Roof challenge

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:

• 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.

• 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 on going recharging of batteries.

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

To create the Roof Given data is-

Roof Skin

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

Center Roof Rail View-

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

Center and Reinforcement Roof Rail View

Side Flanges on Bow Roof designed/created is for Mastic Sealant application

Center and Reinforcement Roof Rail Intersection View

Bow Roofs View

Mastic Sealant application point is created

Bow Roof 2

Bow Roof 3

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 Rails Position

Outer Panel Flanges are created

Complete Assembly View

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 Value Obtained

RX 2 Value Obtained

RX 3 Value Obtained

RX 4 Value Obtained

RY 1 Value Obtained

RY 2 Value Obtained

RY 3 Value Obtained

RY 4 Value Obtained

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

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

Rx 1 Value Obtained

Rx 2 Value Obtained

Rx 3 Value Obtained

Rx 4 Value Obtained

Ry 1 Value Obtained

Ry 2 Value Obtained

Ry 3 Value Obtained

Ry 4 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

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, Bow 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 strength, the embosses are created.

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.

Front Roof rail

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

S=I/Y

S=

I=768.609mm4

Y=126.876/2=63.438

S=12.11mm3

Center Roof rail and 1st Bow Roof

S=I/Y

S=

I=476.754mm4

Y=119.873/2=59.936

S=7.954mm3

2nd Bow Roof rail

S=I/Y

S=

I=38.944mm4

Y=39.946/2=19.973

S=1.949mm3

3rd Bow Roof rail

S=I/Y

S=

I=38.749mm4

Y=39.956/2=19.977

S=1.939mm3

Rear Roof rail

S=I/Y

S=

I=2.06032mm4

Y=181.558/2=90.779

S=22.696mm3

Draft Analysis

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

Front Roof rail

Center Roof rail

1st Bow Roof

2nd Bow Roof

3rd Bow Roof

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.

conclusion

The Roof was created in Siemens NX Crack Version with all the required parameters and different Calculations were done for safety concerns.