Roof challenge

ANSWER :

A dumb solid body of roof CAD model is provided for which following points to be designed and analysed are given below

• Extract the styling surface and for the given car roof styling and design the integral flanges and reinforcement, namely Ditch Area, Front Roof Rail, Rear Roof Rail, Centre Roof Rail, and Bow-Roofs.
• Do a curvature study on the roof and perform calculations for Heat Distortion and Snow load criterion to determine and justify the position of the Bow-roofs.
• Finding a moment of inertia and Section Modulus for Bow Roof Rails and Centre Roof Rail.
• Perform a Draft Analysis for each reinforcement.

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 that contribute to its strength and stability. The number of such reinforcement structures/supports differs with respect to the size of the vehicle. They are primarily installed on flatter areas of the roof to enhance strength.

These are the Front Roof Rail, Rear Roof Rail, Centre Roof Rail, and Bow Roof Design which are given Mastics and spot-welded to roof flanges & ditch areas and improve NVH value by increasing the strength of the roof using mastic sealants.

Design consideration of roof :

• Visibility Criteria
• Head clearance
• Curvature study
• Heat 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 in a rollover 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.

Input surface :

Styling Surface to Extracted Surface :

The styling team has provided the solid dump outer panel from that we are extracting the surface to model the necessary reinforcements like Ditch area, Front Roof Rail, Bow Roof Rails, Centre Roof and Rear Doof Rail.

Design a Ditch Area for Outer Roof Panel :

The 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, headroom clearance and other important parameter dimensions like fitment of the 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 the front and rear sections.

Front Ditch Area :

From dump solid part class A surface is extracted and ditch areas are developed considering the parameter specified in the following master section.

The flange of around 15mm was created with the law extension command, corner relief at rear flanges and a positive draft of more than 3 degrees considering for the drawing operation process. The overall thickness of sheet metal considers 0.75mm throughout.

Rear Ditch Area :

The surface is offset from the outer panel to form the ditch area and with the law extension command, corner relief at rear flanges and positive draft in required degree considering for the drawing operation process. The overall thickness of sheet metal for the outer panel consider 0.75mm throughout.

Completed roof pannel :

Front Roof Rail :

The front rail underpins the rooftop external shell at the front end. It is mounted with support rooftop rail fortification on the two closures. The rails are furnished with embosses to improve the strength over the length and the rail is associated with the external board utilizing mastic sealants on one side and the other joinery is spot welded.

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 0.65 mm. Similar to the Front Roof Rail design, we have to take into consideration the rear Headroom Clearance and the rear visibility criteria while designing the Rear Roof Rail.

Bow Roof 1 :

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. The longer the roof more the bow roofs are integrated into the 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 a curvature study. The heat distortion criteria and snow load criteria are checked to analyse the position of the bows in various conditions.

The thickness of the Bow Roof is 0.75mm and also mastic area has been created.

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.

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 0.75 mm thickness.

complete roof :

Curvature Study on the Roof:

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.

Bow – roof prediction Formula

Heat Distortion Criteria :

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

Judgement Condition : OK< 2.7> 3.5

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]

Thickness for the Roof :

Front Roof Rail, t = 0.75 mm

Roof Outer Panel, t = 0.75 mm

Bow Roof Rail, t = 0.75 mm

Centre Roof Rail, t = 0.75 mm

Bow Roof Rail 2, t = 0.75 mm

Rear Roof Rail, t = 0.75 mm

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.

Snow load Prediction formula

Qr = [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 10-8]

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

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]

Roof panel X curvature radius of length s in Side view

Rx & Ry (X & Y Curvature):

• 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.
• Project the intersection points in the sketch to roof surface
• Join the projected curves using arc curve and measure the minimum radius and linear dimension.
• 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.
• Put all the linear and radial dimension in following formula to calculate Qr Value.