Roof Design

Roof Design

Aim:

• The aim of this project is to design a car roof with the help of given skin data and master-section.
• Design essential flanges and reinforcements like ditch area , front roof rail ,bow roofs, central roof rail and rear roof rail with the help of given master section.
• Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct position or not.
• Perform the draft analysis for each reinforcements

INTRODUCTION :

Roof of an automobile is that commodity which sits above the passenger compartment. Roof houses the vehicle as shielding part which safeguards the occupants from sunlight, heat, rain, wind, dust, etc.

In the past roof was introduced as cover for passenger compartment but later on, it also used as a luggage carrier, sunroof and in recent past years with the introduction of the solar panelled electric vehicle got a major boost in the automotive industry.

Roof has the following functions:

• protects from external damage
• prevents passenger and vehicle from accidents
• luggage carrier
• design of the roof also affects in giving aerodynamic shape to vehicle
• sunroof
• solar panelled roof in EV

Since the roof has numerous functions so the roof has to be stronger enough to withstand the damage experienced by it. To have good strength and stiffness, the roof is made with different components such as front roof rail, rear roof rail, bow roofs, center roof reinforcement.

Components Of Roof:

The components of a roof are outer panel, front roof rail, bow roofs, center roof rail, rear roof rail.

• The roof has an outer panel which is the styling part.
• Bow roofs and center roofs are mainly used as a reinforment to increases the strength the roof.
• The front roof rail help to fix the windshield glasses. Visibility criteria and headroom clearance have to be followed while designing the front roof rail.
• The Rear roof rail has hinge mounting for the back door. It is attached at the portion where the back door meets with the roof.
• Bow roofs are provided in between the front roof rail and center roof rail as well as the Rear roof rail and center roof rail. They are attached directly to the outer panel of the roof with the help of Mastics.
• Reinforcement center or center roof rail is used as a cross member between Left hand and right-hand bodysides.

Design Considerations:

• While designing of roof following considerations have to be taken into account.

  • Visibility Criteria
  • Head clearance
  • Head Distortion and snow load criteria
• Clearance of 5 mm should be present between where mastics are used and for spot welding there should be no gap between to mating parts.
• Thickness for all sheet metal parts - 0.75 mm.
• All edges should have fillets and there should not be any sharp edges present.
• A minimum draft of 7 degrees is required for all the faces.

Design Methodology

Roof Outer Panel

This is the outer surface of the car roof. It is the styling part of the roof. This part is designed with the help of given skin data and master section.

Here the ditch area is the extended portion of the roof outer panel 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 or mastic. 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 (FRR)

Front Roof Rail is a reinforcement, 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.

Bow Roof Rails (BR1&BR2)

Bow Roofs are also used as a reinforcement. The development of the BR1 & BR2 are based on the given master section. The span between the bow roofs depends on the length of the roof surface, if the roof surface is too large then we need more bow roofs.

We use heat distortion and snow load prediction criteria to predict the bow roofs.

Center Roof Rail (CRR)

Central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load. Generally central roof rail is placed at the center of the roof which is connected to the B-pillar support structure. This central roof rail helps in the strength to the roof during the roll-over test.

If there is a sun roof then there is a extra reinforcement roof rail present in between the outer and inner roof rail.

The master section

Rear Roof Rail (RRR)

Just as the front rail roof, the rear rail roof provides reinforcement to the rear part of the car roof. The design of the rear roof rail also depends on the back door opening position.

Analysis and Calculation:

Draft Analysis:

The Draft Analysis allows the engineer to easily analyze a part for proper draft angle. A Draft Angle describes the amount of taper for molded or cast parts perpendicular to the parting line.

Draft Analysis for Outer Panel

Draft Analysis for Front Roof Rail

Draft Analysis for Rear Roof Rail

Draft Analysis for Center Roof Rail

Draft Analysis for Bow Roof_1

Draft Analysis for Bow Roof_2

Curvature study

Heat Distortion Criteria Prediction Calculations

Heat distortion criteria is useful to justify the bow roof rail position in extreme heat regions. Due to extreme heat flow there will be a chance of sagging of outer roof. To prevent the bending we need to design bow roofs at such position which will provide more strength to roof.

Bow–Roof Prediction Formula for heat distortion

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

Here I attached some snap shots to understand the parameters

Snow Load Criteria Prediction Calculations

Snow load criteria is useful to justify the bow roof rail position in snow fall regions. Due to snow fall snow will get collected on the top roof which will tend to bend the roof. Bow roof will be at such position which will provide more strength to roof against bending.

Snow Load Criteria Prediction 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]

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

Also here I attached some snap shots to understand the parameters

Conclusion:

• The Design of Roof Outer Panel, Front roof rail, Rear Roof Rail, Bow Roofs and Center roof reinforcement by following the Master section with the design parameter for the given class A surface is done.
• As per curvature study and analysis the design of bow roofs for Heat distortion criteria are found OK.
• As per curvature study and analysis the design of bow roofs for Snow load criteria are found OK.
• Also we can provide emboss in the roof rails to enhance the strength of the roof rail.
• The draft angles are analyzed and the results are satisfactory.