Roof Design

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.

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.

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 utomobile 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: It is 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.

Design consideration of Roof:

•  Rear visibility Criteria
•  Front visibility Criteria
•  Head room Clearance
•  Curvature Study
•  Heat Distortion and Snow Load Criteria
•  Draft Analysis
•  Safety Parameters

 Roof Crush Test:

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

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. 

Car Roll over 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.

HEAT DISTORTION STUDY ON THE ROOF :

The heat distortion study plays a major role in sheet metal usage . heat distortion temperature is a temperature  limit above which the (MATERIAL) cannot be used for the 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 the roof, based on the heat distortion  temperature , this study will predict the bow roof position which helps to strengthen the roof .

PROCEDURE:

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.

Design a Ditch Area for Outer Roof Panel:

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 7 degrees considering the drawing operation process. The overall thickness of sheet metal considers 0.75mm throughout.

 Design of Rear Roof Rail: 

 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.

 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.

 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.

 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 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.

      

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.

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]

 Judgement Condition : OK< 2.7> 3.1

APPLY THE VALUES IN THE FORMULA :

In order to find the heat distortion apply all the necessary values from the roof to the give formula

 CALCULATIONS :

From the above table it can be concluded that all values of w<2.7  and also not w>3.1 so thus infers that current positioning of bow row are good in state  as per design  and found ok

SNOW LOAD CRITERIA :

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

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

                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]

APPLY THE VALUES IN THE FORMULA :

In order to find the snow load criteria  apply all the necessary values from the roof to the give formula

 CALCULATIONS :

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).

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 design include area  for tension and shear, radius of gyration  for compression, and moment of inertia and polar moment of inertia  for stiffness. Any relationship between these properties is highly dependent on the shape in question. Equations for the section moduli of common shapes are given below.

There are two types of section moduli :

The elastic section modulus and

The plastic section modulus. 

 The elastic section modulus is defined as

S = I / y

where :

S= Section modulus

I = second moment of inertia  (or area moment of inertia, not to be confused with moment of inertia)

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

BOW ROOF 1

Create an intersection curve with the help of the plane and select the bow roof 1

  

Moment of inertia maximum,( MAX)  I _max  =2.8439 × 10⁴ ( mm⁴)

Moment of inertia maximum ,( MIN)  I _min =1.921e^+03   ( mm⁴)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 34.94 mm

 FOR MAXIMUM MOI :

S=2.8439 × 10⁴ ( mm⁴)  / 34.94 mm

S _max  = 813.9381 mm³

 FOR MINIMUM MOI :

S=  1.921e^+03  ( mm⁴)/ 34.94 mm

S _min  = 54.98mm³

BOW ROOF 2

Moment of inertia maximum,( MAX)  I _max  =2.85417× 10⁴ ( mm⁴)

Moment of inertia maximum ,( MIN)  I _min =1.937682764e+03 ( mm⁴)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 34.98 mm

 FOR MAXIMUM MOI :

S= 2.85417× 10⁴( mm⁴)  / 34.98 mm

S _max  = 815.944 mm³

 FOR MINIMUM MOI :

S= 1.937682764e+03/ 34.98 mm

S _min  =55.39 mm³

CENTRAL ROOF RAIL :

Moment of inertia maximum,( MAX)  I _max  =5.59197× 10⁴ ( mm⁴)

Moment of inertia maximum ,( MIN)  I _min =2.609370860e+03( mm⁴)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 44.96 mm

 FOR MAXIMUM MOI :

S= 5.59197× 10⁴( mm⁴)  / 44.96  mm

S _max  = 1243.7267 mm³

 FOR MINIMUM MOI :

S= 2.609370860e+03/ 44.96mm

S _min  = 58.0376 mm³

FRONT ROOF RAIL:

Moment of inertia maximum,( MAX)  I _max  =1.38198× 10⁵ ( mm⁴)

Moment of inertia maximum ,( MIN)  I _min =9.243784 × 10³( mm⁴)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 120.30 mm

 FOR MAXIMUM MOI :

S= 1.38198× 10⁵( mm⁴)/ 120.30 mm

S _max  = 1148.77 mm³

 FOR MINIMUM MOI :

S= 9.243784 × 10³( mm⁴)/120.30mm

S _min  =76.83 mm³

REAR  ROOF RAIL:

Moment of inertia maximum,( MAX)  I _max  =5.94045× 10⁵ ( mm⁴)

Moment of inertia maximum ,( MIN)  I _min =4.42739 × 10⁴( mm⁴)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 100.47 mm

 FOR MAXIMUM MOI :

S= 5.94045× 10⁵( mm⁴)  /100.47 mm

S _max  = 5912.66mm³

 FOR MINIMUM MOI :

S= 4.42739 × 10⁴( mm⁴)/100.47 mm

S _min  = 440.66mm³

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.

Minimum draft angle of 7⁰ is considered for analysis . green colour infer on the parts that all face along the tooling direction has positive draft angle greater than 7⁰ and passed in analysis

DRAFT ANALYSIS FOR FRONT ROOF RAIL :

DRAFT ANALYSIS FOR BOW ROOF RAIL - 1:

DRAFT ANALYSIS FOR CENTRAL ROOF RAIL :

DRAFT ANALYSIS FOR BOW ROOF RAIL - 2:

DRAFT ANALYSIS FOR FRONT REAR  ROOF RAIL :

ROOF IMAGES :

TOP VIEW :

SIDE VIEW :

ISOMETRIC VIEW :

CONCLUSION :

Hence the  roof , front roof rail, bow roof rails , central roof rail and the rear roof rail is designed by following the master sections and the moment of inertia , section modulus  and draft analysis are carried out to check feasibility manufacturing.