Roof Design
Roof Design
Explanation
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.
When it is seeing from outside of the car, it is just closing the top portion. But in design point of view, it is hard to create the design. Because of you have to create no of support rails for enough strength to withstand the load. Bow roofs are primarily fixed in flatter region to enhance strength.
Front roof and rear roof rails are fixed to the ditch area using spot weld operation. But front, rear bow roof and center roof rails are fixed to roof using mastic sealants. We don’t want so much gap between roof rails and bow roof area. So, we reducing the NVH issues.
DESIGN CONSIDERATION OF ROOF
- Visibility criteria
- Head room clearance
- Bow roof position
- Heat distortion and Snow load criteria
- Draft analysis
Master section data for front roof rail, rear roof rail, bow roof front, bow roof rear, Center roof rail and reinforcement.
Using these master section as referance we were going to develop the front and rear roof rails for roof design
FRONT DITCH AREA
Front ditch area is creating with help of the dump solid of class A surface developed by considering the parameters to design the front ditch area,
We have to create the extension sheet with minimum 15mm, and give the positive draft angle 7 degree considering the following drawing operation
FRONT ROOF RAIL:
Front roof rail is used for supporting the roof for externally applied forces occurring at accidents. When we increasing the front roof rail is good for strength but also it is not for drivers because of it hide visibility of road.
When considering the parameters of roof rail, give them 60-80mm length, proper draft angle and also using the embosses for increasing the strength. Front section of front roof rails spot welded to the front ditch area and other end is fixed with help of mastic sealants.
Sheetmetal thickness t = 0.75mm
REAR DITCH AREA:
It’s same to the front roof ditch area but we have to fix the rear door in this area, when considering the parameters to design the rear roof ditch area.
REAR DITCH AREA:
It’s same to the front roof ditch area but we have to fix the rear door in this area, when considering the parameters to design the rear roof ditch area.
REAR ROOF RAIL:
Rear roof rail is creating to length 80-100mm with proper draft angle. Give embosses on the sheet to ensure the enough strength for holding the roof structure. When considering the all parameters like length, width, sealants, rubber materials, wind shield to create the design properly.
Head room clearance = 120mm
Sheetmetal thickness = 0.75mm
BOW ROOF FRONT:
The bow roofs are given to the improve the torsional stiffness and load bearing capacity of the roof structure. When number of bow roofs are given by the roof dimension. Longer the roof area we have to give the more bow roof to the structure.
Positioning of the bow roofs are decided by the curvature study. The Heat distortion and Snow load criteria are checked to analyze the bow roof position by various condition.
We also give the Mastic area for fixing the Bow roof to the roof outer. Each mastic sealants are having 80mm circular effective area of surroundings. Thickness of the Bow roof is 0.75mm.
The bow roofs are given to the improve the torsional stiffness and load bearing capacity of the roof structure. When number of bow roofs are given by the roof dimension. Longer the roof area we have to give the more bow roof to the structure.
Positioning of the bow roofs are decided by the curvature study. The Heat distortion and Snow load criteria are checked to analyze the bow roof position by various condition.
We also give the Mastic area for fixing the Bow roof to the roof outer. Each mastic sealants are having 80mm circular effective area of surroundings. Thickness of the Bow roof is 0.75mm.
REINFORCEMENT CENTER ROOF RAIL:
Reinforcement Center roof rail is effective support to the flat area structure roof. Because of center area of roof is more susceptible to failure under action of load. Usually, Reinforcement center roof rails are fixed at the center of the roof for connecting to the B – Pillar support structure, it will used for roll-over test.
Center bow roof and reinforcements are placed at the center of the roof structure. They two are connecting to help of spot - welding operation. Thickness of the Reinforcement center roof rail is 0.75mm.
HEAT DISTORTION:
The heat distortion study the plays an important role in Sheetmetal application. Heat distortion is temperature limit above which material cannot be used for structural application. This study is useful to fin the where the material starts to soften when exposed to a fixed load at elevated temperature. In order to avoid the bending, damage to the roof based on the heat distortion temperature. This study will provide the which position of the bow roof give enough strengthen to roof.
Bow Roof Prediction Formula:
HEAT DISTORTION CRITERIA:
W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/2] + [1.10 x 10^(-3) x I]-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]
I = Bow roof span [mm]
Thickness for the Roof(t):
Roof Outer Panel, t= 0.75mm
Front Roof Rail, t = 0.75mm
Rear Roof Rail, t = 0.75mm
Bow Roof Rail Front, t = 0.75mm
Bow Roof Rail Rear, t = 0.75mm
Reinforcement roof rail, t = 0.75mm
| 1 | FRR & BR1 | 5693.244 | 3007.084 | 3935.5 | 514.6 | 2029.24 | 0.75 | 2,31,07,042.71 | 1.777 | OK |
| 2 | BR1 & CRR | 4723.685 | 7194.126 | 5702.856 | 393.6 | 2029.24 | 0.75 | 5,78,16,760.60 | 2.68 | OK |
| 3 | CRR & BR2 | 3837.294 | 11214.1 | 5718.026 | 403.2 | 2029.24 | 0.75 | 10,75,32,923.50 | 2.38 | OK |
| 4 | BR2 & RRR | 3235.521 | 14231.86 | 5272.662 | 284.1 | 2029.24 | 0.75 | 4,94,23,899.51 | 2.05 | OK |
From the above table we, conclude that all the values are of W< 2.7 and also W> 3.1. So, that all bow roofs areas are placed at the right place.
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
From the above results it can be seen that the designed roof passes Snowed Criteria.
SECTION MODULUS:
The moment carrying capacity of an object is directly dependent on geometrical property (I) and material property (E) of an object, which is collectively termed as flexural rigidity (EI). Geometry of an object plays an important role in load bearing capacity of an object which is indicated by moment of inertia of a section.
Therefore, section modulus is the predominant factor which evidences the strength of an object and is defined as the ratio of the moment of inertia of the object about its centroidal axis to the distance of the extreme fibers of the object from the neutral axis.
There are two types of section modulus we have,
- Elastic section modulus
- Plastic section modulus
Elastic modulus is defined as,
S = I/Y
S – Section modulus,
I – Second moment of area
Y – distance from the neutral axis to outer most fiber of the object.
BR2
BR1
CRR
DRAFT ANALYSIS:
The Draft Analysis command enables you to detect if the part you drafted will be easily removed. 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.
Minimum draft angle is 7 degree is considered for analysis. Green colour show positive draft angle of the part more than 7 degree
Draft analysis for TRR
Draft analysis for FRR
Draft analysis for CRR
Draft analysis for BRR2
Draft analysis for RRR
