Roof Design

Aim-

To develop the essential flanges and reinforcements with appropriate dimensions and checking draft by performing Draft analysis, Calculating Section inertia, Heat distortion and Snow load on the roof.

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.

In later years, many variations on the automobile roof developed. These include:

• Hardtops - A hardtop is a rigid form of automobile roof, which for modern cars is typically constructed from metal.

• Convertible roofs - A convertible or cabriolet is a passenger car that can be driven with or without a roof in place. The methods of retracting and storing the roof vary between models. A convertible allows an open-air driving experience, with the ability to provide a roof when required. Potential drawbacks of convertibles are reduced structural rigidity (requiring significant engineering and modification to counteract the effects of removing a car's roof) and cargo space.

• HARD TOP COVERTABLE ROOF

SOFT TOP CONERTABLE ROOF :

• Sunroofs - A sunroof - is a movable panel that opens to uncover a window in an automobile roof, allowing light and/or fresh air to enter the passenger compartment. Sunroofs can be manually operated or motor driven, and are available in many shapes, sizes and styles. While the term sunroof is now used generically to describe any glass panel in the roof, the term "moonroof" was historically used to describe stationary glass panes rigidly mounted in the roof panel over the passenger compartment. A moonroof has a glass panel that is transparent and usually tinted. Previous terms include Sunshine Roof, Sliding Head and Sliding Roof.

• T-Tops - 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 Tops - Targa top, or targa for short, is a semi-convertible car body style with a removable roof section and a full width roll bar behind the seats. The rear window is normally fixed, but on some targa models, it is a removable plastic foldable window, making it a convertible-type vehicle. Any piece of normally fixed metal or trim which rises up from one side, over the roof and down the other side is sometimes called a targa band, targa bar, or a wrapover band.

• Targa tops are different from T tops, which have a solid, non-removable bar running between the top of the windscreen and the rear roll-bar, and generally have two separate roof panels above the seats that fit between the window and central t-bar.

• Vinyl Roofs - This covering 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. When leather, canvas and vinyl were sometimes used along with landau bars, to give a fairly accurate reproduction of a horse-drawn carriage's movable top.
• Oper Manta with Vinyl Roof
• 

Roof is designed by following some criteria-

1. Visibility criteria as safety and functionality
2. Head clearance for ease and comfort
3. Draft analysis fro easy manufacturing and tooling
4. Safety parameters for users
5. Heat distortion
6. snow load criteria

ROOF INTRODUCTION :

An automobile roof is considered as an integral part of BIW (body in white) design. A car roof is responsible for protecting the passenger from rain, wind, sunlight, accidents etc. It also offers aerodynamic support to the car when it is in motion. A roof panel assembly includes a roof outer panel/ditch area and inverted hat like reinforcements rails which underlie the outer side edges of the roof outer panel. Roof rails are provided to give strength to the outer panel and also provide support to other commodities present in a car.

There are different rail reinforcements in a roof panel like:

1. Front roof rail
2. Rear roof rail
3. Bow roofs

CLASS A SURFACE MODIFICATION OR STYLING SURFACE MODIFICATION :

it is necessary to remove any discontinuity in the styling surface which may result in faulty surfaces.Also it helps to generate smooth surfaces while performing complex operations. Surfaces were extracted from the given styling data and untrimmed to get the original surface back.

CREATION OF FRONT ROOF RAIL OUTER AND I NNER SURFACES :

By taking consideration of dimensions and given mastersection i designed front roof rail

Embosses are created to give strength to the roof rail and mounting for brackets are given in which lights and switches will position.

Front Roof Rail Mastersection

SECTIONAL VIEW OF FRONT ROOF RAIL

CREATION OF BOW ROOF :

In my project i created two BOW ROOFS ( BOW Roof 1 and BOW Roof 2 )

MASTER SECTIO0N WHICH I FOLLOWED

BOW ROOF RAIL 1

SECTION VIEW OF BOW ROOF RAIL 1

BOW ROOF RAIL 2

I followed the same mastersection to create BOW ROOF 2

SECTION VIEW OF BOW ROOF RAIL 2

CREATION OF CENTRE ROOF RAIL :

MASTER SECTION WHICH I FOLLOWED

SECTION VIEW

REAR ROOF RAIL :

MASTER SECTION WHICH I FOLLOWED

Embosses are created to provide strength to the material and holes are created to somehow reduce the vehicle weight along force distribution.

Roof Rails Position

Outer Panel Flanges are created

REAR FLANGE IS CREATED IN WHICH BACKDOOR HINGES WILL POSITION AND BACKDOOR WILL SIT OVER

OUTER FRONT FLANGE

Outer Ditch AREA FLANGE

COMPLETE ASSEMBLEY :

After completing design there are some calculation can be done by us to calculate the strength of the Designed Roof.

Roof Bow Calculations

Heat Distortion Calculation

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> 3.1

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]

Except Rx and Ry all values can be measured on the model.

To get the Rx and Ry curvature a sketch was required-

• A sketch was created on a plane on top of the Roof
• 0-Y line was created in the X-Direction in center of roof
• Then a Horizontal line below 300mm was created named as -3Y Line
• Two lines on both sides at 100mm were created from -3Y Line
• Intersection point on line was created and projected on outer Panel of Roof

Projected Points

Then Three point Arc was created by using these points

Then Minimun Radius of curvature was measured to get the Rx and Ry Value

RX 1, Rx2 , Rx3 ,Rx4 value obtained values

RY1 , RY2 , RY3 , RY4 obtained minimum values

By using all Values Heat Distortion was Calculated in the MS Excel

According to Judgement Condition : OK< 2.7> 3.1

All Values are under Judgement Condition.

Snow Load Calculation

During Winter Season the snow gets clogged over the top surface/roof of the car and the snow has some mass on it. Due to this, the roof can fail if the snow load exceeds the capacity of the roof that was designed to sustain.

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 front and rear reference point.

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

To get the values a sketch was created on top of Roof

• O-Y line was created in the center of roof
• A line between First and center Roof rail was created
• Then two lines on both sides at 100mm was created
• Similar lines were created between other center roof rail, both Bow Roof rails and rear roof rails
• Intersection points were created and projected on the roof

SECTIONAL VIEW OF BOW ROOF RAIL 1

SECTIONAL VIEW OF CENTRE ROOF RAIL

SECTIONAL VIEW OF BOW ROOF RAIL 2

Projected points for Snow load Calculation

Then Three point Arc was created by using these points

Then Minimun Radius of curvature was measured to get the Rx and Ry Value

Rx1 , RX 2 , RX3 , RX Value Obtained

RY 1 , RY 2 , RY 3 , Value Obtained

By using all the values Snow load was calculated in the MS Excel

Snow Load Calculation

So, according to the Judgement condition = Qr ≥ 3.1

Section Modulus:

Section modulus is a geometric property for a given cross-section used in the design of beams. Other geometric properties used in the design include an area for tension, radius of gyration for compression, and moment of inertia for stiffness. The Section modulus is defined as

S=Iy

Where

S = Section Modulus

I = Moment of Inertia

y = Distance from the neutral axis to any given part.

The intersection was created and then Section Inertia command was used to obtain MO

Draft Analysis

Draft analysis was performed with 7 Degrees Value(generally given in sheet metal)

Front Roof rail

Center Roof rail

first Bow Roof Rail

Second Bow Roof Rail

Rear Roof rail

Outer Panel

After Completing the Design there are physical test of vehicle is done to check the strength of the Roof with vehicle.

There are various Car Roof Designs to check their strength some tests are done which are:

• Car Roll-over Test - The Roll-over 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.

The vehicles which have a higher center of gravity like SUV or Small Trucks are more prone to Roll-over. 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 effects 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, and cuts, scrapes, and bruises. These injuries are caused from slamming into another vehicle or obstacle, impact with unsecured cargo, contact with broken glass or torn metal, or from being tossed within the vehicle or ejected from the vehicle.

Volvo Roll-Over Test

• Roof Crush Test - In this test the load is applied on the roof to test the strength in static car postion. The standard does not apply to school buses and passenger cars that conform to the dynamic rollover test requirements of FMVSS 208, Occupant Crash Protection, and S5.3 by means that require no action by passenger car occupants. It also does not apply to convertibles, except for optional compliance with the standard as an alternative to the rollover test requirements in S5.3 of FMVSS. In April 1991, the National Highway Traffic Safety Administration (NHTSA) announced that FMVSS 216 was to be extended to light trucks, buses and multi-passenger vehicles not exceeding 6000 pounds, and that requirement came into force in September 1993. American statistics have long indicated that the fatality rate when light trucks (including four-wheel drives and utility vehicles) are involved in rollover accidents is twice that for passenger cars.
  

FMVSS 216 Roof Crush

The purpose of the FMVSS 216(Federal motor vehicle safety standard) is to reduce the head injuries, neck injuries and deaths by the roof crush of the car in to the passenger compartment in rollover crashes.

The resistance of the vehicle roof to deformation is dictated by a static test, in which the force of 1.5 times the unloaded vehicle weight or 5000 pounds, which is less, is progressively applied to the roof of the vehicle in the region of the “A” pillar. Force is applied to the plate at a roll angle of 25 degree and a pitch angle of 5 degree to the direction of the forces simulation can be experienced in rollover crashes. At the t time of the test, displacement of vehicle roof should not more than 5 inches of intrusion, as estimated by the movement of the test device.

Some Views of Roof created

Front View

Side View

Back View

Isometric View

Conclusion

1. Using all input data we have successfully designed Ditch area flanges, Front roof rail, Bow roof rail 1&2, Rear roof rail as per the requirement
2. As per heat distortion criteria we come to know all bow roof positions are OK
3. As per Snow load criteria we come to know there is improvement required
4. All roof rails draft analysis are found OK