Roof Design

Design of roof and implemention of reinforcements and flanges with appropriate dimensions and verify the design by performing Heat Distortion, Snow Load.

Roof:- 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,” and helping ensure their safety in the event of vehicle rollover.

Types of roof can include (with variations):

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

Convertible roofs – According to Wikipedia, “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 among eras and manufacturers. A convertible car's design allows an open-air driving experience, with the ability to provide a roof when required. A potential drawback of convertibles is their reduced structural rigidity =

Hardtops – According to Wikipedia, “a hardtop is a rigid form of automobile roof, which for modern cars is typically constructed from metal. A hardtop can either be fixed,” or non-detachable, or detachable for separate storing.

Sunroofs – As per Wikipedia, “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 variation on the sunroof is the moonroof, a transparent panel on the roof that is either non-moveable or only partially movable, such as tiltable. According to Wikipedia, “a moonroof has a glass panel that is transparent and usually tinted. Previous terms include Sunshine Roof, Sliding Head and Sliding Roof.”

T-Tops – Wikipedia indicates that 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 – According to Wikipedia, the 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.

Vinyl Roofs - A final mainstream type of roof in production vehicles is the Vinyl roof. According to Wikipedia, “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.”

Solar Roof

An additional type of roof, not yet in widespread commercial use, is the Solar Roof, or a roof with a built-in solar panel. Specially-designed, lightweight solar-powered cars have traversed the entire United States and the entire continent of Australia, powered only by the Sun.

Roof Design Criteria

Roof design follows several key criteria, as below:

1. Safety

• Visibility
• Crash-worthiness

1. Durability

• Heat distortion
• Snow load criteria

3. Comfort
   • Head clearance
   • Visibility and NVH

1. Manufacturability
• Draft analysis

Roof Skin

We were provided a roof skin, or Roof Outer Panel, from the design team.

Front Roof Rail Master Section

Center Reinforcement Roof Rail Master Section

Bow Roof Master Section

Rear Roof Rail Master Section

Front Roof Rail

We designed the front rail as per the Master Section, and added Embosses for Strength as well as to attach lighting and other parts and components, and also added holes for component attachment.

Center Roof Rail View-

Front Bow Roof Rail

The front bow roof rail has tabbed flanges for mastic sealant application.

Rear Bow Roof Rail

The rear bow roof is similar to the front, with slightly different notching for Mastic Seal application.

Rear Roof Rail View

The rear roof rail contains numerous flanges for rear door attachment. It also has large embosses to provide strength, and slots to reduce weight and absorb rear impact.

Roof Rails Position

In our final design, the three central rails are positioned from the mid-section of the roof to the rear. As the front of the roof has larger curvature, it is is stronger and thus should require less support than the flatter rear section of the roof.

Outer Roof Panel Flanges

Rear end rail corner clearance

Corner relief where the rear extension meets the roof ditch area side flanges.

Subsequent to completing the roof design, we performed calculations to verify the strength and durability of the Roof. In particular, we conducted a Heat Distortion Study, and a Snow Load Study.

Heat Distortion Calculation

We used the following Heat Distortion Calculation as given:

W = [1.73 x 10^-3 x L] + [1.85 x 10^-8 x (R²)/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]

To determine Rx and Ry we created a sketch to measure these curvature values-

• We created the sketch on the Top Plane of the Roof
• We created a 0Y line along the x-axis in the center of roof
• We offset the 0Y line by 300mm to create a parallel horizontal line below the 0-Y line (-3Y line)
• We added two lines on both sides of the -3Y line, at 100mm offsets
• We created vertical lines running through the mid-point of the sections between each consecutive pairs of rails.
• We offset these vertical lines by 100mm on either side, to create a “grid” pattern.
• We added intersection points at the vertices of the grid.
• We projected the sketch points onto the roof surface, and then created “X” or “+” arcs on the surface to measure the curvature (Rx and Ry)

Projected curves used to calculate curvature

Study Results

S NO Rx Ry R l W Condition

1 200.3 201.5 200.9 414.38 1.27 Pass(<2.7)

2 200 200 200.2 478.5 1.34 Pass(<2.7)

3 200.2 200.2 200.34 409.5 1.27 Pass(<2.7)

4 200.6 200.6 200.64 323.9 1.89 Pass(<2.7)

As the Judgement Condition is: OK< 2.7, all of the values obtained met the Judgement Condition.

Snow Load Calculation

In Winter in cold climates, snow can accumulate on the roof. The roof needs to be strong enough to hold the load, and regain its curvature when the snow is cleared.

Snow load Prediction formula

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 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 obtain the input values a sketch was created on top of Roof

• The OY line was created in the center of roof, along the X axis, with horizontal offsets of 100mm on either side.
• We created vertical lines midway between each successive pairs of rails.
• We offset vertical lines by 100mm on either side of the midway lines above
• We created intersection points on the grid created above, and projected these points onto the roof surface.
• Using the projected points, we constructed “X: or “+” shaped arcs on the roof surface and used Radius Analysis to measure the curvature of these arcs.
• We took cross-sections of the three central rails, and projected these into sketches, then performed Section Inertia Analysis to obtain the Section Inertia inputs into the Snow Load equation.

Section Inertia Analysis:

Section Inertia Analysis, Bow Roof Front

Section Inertia Analysis, Center Reinforcement Rail

Section Inertia Analysis, Bow Roof Rear

Snow Load Calculation

S No Iy Y R Lx alpha S Q Status

1 3.75 881.5 40322.64 1094.5 0.7211 440.07 16.48 Pass(>3.1)

2 2.12 1431.6 40008 1106.66 1.086 508.52 5.39 Pass(>3.1)

3 3.66 1887.74 40042.01 1132.5 1.222 505.22 8.32 Pass(>3.1)

The Judgement condition is: Qr ≥ 3.1

Based on the Judgement condition, all the sections passed the condition for snow load criteria.

Conclusion: We successfully completed the design. The structure as given pass the Snow Load test.