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

THEORY

What is a Roof?

• The component of an automobile that lies above the passenger compartment and protects the occupants from the sun, wind, rain, and other external elements is known as the roof or car top.

• Along with stylish aesthetics, there is also involved a lot of engineering behind the design of the roof.

Main Parts Of Roof

• The roof has an outer panel which is the styling part.
• Strengthening parts are present below an outer panel of a roof, which are front roof rail, Bow roof, Centre roof rail, and Rear 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 rearview mirror is provided on the front 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.
• Bow roofs are provided in between the front roof rail and center roof rail as well as the Rear roof rail and center 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.
• Reinforcement center or center roof rail is used as a cross member between Left hand and right-hand bodysides.
• Both the front roof rail and rear roof rail provided with the Master section for the design.
• The below image shows the location and Material used for all the strengthening parts.

Design and Functional Requirements for the Roof

While designing of roof following considerations have to be taken into account.

• Visibility Criteria
• Head clearance
• Curvature Study
• Head Distortion and snow load criteria

Roll Over Test On Roof

• The rollover test is given by the Insurance Institute of highway safety ( IIHS). After analyzing 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.

MAIN CONTENT

Software Used - Siemens NX 11

Design Considerations

• Clearance of 5 mm should be present between outer roof rail and Front roof rail, Rear roof rail, Bow roof, and Reinforcement center where mastics are used for adhesion.
• Thickness for all sheet metal parts - 0.75 mm.
• All edges should have fillets and there should not be any sharp edges present.
• A minimum draft of 7 degrees is required for all the faces.

Design Of Roof Rail and Bow Roofs

A.Front Roof Rail

• By following the given Master section and dimensions front roof rail has been designed.
• The below image shows the given master section for the front roof rail.

• The below image shows the dimensions used for the front roof rail.

• The below image shows thickened part of the Front Roof rail.

B.Bow Roof 1

• By following the given Master section Bow Roof has been designed.
• The below image shows the given master section for Bow Roof.

• The below image shows thickened part of the Bow Roof.

C.Centre Reinforcement

• By following the given Master section Centre Reinforcement Rail has been designed.
• The below image shows the given master section for Centre Reinforcement.

• The below image shows thickened part of the Centre Reinforcement

D.Bow Roof 2

• By following the given Master section Bow Roof has been designed.
• The below image shows the given master section for Bow Roof.

• The below image shows thickened part of the Bow Roof.

E.Rear Roof Rail

• By following the given Master section Rear Roof Rail has been designed.
• The below image shows the given master section for Rear Roof Rail.

• The below image shows thickened part of the Rear Roof Rail.

F.Roof Outer Panel

• Necessary flanges have been created for the roof outer panel to join a side, front, and rear of the car roof with the main body.
• The below image shows thickened part of the roof outer panel.

Moment of Inertia

A.Bow Roof 1

• The minimum Moment Of inertia of the Bow Roof Section is 357.08 mm4

B.Centre Reinforcement

• The minimum Moment Of inertia of the Centre Reinforcement section is 839.145 mm4.

C.Bow Roof 2

• The minimum Moment Of inertia of the Bow Roof section is 361.73 mm4

Draft Analysis

A.Front Roof Rail

• All the faces of Front Roof Rail have successfully cleared the draft analysis with a minimum draft angle of 7 degrees.

B.Bow Roof

• All the faces of Bow Roof have successfully cleared the draft analysis with the minimum draft angle of 7 degrees.

C.Centre Roof Rail

• All the faces of Centre Roof Rail have successfully cleared the draft analysis with a minimum draft angle of 7 degrees.

D.Centre Reinforcement

• All the faces of Centre Reinforcement have successfully cleared the draft analysis with the minimum draft angle of 7 degrees.

E.Rear Roof Rail

• All the faces of Bow Roof Rail have successfully cleared the draft analysis with a minimum draft angle of 7 degrees.

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.

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

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]

Annotations

Drawing of Arcs to find out Values for Rx and Ry

Calculations

Snowload Analysis

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

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 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

Annotations

Drawing of Arcs to find out Values for Rx and Ry

Calculations

Assembly Views

RESULTS

• 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.

CONCLUSION

The roof has been successfully designed considering all the design Rules, Master sections, Manufacturing feasibility, Draft analysis, Heat distortion criteria, and snowload analysis.