Roof Design

OBJECTIVE:-

Design a Roof with its components by following the master section of the front roof rail with all the design parameter and do the curvature study and calculate the heat distortion study and snow load criteria, section modulus and performe draft analysis of each reinforcement.

INTRODUCTION:-

An automobile roof or car top is the part of an automobile that positioned above the passenger compartment,It covers and protect the vehicle occupants/passenger from sun, wind, rain, and other external elements.It has given some reinforcement to give strength to this part.It is made curved because if the roof is flat then it will more prone to bend if some load applied.

Reinforcement of Roof:-

1) Front Roof Rail

2) Bow Roof Rail

3) Center Roof Rail

4) Rear roof Rail

FRONT ROOF RAIL

Front roof rail is provided at the front end of the roof to provide strength to that area of roof, while designing this part we should maintain the visibility and headroom clearance. Embosses are created to strengthen the front roof rail.

BOW ROOFS

It is provided to give strength to the roof at that areas,it can be created at any number depending upon the length of the roof. Mastic points are provided to join this bow roofs to the outer panel.

CENTER ROOF RAIL

It is provided at the center of the roof to strengthen that area and it is joined to the outer panel by mastic.

REAR ROOF RAIL

It is provided at the rear part of the roof and it is joined with the outer roof by spot welding. We should take consideration of rear headroom and visibility criteria while desiging this. Embosses are created to strengthen the rear roof rail.

Heat Distortion Criteria

This calculation is performed to check the strength of the roof at a certain load with a certain temp. This study will predict the position of bow roofs.

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]

Judgement Condition - OK<2.7

Procedure

1) We create a 0Y line on the top of the roof.

2)We sketch a -3Y which is 300m below from the 0Y line

3) We sketch 2 line on the both side of the -3Y line at 100mm distance

4) We sketch a line in the middle of the FR and BOW ROOF 1,BOW ROOF 1 and CR, CR and BOW ROOF 2,BOW ROOF 2 and RR.

5) We sketch 2 line on the both sides of the middle line.

6)we create intersection points of the line and project it on the roof then measure the Rx and Ry distance.

This model has passed the heat distortion criteria.

SECTION MODULUS CALCULATION:-

It measure the strength of any area of an object,Greater the value of section modulus, greater will be the strength of the section.The section modulus depends upon the moment of inertia which again depends not only on the total area of the cross-section but also on the disposition of the area with respect to neutral axis.

Section Modulus Formula (S) =I/Y
Where I =Second Moment of Inertia
Y= Distance from the neutral axis to any given fiber

Section Modulus of each part of the roof are calculated below:-

Front Roof Rail

I= 669.75mm^4

Y= 136.51/2= 68.25mm

S= I/Y= 9.81mm^3

Bow Roof Rail 1

I= 278.51mm^4

Y=69.88mm/2 =34.94mm

S= I/Y= 278.51/39.94= 6.97mm^3

Center Roof Rail

I= 354.04mm^4

Y= 87.93mm/2= 43.96

S= I/Y= 354.04/43.96= 8.05mm^3

Bow Roof Rail 2

I= 281.42mm^4

Y=69.96mm/2= 34.98mm

S= I/Y= 281.42/34.98= 8.04mm^3

Rear Roof Rail

I= 3.02*10^3

Y= 267.88mm/2= 133.94mm

S= I/Y= 22.54mm^3

Snow Load Prediction Formula

This calculation is done to check the strength of roof with snow load. In some country snow falls happens so for that reason this calculation is necessary,if the roof will fail in this calculation we create embosses on the roof to give strength.

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

1) we create 2 line on the both sides of the 0Y line at 100mm distance

2) we create a middle line in between FR and BOW ROOF 1, BOW ROOF 1 and CR, CR and BOW ROOF 2, BOW ROOF 2 and RR.

3) we create 2 line on the both sides of these middle lines at 100 mm distance

4) we create intersection points of the line and project it on the roof then measure the Rx and Ry distance.

As the calculation show,This model of roof has not passed the snow load criteria so we created embosses on the top to provide strength for better load resistance capacity.

DRAFT ANALYSIS:-

Draft Analysis allows the to easily analyze a part for proper draft angle. It was performed with 7 degree draft angle.

Front Roof Rail

Bow Roof Rail 1

Center Roof Rail

Bow roof Rail 2

Rear Roof Rail

CONCLUSION

This model of roof and its reinforcements are designed according to the given master section and calculated all the curvature study to determine its strength and it is given embosses on the roof as it failed at snow load prediction.