Roof Design
DESIGN OF AUTOMOTIVE ROOF
OBJECTIVE
The mian objective of this project is to understand the design process to design the roof with all the roof rails and to understand the calculations behind it. We also get to learn about the different tools used in NX cad for designing.
INTRODUCTION
WHAT IS AN AUTOMOTIVE 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.
TYPES OF ROOFS
CONVERTIBLES
A convertible is a passenger car that can be driven with or without a roof in place.Convertibles are one of the most popular car types in the automotive industry. The free open space above the head attracts a large number of buyers. Convertibles have an aesthetically pleasing design that excites most car enthusiasts. The way the roofs in the convertibles open has also evolved over the decades as per popular demands. The technology has changed, and so has the look, and thanks to technology. Based on the materials used in roof they are further classified into Hard top and soft top convertibles. These are operated electrically. While there is another category which is hand operated manual convertibles.
HARDTOPS
A hardtop is a rigid form of automobile roof , which for modern cars is typically constructed from metal. A hardtop roof can be either fixed (i.e. not removable), detachable for separate storing or retraceable within the vehicle itself.
SUNROOFS
A sunroof is a movable panel that opens to uncover a window in an automobile roof, allowing light and 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.
MATERIAL USED
- Soft-tops are made of vinyl, canvas or other textile material
- Hard-top convertibles have roofs made from steel, aluminium, plastic or carbon fibre composites.
MAIN PARTS OF THE ROOF & DESIGN CONSIDERATIONS
- The roof has an outer panel which is the styling part.
- Strengthening parts are present below an outer panel of a roof, which are front roof rail, Bow roof, Centre roof rail, and Rear roof rail.
- The front roof rail provides provision for the windshield glasses. Visibility criteria and headroom clearance have to be followed while designing the front roof rail. Provision for windshield mounting and inside rearview mirror is provided on the front roof rail.
- The Rear roof rail has hinge mounting for the back door. It is attached at the portion where the back door meets with the roof.
- Bow roofs are provided in between the front roof rail and center roof rail as well as the Rear roof rail and center roof rail. They are provided to enhance the localized strength of the roof and are attached directly to the bottom of the roof with either spot welding or with the help of Mastics.
- Reinforcement center or center roof rail is used as a cross member between Left hand and right-hand bodysides.
While designing of roof following considerations have to be taken into account.
- Visibility Criteria
- Head clearance
- Curvature Study
- Head Distortion and snow load criteria
- Clearance of 5 mm should be present between outer roof rail and Front roof rail, Rear roof rail, Bow roof, and Reinforcement center where mastics are used for adhesion.
- Thickness for all sheet metal parts - 0.75 mm.
- All edges should have fillets and there should not be any sharp edges present.
- A minimum draft of 7 degrees is required for all the faces.
- Mastersections provided should be strictly followed.
WHY REINFORCEMENTS?
If the roof is flat it is very weak and cannot able to withstand the forces like snow, so inorder to increase the strength of the roof these reinforcements like front roof rail, centre roof rail, rear roof rails and the bow roofs are introduced and joined to the flat surface of the roof with the different joing process.
Front roof rail
- It is used to join the roof with front glass, and chassis via A-pillars.
Rear roof rail
- It is used to join the roof with back door and chassis via C-pillars.
Reinforcement centre
- It is used to join the roof with the B-pillar.
Bow roofs
- It is used to increase the strength of the roof.
- A number of bow roofs are decided based on the length of the roof and strength required.
SAFETY TESTS
Roll Over Test On Roof
- The rollover test is given by the Insurance Institute of highway safety ( IIHS). After analyzing many accidents IIHS has fixed some regulations about how to perform the test.
- IIHS rating is based on the following things which are how fast a car can reform its original shape after rollover and how deep a protrusion is.
- The aim of this test is to minimize the protrusion of the roof.
- Two conditions need to be followed while performing this test. One is to lock all the doors of the car properly and the second is to close all the windows of a car.
- Also, it is mandatory to fix the chassis on the hard lever surface horizontally.
- Load is applied at 25 degrees tilt when view from the front over the length of 762 mm.
- Load is applied at 5 degrees tilt when view from the side, over the length of 1829 mm.
MODELLING & DRAFT ANALYSIS
Front roof rail
- It is used to join the roof with front glass, and chassis via A-pillars.
- All the faces of Front Roof Rail have successfully cleared the draft analysis with a minimum draft angle of 7 degrees.
Bow roofs
- It is used to increase the strength of the roof.
- A number of bow roofs are decided based on the length of the roof and strength required.
- All the faces of Bow Roof have successfully cleared the draft analysis with the minimum draft angle of 7 degrees.
Rear roof rail-
- It is used to join the roof with back door and chassis via C-pillars.
- All the faces of Rear Roof Rail have successfully cleared the draft analysis with a minimum draft angle of 7 degrees.
SECTION INERTIA ANALYSIS OF ROOF RAILS
| S= I/y |
Where,
S = Section Modulus(mm^3)
I = Moment of Inertia(mm^4)
y = distance between the neutral axis and the extreme end of the object (mm)
FRONT ROOF RAIL
Rear ROOF RAIL
1st Bow Roof Rail
Center Bow Roof Rail
2nd Bow Roof Rail
CALCULATIONS
Heat Distortion Analysis
- Heat distortion analysis plays a pivotal role in the strength of the roof panel. It is the temperature above which material cannot be used for structural applications.
- This study is used to predict the heat distortion temperature where the material starts to soften when exposed to a fixed load at elevated temperature.
- In order to avoid any bending or damages on the roof, heat distortion temperature can be recalculated using the trial and error method by changing the position of bow roofs.
Bow–Roof Prediction Formula
W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68
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]
Judgment Condition
OK< 2.7
Snowload Analysis
- The purpose behind this analysis is to check the strength of the roof when snow is present on it acting as a load.
- A roof should be designed in such a way that it should rebound to its original shape after the removal of snow.
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)
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]
The 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 = L1/2 + 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
Judgment condition
Qr ≥ 3.1
250 ≤ s ≤ 380
Now since we these values for the snow load values it does not satisfy the condition.
So to make the roof stronger we create embossses on the roof to strengthen the roof.
FINAL OUTPUT
CONCLUSION
We have sucessfully designed the automotive roof & all the necessary roof rails for better strength, stability and support. Various calculations were carried out to check the feasibilty of the roof rails positioning and snow load criteria. On calculating we found out that the roof did not pass the snow load test. To counter this we created embosses on the roof outer panel to add strength the roof.