Roof challenge
AIM
Design a BIW of car roof.
OBJECTIVE
- Create an assembly of car roof for the given roof styling with essential flanges of roof outer surface and reinforcements like ditch area of outer roof, front, centre & rear roof rail and Bow roofs.
- Perform the draft analysis of each reinforcements
- Perform heat distortion and snow load criteria for Roof.
THEORY
- Curvature study i.e. Bow-Roof and Snow load criteria
Design Roof Ditch Area and its Reinforcements
Roof Ditch area
To Develop the ditch area of roof, considered the visibility criteria from front side and also provide backdoor clearance from the backside.
The angle of roof front ditch area restricted to 3 degree (positive draft) for better visibilty criteria but for rear side, provide negative draft angle to easily opening of backdoor.
Front roof rail
While designing the front roof rail, head cleanance and visibility criteria should be considered.Also consider the mounting of windshield and rearside mirror inside cabin.
Master Section of front roof rail
Design image of front roof rail
Rear roof rail
Rear roof rail has hinge mounting for the back door.
Master Section of rear roof rail
Design image of front roof rail
Bow roofs
To enhance the strength of roof, bow roofs are provides between roof rails as per requirements.
Master Section of Bow roofs
Design image of Bow roof 1
Center roof rail
Bow roof 2
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.
Front roof rail
Front bow roof
Centre roof rail
Centre Reinforcement
Bow roof 2
Rear roof rail
HEAT DISTORTION CRITERIA
Heat distortion 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
Calculation
From the above table, the value of W between the first bow roof and centre roof rail is failed to satisfy so to overcome this we introduced embosed on roof outer panel between Bow rails to increase the strength and stiffness of roof panel or another option is to shift the bow toward front to get the value of W less the 2.7
Here, shift the front bow 80mm toward front then calculate the value W.
From the above table we concluded that the value of W less than 2.7
SNOW LOAD CRITERIA
Snow load is the downward force on a building's roof by the weight of accumulated snow and ice. The roof or the entire structure can fail if the snow load exceeds the weight the building was designed to shoulder.
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)
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
Calculation
After calculating the Qr value, the value between all reinforcements are failed to satisfy snow load criteria so to get the value of Qr above 3.1 here, increased 2mm depth of each reinforcements to improves the Area MOI of all reinforcemnts sections then again calculating the value of Qr.
From the above table it can be concluded that all values of Qr are satisfy the snow load crietria.
RESULT
Position of all roof outer panel reinforcements.
CONCLUSION
From the given roof styling, design essential flanges and reinforcemnets and perform draft analysis of reinforcements and also study the position of bows and curvature of roof top by calculating Heat Distortion and Snow Load criteria. Thus design the BIW of car roof with considerartion of visibility and head clearance.