DESIGNING A BIW ROOF OF A CAR WITH HEAT DISTORTION AND SNOW LOAD PREDICTIONS

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 the engineering standards. The roof must be subjected to Draft Analysis for verifying whether it satisfies the manufacturing requirements. Curvature Study and Section Modulus Study needs 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, 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 degree 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 was 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

Bow Roof Rail 3

CENTER ROOF RAIL AND ITS REINFORCEMENT

• Center roof rail was place after the Bow Roof 1 in my model but it can also be swapped between the Bow Roofs but it should be positioned as 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 Head room 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 taking the back door clearance into consideration 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

ANALYSIS

Three types of analysis were carried out to get a successful design of the BIW roof of a car which satisfies the Manufacturing Standards.

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 color ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.

Roof with Ditch Area and Embosses

Front Roof Rail

Bow Roof Rail – 1

Center Roof Rail With its Reinforcement

Bow Roof Rail – 2

Bow Roof Rail – 3

Rear Roof Rail

SECTION MODULUS ANALYSIS

FRONT ROOF RAIL

BOW ROOF RAIL – 1

CENTER ROOF RAIL WITH REINFORCEMENT

BOW ROOF RAIL – 2

BOW ROOF RAIL – 3

REAR ROOF RAIL

CURVATURE STUDY

The above picture will show the standard for calculating the curvature study,

• [0-Y] – Considered for the Snow Load Prediction
• [-3-Y] – Considered for the Heat Distortion Criteria

Rx

Ry

ROOF SPAN ( ‘l’ and ‘y’ )

HEAT DISTORTION CRITERIA

Heat Distortion 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 temperature with respect to the positioning of the reinforcements which contributes to increase the strength.

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 Criteria study is done to check the withstand ability when the snow loads over the roof during winter in snow fall regions. This study will also analyze the roof that whether the permanent deformation occurs after removing the snow load. That it may bend but needs to be bounce back to its original position.

Qr = [Iy x t^2] / [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]

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm] i.e (Width of the roof panel 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

CALCULATION TABLE

• From the above results, the last two cases were failed to satisfy the judge mental condition.
• Hence to increase the strength and stiffness over the roof from center roof rail to rear roof rail where the Qr results were very low embosses were provided.
• The embosses over there will increase the roofs strength and stiffness drastically.

CONCLUSION

For the Given Roof styling, the essential flanges and reinforcements were developed and checked for Draft analysis. The curvature study and Section Modulus on the roof was carried out and to determine the position of the Bow-roof the Heat Distortion and Snow Load Criteria calculations were also performed. Thus the BIW roof of a car was designed and achieved a better result by applying embosses over the roof.