Roof Design

Objective:

• Study the different types of roofs available in the market.
• Study the heat distortion criteria to adjust the position of bows and center roof rail.
• Study snow load conditions to check the strength of the roof according to bows positions.

Introduction:

An automobile roof or car roof is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from the 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.

Roof Structure:

The use of aluminum for the car roof is well established in all aluminum car body designs. But there are also significant aluminum applications in the roof of vehicles with steel car body structures. Weight reduction in the roof (i.e., lowering the center of gravity) has always been a high-priority topic in automotive design. However, the roof is generally an integral part of the BIW. Therefore, the integration of structural aluminum components into the roof of a steel body structure requires an in-depth consideration of different aspects.

Roof Rails:

Roof rails are placed on the top of the vehicle and are used to carry additional luggage and types of equipment. Aluminum extrusions are used for manufacturing roof rails because they enable visually attractive, low-weight & high-strength designs. Roof rails have two principal functions: to enhance the design features of the vehicles & to carry extra loads placed on the roof, often with the help of another structure, eg.: transverse load bars or a roof box.

Type of roofs:

                                         

Methodology for designing the parts of the Roof:

1. Select the type of roof from the above list for the given skin data of the roof surface.

2. Deciding the number of bows used to support the roof surface according to conditions.

3. Select the joining method used to join the roof surface with the front roof rail. rear roof rail, central roof rail, and bows.

3. Designing the front roof rail and rear roof rail according to the given master sketch.

4. Designing the central roof rail and bows(as required) according to the given master sketch.

5. Calculating the parameters required for heat distortion criteria and checking the values of distortion in the roof are within range or not.

6. If the value of distortion in the roof body is not within the range, we need to change the position of the bows and central rail roof so that the distortion in the roof should be within range.

7. Next we have to do an iteration for the snow load condition to check the strength of the roof again if the value from the snow load condition is not in the range we have to do modifications in the bows section area and their position on the roof body.

Design Consideration and criteria:

1. Roof:

a.We are making the hardtop type of roof from the given master section so we are not adding any reinforcement for the central roof rail.

b.We are adding the flanges to all sides of the roof for spot welding of the front roof rail, central roof rail, rear roof rail, and bows.

c.The length and angle of the flange are decided based on the visibility criteria and according to the draw angle required for manufacturing the sheet metal.

d.The thickness of the roof is decided at 0.75mm

2. Front roof rail:

a.The front roof rail is used as the reinforcement to the roof surface to increase the strength and also used to mount the brackets for some features such as room lamp or map lamp and rear mirror.

b.The depth of the front roof rail is decided based on the visibility criteria line decided for the driver visibility.

c.The width of the front roof rail is decided in such a way that there should be enough headroom clearance between the head of the driver and the end surface of the roof rail to avoid injuries to the person.

d.At the front side we are doing spot welding and on the rear side of the front roof rail, we are using mastic points to join it with the roof surface.

e. The minimum radius for the fillet at edges is 3mm

3. Rear roof rail:

a.The rear roof rail is used as the reinforcement to the roof surface at the rear side.

b.The joining method used for this rail is the same as that of the front roof rail.

c.The minimum radius for the fillet is 3mm.

4. Bows:

a.We are making two bows one is in between the front roof rail and central roof rail and another is in between the central roof rail and rear roof rail.

b.Both ends of the bow are joined to the roof outer flanges by the spot welding and the inner sides of the bow are joined to the inner surface of the roof with the help of mastic points.

c.The minimum radius of the fillet is 3mm.

d.The cross-section of the bow is made in such a way that we should get the maximum value of the minimum moment of inertia at which the bow fails.

5. Heat distortion criteria:

 In this criteria, we check the heat distortion capacity of the roof at higher temperatures with the fixed load on it. So when the surrounding temperature is high and the roof is loaded with some weight on it then if the roof is deformed it fails, so to calculate whether the roof deforms or not, according to the position of the bow we do these calculations as follows

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]

Judgment Condition

OK< 2.7

Snow Load Prediction Formula

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

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

Where,

α = My x Lx² 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 front and rear reference point.

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 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

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380