Roof challenge

AIM - Design of an Automotive Roof

OBJECTIVES: -

To design Front Roof Rail

To design Rear Roof Rail

To design Centre Roof Rail

To design Bow Roofs

To find Moment of Inertia of Bow Roofs

To perform Draft analysis on all the Rails and Bow Roofs

To check the Roof for Heat Distortion criteria

To check the Roof for Snow Load criteria

Master Section

THEORY.

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

Type of Roof in Industry.

1. Sun roofs: -
   
   An automotive sun roof is an operable (tilting or sliding) opening in an automobile roof which allows light and/or fresh air to enter the passenger compartment. Sun roofs may be manually operated or motor driven, and are available in many shapes, sizes and styles.
2. Detachable hardtops: -
   

A convertible is a type of automobile that can convert from an enclosed to an open-air vehicle. In the past, some convertibles featured as standard or optional equipment fully rigid, manually installable hardtops. These hardtops provided acoustic insulation, but also required space consuming off-season storage and a cumbersome two-person installation. Lightweight aluminium detachable hardtops facilitated the installation significantly compared to steel versions.

3. Retractable roofs: -
   

A prominent option for a convertible is the retractable roof. Retractable roof designs vary widely, but a few characteristics are common to all convertibles. The roofs are affixed to the vehicle body and are usually not detachable. Instead, the roof is hinged and folds away, either into a recess behind the rear seats or into the boot of the vehicle.

Roof Component.

1. Roof Outer Panel.
   

The roof has an outer panel which is the styling part.

2. Front Roof Rail.
   

The front roof rail provides provision for the windshield glasses. Visibility criteria and headroom clearance

have to be followed while designing the front roof rail. Provision for windshield mounting and inside rear-

view mirror is provided on the front roof rail.

3. Bow Roof rail / Centre Roof Rail.
   

Bow roofs are provided in between the front roof rail and centre roof rail as well as the Rear roof rail and centre roof rail. They are provided to enhance the localized strength of the roof and are attached directly to the bottom of the roof with either spot welding or with the help of Mastics.

4. Rear Roof Rail.
   

The Rear roof rail has hinge mounting for the back door. It is attached at the portion where the back door meets with the roof.

Section View: -

Moment of Inertia of Bow Roofs: -
Bow Roof First

Centre Roof Rail

Bow Roof Second

Draft analysis on all the Rails and Bow Roofs: -

1. Roof Outer Panel.
   
2. Front Roof rail.
   
3. Bow Roof rail first.
   
4. Centre Bow Roof rail.
5. Bow Roof rail Second.
6. Rear Roof rail.

All the part of Roof has successfully cleared the draft analysis with a minimum draft angle of 7 degrees.

Roof for Heat Distortion criteria: -

Heat Distortion Analysis

Heat distortion analysis plays a pivotal role in the strength of the roof panel. It is the temperature above which material cannot be used for structural applications. This study is used to predict the heat distortion temperature where the material starts to soften when exposed to a fixed load at elevated temperature. In order to avoid any bending or damages on the roof, heat distortion temperature can be recalculated using the trial-and-error method by changing the position of bow roofs. In this way, it can come in the range which is shown below.

Roll Over Test on Roof

• The rollover test is given by the Insurance Institute of highway safety (IIHS). After analysing many accidents IIHS has fixed some regulations about how to perform the test.
• IIHS rating is based on the following things which are how fast a car can reform its original shape after rollover and how deep a protrusion is.
• The aim of this test is to minimize the protrusion of the roof.
• Two conditions need to be followed while performing this test. One is to lock all the doors of the car properly and the second is to close all the windows of a car.
• Also, it is mandatory to fix the chassis on the hard lever surface horizontally.
• Load is applied at 25 degrees tilt when view from the front over the length of 762 mm.
• Load is applied at 5 degrees tilt when view from the side, over the length of 1829 mm.

Img: -1 (Roll over test)

Img: -2 (Roll over test)

Img: -3 (Roll over test)

Img: -4 (Roll over test)

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

Judgement Condition using w: OK< 2.7> 3.1

Were

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]

Roof for Snow Load criteria: -

Snowload Analysis

• The purpose behind this analysis is to check the strength of the roof when snow is present on it acting as a load.
• A roof should be designed in such a way that it should rebound to its original shape after the removal of snow.

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.

Were,

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 centre 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

CONCLUSION

• All the parts have successfully cleared the draft analysis.
• The proposed Roof design has successfully cleared the heat distortion analysis.
• The portion between bow roof 1 and Rear Roof Rail is failing in Snowload analysis.
• Embosses have been provided onto the roof outer panel portion where it is failing in snowload analysis. Embosses increase the strength of the roof panel.
• The roof has been successfully designed considering all the design Rules, Master sections, Manufacturing feasibility, Draft analysis, Heat distortion criteria, and snowload analysis.

End of Report---------