Roof Design
ROOF DESIGN
For the given roof styling, develop the essesntial flanges and reinforcements, provide appropriate dimensions and check for draft analysis and to perform roof curvature study using NX-CAD
Introduction:
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.
The roof is a styling surface part which is fixed with the BIW body through the rails and panels inside the upper roof to provide it strength and stiffness.
Ditch area plays an important role here to fix the reinforcement on the component.
The rails and reinforcements are joined to the ditch area with a spot weld where it's not the styling part.
The required tool opening angle is minimum 3 Degrees.
Factors which is considered before creating Ditch area,
- Headroom clearance
- Visibility criteria
- Back door assembly
- Windshield assembly
Software Using NX CAD:
Class A Styling Input
- After the file is opened, the roof part is not joined together but as the individual surfaces.
- It is also seen as the surface is perfectly symmetrical in shape which helps us to work on the half of the portion and reflect it which creates the whole roof. We work on the half of the surface and mirror to complete the rest.
Components of roof:
- ront roof rail
- Rear roof rail
- Bow roof
- Centre roof rail
- Ditch area
Front roof rail:
It is created using Given reference master section
Front roof rail is used to provide strength to the roof panel front and it also acts as link between windshield glass, roof panel and headliner. Spot welds are applied at wind shield end of the rail and the other end is joined with roof using mastic sealent which also increases the strength.
Rear roof rail:
It is created using Given reference master section
Center Roof rail:
Center roof rail will be at the centre of the roof to provide strength and stability to the roof. Here the centre roof rail is joint with the roof through mastic sealent.
Bow roof rails:
The bow roof rails provide additional support to the roof and are at least 1mm thick, they are positioned in such a way that they provide maximum support to the roof, Care must be taken so that they do not intrude into the headroom requirements of the car. Bow roof rails are an optional feature provided as required by the design. Small cars may have none while large ones may have 3 or more, they may also be integrated into the centre roof rail in some cases. The roof is fixed to the roof rails with the use of mastics while the ends are spot welded
Final Model:
Curvature study on the Roof:
After getting the final body it has to be checked and justify that it passes the standards.
Analysis:
It is done to justify the position of the bow roofs.
1.Roof crash test.
2.Snow load test.
3.Heat distortion criteria
Heat distortion criteria:
Heat distortion temperature is a temperature at which the metal softens and deform under a specific load.This study is done to check the ability of the roof to withstand high temperature with respect to the position of the reinforcement which contribute to the increase strength.
- To calculate the heat distortion criteria by the bow roof position formula,
- We need to create a sketch , draw a line on the center of the roof on OY axis (mid horizontal axis)
- Next draw a line from the center line by a 300mm distance
- After that draw a line from 100mm distance on both sides of the 300mm line
- Then draw a line between the center of roof rails and draw the lines from the center line by the distance of 100mm on both sides.
- Atlast make an intersection on these lines from a coincident line to get points.
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
The test value infer that the design is good and under the limit of standards.
Snow Load criteria:
Snow load criteria study is done to check the ability of the to withstand snowfall and bounce back to its orginal shape without any permenant deformation once the load is removed.
- To calculate the snow load condition, we have a snow load formula
- To measure the required values
- Created a sketch and draw a line on OY axis from the center of the roof
- From that line draw a parallel line distance of 100mm on both sides
- Then make intersection points of these lines and draw a three point arc
SNOW LOAD:
Snow load criteria study is used to check the durability of the roof to withstand the snow fall on the roof and reflex it is original shape without any permanent deformation after load being removed.
SNOW LOAD CRITERIA:
Qr = [IY x t2] / [α x s x [(RX + RY)/2]2 x 10-8]
Where,
α = My x LX2 x10-12, 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 rails 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],
With of the roof panel exposed on the surface.
Y = Distance between Front roof rails 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 [mm4],
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[mm4],
Roof panel X curvature radius of length s in side view.
RX & RY (X & Y Curvature):
At 0-Y axis on perpendicular plane to roof. Draw 100mm lines offset to either side of the of 0Y axis line. Now center lines are drawn between each adjacent roof rail respective. Intersection points are marked on the sketch. Make all the geometry in construction lines except points.
Project the intersection points in the sketch are projected to roof surface.
Join the project points using arc curve and measure the radius value using minimum radius comment.
| S.N0 | BOWS | IY | α | Lx | Ly | y | s | Rx | Ry | t | QR | RESULT |
| 1 | FR&BOW1 | 310 | 0.80 | 1018.28 | 2028 | 514 | 366.23 | 4457.20 | 2708 | 0.75 | 107.18 | 0K |
| 2 | BOW1&CEN | 338.6 | 0.76 | 1052.78 | 2028 | 436 | 222 | 3777.80 | 6362.93 | 0.75 | 76.458 | 0K |
| 3 | CEN&B0W2 | 312 | 0.21 | 1039.64 | 2028 | 444 | 226.443 | 4237.01 | 10606 | 0.75 | 27.603 | 0K |
| 4 | BOW2&RR | 2028 | 350 | 350.23 |
Draft Analysis
It is done to check if the features designed clear the minimum draft 5 degree for manufacturabililty. This type of analysis is performed based on color ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.
It is seen that the features designed clears the minimum draft angle which denoted in 'green'
Conclusion:
- Essential roof components such as bow and center rails required with flanges and reinforcements are designed.
- Curvature study on roof was analysied with heat distortion and snow load standards
- Draft analysis was done to check toolablilty.