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. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure and to with stand the over head forces like snow.

If the roof is flat it is very weak and cannot able to with stand the forces like snow, so in order to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process.

MASTER SECTION FOR FRONT ROOF RAIL:

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.

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. We also give the Mastic area for fixing the Bow roof to the roof outer. Each mastic sealants are having 80mm circular effective area of surroundings. Thickness of the Bow roof is 0.75mm.

REINFORCEMENT CENTER ROOF RAIL:

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. Center bow roof and reinforcements are placed at the center of the roof structure. They two are connecting to help of spot - welding operation. Thickness of the Reinforcement center roof rail is 0.75mm.

BOW ROOF REAR:

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. But, remember we place bow roof in flatter region for enhance strength of the roof.

HEAT DISTORTION CRITERIA:

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

R_X = X Curvature

R_Y = Y Curvature

t = Roof plate thickness [mm]

I = Bow roof span [mm]

Thickness for the Roof(t):

Roof Outer Panel, t= 0.75mm

Front Roof Rail, t = 0.75mm

Rear Roof Rail, t = 0.75mm

Bow Roof Rail Front, t = 0.75mm

Bow Roof Rail Rear, t = 0.75mm

Reinforcement roof rail, t = 0.75mm

Roof length L - 2127.6

BOW ROOF SPAN (I):

R_X Value:

R_Y Value:

FINAL RESULTS OF HEAT DISTORTION:

For front Bow roof -

W= 1.536

For Front centre roof -

W= 1.465

For rear bow roof-

W= 1.511

for rear roof-

W= 1.601

From the above table we, conclude that all the values are of W< 2.7 and also W> 3.1. So, that all bow roofs areas are placed at the right place.

SNOW LOAD:

Snow load criteria study is used to check the durability of the roof to withstand the snow fall on the roof and reflex it is original shape without any permanent deformation after load being removed.

SNOW LOAD CRITERIA:

Q_r = [I_Y x t²] / [α x s x [(R_X + R_Y)/2]² x 10^-8]

Where,

α = M_y x L_X² x10^-12, M_{y =} Y(L_y–Y)

Judgement condition = Q_r > 3.1

250 < s < 380

t = Roof plate thickness[mm]

L_Y = 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 rails front/ rear as the front and rear reference point.

L_X= Distance between the left and right end of the roof on the roof bow[mm],

With of the roof panel exposed on the surface.

Y = Distance between Front roof rails to roof bow [mm],

s = Distance for which roof bow bears divided load [mm],

s = L₁/2 + L₂/2

I_y = Geometrical moment of inertia of roof bow (Y cross – section) [mm4],

R_X = Lateral direction curvature radius of roof panel Y cross section on roof bow [mm4],

Roof panel curvature radius of the length L_X in front view.

R_Y = Longitudinal direction curvature radius of the roof panel X cross section on roof bow[mm4],

Roof panel X curvature radius of length s in side view.

L_X Values:

MOMENT OF INERTIA:

BOW ROOF FRONT:

α = 0.9925

Qr = 17.2

CENTER REINFORCEMENT RAIL:

α = 0.619

Qr = 12.61

BOW ROOF REAR:

α = 0.5948

Qr = 4.89

SECTION MODULUS:

The moment carrying capacity of an object is directly dependent on geometrical property (I) and material property (E) of an object, which is collectively termed as flexural rigidity (EI). Geometry of an object plays an important role in load bearing capacity of an object which is indicated by moment of inertia of a section.

Therefore, section modulus is the predominant factor which evidences the strength of an object and is defined as the ratio of the moment of inertia of the object about its centroidal axis to the distance of the extreme fibers of the object from the neutral axis.

There are two types of section modulus we have,

• Elastic section modulus
• Plastic section modulus

Elastic modulus is defined as,

S = I/Y

S – Section modulus,

I – Second moment of area

Y – distance from the neutral axis to outer most fiber of the object.

FRONT ROOF RAIL:

Moment of inertia (Max), Imax = 1.621 x 10⁴ mm⁴

Moment of inertia (Min), Imin = 911.62 mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 78 mm

For maximum MOI,

S = 1.621 x 10⁴ / 78

S_MAX = 207.82 mm³

For minimum MOI,

S = 911.62 / 78

S_MIN = 11.6874 mm³

^{REAR ROOF RAIL:}

Moment of inertia (Max), Imax = 9.705 x 10⁴ mm⁴

Moment of inertia (Min), Imin = 6.800 x 10³ mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 103 mm

For maximum MOI,

S = 9.705 x 10⁴/103

S_MAX = 942.23 mm³

For minimum MOI,

S = 6.800 x 10³/103

S_MIN = 66.019 mm³

^{REINFORCEMENT CENTER ROOF RAIL:}

Moment of inertia (Max), Imax = 8.391 x 10³ mm⁴

Moment of inertia (Min), Imin = 1.672 x 10³ mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 43.5 mm

For maximum MOI,

S = 8.391 x 10³/43.5

S_MAX = 192.896 mm³

For minimum MOI,

S = 1.672 x 10³/43.5

S_MIN = 38.436 mm³

^{BOW ROOF FRONT:}

Moment of inertia (Max), Imax = 4.033 x 10³ mm⁴

Moment of inertia (Min), Imin = 1.2829 x 10³ mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 33 mm

For maximum MOI,

S = 4.033 x 10³/33

S_MAX = 122.212 mm³

For minimum MOI,

S = 1.2829 x 10³/33

S_MIN = 38.8757 mm³

^{BOW ROOF REAR:}

Moment of inertia (Max), Imax = 4.7791 x 10³ mm⁴

Moment of inertia (Min), Imin = 1.2328 x 10³ mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 28.7 mm

For maximum MOI,

S = 4.7791 x 10³/ 28.7

S_MAX = 166.519 mm³

For minimum MOI,

S = 1.2328 x 10³/ 28.7

S_MIN = 42.954 mm³

CONCLUSION:

• I did the My roof design and check heat distortion and snow load condition for bow roofs are placed at right place or not. So, after finishing my design I did the heat distortion and snow load condition.
• After, finishing the curvature study. Then go to check the section modulus. And last, we check the draft analysis. Because, whether part is manufacturable or not. That, why we doing this analysis. That’s all my design pass the all requirements for placing the roof on the car.