Roof Design
Introduction:
Roof is an integral part of the car or any automobile because the passangers requires a shield which protects them from varying weather conditions. The passangers sort to automobiles like cars for the comfort it offers during travelling from one place to another .
Designing of Roof:
The starting point of the roof design is the roof styling surface which is got from the styling team who take into consideration of all the aesthetic considerations of the design
The roof design engineer must take into consideration of three important factors before proceeding with the design .They are
1.Front visibility criteria
2.Rear visibility criteria
3.Head room clearance
The roofs are not the load carrying member but they should be designed to carry loads in order to remain strong without any dents or permanent deformation during the periods of unforseen situations.
Reinforcements for roof
The roofs are reinforced with five structures which acts as support members.They are
1.Front roof rail
2.Rear roof rail
3.Center reinforcement roof rail or Center bow roof rail
4.Front bow roof rail
5.Rear bow roof rail.
Design considerations:
The Draft angle should be greater than 7 degrees.
Since the head room clearance should be taken into considerations, the desirable height of the bow roof rails added to support the roof surface should not be greater than 20 mm and the number of bow roof rails and the location of bow roof rails are optimized by taking into consideration of the empirical formulae like Bow load prediction formula and Snow load prediction formula.
Rear roof rail master section reference image
Formulas :
Bow–roof prediction:
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
R = 2(Rx*Ry)/(Rx+Ry)
L = Distance between the front end to the rear end of the roof surface, excluding the roof rails . Roof Length in X-Direction[mm](Roof dimension in 0-Y)
R = Roof curvature which is the geometric average of the roof's radius i
Rx = Radius of the roof surface in X curvature
Ry = Radius of the roof surface in Y curvature
t = Roof plate thickness [mm]
l = Bow Roof Span [mm]
Snow load Prediction:
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)
s = L1/2 + L2/2
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]
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
Roof Design Flanges and reinforcements:
All the roof rails and reinforcements are designed by taking into consideration of the draft angle and also made sure that all the sharp corners are avoided by applying blends of the surfaces
Front Roof Rail:
Rear Roof Rail:
Center Reinforcement:
Bow Roof 1:
Bow Roof 2:
Assembly:
Heat Distortion Criteria:
Bow Roof prediction
W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68
R = 2(Rx*Ry)/(Rx+Ry)
Judgement Condition : OK< 2.7> 3.1
The values that must be got from the Design are L,R,t and l
Snow Load Prediction:
This empirical formula is used to predict whether the car's roof surface is designed strong enough to carry the accumulated weight of the snow or ice
Procedure:
Create a center line in oy direction from there offset 100mm on both side and make centerline between the FRR to BR1 and offset by 100mm on both side so we get three points on horizontal and vertical direction.
Formula:
Qr = [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 10-8]
α = My x Lx2 x 10-12 , My = Y(Ly -Y)
s = L1/2 + L2/2
Judgement condition = Qr ≥ 3.1 ; 250 ≤ s ≤ 380
Moment of Inertia :
Iy for Section Bow Roof 1
Iy for Section Center Roof Reinforcement
Iy for Section Bow Roof 2
Draft Analysis:
Draft analysis is used to predict the manufacturable feasibility of the part.
The draft angle provided for design is around 7 degrees.
Roof Styling surface
Front Roof rail
Rear roof rail
Center roof reinforcement
Bow Roof1
Bow Roof 2
Success rate of Criteria:
The Design passes both the Bow load prediction as well as the snow load prediction criteria.So there is no modification needed in the roof design
Conclusion :
Roof design is made as per the design and the engineering standards.