Roof Design

Aim:

To design various sections of a Car Roof based on the given inputs, such as roof styling, design parameters & standards and to do a curvature study on the roof and perform calculations to validate and determine the position of the Bow-roofs using NX CAD.

Introduction:

A car roof or car top is the portion of a car that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements.The external sheet metal part is called roof panel which is mounted on and supported by roof bows.

In this project, we will be focusing on designing the roof rails & bow-roofs, as well as the roof ditch area by developing necessary flanges & reinforcements and doing draft analysis of individual roof rails, analysing the curvature of the roofs by performing calculations, to obtain the right positioning of the bow roofs.

Roof Design & Assembly:

The roof is an assembly of roof rails such as front roof rail, rear roof rail, reinforcement centre & bow roofs. The number of bow roofs is directly proportional to the length of the roof. The bow roofs are usually positioned where the roof curvature is considerably flat. The ultimate purpose of placing the roof rails & bow roofs is to support the roof & to add strength to it.

The roof rails, bow roofs and the roof ditch area were predominantly designed using the following commands

• Offset Surface : This command is used to create a surface or a body by offsetting a set of faces.
• Offset Curve in Face : Offsets curves along the faces on which they lie.
• Trim Sheet : Trims off a portion of a sheet body.
• Extend Sheet : Extends a sheet body by distance or to the intersection with another body.
• Trim Body : This command is used to trim a portion of a body along a plane or a face.
• Through Curves : Creates a body through multiple sections where the shape changes to pass through each section.
• Law Extension : Creates a law-controlled extension from base sheet, dynamically or based on laws for distance and angle.
• Face Blend : Adds tangent blend faces between selected set of faces based on the radius specified.
• Edge Blend : Rounds sharp edges between faces based on the radius specified.
• Projected Curve : Projects curves, edges, or points onto faces or planes.
• Emboss : Modifies a body with faces made by projecting a section along a vector, with options for end cap location and shape.
• Curve on Surface : Creates a surface spline feature directly on a face.
• Mirror Geometry : Copies geometries and mirrors them across a plane.
• Sew : Combines sheet bodies by sewing common edges together, or combines solid bodies by sewing common faces.
• Thicken : Creates a solid body by adding thickness to a set of faces.

Front Roof Rail:

Bow Roof 1:

Centre Roof Rail Reinforcement:

Centre Bow Roof:

Bow Roof 2:

Rear Roof Rail:

Positioning of Roof Rails:

Assembly of Roof Rails:

Draft Analysis :

Draft analysis is the analyzing command which is used to determine whether a model or design has draft angle in a given tool direction.

The Draft analysis enables the user to detect if the part drafted will be easily removed. This type of analysis is performed based on color ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.

The draft analysis results of all the roof rails are as follows:

Section Modulus Study:

Section modulus is the geometric property of a given cross-section used in the design of flexural members. Section modulus depends only on the cross section shape of the beam or structure.

Section modulus may be defined as the ratio of total moment resisted by the section to the stress in the extreme fibre which is equal to yield stress.

Section Modulus is given by,

S = I/Y

Where,

S = Section Modulus

I = Moment of Inertia

Y = distance between the neutral axis and the extreme end of the object

Front Roof Rail:

Here, I= 1.6755e+03 mm^4

Y= 70mm

S = 23.935 mm^3

Bow Roof 1:

Here, I= 2.0600e+03 mm^4

Y= 100 mm

S = 20.600 mm^3

Centre Roof Rail:

Here, I= 2.7716e+03 mm^4

Y= 120 mm

S = 23.096 mm^3

Bow Roof 2:

Here, I= 2.5126e+03 mm^4

Y= 100 mm

S = 25.126 mm^3

Rear Roof Rail:

Here, I= 4.1072e+03 mm^4

Y= 80 mm

S = 51.34 mm^3

0Y Section:

Here, I= 1.9226e+05 mm^4

Y= 1056.5 mm

S = 181.984 mm^3

Roof Bow Calculation (Curvature Study) :

The Roof Bow Calculation which is otherwise known as Curvature Study is an analysis performed to validate and determine whether the bow roofs & roof rails are positioned correctly in accordance with the curvature & length of the roof outer panel.

This study is done based on the following two criteria :

1. Heat Distortion Criteria
2. Snowed Criteria

Heat Distortion Criteria:

Bow–roof prediction Formula : W = 1.73x10^-3 x L+1.85x10^-8 x (R^2)/t + 1.10x10^-3 x l-2.68

Judgement Condition : OK<2.7mm<NG

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]

Snowed Criteria:

The roof design satisfied both the above conditions & the position of the bow roofs & roof rails were thus, validated & verified.

Result:

Therefore, the various sections of a Car Roof were designed & a curvature study was performed to validate the positioning of the bow-roofs and roof rails using NX CAD.