Roof Design

Report on 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 includes:

• Convertible roof: A convertible or cabriolet is a passenger car that can be driven with or without a roof in place. The methods of retrcating and storing the roof vary between models. A convertible allows an open air driving experience, with ability to provide a roof when required. Potential drawbacks of convertibles are reduced structural rigidity and cargo space.

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

• Sunroof: 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 bemanually 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 panels rigidly mounted in the roof panel over the passenger compartment. A moonroof has a glass panel that is transparent and ususally tinted.

• 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 pillar to the center of the next structural bar. The panels of a traditional T-tops are usually made of auto grade safety glass.

• targa Tops: targa tops is an semi conventional 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 seperate roof panels above the seats that fit between the window and central t-bar.

• Vinyl Roofs: This covering was originally designed o give the apperance 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.

• Solar Panel roof: The solar panel roofs are newly developed for electric cars to charge the batteries by using sunlight without stopping at charging stations which consumes a good time to recharge the batteries. So that the solar panel roofs were developed for ongoing recharging of batteries.

Roof is designed by following some critieria:-

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

To create the roof, Given data is:-

Roof skin

Front Roof Rail Mastersection

Center and reinforcement roof rail mastersection

Bow roof mastersection

Rear roof mastersection

Front roof Rail view

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

Bow roof Rail view

Center roof Rail view

Mounting holes for brackets are created for cabin/reading light positioning.

Bow roof 2

Rear roof rail view

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

Roof rail Positions

Outer Panel Flanges are created

Rear flange is created in which backdoor hinges will position and backdoor will sit over it

Complete assembly view

After completing design there are some calculation can be done by us to calculate the strenght of the designed roof.

Roof Bow calculation

Heat Distortion caculation

W = [1.73 x 10 ^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [1.10 x 10^(-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 on 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 minimum radius of curvature was measured to get the Rx and Ry value

By using all values Heat distortion was calculated in the MS Excel

According to judgemental Condition : OK<2.7>3.1

All values are under judgement condition.

Snow load calculation

During winter seasons the snow gets clogged over the top surface 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 = [ly x t^2] / [α x s x [(Rx+Ry)/2]^2 x 10 ^-8]

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

Where

α = My x Lx^2 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 Roof Rail to Roof BOW [mm]

S = Distance for which roof BOW bears divided load[mm]

s = L1/2 + L2/2

Ly = 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 = Longtitude Direction curvature radius of the roof panel X cross section on roof 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

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

And, here one result of Qr is not fulfilling the demand. So, there are some solutions, that the model can pass over to the styling team to change the curvature to improve the strength, BoOW roof rails position can be changed to the weaker side of roof or embosses could be created on the outer panel to improve the strength.

To improve the strengt, the embosses are created.

Outer panel

After completing the design there are physical test of vehicle is done to check the strength of the roof wih vehicle.

There are various car roof design 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. Roll over have a higher fatality rate than other types of vehicle collisions.

Vehicle roll over 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 crasches are the result of steering input, speed, and friction with thevground.

The vehicle which have a higher center of gravity like SUV 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 shooulders, uneven ground, collision with curbs or groundrails after the vehicle 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 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. Thses 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.

• Roof Crush test: In this test the load is applied on the roof to test the strength in static car position. The standard does not apply to school buses and passenger cars that conform to the dynamic rollover test requirement 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 compliances 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, busses and multi passenger vehicle 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. Forces 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 crashed. At the 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 view of roof created

Front view

Side view

Back view

Isometric view

Conclusion

The roof was created in siemens NX student version with all the required parameters and different calculations were done for safety concern.