Roof Design

Aim:

To a given roof stylling, create essential flanges and reinforcement and calculate the Heat distorsion calculation and snow load predition of the Roof.

Objective:

1. To design the automotive roof based on the given styling surface and to analyze the prepared model to check the manufacturability and the feasibility of the design.

2. To perform the section modulus study for the master sections and create a front roof rail, bow roof rail and center roof rail.

3. To create the ditch area and to perform the calculation of Heat distortion area and snow load prediction of the roof by the given formulas and study for draft analysis for min draft angle of 7 degrees.

Introduction to Roof:

An automotive roof or car roof top is the portion of the automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, dust and other externel elements.

under the styling surfaces that is seen from the outside, there are many other structures under the roof that contributes to its strength and stability. The number of such reinforcement structures supports differ with respect to the size of the vehicle they are primarily installed on the flatter areas of the roof to enhance the strength.

They are the Front Roof Rail, Rear Roof Rail, Center Roof Rail and Bow Roof design which are given Mastics and spot welded to the roof flanges & ditch are and improve NVH value by increasing the strength of the roof of using mastics sealents.

Types of Car Roofs:

1. Convertible Roofs.

2. Hard Roofs.

3. Sun Roofs.

4. T- Tops.

5. Target tops.

6. Vinyl Roofs.

Design consideration of roof:

1. Visibility Criteria

2. Head clearance

3. Curvature study

4. Heat distortion and snow load criteria

5. Draft analysis

6. Safety parameters

Roof crush test for passenger car:

Safety point of view vehicle roof pays a very crutial role especially roll over kind of accidents, in which vehicle tips over onto its side of the roof vehicles roll over crashes are the cause of many fatalities of head, neck and spine trauma around the world.

Strength of the roof structure to occupant protection in real world roll over crashes, requiring the maximum moving distance of the roof structures should be less than 127mm where the indused load is 1.5 times the vehicle weight.

For the test the passenger car shall be rigidly placed or positioned on a horizontal surface with the doors locked and the window closed. Further more , a flat, riged block with a lower surface a rectangle measuring 762mm wide & 1829mm shall not move more than 127 mm measured 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 unload vehicle weight or 5000 lbs, whichever is less. To either side or forward edge of the vehicle.

This test can be conducted on either side of the roof structure, the front left or the front right side but not on the both sides in one single test and still meet the requirement of the test. The regid plate shall not move faster than 0.5 inches/sec and the force applied on the plate shall not exceed the lesser of 1.5 times the unloaded vehicle weight expressed in kilograms multiplied by 9.8 or 5000lbs

Also, the direction of the forceshall be downwards and perpendicular to the lower surface of the rigid plate and such that it moves in a straight line without rotation. The duration of the test shall not exceed 120 sec. The longitudinal axis of the plate is pitched forward at a 5' angle as viwed from the side of the passenger car as shown.

The lateral axis or the roll of the plate is such that it forms a 25' outboard angle with a horizontal surface as viewed from the front of the passenger car as shown in below, fig

section and its parameters:

Joining process:

Mastics are introduced between the roof outer panel and the reinforcements for joining. Where ever the mastics introduced there is remarkable strength. Spotwelding is also introduced to join the reinforcement and the outer panel.

Reinforcement and its types:

Ifthe roof is flat, it is weak and cannot able to with stand the external load like snow load, roll over and etc. So in order to increase the strength of the roof, reinforcement like front roof rail, central roof rail, rear roof rail and the bow roof are introduced and joined in the flat surface of the roof with different joining process. In general there are four types of reinforcement are used in the hard top they are

• Front roof rail
• Central Roof rail
• Rear roof rail
• Bow roof rail

Roof outer panel:

Front Roof Rail:

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

Rear Roof Rail:

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

Central Roof Rail:

Central roof rail helps in providing effectively supportto the flat area of the roof as it more susceptible to failure under the action of load. Generally Central roof rail is placed at the center of the roof which is connected to the b-piller support structure. This central roof rail helps in strength to the roof during the roll over test.

Bow Roof:

The Bow Roof is given to improve the torsional stiffness and load bearing capacity of the roof structure. The number of roof presented is depends on the overall size of the roof. Presently two roofs are calculated.

Different views of complete Roof Design:

Design calculation of the 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 as follows:

• Heated Distortion study on the roof
• Snow load condition

Heat Distortion study on the roof:

This study plays a major role in sheet metal usage. Heat distortion temperature is the temperature limit above which the material cannot be used for the structural applications. This study is used to predict the heat distortions temprature of the material where it starts to soften when exposed to a fixed load at elevated temperature. In order to avoid bending and 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 study:

W=[1.73x10^(-3)xL]+[1.85x10^(-8)x(R^2)/t]+[1.10x10^(-3)x1]-2.68

Where

L = Roof strength 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 is OK when its value is<2.7

Here we have to find out Rx and Ry values:

• Create a sketch on top roof and draw a horizontal line at a center.
• Then offset this line below 300mm and caled it as -3y line.
• Fron -3y line offset 100mm above and below and convert all these lines into referance lines.
• Draw the vertical lines at the middle of each reinforcements and offset it at 100mm both sides.
• intersection point is created where the lines are met.
• Project these points on the outer roof panel
• 

Table of Heat Distortion calculation Result

From the above table it can be concluded that all the values of 'W' is <2.7. So, thus infers that current position of bow row ae good in state as per design and found ok.

Snow load condition:

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

Qr = [ly x t^2]/[a x s x [(Rx + Ry)/2]^2 x 10^-8]

where

a=My x Lx^2 x 10^-12,

My = Y(Ly-Y)

judgement condition = Qr is >/=3.1

T=Roof plate thickness(mm)

Ly=Distance between the front and rear roof rails on the vehical along with oY [mm]

Length of roof panel with the center point between front roof rail/rear roof rail as the referance point of the front and the rear.

Lx = Distance between the left and right end of the roof on the bow roof [mm]

width of the roof panel exposed on the surface.

Y = distance of frnt roof rail to bow roof[mm]

s = Distance for which roof Bow bears divided by load[mm]

s = L1/2 + L2/2

ly = Geometrical moment of inertia of bow roof(Y cros section)[mm^4]

Rx = lateral direction curvature radious of roof panel Y cross section on Bow roof[mm]

Ry = Langitudinal direction curvature radious of the roof panel x cross section on roof bow[mm]

Table of Snow Roof prediction calculation Result

Hence we arrived to decision after result from calculation, one of the Qr result is not acceptable. so, this model will pass over to the styling team to change the curvature to improve the strength.

Section Modulus:

Section Modulus is a geometric property for a given cross section used in the design of beams or flaxtural 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 stifness. Any relationship between these properties is highly dependent on the shape in question. Equation for the section moduli of common shapes are given below.

s = I/Y

s = section modulus

I = second moment of inertia

Y = distance from the neuteral axis to any given fiber. It is often reported using Y=C, where C is distance from the neutral axis to the most extreme fibre.

section Modulus calculation for Bow roof-1:

Moment of inertia

Imin = 6.7244 x 10^3 (mm^3)

Section Modulus (S) =I/Y

Y = L/2 = 32.91mm

S = 0.1021 x 10^3 (mm^3)

section Modulus calculation for center roof rail:

Moment of inertia

Imin = 9.1202 x 10^3 (mm^3)

Section Modulus (S) =I/Y

Y = L/2 = 41.65mm

S = 0.2189 x 10^3 (mm^3)

section Modulus calculation for Bow roof-2:

Moment of inertia

Imin = 7.1484 x 10^3 (mm^3)

Section Modulus (S) =I/Y

Y = L/2 = 32.91mm

S = 0.2171 x 10^3 (mm^3)

Drft Analysis:

Draft analysis is a command enables you to detect if the part you drafted will be easily removed. this type of anlalysis is performed based on colour ranges identifying zones on the analyzed elements where the viation from the draft direction at any point, corresponds to specified values.

minimum draft angle of 7 degrees 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.

Conclusion:

Thus the required reinforcement and the ditch area for the roof were designed and developed from the given styling. The curvature study was done after performing calculation for the Heat distortion and snow load conditions with the position criteria met for all reinforcements

Further the section modulus study was conducted and Draft analysis is done for each reinforcement successfully.