Roof challenge
Mayur Vispute
Project- Roof Design
Aim-
To develop the essential flanges and reinforcements with appropriate dimensions and checking draft by performing Draft analysis, Calculating Section inertia, Heat distortion and Snow load on the roof.
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 element 
Roof is designed by following some criteria-
- Visibility criteria as safety and functionality
- Head clearance for ease and comfort
- Draft analysis fro easy manufacturing and tooling
- Safety parameters for users
- Heat distortion
- snow load criteria
The Automobile roof consists of the following parts:
- The outer panel
- Front roof Rail
- Bow Roof
- Centre Roof Rail
- Rear Roof Rail
FRONT ROOF RAIL:
The front roof rail is created at the front side of the roof to provide strength to the roof. Headroom clearance and the driver visibility are the two important factors to be considered while designing the front roof rail. the front roof rail is connected to the roof outer panel with the help of mastic seal.
BOW ROOF 1:
It is used to give strength to the area between the front roof and the centre roof rail. Also it is created in the area between the centre roof rail and the rear roof rail to provide enough strength. It can be created in any numbers depending upon the roof length but the position of the bow roofs is to be justified by curvature study.
BOW ROOF 2:
CENTRE ROOF RAIL:
It is created at the centre of the roof. It is connected to the roof by mastic seal.
REAR ROOF RAIL:
It is created at the rear end of the roof. The fitment of the backdoor is to be considered while designing the rear roof rail.
ROOF:
Roof Assembly:
Roof Bow Calculations
Heat Distortion Calculation
W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/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]
Except Rx and Ry all values can be measured on the model.
To get the Rx and Ry curvature a sketch was required-
- A sketch was created on a plane on top of the Roof
- 0-Y line was created in the X-Direction in center of roof
- Then a Horizontal line below 300mm was created named as -3Y Line
- Two lines on both sides at 100mm were created from -3Y Line
- Intersection point on line was created and projected on outer Panel of Roof
Projected Points
Then Three point Arc was created by using these points.
TO MEASURE Rx:
TO MEASURE Ry:
TO MEASURE l (BOW ROOF SPAN):
TO MEASURE L:
By using all Values Heat Distortion was Calculated in the MS Excel
According to Judgement Condition : OK< 2.7> 3.1
All Values are under Judgement Condition.
Snow Load Calculation
During Winter Season the snow gets clogged over the top surface/roof of the car and the snow has some mass on it. Due to this, the roof can fail if the snow load exceeds the capacity of the roof that was designed to sustain.
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 get the values a sketch was created on top of Roof
- O-Y line was created in the center of roof
- A line between First and center Roof rail was created
- Then two lines on both sides at 100mm was created
- Similar lines were created between other center roof rail, both Bow Roof rails and rear roof rails
- Intersection points were created and projected on the roof
Projected points for Snow load Calculation
Then Three point Arc was created by using these points.
TO MEASURE Ly:
TO MEASURE Lx:
TO MEASURE Ry:
TO MEASURE Rx:
TO MEASURE s:
For Section 1, s = L1/2 + L2/2
For Section 2, s = L2/2 + L3/2
For Section 3, s = L3/2 + L4/2
TO MEASURE Y:
By using all Values Snow Load was Calculated in the MS Excel
So, according to the Judgement condition = Qr ≥ 3.1
And, Here two result of Qr is not fulfilling the demand. So, there are some solutions, that the model can pass over to the styling team to change the curvature to improve the strength, Bow Roof rails position can be changed to the weaker side of roof or Embosses could be created on the outer panel to improve the strength.
To improve the strength, the embosses are created.
Section Modulus:
The section modulus of the cross-sectional shape is of significant importance in designing of structures. It is a direct measure of the strength of the structure. A structure that has a larger section modulus than another will be stronger and capable of supporting greater loads.
BOW ROOF 1:
S=I/Y
I=178.1314mm4
Y=605.4
S=0.2942mm3
CENTER ROOF:
S=I/Y
I=709.495mm4
Y=1046.4
S=0.678mm3
BOW ROOF 2:
S=I/Y
I=189.314mm4
Y=1547.3
S=0.1223mm3
Draft Analysis:
Draft analysis was performed with 7 Degrees Value(generally given in sheet metal)
Front Roof rail
Center Roof rail
1st Bow Roof
2nd Bow Roof
Rear Roof rail
Roof Outer Panel
Conclusion:
The Roof was created in Siemens NX Student Version with all the required parameters and different Calculations were done for safety concerns.