Roof Design

Introduction:

Roof:- 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.

The roof is a styling surface part which is fixed with the BIW body through the rails and panels inside the upper roof to provide it strength and stiffness.

Ditch area plays an important role here to fix the reinforcement on the component.

The rails and reinforcements are joined to the ditch area with a spot weld where it's not the styling part.

The required tool opening angle is minimum 3 Degrees.

Factors which is considered before creating Ditch area,

• Headroom clearance
• Visibility criteria
• Back door assembly
• Windshield assembly

Software Using NX CAD:

Class A Styling Input

Components of roof:

• Front roof rail
• Rear roof rail
• Bow roof
• Centre roof rail

FRONT ROOF RAIL :

Front roof rail is the one which joins the wind shield glass , body side outer and the inner panel also. Here the front roof rail is designed based up on the master saection given.

REAR ROOF RAIL :

Rear roof rail is the one which joins the back door and the body side outer

CENTRAL ROOF RAIL :

central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load . generally central roof rail is placed at the center of the roof which is connected to the B-pillar support structure . This central roof rail helps in adding the strength to the roof during the rool- over test. The thickness of the central roof rail is 0.75 mm.

BOW ROOFS :

The bow roofs are give to improve the torsional stiffness and load bearing capacity of the roof structure . The number of bow roof present is depends on the overall size of the roof . Presently in this project two bow roofs are added.

BOW ROOF 1

BOW ROOF 2

Final Model with Assembly

Assembly section view

HEAT DISTORTION STUDY ON THE ROOF :

The heat distortion study plays a major role in sheet metal usage . heat distortion temperature is a temperature limit above which the material cannot be used for the structural applications . This study is used to predict the heat distortion temperature at where the material starts to soften when exposed to a fixed load at elevated temperature . In order to avoid bending or damages on the roof, based on the heat distortion temperature , this study will predict the bow roof position which helps to strengthen the roof .

Bow – roof prediction Formula

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

Solution:-

L= 2026.09

T= 0.75

For Front RR to Bow RR1:=

W= (1.73E-3*2026)+(1.85E-8*(2784.1^2)/0.75)+(1.10E-3*473.455)-2.68

=3.508078+0.000000000000000000000000000000000000000000000002+0.5208015-2.68 =1.3488795

For Bow RR1 to Centre RR:=

W=(1.73E-3*2026)+(1.85E-8*(2860.95^2)/0.75)+(1.10E-3*428.656)-2.68

=3.508078+0.000000000000000000000000000000000000000000000002+0.4715216-2.68 =1.2996

For Centre RR to Bow RR2:=

W=(1.73E-3*2026)+(1.85E-8*(2920.55^2)/0.75)+(1.10E-3*326.853)-2.68

=3.508078+0.000000000000000000000000000000000000000000000002+0.3595383-2.68 =1.1896163

For Bow RR2 to Rear RR:=

W=(1.73E-3*2026)+(1.85E-8*(3501.83^2)/0.75)+(1.10E-3*346.909)-2.68

=3.508078+0.000000000000000000000000000000000000000000000002+0.3815999-2.68 =1.2116783

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 .

Snow Load Prediction Formula

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)

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

250 ≤ s ≤ 380

Solution:-

Ly= 1950.1

Iy= 7.101110697e^-3 mm⁴

DRAFT ANALYSIS

• Roof

• Front roof rail

• Rear roof rail

• Bow roofs

• Centre roof rail

CONCLUSION :

Hence the roof , front roof rail, bow roof rails , central roof rail and the rear roof rail is designed by following the master sections and the moment of inertia , section modulus and draft analysis are carried out to check feasibility manufacturing.