Roof Design
ROOF DESIGNING
Objective : To design the roof outer and it's supportive components from given skin data by following the given master section for a vehicle (car) as per designing standard ,in such a way that it should be able to withstand the effect of snow load and heat distorsion.
Introduction : Roof is an integral part of a car or any automotive vehicle because we have required a shield which protects us from varying weather conditions like as rain, snow fall, air, dust particle, high temperature etc. Roof also protect us during rollover crash of the vehicle.
Car with Sunroof Feature Car without Sunroof Feature
Car Roof During Snow Fall
Car Roof During Rollover Crash
Designing of roof :
For roof designing we get the skin data from styling team who took the consideration on various point like as aesthetic view, curvature ,aerodynamics shape etc. The roof design engineer must take the mandatory consideration for roof design , which are as follow :
1) Visibility Criteria ( Front Visisbility Criteria and Rear Visibility Criteria )
2) Head Clearance Criteria
3) Curvature Study
4) Heat Distorsion Criteria and Snow load Criteria
The roof designing or roof manufacturing is not done in a singal part. In complete roof the roof outer is assembled with some additional part , these additional part improves the strength of the roof and also helpfull to assemble the other parts of vehicle with roof assembly. All component and reinforcement of car roof assembly are as follow :
1) Roof Outer Panel
2) Front Roof Rail
3) Rear Roof Rail
4) Center Roof Rail
5) BOW Roof 1
6) BOW Roof 2
1) Roof Outer Panel : The roof outer panel is the outermost sheet metal component of the roof which is aeroddynamic in shape and it consist of some other feature like as sunroof cut out, some fixtures or carriages to carry luggage on the top of the car. The thickness of the part should be around 0.75 mm and minimum draft angle is 7 degree. All the sharp edges are filleted with appropriate radius as per master section.
Master Section Used for Roof Outer Panel
Roof Outer Panel Draft Analysis
2) Front Roof Rail : A front roof rail is a reinforcement placed in the front side of the roof. Front roof rail attached with the roof outer panel by the help of spot welding which is a type of resistance welding. This reinforcement connects with side sections of the car and front windshield. Designing of the front roof rail is done on the basis of given master section . Some embosses are also created to improve the strength of the reinforcement.
Given master section for Front Roof Rail
Draft Analysis of Front Roof Rail
3) Rear Roof Rail : Rear roof rail is placed in the back side of the roof. Designing of the rear roof rail is done on the basis of given master section. Some embosses are also provided to increase the strength of the part. Minimum draft angle is 7 degree and all sharp edges are filleted with appropriate radius. Rear roof rail will attached with the roof outer panel by the help of spot welding. For spot welding the minimum width of the flange should be 10 mm.
Given master section for Rear Roof Rail
Draft analysis of Rear Roof Rail
4) BOW Roof-1 and BOW Roof -2 :
BOW Roof-1 and BOW Roof-2 are used to provide additional strength to the roof . We can use more numbers of BOW roof but we have create minimum two BOW roof . If the length of the bow roof is large then we can create more numbers of BOW roof. Generally BOW roof are positioned where the curvature of the roof is less, because in that portion the strength of the roof will be less. Here BOW roof 1 is provided between the front roof rail and center roof rail, and BOW roof-2 is provided between the center roof rail and rear roof rail. Correct position of BOW roof is depend on the snow load calculation and heat distortion calculation. Bow roofs are attached with the roof outer panel by the help of spot welding in ditch area and by the help of mastic sealent in mastic cut out area.
Given Master Section For The BOW Roof Rail
BOW Roof Rail 1 Bow Roof Rail 2
Draft Analysis of BOW Roof Rail 1 Draft Analysis of BOW Roof Rail 2
5) Center Roof Rail : Center roof rail is also used to increase the strength of the roof .The position of the center roof rail is at the middle of the roof. Center roof rail is more important because in this area the flatness of the roof is more hence the strength will be less. Also during the rollover accident that middle portion comes in the contact of the road more. The center roof rail also attached with roof outer panel by the help of spot welding in the ditch area and by the help of mastic sealent in the mastic cut out area.
Given master section for Center Roof Rail
Draft Analysis for Center Roof Rail
Assembly Of The Roof With All Reinforcements :
CALCULATION OF BOW ROOF REINFORCEMENT PREDICTION :
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
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]
Heat Distorsion Calculation Table :
Snow Load Prediction Criteria :
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
Case : 1 : BOW Roof Rail 1 :
For BOW Roof Rail 1
MOI min = 882.43 mm^4
Case : 2 : Center Roof Rail :
For Center Roof Rail
MOI min = 1125.95 mm^4
Case : 3 BOW Roof Rail 2 :
For BOW Roof Rail 2
MOI min = 900.45 mm^4
Snow Load Prediction Calculation Table :
Conclusion :
From above analysis we found that our roof design following the criteria of snow load prediction in all three conditions. But in heat distortion criteria the roof design following the criteria in three cases out of four cases. In the case of BOW RR1 -CRR the roof design is not following the judgement criteria. To overcome from this problem we created embosses in outer panel of the roof. These embosses will provide additional strength to our roof.