Roof Design
A REPORT ON ROOF DESIGN
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.
Variations
In later years, many variations on the automobile roof developed. These include:
SUNROOF
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. Previous terms include Sunshine Roof, Sliding Head and Sliding Roof.
QUESTION –
For the Given Roof styling, create an ASSEMBLY by developing the essential flanges for the outer roof and creating the reinforcements.
Provide appropriate dimensions and check for Draft analysis. Do a curvature study on the roof and perform calculations to determine the position of the Bow-roofs. Suggest and implement remedies in the design if the criterion is not full-filling . Start with the creation of a roof ditch area with a tool opening angle of 7 degrees.
Develop the reinforcements as separate parts.
INPUT –
Class A Surface
DESCRIPTION
DESIGN CRITERIA FOR ROOF DESIGN
- Visibility criteria
- Head room clearance
- Mastic distance
- Material selection
- Depth Hight of the Reinforcements
- Weight and Strength
- Positioning of Bow Roof
- Back Door Position
- Load Criteria
VISIBILITY CRITERIA
Visibility of the driver in an automotive is one of the major design consideration. Thus the Visibility criteria lines should be given to a Design engineer from the project team in order to consider it in his/her designs.
While designing the Roof outer panel and the front roof rail we need to consider the visibility criteria. The picture below gives a clear understanding about this visibility criteria Here as the length L ‘which is the length of the front ditch area increases its going to reduce the visibility. So it is one of the consideration here Also as the depth of the Front roof rail D increases , again the visibility decreases. This is the second consideration So its mandatory to maintain the visibility criteria which is given from the project management team in Both of these designs.
HEAD ROOM CLEARANCE
Here the Factor which effects the head room clearance is the width of the Front Roof Rail. As the width of the Front Roof rail increases ,the Head room clearance is getting reduced.so its necessary to maintain the width of the Front Roof Rail according to the Head room clearance data given by the project team.
POSITIONING OF REINFORCEMENTS
Positioning of the bow roofs have the major role in the strength of the Roof. The roof must be capable to withstand high temperature and load with respect to the positioning of Reinforcements which contributes more strength. So we have two criteria to check whether the position of the Bow roof are correct or not.
HEAT DISTORTION CRITERIA
The heat distortion value ‘W’ must be less than 2.7 to ensure that , the roof can withstand high temperature .
SNOW LOAD CRITERIA
In the winder season , in some countries there will be a huge load of snow on the roof top of the car. So for the roof it is necessary to have enough strength to bear this snow load. Thus the snow load value ‘Qr’ for the roof must be grater than 3.1
Taking all these design consideration we are creating all the components of the roof one by one
The first part to be created is the Roof outer panel
PROCEDURE
1) ROOF OUTER PANEL
Here I have the styling surface as reference, so I only need to create the side flange and the ditch area in the roof. As I doesn’t have any data from any other parts of the car , I am taking a given master sketch as another reference. So using the master sketch I have created a side flange with an opening angle of 7 degree and another flange , which is the ditch area od length 22 mm
Also I Added some patterns on it to increase its strength. After creating the part I have checked Draft Analysis of it.
Draft analysis
For draft analysis I have defined a tooling direction which is perpendicular to the part
Here I could find a negative draft angle at the rear side of the roof. But it is negligible in this case because I have assumed a car having backdoor with the spoiler in it . So while opening the back door the Spoiler should go inside this space. Thus a positive draft angle is cannot be achieved
2) Front Roof Rail
The same master sketch taken for creating the ditch area in the roof is taking here as the reference. I have used wave geometry linker to take the roof outer surface and offset it to needed values and them trimmed of the extra materials.
One of the major consideration is the depth/height of the Front roof rail as it effects the visibility criteria. Here we doesn’t have any data rather than the master sketch. So I am taking 32 mm the height of the Front roof rail.
Draft analysis
Same as in the Front roof rail, I have drawn a perpendicular line and then it is tilted to 7 degrees for achieving an accurate tooling direction. After analysis there are no negative draft angles shown.
3) Centre Reinforcement
As in the Front Roof rail I have used wave geometry linker to take the outer surface of the roof. Then offset it to certain values and trimmed the extra materials to create the part. Total width of the Center reinforcement I have created is 92 mm and the height/depth is 15 mm .
Master section of Center Reinforcement Rail
Draft analysis
Here I have taken the Y axis as the tooling direction and after the draft analysis I could find zero negative draft angle
4) Bow Roof 1&2
Apart from the center reinforcement I have created two Bow roofs , one in between the Rear roof rail and the center reinforcement and the other is in between Rear roof rail and the center reinforcement. Method of creation for each Bow roof are same as the center reinforcement . But here the width and the Height varies , Width is 70 mm and the depth is 20 mm. Foe both the Bow roofs these values are same. Also to reduce the weight of the part I have done some cut outs From the top surface of the Bow roof as shown in the figure.
Draft analysis
There are no negative draft angles
5) Rear Roof Rail
Designing of the Rear roof rail is exactly same as the Front roof rail.
Draft analysis
Here I have taken the y axis as the tooling direction and after analysis there is no negative draft angles After designing and the assembly is complete , we should check whether the positions of the reinforcements are correct and whether they can give enough support to the roof to bear enough load .Also whether the roof can withstand high temperature with out any deformation .For that we need to do two calculation .
Heat Distortion calculation
This is the calculation to check whether the roof can withstand high temperature with respect to the positioning of the reinforcements with out any deformation .
The Heat distortion criteria is W < 2.7
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]
|
Sr no |
Bows |
Rx |
Rx |
R |
l |
L |
t |
W |
Result |
|
1 |
FR-1st |
2722.684 |
5726.22 |
3690.582 |
432.0282 |
2030.1 |
0.75 |
1.643273 |
Pass |
|
2 |
1st -CTR |
6765.348 |
4829.862 |
5636.068 |
452.4177 |
2030.1 |
0.75 |
2.113275 |
Pass |
|
3 |
CTR-2nd |
11109.44 |
3874.178 |
5744.935 |
391.9502 |
2030.1 |
0.75 |
2.077323 |
Pass |
|
4 |
2nd-RR |
14755.34 |
3253.913 |
5331.992 |
308.002 |
2030.1 |
0.75 |
1.872151 |
Pass |
The measured values and the calculations are depicted in the table below Here in each cases this model pass the heat distortion criteria.
Snow Load calculation
In the winder season , in some countries there will be a huge load of snow on the roof top of the car. So for the roof it is necessary to have enough strength to bear this snow load.
The snow criteria is Qr ≥ 3.1
We have the equation
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]
Geometrical moment of inertia of Roof BOW (Y cross-section )[mm4]
FR ROOF RAIL
CENTRE ROOF RAIL
BOW ROOF 1
BOW ROOF 2
RR RAIL
The measured values and the calculations are depicted in the table below
Here we can see that in the two cases the Qr value doesn’t meet the Snow load criteria , ie the QR value is not grater than or equal to 3.1. This means that ,the bow roofs cannot give enough strength to the roof with respect to that position. So to pass the snow load criteria its needed to recreate the Bow roof by changing its position .
So here I have created Some embosses as shown in the picture with out affecting the good looks of the Roof
The Final model after doing the embosses is given below
CONCLUSION -
Roof assembly has been made by following all the engineering standards and considerations.
Heat distortion criteria is been successfully passed.
Snow load criteria has been failed in some cases and to over come that Embosses are added on the Roof outer panel
Please find attached zip file