Roof Design

AIM:

To design a roof with all the components like front roof rail, rear roof rail, and bow roofs by following the given master sections and the high-quality surface, considering all the design requirements as per the OEM standards.

ROOF

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 roof is not a single panel as seen from the exterior it consists of front roof rail, bow roofs, center roof rail, and rear roof rail with different thicknesses which act as a reinforcement for the top.

OBJECTIVE

• To design a roof of the car with all the reinforcement from the given styling surface.
• To perform the curvature study on the roof by considering Heat Distortion and Snow load criteria.
• To calculate the moment of inertia for the reinforcements.
• To Perform the draft analysis for analyzing the components' manufacturing feasibility.

TYPICAL SECTIONS

• Front Roof Rail.
• Rear Roof Rails.
• Center Roof
• Bow Roof Rails.

GIVEN MASTER SECTION

DESIGN METHODOLOGY

• Ditch Area: The ditch area is the extended portion of the main roof which is concealed by the other BIW components followed by the other aesthetic surface. It also serves the joining purpose for the reinforcement which is joined using the spot welding. Visibility, Clearance for windshield and back door mountings are some of the factors needed to be considered while designing the ditch area

• Front Roof Rail: Front Roof Rail is a reinforcement that features to back of the bracket lamp and the room lamp and it also serves the purpose of attaching the windshield. The development of the FRR is based on the given master section and also references from the given styling and the front ditch area are taken into consideration. Embosses and darts are incorporated for strengthening the part.

• Rear Roof Rail: It is a reinforcement provided to the rear portion of the roof and it also features the tailgate mountings. The development of RRR is dependent upon the rear headroom clearance and rear visibility criteria. The minimum clearance from the headliner to the roof is should be 80mm which will vary from variant and region to region.

• Bow Roofs: These are the reinforcements provided on the most flatter regions of the roof which are more susceptible to failure. The number of bow roofs depends upon the length of the roof i.e, longer roofs have more bow roofs like vans. For the development of the bow roofs, the styling surface and side ditch area are taken as references

Center roof rail

A center roof rail is a structural component of some vehicles that runs along the center of the roof. It typically runs from the front of the roof to the back, and provides additional support and rigidity to the roof structure.

CURVATURE STUDY

HEAT DISTORTION CRITERIA

Heat distortion temperature is the temperature at which the metal softens and deforms under a specific load. This study is done to check the ability of the roof to withstand high temperatures concerning the positioning of the reinforcements which contribute to increased strength.

Procedure:

• Create a center line at the 0-Y region of the roof and then offset the same to -3Y.
• Create a symmetric line from the -3Y region at a distance of 100mm.
• Draw a vertical line at the midpoint of the line drawn from the center points of the FRR and BR 1 and followed the same till the rear roof rail.
• Offset the vertical line to a distance of 100mm on each side.
• Create intersection points and project them on the roof.
• From these projected points, make a 3-point arc and the Rx and Ry values.

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]

Judgment Condition

OK< 2.7

From the above table it can be concluded that all values of < 2.7 Ok So thus infers that current positioning of bow row are good in state as per design and found ok.

SNOW LOAD CRITERIA

A snow load study is done to check the ability of the roof to withstand snowfall and bounce back to its original position without undergoing permanent deformation upon the removal of the load.

Procedure:

• Create a center line at the 0-Y region of the roof and then offset the same to 100mm on each side.
• Draw a vertical line at the midpoint of the line drawn from the center points of the FRR and BR 1 and followed the same till the rear roof rail.
• Offset the vertical line to a distance of 100mm on each side.
• Create intersection points and project them on the roof.
• From these projected points, make a 3-point arc and the Rx and Ry values.
• Project the extreme edges of the styling surface fro measuring the width at the respected sections.

Formula:

Qr = [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 10^-8]

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]

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]

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]

Ry = Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW [mm]

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

Hence above all the Qr ≥ 3.1 so my design is accepted

Conclusion:

Thus the required reinforcement and the ditch area for the roof were designed and developed from the given styling. The curvature study was done after performing calculation for the Heat Distortion and Snow load conditions with the position criteria met for all reinforcements.

Further the section modulus study was conducted and Draft analysis were done for each reinforcement successfully.