Roof Design

AIM

To design the Roof with Front Roof Rail, Bow Roof_1&2, Center Roof Rail, and Rear Roof Rail.

Objective

Design a Roof with its components by following the master section of the front roof rail (at least One Front roof rail, two bow roofs, one Center roof rail, and one rear roof rail) with all the design parameters.

Introduction

An automobile roof or car top is the portion of an automobile that covers the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. The roof is the structure above the windshield glass having the reinforcements to provide strength to the overall structure and to withstand overhead forces like snow.

Reinforcement

On any component having less strength and not able to withstand the forces at that case reinforcement will provide the strength in my case I am designing a Roof that the forces are like snow load and Load calculation, so to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail, and bow roofs are introduced and joined to the flat surface of the roof with the different joining process.

Types of Reinforcement

Front roof Bow roofs

Bow roof rail

Central roof rail and

Rear roof rail

CONSIDERATIONS

Driver head clearance

Visibility criteria

SNOW LOAD Calculation

Load Calculations

Curvature study

SOFTWARE

NX-CAD

INPUTS

Class-A surface of the Roof Outer and a master section for the Bow roof, Center Roof, Front Roof, and Rear Roof rail.

Procedure

Start with the outer roof since the Class-A surface was provided and check the class-A if there are any issues with the surface then split the surface into two parts

Front Roof Rail

Starting the front roof rail is the one that joins the windshield glass and the body side outer and the inner panel also. Here the front roof rail is designed based on the master section given.

FRONT ROOF RAIL

SIDE VIEW OF FRONT ROOF RAIL

MASTER SECTION

Rear Roof Rail

The rear roof rail is the one that joins the back door and the body side outer. Based on the Master section.

REAR ROOF RAIL

REAR ROOF MASTER SECTION

Bow Roof Rail

The bow roofs are given to improve the torsional stiffness and load-bearing capacity of the roof structure. The number of bow roofs present depends on the overall size of the roof. Presently in this project, two bow roofs are added.

BOW ROOF

BOW ROOF MASTER SECTION

Center Roof Rail

central roof rail supports the flat area of the roof as it is more susceptible to failure under load acting on the roof. Center roof rail connected to the B-pillar support structure. This central roof rail helps in adding strength to the roof during the roll-over test.

CENTER ROOF RAIL

CENTER ROOF MASTER SECTION

Calculations involved On Roof Design

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

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]

Judgment Condition; OK< 2.7.

If the calculations W1, W2, W3, W4 all the W values should be less than 2.7 then the position of the BOW roof was ok or it was more than 2.7 then there are some steps to be taken.

Solution for not reaching the Judgement condition for Bow Roof Prediction:

1. Changing the position for all the Bow roof 1 or 2 or Center roof rail based on the values.
2. Increasing the thickness of the bow roof 1 or 2
3. Use the Embosses on the Bow roof rail to reduce the stress on the Bow roof

I got the values for Bow Roof Prediction

W₁ -1.577mm ; W₂ -1.9846mm; W₃ -1.9699mm; W₄ -1.9327mm.

Snow Load Criteria

This test is done to know how the roof behaves when there are snow-falling regions. When the snow falls heavy on the because of the weighs it will cause a dent in the Roof. The roof should have elastic in nature so that it can regain its original shape when the snow is removed, there are basic requirements for snow load criteria.

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 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

If the failure occurs the same iterations can be taken from the Bow-Roof Prediction iteration.

BOW ROOF MOI

CENTER ROOF MOI

Result for Snow Load Qr₁ = 1.918mm, Qr₂ = 2.2774mm, Qr₃ = 0.4452mm

Testing Performed on the Roof

ROOF CRUSH TEST:

The roof crush test is the design given by the IIHS (insurance institute of highway safety). After analyzing the many accidents IIHS has then fixed some regulations, on how to perform the crush test.

IIHS gives the rating based on this test. For this, they consider two points while giving the rating

How fast the car can reform to its original shape after the rollover and

How deep is the protrusion after the rollover

Draft Analysis

Front Roof Draft Analysis

Rear Roof Draft Analysis

Bow Roof Draft Analysis

Center Roof Draft Analysis

Goal Of Roof Crush Test:

To minimize the protrusion into the occupant compartment, the seat belts and airbags can do their job of protecting the occupant

CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST:

There are two conditions that should be followed for the roof crush test, they are

INITIAL CONDITION:

Close and lock all the doors

Close all the windows

SECOND CONDITION:

Fix the body sill or chassis frame on a hard lever surface horizontally

METHOD OF APPLYING THE FORCE:

The load is applied in such a way that 50 is tilted from the front end to the rear end and the load will be 1800 mm in length and 750 mm in width. The load is applied at the position of 250 from the flatness of the roof. The load is applied on the roof for almost 120 sec and the crushing speed at which the load is applied is 13 mm/sec or less than that.

RESULTS:

If the displacements go above the requirements level, then we have to increase the bow roof thickness or add more mastics points to increase the strength. the strength the roof the better it can protect the occupants in rollover crashes.

HEAT DISTORTION STUDY ON THE ROOF:

The heat distortion study plays a major role in sheet metal usage. heat distortion temperature is a temperature limit above which the material cannot be used for structural applications. This study is used to predict the heat distortion temperature at where the material starts to soften when exposed to a fixed load at an elevated temperature. In order to avoid bending or damage to the roof, based on the heat distortion temperature, this study will predict the bow roof position which helps to strengthen the roof.

Conclusion

Hence Roof Design was completed with the FRONT ROOF RAIL, BOW ROOF RAIL, CENTER ROOF RAIL, and REAR ROOF RAIL.