Roof challenge

AIM

To design & develop the Roof of a car with its supporting components by following the master section and design parameters.

INTRODUCTION

An automobile roof is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. Under the styling surface that is seen from the outside, there are many other structures under the roof which contribute 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 flatter areas of the roof to enhance the strength. Front Roof Rail, Rear Roof Rail, Centre Roof Rail and Bow Roof Design which are spotwelded to roof flanges & ditch area and improve NVH value by increasing strength of roof using mastic sealants.

Design consideration of roof:

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

ROOF CRUSH TEST FOR PASSENGER CAR

Safety point of view vehicle roof plays a very crucial role especially roll over kind of accidents, in which vehicle tips over onto its side of the roof. And vehicle rollover crashes are the cause of many fatalities and head, neck, and spine trauma around the world. Therefore, passenger safety is also a very important parameter in front of OEMs. It is not only a marketing strategy but also it is an obligation stipulated by international standards that are now placed in several countries, as well as a governmental requirement. For this reason, strength is a very important criterion for designing a vehicle roof.

The strength of the roof structure to occupant protection in real-world rollover crashes, requiring the maximum moving distance of the roof structure should be less than 127 mm (5 in.) when the induced 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. Furthermore, a flat, rigid block with a lower surface a rectangle measuring 30 inches wide by 72 inches long (762mm X 1829mm) shall not move more than 5 inches(127mm), 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 unloaded vehicle weight (UVW) or 5000 lbs(22224N), 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 both sides in one single test and still meet the requirements of the test. The rigid plate shall not move faster than 0.5 inches/second and the force applied on the plate shall not exceed the lesser of 1.5 times the unloaded vehicle weight (UVW) expressed in kilograms multiplied by 9.8 or 5000lbs (22240N).

Also, the direction of the force shall be downward 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 seconds. The longitudinal axis of the plate is pitched forward at a 5 º angle as viewed from the side of the passenger

DESIGN METHODOLOGY

Styling 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 important parameter dimension like fitment of windshield, sealant rubber etc. All these data are well calculated in the planning stage to design the peripheral component accordingly. Thus, here designing is done just to replicate those input data considering engineering parameters which should be feasible for manufacturing.

Master section of Front Rail

Master section of Rear Rail

DESIGN OF MAIN ROOF WITH FLANGES

Extract the surface from the input data and falnges are created. Flange of around 17mm created with the law extension command, corner relief at rear flanges and positive draft more than 7 degree considering for the drawing operation process.

DESIGN OF FRONT ROOF RAIL

For designing front roof rail we need to consider some parameter which are as discussed below:

1.Visibility of the driver and passenger

We always keep in mind while designing the front roof we do not reduce the visibility available to driver. While designing front roof one must stick to the master section dimensions because little change in dimension will have impact on the visibility of the driver (e.g., small change in angular dimension will lead to large change in linear dimension)

2.Head room

There should be enough space between roof rail and head position of driver so when accident will happen head will not hit to the front roof rail.

DESIGN OF REAR ROOF RAIL 

It is the support which is joined to the Back Door and the Body Side Outer and the inner panels. It is also usually made from Body Side Outer scrap and therefore the thickness of the rail is 0.75 mm.

DESIGN OF BOW ROOF 1 AND BOW ROOF 2

The Bow roofs are given to improve the torsional stiffness and load-bearing capacity of the roof structure. The number of bow roofs provided will depend upon 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 80 mm circular effective area. The positioning of bow roofs is done based on curvature study. The heat distortion criteria and snow load criteria are checked to analyse the position of the bows in various conditions. Special notches on the flanges are provided for the application of mastic sealant. Either end flanges of the bow roof are flat offset to the roof flanges in order to join together by spot welding. Both bow roof 1 and bow roof 2 are similar in dimension and pattern only curvature profile is different at respective positions.

DESIGN OF CENTRE ROOF RAIL

Centre roof rail is provided to effectively support the flat area of the roof as it is more susceptible to failure under the action of the load. Usually, the centre reinforcement roof rail will be placed at the centre of the roof connecting the B pillar support structure which results in added support in the roll-over test. The thickness of the centre bow roof is 1.25mm thickness.

ASSEMBLY MODEL VIEW

Heat Distortion Curvature Study

The heat distortion study plays an important role in sheet metal usage. Heat distortion 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 avoid 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.

Curvature study on the roof:

Bow – roof prediction Formula is given as

W = 1.73x10^-3 x L+1.85x10^-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 (minimum radius in mm)

Ry = Y curvature (minimum radius in mm)

t = Roof plate thickness (mm)

l = Bow Roof Span (mm)

Judgement condition: ok < 2.7mm < NG

Steps followed:

1. From 0-Y axis, at -3Y about distance 300mm is drawn parallel on the perpendicular plane as shown in fig. after that 100 mm lines are offset to either side of -3Y line. Now centre lines are drawn between each adjacent roof rail edges respectively. Intersection points are marked on the sketch and exit the sketch. Make all geometry in construction lines except the points.
2. Project the intersection points in the sketch to roof surface as shown in fig.
3. Join the projected curves using arc curve and measure the minimum radius and linear dimension as follow  & put into  the formula to calculate “W” .

WORKOUT CALCULATIONS

From the above table it can be concluded that all values of  W< 2.7 so thus infers that current positioning  of Bow roof are optimum as per design and found OK.

Snow Load criteria

Snow load criteria study is done to check the durability of the roof to withstand snowfall and reflex back to its original shape without any permanent deformation once the load is removed.

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

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 centre 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]

Roof panel X curvature radius of length s in Side view.

Steps followed:

1. At 0-Y axis on perpendicular plane to roof as shown in fig. draw 100 mm lines offset to either side of 0Y axis line. Now centre lines are drawn between each adjacent roof rail edges respectively. Intersection points are marked on the sketch and exit the sketch. Make all geometry in construction lines except the points.
2. Project the intersection points in the sketch to roof surface as shown in fig.
3. Join the projected curves using arc curve and measure the minimum radius and linear dimension.
4. Project the intersection plane bow roof and centre roof rail on the horizontal plane to find the moment of inertia of the respective section to find Iy values.
5. Put all the linear and radial dimension in following formula to calculate Qr Value.

MOMENT OF INERTIA DETAILS

WORKOUT CALCULATIONS

It is observed from the values that obtained values of QR> 3.1, thus it infers that the designed roof rails and bow roof are safe for snow load condition and found ok.

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 at any point, corresponds to specified values.

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

Main Roof Draft analysis

Front rail Roof Draft analysis

Rear rail Roof Draft analysis

Bow Roofs Draft analysis

Center Roof Reinforcement Draft analysis

CONCLUSIONS 

Thus, the Design of Roof, Front roof rail, Rear Roof Rail, Bow Roofs, and reinforcement roof centre is developed by following the Master section is done.

• All reinforcement BIW parts of roof are designed according to the input data provided by the styling team. Designed parts will be submitted to CAE team to carry out further FEA analysis to obtain numerical data to improve the design.
• As per heat distortion criteria all bow roof positions are OK
• As per Snow load criteria there should be some improvement required. So, in order to overcome this challenge Embosses are created at top in order to provide more rigidity, this will improve in holding the load. The main thing to do is to give it to styling team where the snow load is failing and decrease the curvature so that Bow roof position will not create problem