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.

The Automobile roof consists of the following parts:

• The outer panel
• Front roof Rail
• Bow Roof
• Centre Roof Rail
• Rear Roof Rail

FRONT ROOF RAIL:

The front roof rail is created at the front side of the roof to provide strength to the roof. Headroom clearance and the driver visibility are the two important factors to be considered while designing the front roof rail. the front roof rail is connected to the roof outer panel with the help of mastic seal.

                                                  

RAIL

BOW ROOF:

It is used to give strength to the area between the front roof and the centre roof rail. Also it is created in the area between the centre roof rail and the rear roof rail to provide enough strength. It can be created in any numbers depending upon the roof length but the position of the bow roofs is to be justified by curvature study.

                           

CENTRE ROOF RAIL:

It is created at the centre of the roof. It is connected to the roof by mastic seal.

                                                           

REAR ROOF RAIL:

It is created at the rear end of the roof. The fitment of the backdoor is to be considered while designing the rear roof rail.

                                                   

ROOF OUTER:

               

          

HEAT DISTORTION CRITERIA:

Heat distortion temperature is the temperature at which metal softens and deforms under a specified load. While designing roof, this criteria is taken into consideration. We create bow roofs when the roof is long and flat to increase its strength. The position of the bow roofs is to be justified and this justification is done by curvature study:

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]

TO MEASURE L:

TO MEASURE Rx:

TO MEASURE Ry:

TO MEASURE l (BOW ROOF SPAN):

HEAT DISTROCTION CRITERIA:

Here we can see that the roof is completely satisfying the conditions of heat distortion.

SNOW LOAD PREDICTION FORMULA:

Snow load is the downward force on a car's roof by the weight of accumulated snow and ice. The roof can fail if the snow load exceeds the weight the roof was designed to shoulder. Or if the roof was poorly designed or constructed.

So there is a formula to predict the strength of the roof.

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

TO MEASURE Ly:

TO MEASURE Lx:

TO MEASURE Ry:

TO MEASURE Rx:

TO MEASURE s:

                        

TO MEASURE Y:

After calculating all these values the Qr is found out.

It is observed that areas it is not satisfying the condition. So in order to strengthen the roof at these areas embosses are created on the outer roof.

SECTION MODULUS STUDY OF THE SECTION OF THE ROOF RAILS:

The section modulus of the cross-sectional shape is of significant importance in designing of structures. It is a direct measure of the strength of the structure

A structure that has a larger section modulus than another will be stronger and capable of supporting greater loads.

Section modulus (S) = I/y

Where,

I = Moment of inertia in (mm⁴)

y = Distance between the neutral axis and the extreme end of the object (mm)