without any permanent deformation once the load is removed.

snow load prediction formula

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 front and rear reference point.

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]

From the above table it is concluded that the designed roof does not satisfies the snow load criteria.so in order to increase the strength of the roof embosses are created on the outer panlel.

Section modulus calculation

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 design include area for tension and shear, radius of gyration for compression, and moment of inertia and polar moment of inertia for stiffness. any relationship between these properties is highly dependent on the shape in question.

Formula to calculate section modulus

S=I/Y

where,

S-section modulus

Y-moment of inertia

I-maximum distance between neutral axis andone of the extreme end of the object.

1. bow roof 1

I=49.991mm^4

Y=34.936mm

S=I/Y=1.430mm^3

2. Centre bow roof

I=92.6015mm^4

Y=64.943mm

S=92.6015/64.943=1.425mm^3

3. bow roof 2

I=50.7471mm^4

Y=27.188mm

S=50.7471/27.188=1.866mm^3

Draft analysis

Parts cannot be ejected from the tool if it is 90 degree for this reason, a small draft is to be provided for each surface for the components in order to easily remove the components from the mould without any defects. draft is the angle in which the part is tapered to allow it to release. most of the sheet metal part require minimum 7 degree to 10 degree however 7 degree is widely accepted in sheet metal components design.

Conclusion

1. Using all input data we have successfully designed ditch area flanges, front roof rail,bow roof rails, rear roof rail as per the requirement.

2. As per the heat distortion criteria we come to know all bow roof positions are ok.

3. As per snow load criteria we come to know there is improvement required.

4. All roofs draft analysis are found ok.