Challenge

Roof challenge

DDeepak Sharma13 Jul 2021

Roof challenge-

Objectives:-

Design the roof from the given styling surface with front & rear roof rails.
Create the Bow roof rails & center Bow with reinforcement.
Do a curvature study on the roof & validate the design by applying the Heat distortion criteria & snow load criteria.
Perform the section modulus.
perform the draft analysis.

Introduction:-

An automobile roof or cat top is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain & other external elements.

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

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

The rails & 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

And 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.

Bow roof-

It is used to give strength to the area between the front roof & the centre roof rail. Also it is created in the area between the centre roof rail & 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 images-

Heat distortion Criteria-

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

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(Rx*Ry)/(Rx+Ry)

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness [mm]

l = Bow Roof Span [mm]

Snow load prediction formula-

Snow load is the downward force on a car's roof by the weight of accumulated snow & 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]²x 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

250 ≤ s ≤ 380

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 reference point of the front and the rear.

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

Section modulus study 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 & capable of supporting greater loads.

S=I/y

where,

I= moment of inertia

y= Distance between the neutral axis & the extreme end of the object.

Front roof rail-