Roof Design

ROOF DESIGN WITH REINFORCEMENTS, ANALYSING HEAT DISTORTION CRITERIA & SNOW LOAD PREDICTION

OBJECTIVE:

The objective of this project is to design a BIW Roof of a car with Front Roof Rail, Bow Roof Rail, Center Roof Rail, and Rear Roof Rail by following engineering standards. The roof must be subjected to a Draft Analysis to verify whether it satisfies the manufacturing requirements. Curvature Study and Section Modulus Study need to be carried out for the designed roof.

INTRODUCTION

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. Convertible roofs, Roof modules, Roof modules, Hardtops, Sunroofs, T-tops, Targa tops, and Vinyl roofs are the different variants in an Automobile.

DESIGN METHODOLOGY

MASTER SECTION

Front Roof Rail

Rear Roof Rail

DESIGNING THE REINFORCEMENTS

ROOF – DITCH AREA

• The Ditch Area of the Roof was designed by taking the considerations provided by the styling team.

• The ditch area was designed to have only 3 degrees of maximum draft angle and 0.75mm thickness.

• The ditch area is the extended portion of the main roof concealed by other BIW panels.

• The rails of the roof were attached by using Spot welds and Mastic Sealants.

• Visibility Criteria, Head Room Clearance, windshield clearance, and back door clearance were also taken into the design consideration of the Roof.

FRONT ROOF RAIL

• The front roof rails will be joining the wind shield, Body Side Inner and Outer panel, so the front roofs must be designed to have the visibility clearance for the Wind Shield.

• By considering the Deep Drawing of the part the front roof was designed to have the draft value of more than 5 degree.

• The front roof rail has the same offset surface for the ditch area where both will be connected using Spot Welds.

• The front roof rail was provided with embosses to increase the strength and stiffness of the rail.

BOW ROOF RAILS

• All the bow roof rails were placed between the front and rear roof rails.

• Bow roof rails were added to increase the stiffness of the roof since it has the large flat surface.

• To increase the curvature result of the roof where it needs to withstand the heat distortion and snow load criteria the bow roofs were positioned as per the standards.

• The bow roofs were provided with mastic points since the joining process will be done using the mastic sealants.

• Spot welds also possible but it will damage the aesthetic appearance of the car roof.

• Embosses were also provided all over the bow roofs to increase the stiffness rapidly.

• The bow roof position needs to satisfy the head room clearance and draft analysis for Deep Drawing.

Bow Roof Rail 1

Bow Roof Rail 2

CENTER ROOF RAIL AND ITS REINFORCEMENT

• The Center roof rail was placed after the Bow Roof 1 in my model but it can also be swapped between the Bow Roofs but it should be positioned as the center of the roof as much as possible.

• The center roof rail was designed to have a reinforcement roof rail which helps the roof of a car to increase the efficiency in curvature study.

• For creating the Mastic Points and Embosses on the Center Roof Rail again the Headroom clearance must be taken into account.

• The reinforcement provided will increase the Geometric MOI of the part drastically.

REAR ROOF RAIL

• The Rear Roof Rail was designed by considering the back door clearance since the opening and closing of the back door will effect on the roof.

• The ditch area on the roof was connected with the rear roof by using the mastic sealant or spot welding.

• Spot welding will be used for joining the hinges of the Back Door with the Roof.

• Embosses were added to increase the stiffness and strength of the rail without affecting the head room clearance.

POSITION OF THE ROOF RAILS

1. FR – Front Roof Rail
2. BR1 – Bow Roof Rail 1
3. CR – Center Roof Rail
4. BR2 - Bow Roof Rail 2
5. RR – Rear Roof

ANALYSIS

Heat Distortion Criteria:

Heat distortion temperature is the temperature at which the metal softens and deforms under a specific load. This study is done to check the ability of the roof to withstand high temperatures concerning the positioning of the reinforcements which contribute to increased strength.

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

Judge mental Condition : 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]

CALCULATION TABLE

Snow Load Prediction:

Snow Load Prediction is a type of study done to check the ability of roof whether it can withstand to different temperatures like cold weathers, and also bear the atmosphere where the snow falls and rebound to its original position when a load of snow load is removed.

• Formula: Qr=Iy×t2α×s×(Rx+Ry2)2×10-8">Qr=Iy×t2α×s×(Rx+Ry2)2×10−8𝑄𝑟=𝐼𝑦×𝑡2𝛼×𝑠×(𝑅𝑥+𝑅𝑦2)2×10-8
• Condition: Qr=≥3.1">Qr=≥3.1𝑄𝑟=≥3.1

where,

s=Distance for which Roof BOW bears divided load in mm

s=L12+L22">s=L12+L22𝑠=𝐿12+𝐿22

α=My×Lx2×10-12,My=Y(Ly-Y)">α=My×Lx2×10−12,My=Y(Ly−Y)𝛼=𝑀𝑦×𝐿𝑥2×10-12,𝑀𝑦=𝑌(𝐿𝑦-𝑌)

Lx= Distance between the Left and Right end of the roof on the Roof BOW in mm

Ly= Distance between the front and rear roof Rails on the Vehicle along with OY in mm

Y= Distance front Front Roof Rail to Roof BOW in mm

Iy= Geometrical moment of inertia of Roof BOW (Y cross-section ) in mm4">mm4𝑚𝑚4

t= Roof Thickness in mm

Rx= Lateral Direction curvature radius of the Roof panel Y cross-section on Roof BOW in mm

Ry= Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW in mm

From the above results, these values for the snow load values it does not satisfy the condition.

To satisfy this condition we make the roof stronger we create embosses on the roof to strengthen the roof and increase the thickness of Bow.

Section Modulus:

FRONT ROOF RAIL

BOW ROOF RAIL – 1

Reinforcement Center:

BOW ROOF RAIL – 2

Draft Analysis:

While designing a component a design engineer must make sure that the component can produced by any means of manufacturing methods with least amount of time, material, manpower and power. So that we can reduce the overall cost of car.

In NX draft analysis can be done after modelling so that we can verify whether the product is manufactured by pressing or not. One side of the surface is highlighted by green and other side by Red colour means that the model can be manufactured. Result of the draft analysis is shown in the figure below

Ditch Area:

Front Roof Rail

Bow Roof 1

Reinforcement Center

Bow Roof 2

Rear Roof Rail

CONCLUSION

Thus the required Ditch Area, Bows, and Reinforcements for the Roof were designed and developed from the given styling. Calculations for the Heat Distortion criteria and Snow load predictions were also performed. The BIW roof of a car was designed and achieved. For a better result apply Thickness in Bows and create Embosses over the Roof.