Roof challenge

Aim:

To design a Roof of a car & do a curvature study to resist hest distortion & snow load condition.

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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.

In later years, many variations on the automobile roof developed. These include:

• Convertibleroofs
  • Roof modules
  • Roof modules
• Hardtops
• Sunroofs
• T-tops
• Targa tops
• Vinyl roofs

Materials used:

Steel is the most used of the materials, making it less expensive and affordable as a metal roofing option for homeowners as well as commercial builds. Tin had its day, but now often refers to galvanized steel when spoken about in terms of roofing.

5 Types of Metal Roofing

1. Copper - Extremely long-lasting, very soft with low melting temperature
2. Aluminum - Long-lasting, resistant to salt water corrosion
3. Zinc - Extremely long-lasting, resistant to corrosion and lowest melting point
4. Steel - Three variations: galvanized, galvalume, and weathering steel (corten)
5. Tin - Often referring to steel, used prior to World War II. No longer commonly applied.

Objectives:

The main objective of this project is to come up with a new design for CAR ROOF from styling surfaces that satisfies,

1. The design requirements for the roof and its components.
2. Functional requirements.
3. Roll over test regulations & requirements.
4. Bow Row position justification.
5. Joining techniques like welding and masting.

Data for Roof design:

1. Roof panel thickness = 0.75mm.
2. Front roof rail thickness = 1.2mm.
3. Rear roof rail thickness = 1.2mm.
4. Bow roof rail thickness = 1.2mm.
5. Centre roof rail thickness = 1.2mm.

Roof Design:

• First thing to consider while designing the roof is the opening angle, of the flanges such that there is no negative draft.
• If the roof rail is flat it is weak and if it is tilted or curved then it has more strength.
• As the roof is large so to increase the strength of the roof where it appears flat due to its long distance Roof Bow are given to increase the strength of the roof.
• Since the roof has numerous functions so the roof has to be stronger enough to withstand the damage experienced by it. To have good strength and stiffness, the Roof consists of components:

1.Front roof rail &Rear roof rail

2.CTR Bow roof

3.Bow roof.

• The Rear roof rail has joints with the backdoor, bodyside outer where as the Front roof rail has joints with windshield glass, bodyside outer and inner panels.
• From the ergonomics team we get to know where the front driver is sitting, hence there should be minimum head clearance.
• There should be clearance of 5-10mm b/w headliner and BIW.
• There should be clearance of 85mm b/w rear roof rail plus headliner and the passenger.
• Mastics are applied between roof and the roof rail and bow roof. When the mastic gooses out it joins the roof rail, bow roof with the Roof.
• As we have curvature in front and more flat in the back side of the roof, it is sufficient to have reinforcement centre alone in front and both the roof bows behind.
• If the roof was small with better curvature we use only one centre reinforcement roof rail which also acts as the roof bow.
• Number of bow roof increases if the car size becomes longer and taller. More taller car means more flat region means more Bow roofs.
• If the styling surface does not with stand the Heat Distortion criteria and snow load condition or the strength of the roof is less then in styling surface modify the radius of curvature of the roof and send it to the styling team if they accept it then proceed.
• So as to propose a value of that curvature radius will have to do trial error for the values of Rx and Ry.
• Always gradually increase the curvature radius not drastically.
• If the styling team does not accept the change in radius of curvature of the surface then embossing is done on the roof of different shapes so as to increase the strength of the roof.
• Always place Roof Bows on the flat region and not on the curvature region.
• Also there is a bracket placed on the reinforcement centre so as to mount the room lamp in the car.

Detailed design

Front roof rail

• It is used to join the roof with front glass, and chassis via A-pillars.
• The thickness of 1.2 mm is given to the surface.
• Necessary embosses are provided on the surfaces, if required.

Rear roof rail

• It is used to join the roof with back door and chassis via C-pillars.
• The thickness of 1.2 mm is given to the surface.
• Necessary embosses are provided on the surfaces, if required.

Reinforcement centre

• It is used to join the roof with the B-pillar.
• The thickness of 1.2mm is given to the surface.
• Reinforcement centre consists of two parts- The inner part and outer part.

Bow roofs

• It is used to increase the strength of the roof.
• A number of bow roofs are decided based on the length of the roof and strength required.
• The thickness of 1.2 mm is given to the surface. ◦ Strength of the roof will increase with the number of bow roof.
• In some cases, the bow roof itself is used as reinforcement centre.
• In this project, two bow roofs and one reinforcement are used.
  

Section Modulus calculation of the Roof

Section modulus is a geometric property for a given cross-section used in the design of beams or flexural members. The elastic section modulus is define d as S = I / y, where I is the second moment of area (or area moment of inertia) and y is the distance from the neutral axis to any given fibre.
We have,
S = I / y
where I - second moment of area (or area moment of inertia)
Y - the distance from the neutral axis to any given fibre.
I – for the above section
Section modulus for various sections are given in the table below,

Roof Curvature Study

1.Heat Distortion Criteria Calculation of the Roof

• Heat distortion test is a type of static load test which plays an important role in maintaining roof strength.Heat distortion temperature is the temperature at which metal softens and deforms under a specified load. From Heat distortion criteria, roof strength can be enhanced by appropriate positioning of bow roof rails.
  

Calculations

Observation

• As per the heat distortion criteria, positions of all the bow roof and reinforcement centre are under acceptable conditions.

2. 
   Snow Load Criteria Calculation of the Automobile Roof

The snow load test is a type of static load test used to verify that roof assembly exhibits no permanent deflection due to a distributed load representing snow load: the distributed pressure applied is in the range 10-15 N/dmᶺ2.

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 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]

· Roof panel X curvature radius of length s in Side view

Observation

As per the heat distortion criteria, positions of all the bow roof and reinforcement centre are under acceptable conditions.

Conclusion

• All the parts of the roof are designed based on the required parameters and are assembled together to form the roof using NX CAD.