Roof challenge

Aim:

To design & develop the Roof of a car with its components by following the master section and design parameters.

Objective:

• To develop the essential flanges and reinforcements using roof styling as input.
• To do a curvature study on the roof and perform calculations to determine the position of the Bow-roof.
• To determine the Heat Distortion Criteria of the car's roof.
• To find the section modulus on the O-Y section of the roof and also for all the other components of it.
• To determine the Snow Load Criteria for a car's roof.
• To perform a Draft Analysis on the final CAD model of the roof.

Introduction:

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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.

Types of Car roofs

• Convertible roofs
• Hardtops
• Sunroofs
• T-tops
• Targa tops
• Vinyl roofs

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

Rollover crashes, especially in the country, are usually very destructive events. Vehicle damage often includes the deformation of the roof and its supporting structures. Head and neck injuries are common and associated with roof deformation. Strengthening of the roof is suggested as an appropriate countermeasure for such injuries.

Types of Rollovers

1. Tripped-NHTSA data show that 95% of single-vehicle rollovers are tripped. This happens when a vehicle leaves the roadway and slides sideways, digging its tires into soft soil or striking an object such as a curb or guardrail. The high tripping force applied to the tires in these situations can cause the vehicle to roll over. E.g. Soft soil, Guardrail, Steep Slope
2. Un-Tripped -Un-tripped rollovers are less common than tripped rollovers, occurring less than 5% of the time, and mostly to top-heavy vehicles. Instead of an object serving as a tripping mechanism, un-tripped rollovers usually occur during high-speed collision avoidance maneuvers.

For this reason, strength is a very important criterion for designing a vehicle roof.              

FMVSS 216 Roof crush standards

FMVSS 216, Roof Crush Resistance, establishes strength requirements for the passenger compartment roof of passenger cars, multipurpose passenger vehicles, trucks, and buses with a GVWR of 2722 kilograms or less. The purpose of the standard is to reduce deaths and injuries due to the crushing of the roof into the passenger compartment in rollover accidents. 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.

The prescribed static loading device is a rigid unyielding rectangular block 762 millimeters by 1,829 millimeters. It shall not move more than 127 millimeters to achieve the specified resistive load when applied to the forward edge of a vehicle’s roof – see Figures 1 and 2. The resistive load to be achieved is 1½ times the unloaded vehicle weight (UVW) of the test vehicle or 22,240 Newtons, whichever is less.

The force shall be applied in a downward direction perpendicular to the lower surface of the static loading device at a rate of not more than 13 millimeters per second until reaching the compliant resistive load. The test shall be completed within 120 seconds. The static loading device shall be guided throughout the test so that it moves, without rotation, in a straight line with its lower surface oriented as follows and as shown in Figures

                                   

Following are the design considerations of the roof-

1. Visibility criteria
2. Head clearance
3. Heat distortion and snow load criteria
4. Draft analysis
5. Safety parameters

 Input data from styling team-

We will be having the Aesthetic Surface data which is provided by the styling team.

Taking it as a reference we can start the design and at the same time we need to follow the Master Section

Initially, we need to create the ditch area for the outer panel in the front and back

 At the corners use a multisection Surface to get smooth corners which will increase the strength and easy to manufacture

After making the ditch area give all the required fillets at the sharp edges and do a symmetry with the mid plane

Next Creating inner Front and Rear Roofs

 Front Roof Rail Master Section

Design Procedure of Front Roof Inner Panel

• Offset the roof to 2.7mm and split it with a distance of 5mm.
• Similarly, create an offset of 4.7mm and cut it.
• Now create a base offset of 32mm from the roof.
• Create the respective sweeps for these offsets and trim them together.
• Give the Edge fillets to it.
• After that embosses are created on this to provide more strength.

Design Procedure of  Rear Roof inner Panel

For the rear roof, rail follows a similar procedure as the front roof and the values for it are given in the diagram.

• Make sure that the Rear roof has a positive draft angle.
• After that embossed & slots are created on this to provide more strength and reduce the weight of the BIW component.

Master Section given

Creating the Centre Reinforcement:

• The Centre Reinforcement has attached at the center of the roof.
• To create the center reinforcement as per the diagram, we use the offset and select the parallel curve option and give an offset value of 50mm on both sides and create a cut on the offset.
• Now create the part as per the design standards and make sure that the offset is given considering the thickness of the component.

                                                                                           

• ditch area needs to be created so that the reinforcement is attached to the roof flange.
• To create that area, select the flange and create an offset to the length including the thickness of the roof and the connecting parts. 
• Cut the Offset to the length which is less than the width of the center reinforcement. 
• Create Splines on both edges of the surface. 

                                     

-Use the Multi-section surface option to select the edges and the curves created as the guide curves to create a body.

-Join the surfaces together

                                

Creating the Bow roofs:

• 3 bow roofs are created to give strength to the roof.
• Follow the same method as with the center reinforcement to create the bow roof.
• Now, after the bow roof is created. The Flanges for the mastic points need to be created.

  

Create some slots on the bow roof to increase its strength.

                         

Creating a Solid:

• After Creating the surfaces, use the thicken option to create the solids with the proper thickness.
• The thickness for the Roof And all other parts is Considered 0.75mm.

Heat Distortion Criteria:

The heat radiating from the sun causes the roof of the car to change in the metallic regions. Heat distortion criteria indicate what temperature materials start to "soften" when exposed to a fixed load at elevated temperatures.

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]

Checking the Heat Distortion criteria:

• To calculate the Heat distortion criteria, we have to take note of a few values and note it down.
• To note down, first, we need to create a sketch.
• Create a line at the center of the roof part in the 0-Y axis and then draw another line with a distance of 300mm.
• Now create the line with a distance of 100 on both sides at the line which is at 300mm.
• Now create the line between the rails in the X direction and create 2 lines with a distance of 100mm on both sides.
• Now create a point coinciding with both lines.

                 

Extract the points to the roof and create the arc using 3 points by creating two lines intersecting with each other.

Measure the radius of the arc in the Y direction as Ry, Ry, and X direction as Rx, Rx.

Rx Value:

Ry Value:

Here I’m Calculating the Heat Distortion Criteria by using excel calculator by giving the formula

If the value of W is satisfying this condition OK< 2.7 then it’s represented by OK. It’s Means Roof Fulfilling the conditions of Heat Distortion Criteria.

Or If the value of W is not satisfying this condition OK< 2.7 then it’s represented by NG. This means the Roof not Fulfilling the conditions of Heat Distortion Criteria. So, we need to change the design of the bow roofs

Snow Load Criteria:

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.

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 is 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/L2 + 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

Checking the Snow Load Criteria:

• To calculate the Snow Load criteria, we have to take note of a few values and note it down.
• To note down, first, we need to create a sketch.
• Create a line at the center of the roof part in the 0-Y axis
• Now create the line with a distance of 100 on both sides.
• Now create the line between the rails in the X direction and create 2 lines with a distance of 100mm on both sides.
• Now create a point coinciding with both lines.
• Extract the points to the roof and create the arc using 3 points by creating two lines intersecting with each other.
• Measure the radius of the arc in the Y direction as Ry, Ry, and X direction as Rx, Rx.

Rx Value:

Ry Value:

Section Inertia:

The area moment of inertia is a property of a two-dimensional plane shape that characterizes its deflection under loading. It is also known as the second moment of area or second moment of inertia. The area moment of inertia has dimensions of length to the fourth power.

Checking the Section Inertia:

• To check the section inertia, we need to create the intersection in the Y-axis.
• After the intersection is done, fill the surface with the intersection created.
• Select the Measure Inertia option to measure the 2D inertia of the surface created.
• Note down the values. 

 Section inertia (Iy):-

 For Bow Roof Rail 1:

here M1=1.018 x 10^-9 m^4

         Iy=1.018 x 10^3 mm^4

         Iy=1018 mm^4

For Central Bow Roof with Reinforcement:-

here M1=1.572 x 10^-8 m^4

         Iy=1.018 x 10^4 mm^4

         Iy=1572 mm^4

For Bow Roof Rail 2:

here M1=1.010 x 10^-9m^4

          Iy=1.010 x 10^4 mm^4

          Iy=1010 mm^4

Here I’m Calculating the Snow Load Criteria by using an excel calculator by giving the formula

If the value of Qr is satisfying this condition of Qr  ≥ 3.1  then it’s represented by OK. It’s Means Roof Fulfilling the conditions of Snow Load Criteria.

If the Qr value doesn't lie say the styling team to create some embosses on the outer panel.

Draft Analysis:

The Draft analysis is done on the individual Parts to check if are passing the criteria.

Roof:

Front Roof Rail:

Center reinforcement:-

Center bow roof:-

Bow Roof 1:

Bow Roof 2:

Final Model:

Section Views:

Front Roof Rail:

Center Reinforcement and Bow Roof 1:

Bow Roof 2 and 3:

Rear Roof Rail:

Learning Outcome:

1. Learned how to create rails as per the design.
2. Learned how to calculate and what are the criteria for the Heat Distortion, Snow Load conditions.

Conclusion

• All reinforcement BIW parts of the roof are designed according to the input data provided by the styling team. Designed parts will be submitted to the CAE team to carry out further FEM analysis to obtain numerical data to improve the design
• Did curvature study and analysis of the design for heat distortion and snow load criteria and found OK
• Moment of inertia and draft analysis is carried out to check the feasibility of manufacturing and find OK