Roof Design

Aim :- The objective of this project is to design car roof with essential flanges and reinforcements like ditch area, front roof rail, bow roofs, central roof rail and rear roof rail for the given styling surface and check for Draft Analysis.

Introduction :-

• 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.
• Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure.

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.

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.
• The reinforcement centre & Bow roof centre are joined either with the spot welds or mastic sealants.

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 Panel :-

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:

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]

Measure L of the roof outer panel :-

All the Rx And Ry value :-

Bow Roof Span :-

From front Roof Rail to Bow Roof Rail 1 :-

From Bow Roof Rail 1 to Centre Roof Rail :-

From Centre Roof Rail to Bow Roof Rail 2:-

From Bow Roof Rail 2 to Rear Roof Rail :-

Procedure to measure the parameters:

a) On plane parallel to the top plane draw sketch in which 0Y line is offset by 300 mm.

b) Draw lines perpendicular to 0Y line passing through point exact midpoint of the two adjacent parts (e.g. midpoint between front rail and bow roof 1)

c) Offset these reference lines on both sides by 100 mm

d) Mark intersection points of reference line and 100 mm offset lines.

e) These reference points are then projected on to roof outer panel surface to create curves.

f) Take all necessary measurements required to calculate W value as per mentioned formula.

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

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 and capable of supporting greater loads.
• The moment of Inertia also required to calulating the snowloaded condition of the Roof Design.

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)

Bow Roof Rail 1:-

Centre Roof Rail -

Bow Roof Rail 2:-

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

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

Procedure to calculate necessary parameters:

a) On plane parallel to the top plane draw sketch in which 0Y is projected along the direction perpendicular to top plane

b) Draw lines perpendicular to 0Y line passing through point exact midpoint of the two adjacent parts (e.g., midpoint between front rail and bow roof 1)

c) Offset these reference lines on both sides by 100 mm

d)Mark intersection points of reference line and 100 mm offset lines.

e) These reference points are then projected on to roof outer panel surface to create curves.

f) Take all necessary measurements required to calculate QR value as per mentioned formula.

TO MEASURE Ly:-

TO MEASURE Lx:-

TO MEASURE Rx & Ry:-

TO MEASURE s:-

TO MEASURE Y:-

After calculating all these values the Qr is found out.

• Hence the result of snow load criteria shows there is an improvement required for the bow roof rails placement, So we add Emboss to the Outer Panel Roof For better Strength for the Roof.

Draft Analysis:-

• In NX to check whether the sheet metal part we have designed can be deep drawn, there is an option called draft analysis.
• 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.

Roof Outer Panel :-

Front Roof Rail:-

Bow Roof Rail 1:-

Centre Roof Rail :-

Rear Roof Rail:-

Conclusion-

1. Using all input data we have successfully designed Ditch area flanges, Front roof rail, Bow roof rails, Rear roof rail as per the requirement.
2. As per heat distortion criteria we come to know all bow roof positions are OK.
3. As per Snow load criteria we come to know there is improvement required.
4. All roof rails draft analysis are found OK.