Roof Challenge

Roof challenge

Aim:

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

Automobile 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. Automobile roofs are conventionally supported by pillars on each side of the body.

Design of Automobile roof

The first step in designing an automobile roof is to create an outer surface (ski n surface), which are usually designed by styling team based on parent car design and aesthetics.

Following procedure is followed for designing a roof.

• Using surface modelling techniques, necessary ditch areas are designed on all the sides of the roof.
• In addition to that, Rails and spoilers (if necessary) are designed on the top of the roof.
• The surface is provided with a 0.75 mm thickness.
• To increase strength and to join the roof with the body of the car, some reinforcements are provided on the roof. Such as,

1. Front roof rail
2. Rear roof rail
3. Reinforcement centre
4. Bow roofs

Detailed design

Front roof rail

• It is used to join the roof with front glass, and chassis via A-pillars.
• The thickness of 1.5 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 1.5 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 1.5 mm 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 1.5 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.