Roof Design

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Roof Design

AIM

For the Given Roof styling, create an ASSEMBLY by developing the essential flanges for the outer roof and creating the reinforcements.

Provide appropriate dimensions and check for Draft analysis.

Do a curvature study on the roof and perform calculations to determine the position of the Bow-roofs.

Suggest and implement remedies in the design if the criterion is not full-filling

Start with the creation of a roof ditch area with a tool opening angle of 7 degrees.

Develop the reinforcements as separate parts

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]

Judgment Condition

OK< 2.7

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)

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 front and rear reference point.

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]

The width of the roof panel is 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

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

Here are some Reference images for the Master sections - These images are just to give you a brief idea about the design.

Front Roof Rail Master section

Master section of the Rear Roof Rail

Master section of Bow Roof

Master section of Center Reinforcement Rail

ANSWER

WHAT IS ROOF IN AUTOMOBILES?

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 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

RESULT

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.

Now lets see what are all the parts are there in roof:--

• OUTER ROOF
• FRONT ROOF RAIL
• BOW ROOF 1
• CENTER ROOF
• BOW ROOF 2
• REAR ROOF RAIL

• OUTER 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.

• 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

Front Roof Rail Master section

• BOW ROOF 1

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.

• CENTER ROOF

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 center 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 rool- over test.

REAR ROOF RAIL

Rear roof rail is the one which joins the back door and the body side outer

Roof With roof Rails

Now Lets see what is the procedure to Design the Roof :--

Open the given model in NX that is the styling surface of the car's roof.

In this challenge, we will be creating the Roof's outer panel, Front roof rail, Rear roof rail, Centre reinforcement and Bow roofs.

Roof Outer Panel:

• First, we will be extracting the top half portion of the roof surface. Since the roof surface is symmetrical in shape, we can easily work only on half of the portion and reflect it at the end to create the whole roof.

• Untrim this surface in order to avoid the holes. Extract the outer boundaries of the original half surface and project it on the untrim surface to get the exact outer edges of the roof.

• The corners of the roof surface are sharp so use the bridge curve and trim the sheet and trim the sharp edge

• Now we will be creating the flanges by using the law extension command

• From the edge of the law extension create an another law extension
• Similarly, by following the same procedure we can create a flange on the other edges as well.

• Using the Through Curve Mesh Command to create a surface on the corners by giving guide curves and support of the surfaces.
• Sew these Surfaces together and give the required fillets.

• Now Follow The Master section given for Rear roof rail and create the Additional area
• For the Rear end, there should be an additional area that helps to hold the back door without hitting the roof.

Front Roof Rail:

• First Create the Part file for the Front roof rail in assembly file.
• Following the master section which is given to us as input, we will be creating the Front Roof Rail.

Front Roof Rail Master section

• Use the Command Wave Geometry linker and select the extracted roof and the dich area part
• Now Offset the surface to 32mm and dich area to 1.5mm(For Spot Welding)
• Use Law extension command for dich area to create the lower part of the front roof area
• Extend and attach the offsetted sheet till it touches the law extended sheet
• Trim and extend the sheet and lawextended sheet
• Then use Offset curve in face command and select the lower edge and set it to 80mm and trim the unwanted sheet
• Use law extension to bulid the lower side for the other part at an angle of 25deg
• Now offset the surface for 5+0.75+0.75mm for the mastic points for the first curve the trim and extend
• Then use offset curve in face and set it for 10mm
• Then offset the surface again for 3+0.75+0.75 for the mastic points for the second curve and use trim and extend
• Then use offset curve in face and set it for 5mm
• Now Fillet the edges to 3.75+t thickness
• Now Draw some Sketches and create embosses
• Then Extend the sheet and trim the sheet so that there is no gap after mirroring
• Now Mirror the sheet and sew it
• Thciken it for 0.75mm

Rear Roof Rail:

• First Create the Part file for the Rear roof rail in assembly file.
• Following the master section which is given to us as input, we will be creating the Front Roof Rail.

Master section of the Rear Roof Rail

• Similar operations which were performed in the Front Roof Rail needs to be followed to create the Rear Roof Rail.

Center Roof Rail:

• First Create the Part file for the Front roof rail.
• Following the master section which is given to us as input, we will be creating the Center Roof Rail.

Master section of Center Reinforcement Rail

A center reinforcement is created to provide additional strength to the area where the roof surface is flat.

• Use the Command Wave Geometry linker and select the extracted roof and the dich area part
• Use a datum plane on the curve at 50%
• Use Intersection curve command then select the face and the dataum plane the intersection line will be created
• Then offset curve in face for 20,40,55,75 and 95

• Trim sheet by selecting the curves

• Use law extenstion command and join the sheets at 7deg

• Blend the edges with edge blend command
• Now offset the dich are to 1.5mm in downward direction
• Now use bridge command and sketch a curve which can be use as a guide

• Now use Through Curve mesh command to join these surface

• Now create an Sketch for mastic points
• Now Draw some Sketches and create embosses
• Then Extend the sheet and trim the sheet so that there is no gap after mirroring
• Now Mirror the sheet and sew it
• Thciken it for 0.75mm
• And the over all length should be 80 to 90mm.

Bow Roof Rail 1 and 2:--

• First Create the Part file for the Front Bow roof rail.
• Following the master section which is given to us as input, we will be creating the Bow Roof Rail.

Master section of Bow Roof

• Similar operations which were performed in the Center Roof Rail needs to be followed to create the Bow Roof 1 and 2.

Roof has been desinged by considering all the parameters and considerations:

Bow–Roof Prediction Formula (Heat Distortion 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]

Judgment Condition

OK< 2.7

Calculating the Heat Distortion criteria:

• To calculate the Heat distortion criteria, we have to take a note of few values and note it down.
• To note down the values, first, we need to create a sketch.
• Create a line at the center of the roof part in the 0-Y axis and then draw another line with a distance of 300mm to the O-Y line.
• Now create lines at a distance of 100mm on both sides to the 300mm line.
• Create a new sketch in which we need to Project the Roof Rails curves lines between the bow roofs and rails coming at the Edge. These lines need to be created in X-direction.
• Using these Edge line create lines at a distance of 100mm on both sides.
• Now To find Rx and Ry we have to take the intersection points and meassure them
• so project the points on the surface and use Arc/Circle Command to meassure the curvature
• Now since we got all the values for calculating the Heat Distortion, substitute these values to the below equation.
• W=[1.73×10-3×L]+[1.85×10-8 ×R2t]+[1.10×10-3×l]-2.68">W=[1.73×10−3×L]+[1.85×10−8 ×R2t]+[1.10×10−3×l]−2.68

And My Designed Roof Have Passed Heat distortion criteriaW≤2.7">W≤2.7

Calculating the Snow Load Criteria:

• To calculate the Snow Load Criteria, we have to take a note of few values and note it down.
• To note down the values, first, we need to create a sketch.
• Create a line at the center of the roof part in the 0-Y axis.
• Now using this O-Y line as a center create lines with a distance of 100 on both sides.
• Create a new sketch in which we need to create lines between the bow roofs and rails coming at the center. These lines need to be created in X-direction.
• Using these center line create lines at a distance of 100mm on both sides.
• Now intersect sketch 1 (lines in Y-direction) and sketch 2(lines in X-direction). This result will give us some points where the lines (X & Y)are coinciding with each other.
• Extract the required points and project them on the roof surface. Now using these points create a 3 point arc or circle and note the radius of the arc in the X direction as Rx">Rx and in the Y direction as Ry">Ry.
• Now since we got all the values for calculating the Snow Load Criteria, substitute these values to the below equation.
• Qr=Iy×t2α×s×[Rx+Ry2]2×10-8">Qr=Iy×t2α×s×[Rx+Ry2]2×10−8

Calculating the Section Modulus:

To calculate the section modulus, we need to take the intersection of all the components individually on the 0-Y section.

Bow roof 1:

Center roof

Bow Roof 2

Note: For the Bow Roof 2 - Rear Roof Rail there will be no snow load criteria since the L2 is not available in that region.

Judgement Condition: ROOF RAILS PASS THE CONDITION.

Since the Last Roof have not passed the snow load criteria we can increase the strength of the sheet by creating the embosses on the outer roof and it also increases the drag forces too

Draft Analysis for each part:

Outer Roof Panel

Front Roof Rail:

Bow Roof 1

Center Roof Rail

Bow Roof 2

Rear Roof Rail:

RESULTS:

The Roof of a car has been successfully developed with all the required components and a draft analysis have also been performed for the same.

Final Roof Model:

CONCLUSION:

• The results obtained after doing a curvature study on the roof was adequate.
• The position of the bow roofs can be arranged better (over the flat areas) which helps in increasing the strength of the roof.
• Embossments are added to improve the strength & Aerodynamics of the roof but can be avoided if not necessary as it increases the cost of manufacturing.