Roof Design

 

Roof Design

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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.

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.

 

Front Roof Rail

Front Roof Rail is one which joins the windshield glass, body side outer and the inner panel. 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.

Front Roof View 1

 

Front Roof View 2

 

Front Roof Isometric View

 

Front Roof Draft Analysis

 

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.

Rear Roof View 1

 

Rear Roof View 2

 

Rear Roof Isometric View

 

Rear Roof Draft Analysis

 

Bow Roof 1

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.

Bow Roof 1 View 1

 

Bow Roof 1 View 2

 

Bow Roof 1 Isometric View

 

Bow Roof 1 Draft Analysis

 

Bow Roof 2

It is also same to the Front bow roof but the positioning bow roof only change. 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.

Bow Roof 2 View 1

 

Bow Roof 2 View 2

 

Bow Roof 2 Isometric View

 

Bow Roof 2 Draft Analysis

 

Centre Roof

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.

Centre Roof View 1

 

Centre Roof View 2

 

Centre Roof Isometric View

 

Centre Roof Draft Analysis

 

Curvature Study on the Roof

Heat Load Criteria

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.

 

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8)  x (R^2)/t] + [ 1.10x10^(-3) x I] - 2.68            

Where,

L = Roof Length in X-Direction[mm](Roof dimension in 0-Y) = 2015.7mm

R = Roof curvature

R = 2(Rx*Ry)/(Rx+Ry)

Rx = X curvature

Ry = Y curvature

t  = Roof plate thickness [mm] = 0.75mm

l = Bow Roof Span [mm]

 

Case 1

R = 3499.7348mm

I = 376.8853mm

W = 1.52

 

Case 2

R = 5514.4613mm

I = 419.1163mm

W = 2.01

 

Case 3

R = 5786.7744mm

I = 417.0956mm

W = 2.09

 

Case 4

R = 5355.368mm

I = 372.6776mm

W = 1.92

 

Snow Load Criteria

This test is done to know how is the roof behaving when there is a snow over. normally due to the snow weight the dent will happen. But the roof should be designed in such a way that when the snow is removal the roof should go it its original position. This is the basic requirements for snow load criteria.

 

Qr = [Iy x t²] / [α x s x [(Rx + Ry)/2]² x 10^-8]

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

Where

α = My x Lx² x 10^-12, My = Y(Ly-Y)

Judgement condition = Qr ≥ 3.1           

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

 

Case 1

M_y = 469*(1967.6-469) = 702843.4

L_x = 1139.3

s = (L₁ + L₂)/2 = (469+499.6)/2 = 484.3

I_y = 1.63 *10³

(R_x + R_y)/2 = (5739.7787 + 2517.3121)/2 = 4128.5454

α = My x Lx² x 10^-12 = 0.91

Q_r = 12.20

 

Case 2

M_y = 968.6*(1967.6-968.6) = 967631.4

L_x = 1086.3

s = (L₁ + L₂)/2 = (499.6+499.8)/2 = 499.7

I_y = 3.80 *10³

(R_x + R_y)/2 = (4975.5711 + 6184.2616)/2 = 5579.916

α = My x Lx² x 10^-12 = 1.14

Q_r = 12.05

 

Case 3

M_y = 1468.4*(1967.6-1468.4) =733025.28

L_x = 1058.3

s = (L₁ + L₂)/2 = (499.8+499.1)/2 = 499.45

I_y = 1.88 *10³

(R_x + R_y)/2 = (3995.4699 + 10489.6319)/2 = 7422.55

α = My x Lx² x 10^-12 = 0.82

Q_r = 4.68