Roof Design

ROOF CHALLENGES

OBJECTIVE

We need to design the required reinforcement and flanges for the given Class A surface of Roof. Required reinforcements are Front Roof Rail, Rear Roof Rail, Bow Roof 1,Bow Roof 2.After designing the roof assembly completely we need to perform the Heat distortion analysis and the Snow Load Analysis.

ABSTRACT

Safety point of view the roof plays vital role, specially on accident where cars roll over. Roll over accidents are the causes of many fatalities and several head, neck and spinetrauma around the world. Therefore passenger safety is also very important parameter in front of OEMs now a days.It is not only a marketing strategy but also a obligation stipulated by many government.For this reason the strength of the roof is very crucial while designing the Vehicle roof.

Roof Assembly

• The conventional task of the roof is to provide protection against hard local contact. In case of roll over, compartment integrity is provided by the compartment frames.
• Material Used-:There are wide range of materials are available for making Roofs Vehicle. That are-Steel, Alluminium, Thermosetting Plastic sheet.In case of steel the usually thickness is in the range of 0.65-0.85mm.In all cases any roof panel material requires braces to prevent panel flexing and vertical waivness,commonly known as oil canning.This is the main purpose of introducing the light weight bow roofs which are welded to the body side and bounded to roof panel by smooth flexible adhesive such as MASTIC.Another task of bow roofs is to support the interior roof liner which requires a number of fittings both in central and boundary areas to avoid deflections.

Designing of the Parts of the Roof Assembly :

Front Roof rail

• Front roof rail is an additional part attached to the roof which gives the reinforcement and also having certain features in it which helps in attaching the bracket map lamp and the room lamp.

Rear Roof Rail

• While designing the rear roof rail we need to take care of the head clearance of the passenger & the Rear Visibility Criteria.

Center Roof Rail

• Here the minimum thing we need to take care is the width of the Center Roof Rail should be between 80-110.By following the master section by styling team the physical shape shown below.

BOW ROOFS

• Mastics are applied between roof and the roof rail and bow roof.When the mastic gooses out it joins the roof rail and bow roof with the Roof.
• As we have curvature in front and more flat in the back side of the roof,it is sufficient to have reinforcement in the center of the roof.
• If the roof was small with better curvature we need to use only one reinforcement (center) that wiil act as bow roof.
• No.of bow roof increases if the car size becomes longer and taller.If flat region increases bow roofs also increases.

FORMULAS FOR CALCULATION

1.HEAT DISTORTION

• R = 2(Rx*Ry)/(Rx+Ry)
• W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68
• 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]

• The optimum range needed to fulfill this criteria is less than 2.7 or to the maximum 3.5. So by inferring from the above calculated values the W seems to be in good optimum range. Hence all the bow roofs and rails OK to proceed.
• SNOW LOAD

• α = My x Lx2x 10-12
• My = Y(Ly-Y)
• Qr = [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 10-8]
• 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]
• 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]

CONCLUSION :

Thus the roof is designed with all the necessary reinforcements and roof rails according to the given master section and the styling surface, by considering all the design parameters. Also, the curvature study is done by taking the heat distortion and snow load criteria for determining the strength of the roof.