Roof Design

Car Roof Design and Analysis of Reinforcements

Objective:

A dumb solid body of roof CAD model is provided for which following points to be designed and analyzed are as follow.

1. Extracting the styling surface and for the given car roof styling and design the essential fanges and reinforcment , namely Ditch Area, Front Roof arail, Rear Roof Rail, Rear Roof Rail, Center Roof Rail and Bow- roofs.
2. Do a curvature study on the roof and peform calculations for Heat Distoration and snow load criterion to determine and justify the position of the Bow-roofs.
3. Finding a moment of inertia and Section Modulus for Bow Roof Rails and centre Roof Rail.
4. Perform a Draft Analysis of each reinforcement.

Sotware Used

NX- CAD & MS Excel

Introduction

An automobile roof is the portion of an automobile that sits aove the passenger compoartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. Under the styling surfacethat is seen from the outside , there are many other structures under the roof that contribute to its strength and stability. The number of such reinforcement structures/ supports differ with respect to the size of the vehicle. They are primary installed on flatter areas of the roof to enhance strength.

These are the Front RoofRail, Rear Roof Rail, Centre Roof Rail and Bow Roof Design which are given Mastics and spot-welded to roof flanges & ditch areas and improve NVH value by increeasing the styrength of the Roof using mastic sealants.

Design consideration of roof:

• Visibility Criteria
• Head clearance
• Curvature study
• Heat distortion and snow load criteria
• Draft analysis.
• Safety parameters

Roof crush Test for passanger Car:

Safety point of view vehicle roof plays a very crucial role especially roll over kind of accidents, in which vehicle tips over on to its side of roof. and vehicle rollover crashes are the crashes are the cause of many fatalities and head, neck, and spine trauma around the world.

Therefore, passanger safety is also a very important parameter in front of OEMs. It is not only a marketting strategy but also it is obligation stipulated by international standurds that are now placed in several countries, as well as a govermental requirement. For thisReason, strength isa very important criterion for desigening a vehicle roof.

The strength of the roof structure to occupant protectionin real- world rollover crashes, requiring the maximum moveing distance of the roof structure should be less than 127mm(5in.) when the indused load is 1.5 times the vehicle weight.

For the test, the passanger car shall be rigidly placed or positioned on a horizantal surface with the doors locked and the window closed. further more, a flat, rigid block with a lower lower surface arectangle measuring 30 inches wide by 72 inches long (762mmx1829mm) shall not move more than 5 inches (127mm), measered as the distance between the original location of the lower surface of the test device and its location as the specified force level is reached , when it is used to apply a force of 1.5 times the unloaded vehicle weight (UVW) or 5000 Ibs (22224N), whichever is less, to either side or forward edge ofthe vehicle.

This test can be conducted on either side of the roof structure, the front left or the front right side but not on both sides in one single test and still meet the requirements of the test . Thd rigid plate shal not mve faster than 0.5inches/ secand and the force applied on the plate shall not exceed the lesser of 1.5 times the unloadedvehicle weight (UVW) expressed in kilograms multiplied by 9.8 or 5000lbs (22240N)

Also, the direction of the force shall be downward and perpendiculer to the lower surface of the rigid plate and such that it moves in a stright line without rotation . The duration of the test shall not exceed 120 seconds . The longitudinal axis of the plate is pitched forward at a 5° angle as viewed from the side of the passenger ca as shown in bellow figure.

The lateral axis o r the rolle of the plate is such that it forms a 25° outboard Angle with a horizontal surface as viewed from the front of the passanger car as shown in bellow figure.

Design Methodology

Styling Surface to Extracted Surface:

The styling team has provided the solid dump outer panel from that we are extracting the surface to model the necessary reinforcements like Ditch area. Front Roof Rail. Bow Roof Rail, Centre Roof and Rear Roof Rail.

fig: Styling Surface

Fig: Extracted surface from Dump Body

fig: Roof Panel with and Rear Ditch Area

Design a Ditch Area for Outer Roof panel:

Stying team provided the master section for the roof ditch area . front roof rail and rear roof rail as per the parameter of visibility criteria, head room clearance and other importantparameter dimension like fitment of windshield, sealant rubber etc, All these data are well calclated in the planning stage to design the peripheral component accordingly. thus, her designing is done just to replicate those input data considering engineering parameters which should be feasible for manufacturing. Master section of Front and Rear section.

1. Front Ditch Area:

Front dumpm solid part class a Surface is extracted and ditch area are developed considering the parameter specified in the following master section.

Flange of around Round of 15mm created with the low extension command, corner relief at rear flanges and and positive draft more than 3 degree onsidering for the drawing operation process / overall thickness of sheet metal consider 0.75mm throughout.

2. Rear Ditch Area:

Surface is offset from the outer panel to form the ditch area and with the law extension command, corner relief at rear flanges and positive draft in required degree considering for the drawing operation process . Overall thickness of sheet metal for outer panel consider 0.75mm thought.

Front Roof Rail:

The front rail underpoints the rooftop external shell at the front end. It is mounted with support rooftop rail fortification on the two closures. The rails are furnished with embosaes to improve the strength over length and the rail is assosiated with the external board utilizing mastic sealents on one side and the other joinery is spot welded.

Sheet Metal Thickness:T= 0.75 mm

Emboss Heigh= 2<3T

Flange Length = 13.2> 3T+R

Radius : (R=3mm & 5mm )> T

fig: Master Section Of Front Roof

fig: Section view of front Roof

fig : Front Roof Rail

Rear Roof Rail:

It is Support which is joined to the back Door and the Body side outer and inner panels. it is also usually made from Body side outer scrap and therefore the thickness of the rail 0.75 mm similar to the front Roof Rail design, we have to take into consideration the rear Headroom Clearence and the rear vlisibility creation while designing the Rear Roof Rail. Inputs are taken from the rear master section shown in fig.

fig. Master Section of Rear Roof

fig : Rear Roof Section view

fig: Rear Roof Design

Bow Roof:

The Bow roof given to improve the torsional stiffness and load- bearing capacity of the roots structure. The number of Bow roofs provided will Depend the overall dimensions of the roof. Longer the roof more the bow roofs are integrated to BIW frame they are attached to the outer panel using mastic sealants. Each Mastic Point will have up to 80mm circular effective area. The positioning of bow roofs is done based on curvature study. The heat distortion criteria and snow load createria are checked to analyse the position of the bows in veries conditions.

fig: Master Section of Bow Roof

fig : Section View of Bow Roof Rail

fig: Bow Roof Design

Center Roof Rail:

Venter roof rail is provided to effectively support the flat area of the roof asb it is more skusceptible to failure under the action of the load. Usually the center reinforcement roof rail will be placed at the center of the roof connecting the B piller support structure which results in added support in the rail- over test. The thickness of the center bow roof rail is 0.75 mm thickness.

Fig: Master section of center Roof Rail

fig: Section view of Center roof rail

fig: Center Roof Rail design

Mastic Sealants:

Bow Roof Rails and center Roof Rail are joined with the help of Mastic Sealant. Here we cant's use the spot welding becouse it damages the A class area of the Roof. so mastic sealant is used in this area.

Roof Top View

Roof Rails

Curvature study on the Roof:

The Heat distortion study plays an important role in sheet metal usage. Heat distortion temperature is a 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 elevated temperature . In order to avoide bending or damages on roof, based on heat distortion temperature, this study will predict the bow roof position on the roof to strengthen the roof.

Bow- Roof prediction Formula

Heat Distortion Criteria:

W=[1.73X10^(-3)XL]+[1.85X10^(-8)X(R^2)/2]+{1.10X 10^(-3)X1]-2.68

Judgement Condition: OK,2.7> 3.1

Where

L= Roof Length in X-direction [mm] ( Roof dimension in 0-Y)

R= Roof Curvature

R= 2(RXRy)/(Rx+Ry)

Rx= X curvature

Ry= Y Curvature

t= Roof plate Thickness(mm)

l= Bow RoofSpan(mm)

Escept Rx and Ry all values can be measured on the model.

To get the Rx and Ry curvature a Sketch was required

• A sketch was created on a plane on top of the Roof.
• 0-Y line was created in the X-Direction in center of Roof.
• Then horizontal line below 300mm was created named as- 3Y Line.
• Two lines on both sides at 100mm were created from -3y line.
• Intersection point on line was created and projected on outer panel of Roof.

Fig: Projected Sketch

Projected Points:

Then three point Aec was created by using these points.

Then Minimum Radius of Curvature was measured to get the Rx and Ry Value.

RX1 Value Obtained:

RX2 Value Obtained:

RX3 Value Obtained:

RX4 Value Obtained:

RY 1 Value Obtained:

RY2 Value Obtained:

RY3 Value Obtained:

RY4 Value Obtained:

L Value Obtained:

Bow Roof Span (i) value obtained:

Here want to take distance from the front Roof Rail to Bow Roof 1 to find the Bow Roof Span as Shown in Fig.

By using all the values Heat Distortion was calculated in the MS Excell

Heat Distortion Calculation:

According to Judgement Condition: OK<2.7

Snow Load Calculatin

During Winter Seson the snow gets clogged over the top surface/roof of the car and the snow has some mass on it. Due to this, the roof can fail if the snow load exceeds the capacity of the roof that was designed to sustain.

Snow load Predection 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 type error in the formula given in the video.

Where:

α = My x Lx 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]

width of the roof panel exposed on the surface.

Y = Distance front Roof Rail to RoofBow [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 = Latral direction curvatur radius of Roof panel Y cross- section on Roof Bow [mm]

Roof panel curvature Radius of the Length Lx in Front View

Ry = Longitudianal 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

To get the values a sketch was created on Top Of Roof

• 0- Y line was created in the center of Roof
• A line between First center Roof frail was created
• Then Two lines on both sides at 100mm was created.
• Similar Lines were created Between other Center roof rail, both Bow Roof rails and Rear roof Rails.
• Intersection Points were created and projected on the roof.

Projecte points for snow load Calculation

Then three point arc was created by using These points:

Then minimum Radius of curvature was measured to get the Rx and Ry Value

Rx1 & Ry1 Value Obtained:

Rx2 & Ry2 Value Obtained:

Rx3 & Ry3 Value Obtained:

Rx4 & Ry4 Value obtained:

To find the Lx and Ly values take the styling surface of the Roof

Moment of Inertia and section Modulus for the Roof Rails:

Section modulus is ageometric propety for a given cross section used in the desig of beams .Other geometric properties used inthe design include an area for tension,radius of gyration for compression , and moment of Inertia for stiffness . The section modulus is defined as

S=I/Y

Where

S= Section Modulus

I= Moment of Inertia

Y= Distance from the neutral axis to any given part.

The Intersection was created and then section Inertia command was used to obtain MOI

Front Roof Rail:

S=I/Y

I=8.728687616x10³ mm^4

Y=126.9295/2

Y =63.46475 mm

S=(8.728687616x10³) / (63.46475)

S = 137.536 m­m³

Bow Roof 1:

fig: MOI of Bow Roof Inertia.

S= I/Y

I = 364.154724752 mm^4

Y = 69.5963/2

Y = 34.79815 mm

S= I/Y

= 364.154724752/ 34.79815

S= 10.465 mm³

Center Roof:

fig: MOI of Center Roof

S = I/Y

I= 1327.655742 mm^4

Y= 84.7290 / 2

= 42.3645 mm

S= I/Y

= 1327.655742/ 42.3645

S= 31.338 mm³

Bow Roof 2:

Fig: MOI of Bow Roof 2

S= I/ Y

I= 365.8288936298 mm³

Y = 69.9555 / 2

= 34.97775 mm

S= I/Y

= 365.8288936298 /34.97775

S = 10.4589 mm³

Rear Roof :

S= I/ Y

I = 36,363.633 mm^4

Y= 211.577/2

= 105.7885 mm

S = I / Y

= 36363.633/105.7885

S = 343.73899 mm³

By Using all the values Snow load was calculated in the MS Excel

Snow Load Calculation.

So acording to the Judgement condition = Qr ≥ 3.1

Draft Analysis:

The Draft Analysis command enables you to detect if the part you drafted will be easily removed. This Type of analysis is performed based on colour ranges identifying zones on the analysed element where the deviation from the draft direction atany point, Corresponds to specified values.

Minimum draft angle of 7° is considered for analysis. Green colour infer on the parts that all face along the tooling direction has positive draft angle greater than 7° and passed in analysis.

fig: Roof Outer Panel

Fig: Front Roof Panel

Fig : Bow Roff Rail1

Fig : Center Roof Rail

Fig : Bow Roof Rail 2

Fig : Rear Roof Rail

Manufacturing Process:

• Laser cutting / Punching
• Stamping
• Joining by Spot Welding
• Joining by Mastic Sealant

Clonclusion:

The Design of Roof, Front Roof Rail, Rear Roof Rail, Bow Roofs and Center Roof by folowing the ,master section with th design parameters.

• All reinforcement BIW parts of Roof are Desined according to the input data proided by the styling team.
• Made the curvature study and analysis the design for Heat distortion are found ok.
• Made the Curvature study and analysis the Design for Snow Load are found Ok.
• Moment of Inertia, Section Modulus and Draft analysis are carried out to check the feasability of manufacturing are found ok.