Roof Design using Siemens NX.

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

Objective:

• For the given Car Roof styling, design and develop the essential flanges and reinforcements, namely the Ditch Area, Front Roof Rail, Rear Roof Rail, Centre Roof Rail, and Bow-Roofs with the given master section.
• 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.
• Do a Section inertia analysis for the Master sections created for the Front Roof Rail, Rear Roof Rail, Bow Roof Rails, and Centre Roof Rail.
• Perform a Draft Analysis of each reinforcement.

Theory:

1. 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, many other structures under the roof contribute to its strength and stability. The number of such reinforcement structures/supports differs concerning 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.

Photo of a Roof.

2. Types of Automobile Roofs:

• Convertible: A convertible, cabriolet, or spyder is a passenger car that can be driven with or without a roof in place.
• Hardtop: A hardtop is a rigid form of automobile roof, which for modern cars is typically constructed from metal. A hardtop roof can be either fixed, detachable for separate storage, or retractable within the vehicle itself.
• Sunroof: An Automotive sunroof is a movable panel that is operable to uncover a window in an automobile roof.which allows light and fresh air into the passenger compartment.
• T-top: A T-top is an automobile roof with a removable panel on each side of a rigid bar running from the center of one structural bar between pillars to the center of the next structural bar. The panels of a traditional T-top are usually made of auto-grade safety glass.
• Targa Top: Itis a semi-convertible car body style with a removable roof section and a full-width roll bar behind the seats.
• Vinyl roof: Vinyl roof refers to a vinyl conversion for an automobile’s top. This conversation was originally designed to give the appearance of a convertible to models with a fixed roof but eventually, it evolved into a styling statement in its own right.

3. Design consideration of Roof:

• Visibility Criteria.
• Head Clearance.
• Curvature Study.
• Heat Distortion and Snow Load Criteria.
• Draft Analysis.
• Safety Parameters.

4. Roof Crush Test:

Safety point of view vehicle roof plays a very crucial role, especially in rollover kind of accidents, in which the 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.

Roof Crush Testing Device. Roof Crush Testin of BMW Car.

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 for 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 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 car as shown in the below figure:

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

Loading on the Roof at 5°. Loading on the Roof at 25°.

5. Car Rollover Test:

The rollover test is conducted because a rollover is a type of vehicle crash in which a vehicle tips over onto its side or roof. Rollovers have a higher fatality rate than other types of vehicle collisions.

Vehicle rollovers are divided into two categories: tripped and untripped. Tripped rollovers are caused by forces from an external object, such as a curb or a collision with another vehicle. Untripped crashes are the result of steering input, speed, and friction with the ground.

Vehicles which have a higher center of gravity like SUVs or Small Trucks are more prone to rollover. Rollover accidents typically involve a singular vehicle that loses control due to icy or snowy road conditions, soft road shoulders, uneven ground, collision with curbs or guardrails after a vehicle has lost traction, or hitting an obstacle while driving.

During a rollover accident, the regional body distribution of injuries normally affects five main areas: head and neck, thorax, abdomen, upper limbs, and lower limbs. The majority of rollover crash victims sustain multiple injuries throughout these regions with the head and neck being the most vulnerable area. Some of the most common rollover injuries include traumatic brain injury, spinal cord injury, broken bones, cuts, scrapes, and bruises. These injuries are caused by slamming into another vehicle or obstacle, impact with unsecured cargo, contact with broken glass or torn metal, or being tossed within the vehicle or ejected from the vehicle.

Images of Car Roller Test carried out by Volvo.

6. Manufacturing Techniques:

By outer board machining process, bar-shaped assembly, ring-shaped component processing, and roof cant rail processing technology formed respectively.

Outside plate process: the first step, open the straight snips plate: first select the applicable cold coiling specification, and cold coiling is opened flat; Then according to the production material specification, will open cold coiling after the flat and is cut into the iron plate of the corresponding size; Finally the iron plate after shear is cleaned to iron filings; The first step, oil pressure, punch press: the first iron plate is extended; Secondly to its trimming and punching; Then to its flanging and punching; Finally, after punching completes, it is carried out to the operation of side folding; Completed the making of the outside plate.

Bar-shaped assembly and ring-shaped component process: the first step, blanking, stamps out an iron plate with the outline blank of many roof cant rails, a ring-shaped component, and a bar-shaped assembly; the Second step, carries out annular punching to ring-shaped component; Bar shaped assembly and roof cant rail are carried out to bar-shaped punching.

Car roof anabolic process: the first step, is by a plurality of M6 nuts of a side spot welding of the ring-shaped component; By two M6 nuts of the bar-shaped assembly two ends spot welding; the Second step, spots weld bar shaped assembly the opposite side of the corresponding nut of the ring-shaped component; And circular groups and the punching of bar-shaped assembly are aimed at; The 3rd step, is welded to ring-shaped component the inner side of the outside plate; The 4th step, spots weld roof cant rail the inner side of the outside plate; Finally car roof integral body is carried out to sanding thorn and ending.

Procedure:

Procedure map of Designing Roof.

1. 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 Roof Rail.

Dump part from the Styling team. Extracted Surface from Dump Body.

2. 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 that should be feasible for manufacturing. Master section of the front and rear section. The Ditch Area is the extended portion of the main Roof concealed by other BiW panels. The rail supports of the roof are attached to the Ditch Area using Spot welds since it is not a styling surface and remains concealed. Headroom Clearance, visibility criteria, and clearance for Windshield and Back door are some of the factors to be considered while designing the Ditch Area. It is given a thickness of 0.75 mm.

• Front Ditch Area:

From the dump solid part class, A surface is extracted and the ditch area is 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 the drawing operation process. The overall thickness of sheet metal considers 0.75mm throughout.

Front Ditch Area Master Section. Front Ditch Area of Outer Roof Panel.

• 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 the required degree considering for the drawing operation process. The overall thickness of sheet metal for the outer panel consider 0.75mm throughout.

Rear Ditch Area Master Section. Rear Ditch Area of Outer Roof Panel

Completed Outer Roof Panel with Front, Rear, and Side Ditch Area.

3. Design of 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. It is the support that is joined with the Wind Shield Glass, Body Side Outer, and inner panels. It is usually made from Body Side Outer scrap and therefore the thickness of the rail is 0.75 mm. A draft angle of 3-5 degrees is provided keeping in mind the deep-drawing operation. The Wind Shield end flange is the exact offset of the roof surface. The bottommost flange is decided after taking into consideration the visibility criteria and the Headroom Clearance of the passenger. Spot Welds are applied at the Windshield end of the rail and the other end is joined with the roof using Mastic Sealants which also contributes to increased strength.

Front Roof Rail Master Section. Front Roof Rail Reinforcement. Front Roof Rail.

4. Design of Rear Roof Rail:

It is the support that is joined to the Back Door and the Body Side Outer and 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. Inputs are taken from the rear master section. It is the support that is joined to the Back Door, 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. 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. Inputs from the backdoor design are also taken to accommodate the required clearance. Spot Welds are applied at the Back door end of the rail and the other end is joined with the roof using Mastic Sealants.

Rear Roof Rail Master Section. Rear Roof Rail Reinforcement. Rear Roof Rail.

5. Design of Bow Roof Rail:

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 analyze the position of the bows in various conditions. The thickness of the Bow Roof is 0.75 mm and also mastic area has been created. Two Bow roofs are required for this project so we will have to make 2 bow roofs with the same master section properties.

Bow Roof Rail Master Section. Bow Roof Rail Reinforcement. Bow Roof Rail.

6. Design of Center 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 center reinforcement roof rail will be placed at the center of the roof connecting the B pillar support structure which results in added support in the roll-over test. The thickness of the center bow roof is 0.75 mm thickness.

Center Roof Rail Master Section. Center Roof Rail Reinforcement. Center Roof Rail.

7. Joining Process of the Reinforcements:

• Joining of the Outer Roof Panel and the Front Roof Rail is done by spot welding. The Front Ditch area and the front roof rail are spots welded as it is an easy and efficient process and the welding will be hidden under the windshield area so that it doesn't affect the aesthetic of the car. The same process is done for the joining of the Outer Roof Panel and the Rear Roof Rail, The Rear Ditch area and the Rear Roof Rail are spot welded and they are hidden by the installation of the back door.

Joining of Outer Roof Panel and Front Roof Rail. Joining of Outer Roof Panel and Rear Roof Rail.

• Joining of the middle part of the Outer Roof Rail, Bow Roofs, and Center Roof Rails are done by the use of Mastics Sealants. Here we cant use Spot welding because it damages the A-Class area of the Roof. So, mastic sealant is used in this area. For the application of these Mastics, there must be a gap of at least 5mm between the Roof Outer Panel, Bow Roofs, and Center Roof Rail. The ends of the Bow Roofs and Center Roof Rails are joined by spot welding to the Side Ditch area of the Outer Roof Pan

Clearance of 5mm between the Outer Roof Panel and Bow/Center Roof Rail. Spot welding of the Bow/Center Roof Rail with the Side Ditch Area.

8. Analysis of the Roof:

• Heat Distortion Criteria: 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 an elevated temperature. To avoid bending or damage on the roof, based on heat distortion temperature, this study will predict the bow roof position on the roof to strengthen the roof.

> Roof Length for styling Surface (L) :

For the L we want to take the total of the styling surface and don't include the ditch area as shown below:

Roof Length for styling Surface (L).

> Bow Roof Span (l):

Here want to take the distance from Front Roof Rail to Bow Roof 1 to find the Bow Roof Span as shown in the figure:

Similarly, we will measure the distance of the Bow Roof Span (l) between:

Bow Roof 1 to Center Roof Rail.

Center Roof Rail to Bow Roof 2.

Bow Roof 2 to Rear Roof Rail.

Bow Roof Span (l): Distance from Front Roof Rail to Bow Roof 1.

> Rx & Ry (X & Y Curvature):

• From the 0-Y axis, at -3Y about a distance of 300mm is drawn parallel on the perpendicular plane as shown in the figure. after that 100 mm lines are offset to either side of the -3Y line. Now center lines are drawn between each adjacent roof rail edge 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 the roof surface as shown in the figure.
• Join the projected curves using an arc curve and measure the minimum radius and linear dimension as follow & put them into the formula to calculate “W”.

At -3Y Axis the intersection points are created.

From the above figure, now project the points on the top of the Roof and draw the arc with the three points intersecting the area as shown in below figure:

Projected Points and make the arc.

Find the Rx Curvature and Ry Curvature with the tool Simple Radius. For example, as shown in the figure below:

We will do this procedure to get all the values of Rx and Ry.

Rx and Ry Arc Curvature.

> Heat Distortion Criteria Calculations Formulas:

To find the Heat distortion apply all the necessary values from the Roof to the given formula.

> Table and Calculations are Performed in MS Excel:

Table of Calculation in MS Excel.

From the above table, it can be concluded that all values of W< 2.7 and also not W>3.1 so thus infer that the current positioning of the Bow roof is good in the state as per the 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.

> Rx & Ry (X & Y Curvature):

• At the 0-Y axis on a perpendicular plane to the roof as shown in fig. draw 100 mm lines offset to either side of the 0Y axis line. Now center lines are drawn between each adjacent roof rail edge 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 the roof surface as shown in fig.
• Join the projected curves using an arc curve and measure the minimum radius and linear dimension.
• Project the intersection plane bow roof and center 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 dimensions in the following formula to calculate Qr Value.

At 0Y Axis the intersection points are created.

From the above fig, now project the points on the top of the Roof and draw the arc with the three points intersecting area as shown in below fig,

Projected Points and make the arc.

Find the Rx Curvature and Ry Curvature with the tool Simple Radius. For example, shown in fig below:

Rx and Ry Arc Curvature.

To find the Lx and Ly values take the styling surface of the roof

Lx & Ly Values.

> Moment of Inertia for the Roof Rails:

MOI of Bow Roof 1.

MOI of Centre Roof Rail.

MOI of Bow Roof 2.

> Snow Load Prediction Calculation Formulas:

> Table and Calculations are Performed in MS Excel:

Table of Calculation in MS Excel.

It is observed from the values that obtained values of QR> 3.1

From the calculations we found Bow roof 2 has NG, So we have to ask the styling team to modify the surface with curvature in the y-axis (Ry).

9. 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 the design include an area for tension and shear, the radius of gyration for compression, the moment of inertia, and the polar moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape. There are two types of section moduli

• Elastic section modulus
• Plastic section modulus

The Elastic Section Modulus is defined as, S = I/y

Where,

S: is the Section Modulus

I: is the second moment of area(or area moment of inertia, not to be confused with the moment of inertia)

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

> Bow Roof 1:

Create an Intersection Curve with the help of the plane and select Bow Roof 1.

Intersection Curve made on Bow Roof 1.

When the intersection curve has been created and hides the model, we will get this curve shown in the below figure:

Project the Curve to the XY plane to find the section analysis

Section Modulus of Bow Roof 1.

Moment of Inertia (MAX): Imax = 2.712420687 x 10^4 [mm^4]

Moment of Inertia (MIN): Imin = 1.675772657 x 10^3 [mm^4]

Section Modulus, S = Moment of Inertia (MOI) / Distance between the neutral axis and the end of the object (y)

S = I/y

y = 34.95 mm

For Maximum MOI:

S = (2.712420687 x 10^4 [mm^4]) ÷ (34.95 mm)

Section Modulus: Smax = 621.58 mm^3

For Minimum MOI:

S = (1.675772657 x 10^3 [mm^4]) ÷ (34.95 mm)

Section Modulus: Smin = 47.95 mm^3

> Bow Roof 2:

Create an intersection Curve with the help of the plane and select Bow Roof 1.

When the intersection curve has been created and hides the model, we will get this curve shown in below figure:

Project the Curve to the XY plane to find the section analysis.

Section Modulus of Bow Roof 2.

Moment of Inertia (MAX): Imax = 2.723222592 x 10^4 [mm^4]

Moment of Inertia (MIN): Imin = 1.691894789 x 10^3 [mm^4]

Section Modulus, S = Moment of Inertia (MOI) / Distance between the neutral axis and the end of the object (y)

S = I/y

y = 34.98 mm

For Maximum MOI:

S = (2.723222592 x 10^4 [mm^4]) ÷ (34.98 mm)

Section Modulus: Smax = 778.51 mm^3

For Minimum MOI:

S = (1.691894789 x 10^3 [mm^4]) ÷ (34.98 mm)

Section Modulus: Smin = 48.37 mm^3

> Centre Roof Rail:

Create an intersection Curve with the help of the plane and select Bow Roof 1.

When the intersection curve has been created and hides the model, we will get this curve shown in below figure:

Project the Curve to the XY plane to find the section analysis.

Section Modulus of Center Roof Rail.

Moment of Inertia (MAX), Imax = 4.667420402 x 10^4 [mm^4]

Moment of Inertia (MIN), Imin = 2.096380519 x 10^3 [mm^4]

Section Modulus, S = Moment of Inertia (MOI) / Distance between the neutral axis and the end of the object (y)

S = I/y

y = 42.47 mm

For Maximum MOI,

S = (4.667420402 x 10^4 [mm^4]) ÷ (42.47 mm)

Section Modulus : Smax = 1098.99 mm^3

For Minimum MOI:

S = (2.096380519 x 10^3 [mm^4]) ÷ (42.47 mm)

Section Modulus: Smin = 49.36 mm^3

> Front Roof Rail:

Create an intersection Curve with the help of the plane and select Bow Roof 1.

When the intersection curve has been created and hides the model, we will get this curve shown in below figure:

Project the Curve to the XY plane to find the section analysis.

Section Modulus of Front Roof Rail.

Moment of Inertia (MAX): Imax = 1.539415927 x 10^5 [mm^4]

Moment of Inertia (MIN): Imin = 9.748168632 x 10^3 [mm^4]

Section Modulus, S = Moment of Inertia (MOI) / Distance between the neutral axis and the end of the object (y)

S = I/y

y = 60.83 mm

For Maximum MOI:

S = (1.539415927 x 10^5 [mm^4]) ÷ (60.83 mm)

Section Modulus: Smax = 2530.68 mm^3

For Minimum MOI:

S = (9.748168632 x 10^3 [mm^4]) ÷ (60.83 mm)

Section Modulus: Smin = 160.25 mm^3

> Rear Roof Rail:

Create an intersection Curve with the help of the plane and select Bow Roof 1.

When the intersection curve has been created and hides the model, we will get this curve shown in below fig.

Project the Curve to the XY plane to find the section analysis.

Section Modulus of Rear Roof Rail.

Moment of Inertia (MAX): Imax = 4.958425792 x 10^5 [mm^4]

Moment of Inertia (MIN): Imin = 2.702397076 x 10^4 [mm^4]

Section Modulus, S = Moment of Inertia (MOI) / Distance between the neutral axis and the end of the object (y)

S = I/y

y = 94.02 mm

For Maximum MOI,

S = (4.958425792 x 10^5 [mm^4]) ÷ (94.02 mm)

Section Modulus: Smax = 5273.79 mm^3

For Minimum MOI:

S = (2.702397076 x 10^4 [mm^4]) ÷ (94.02 mm)

Section Modulus: Smin = 287.43 mm^3

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

A minimum draft angle of 7° is considered for analysis. The green color infers the parts that all face along the tooling direction has a positive draft angle greater than 7° and passed in analysis.

Draft Analysis on Outer Roof Panel.

Draft Analysis on Front Roof Rail.

Draft Analysis on Rear Roof Rail.

Draft Analysis on Bow Roof 1.

Draft Analysis on Bow Roof 2.

Draft Analysis on Center Roof Rail.

Results:

Roof Model in Different Views:

Rendered Isometric View of the Roof Assembly.

Rendered Top View of the Roof Assembly.

Rendered Bottom View of the Roof Assembly.

Rendered Bottom Isometric View of the Roof Assembly.

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 Inertia study was conducted and a Draft Analysis was done for each reinforcement successfully.