PASSANGER CAR ROOF DESIGN AND CURVATURE STUDY USING SIEMENS NX_CAD 12.1

BIW COMPONENT - CAR ROOF DESIGN AND CURVATURE STUDY

OBJECTIVE:

• The objective of this project is to design car roof with essential flanges and reinforcements like ditch area, front roof rail, bow roofs, central roof rail and rear roof rail for the given styling surface
• Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct positions.
• Finding moment of inertia and section modulus for Bow roofs, central roof rail. front roof rail and rear roof rail.

INTRODUCTION:

What is BIW?

Body in white (BIW) is the stage in automobile manufacturing in which a car body's frame has been joined together, that is before painting and before the motor, chassis sub-assemblies, or trim (glass, door locks/handles, seats, upholstery, electronics, etc.) have been integrated into the structure. Assembly involves different techniques such as welding (spot, MIG/MAG), riveting, clinching, bonding and laser brazing. Body in white refers to the stage in automotive manufacturing when a car’s sheet metal components are welded together which include:

• Hood
• Trunk lid
• Roof
• Front and rear fender
• Side wall
• Side Doors
• Tail Gate (Back Door)

The term BIW derives from manufacturing practices before steel unibody monocoques — when automobile bodies were made by outside firms on a separate chassis with an engine, suspension, and fenders attached. The manufacturers built or purchased wooden bodies (with thin, non-structural metal sheets on the outside) to bolt onto the frame. The bodies were painted white prior to the final color.

A folk etymology for "body in white" is the appearance of a car body after it is dipped into a white bath of primer (undercoat paint)— despite the primer's actual gray color. BIW could also refer to when car bodywork would be made of timber - all timber products, furniture, etc., are considered to be "in the white" when at the stage of raw timber before finishing or varnishing.

In car design, the "body in white" phase is where the final contours of the car body are worked out, in preparation for the ordering of the expensive production stamping die. Extensive computer simulations of crash-worthiness, manufacturability, and automotive aerodynamics are required before a clay model from the design studio can be converted into a body in white ready for production

WHAT IS CAR ROOF?

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. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure and to with stand the overhead forces like snow

REINFORCEMENT AND ITS TYPES:

If the roof is flat, it is very weak and cannot able to with stand the external loads like snow load, roll over and etc. so in order to increase the strength of the roof, reinforcements like front roof rail, central roof rail, rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process. In general, there are four types of reinforcements use in hard roof top (simple roof top) they are;

• Front Roof Rail
• Central Roof Rail
• Rear Roof Rail
• Bow Roof Rail

FRONT ROOF RAIL

Front roof rail is the one which joins the wind shield glass, body side outer and the inner panel also. Here the front roof rail is designed based up on the master section given

REAR ROOF RAIL

Just as the front rail roof, the rear rail roof provides reinforcement to the rear part of the car roof. Similar to the front rail roof, the rear rail roof is to be designed considering the rear visibility criteria and the rear passenger's headroom clearance.

CENTRAL ROOF RAIL:

central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load. generally central roof rail is placed at the centre of the roof which is connected to the B-pillar support structure. This central roof rail helps in adding the strength to the roof during the roll- over test.

BOW ROOFS:

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

JOINING PROCESS :

Mastics are introduced between the roof outer panel and the reinforcements for joining . where ever the mastic are introduced there is remarkable improve of the strength

Spot welding is also introduced to join the reinforcements and the outer panel

DESIGN CONSIDERATIONS FOR A ROOF :

• Visibility criteria
• Head clearance
• Curvature study
• Heat distortion and snow load criteria
• Draft analysis

ROOF CRUSH TEST :

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 , that how to perform the crush test .

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

• How fast the car can reforms to its original shape after the roll over and
• How deep is the protrusion after the roll over

GOAL OF ROOF CRUSH TEST :

To minimize the protrusion into the occupant compartment , so that the seat belts and air bags can do there job in 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 in 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 crush speed at which the load applied is 13 mm/sec or less than that

RESULTS :

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

Design Calculation of Roof

To make sure the roof design passes the Analysis test some of the criteria to be considered before the design of Bow Rails under the roof. They are,

• Heat Distortion Criteria
• Snowed Criteria

These calculations are to be performed over the styling surface provided by the aesthetics/concept design team.

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 the 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 avoid bending or damages on the roof, based on the heat distortion temperature , this study will predict the bow roof position which helps to strengthen the roof .

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

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]

Judgement Condition : OK< 2.7> 3.1

From the above table it can be concluded that all values of W<2.7 and also not W>3.1 so thus infers that current positioning of bow row are good in state as per design and found ok

SNOW LOAD CRITERIA:

This test is done to know how is the roof behaving when there is a snow over. normally due to the snow weight the dent will happen. But the roof should be designed in such a way that when the snow is removal the roof should go it its original position . This is the basic requirements for snow load criteria .

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 reference point of the front and the rear.

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 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]

SECTION MODULUS:

Section modulus is a geometric property for a given cross-section used in the design of beams or flexural members. Other geometric properties used in design include area for tension and shear, radius of gyration for compression, and moment of inertia and polar moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape in question. Equations for the section moduli of common shapes are given below.

There are two types of section moduli:

The elastic section modulus and

The plastic section modulus.

The elastic section modulus is defined as

S = I / y

where:

S= Section modulus

I = second moment of inertia (or area moment of inertia, not to be confused with moment of inertia)

y = the distance from the neutral axis to any given fibre. It is often reported using y = c, where c is the distance from the neutral axis to the most extreme fibre

SECTION MODULUS CALCULATION FOR FRONT RR:

Moment of inertia maximum, (MAX) I _max =151182.2 ( mm⁴)

Moment of inertia maximum, (MIN) I _min =8272.16 ( mm⁴)

SECTION MODULUS (S)= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 61.41 mm

FOR MAXIMUM MOI :

S= 151182.2 ( mm⁴) / 61.41 mm

S _max = 2461.849862 mm³

SECTION MODULUS CALCULATION FOR REAR RR:

Moment of inertia maximum, (MAX) I _max =295722.6 ( mm⁴)

Moment of inertia maximum, (MIN) I _min =42440.5 ( mm⁴)

SECTION MODULUS (S)= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 75.355 mm

FOR MAXIMUM MOI :

S= 295722.6 ( mm⁴) / 75.355 mm

S _max = 3924.392mm³

SECTION MODULUS CALCULATION FOR BOW RR 1:

Moment of inertia maximum, (MAX) I _max =2785.05 ( mm⁴)

Moment of inertia maximum, (MIN) I _min =44.05 ( mm⁴)

SECTION MODULUS (S)= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 17.45mm

FOR MAXIMUM MOI:

S= 2785.05 (mm⁴) / 17.45 mm

S _max = 159.601 mm³

SECTION MODULUS CALCULATION FOR BOW RR 2:

Moment of inertia maximum, (MAX) I _max =2789.23 ( mm⁴)

Moment of inertia maximum, (MIN) I _min =44.6 ( mm⁴)

SECTION MODULUS (S)= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 17.45 mm

FOR MAXIMUM MOI :

S= 2789.23 ( mm⁴) / 17.45mm

S _max = 159.847mm³

SECTION MODULUS CALCULATION FOR CENTER ROOF RAIL:

Moment of inertia maximum, (MAX) I max =172556.6 (mm⁴)

Moment of inertia maximum, (MIN) I min =5136.09 (mm⁴)

SECTION MODULUS (S)= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 45.25mm

FOR MAXIMUM MOI:

S= 172556.6 (mm⁴) / 45.25mm

S max = 3813.40mm³

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 analysed element where the deviation from the draft direction at any point, corresponds to specified values.

Minimum draft angle of 5 degree is considered for analysis. green colour infers on the parts that all face along the tooling direction has positive draft angle greater than 5 degree and passed in analysis

ROOF OUTER PANAL

FRONT ROOF RAIL

REAR ROOF RAIL

CENTRE ROOF RAIL

BOW ROOF RAIL 1

BOW ROOF RAIL 2

DIFFERENT ANGLES OF VIEWS

TOP VIEW

FRONT VIEW

BOTTOM VIEW

ISOMETRIC VIEW

Conclusion

Thus, the required reinforcements and Ditch Area for the Roof were designed and developed from the given styling. The curvature study was done after performing calculations for the Heat Distortion and Snow load conditions with the position criterion met for all reinforcements.

Further the Section Modulus study was conducted and Draft Analysis were done for each reinforcement successfully.