Roof Design

Roof

The portion of an automobile that lies above the passenger compartment and protects the occupants from the sun, wind, rain, and other external elements is known as the roof or car top.

OBJECTIVE:

• 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
• To 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
• Finding moment of inertia and section modulus for Bow roofs, central roof rail, front roof rail and rear roof rail.
• Perform the draft analysis for each reinforcement.

Design Considerations:

• Clearance of 5 mm should be present between outer roof rail and Front roof rail, Rear roof rail, bow roof, and Reinforcement centre where mastics are used for adhesion.
• 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.

1. Front Roof Rail: The front roof rail is a reinforcement that incorporates the back of the bracket light and the room lamp, as well as the function of attaching the windscreen. Embosses are used to add strength to the part.

2. Rear Roof Rail: It is a reinforcement for the rear portion of the roof that also includes tailgate mountings. RRR development is influenced by the rear headroom clearance and rear visibility parameters. The minimum clearance from the headliner to the roof should be 80mm, which varies by version and region.

3. Bow Roofs

These are the reinforcements built on the roof's flatter areas, which are more vulnerable to failure. The number of bow roofs depends on the length of the roof; longer roofs, such as vans, have more bow roofs.

Bow Roof 1

Reinforcement Roof Rail

Bow Roof 2

Bow Roof 3

Final Assembly

MANUFACTURING PROCESSES

Manufacturing of the fender takes place in 3 Stages namely,

Deep Drawing, piercing and Hemming.

Deep Drawing

The deep drawing process is a forming process which occurs under a combination of tensile and compressive conditions. One of the most popular methods for sheet metal forming is deep drawing. In addition to being used in many other industries, it is applied in the automotive industry for the manufacturing of car body parts.

Hemming

Hemming is a forming operation in which the edges of the sheet are folded or folded over another part in order to achieve a tight fit. Normally hemming operations are used to connect parts together, to improve the appearance of a part and to reinforce part edges. The accuracy of the hemming operation is very important since it affects the appearance of the surface and surface quality.

Piercing

Piercing is a shearing process in which raw metal is pierced with a machining tool, resulting in the creation of a circular or other shaped hole. As the raw metal is pierced, the metal from the newly created hole is considered scrap.

Joining Processes

The Flanges of the front and rear roof rail and flanges of the bow roofs and center roof rail are joined using the spot welding, whereas mastic sealants are provided along the roof rails to improve the strength and also to avoid bad marks on the aesthetic surfaces.

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 centre 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 centre 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

HEAT DISTROTION CRITERIA TABLE

From the above results, the positioning of the bow roofs satisfies the condition for Heat Distortion Criteria.

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 centre 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 centre 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 from 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 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

Moment of Inertia for Front Roof Rail

Moment of Inertia for Bow Roof 1

Moment of Inertia for Reinforcement Bow Roof

Moment of Inertia for Bow Roof 2

Moment of Inertia for Bow Roof 3

SNOW LOAD CRITERIA TABLE

From the above results, the positioning of the bow roofs satisfies from Front Roof Rail to Reinforcement Bow Roof, and is not satisfing for the rest.

CREATING EMBOSS ON OUTER PANNEL

Since Snow Load Condition is not meeting the requirements from Reinforcement Bow Roof to Bow Roof 3, we can overcome the same by creating the embosses i.e., by adding strength where the conditions are not meeting.

DRAFT ANALYSIS

Draft Analysis is carried out to check the manufacturing feasibility of the roof and all the reinforcements rails. All the components should be designed by considering at least the minimum draft value of 7 Deg.

Roof Outer

Front Roof Rail

Bow Roof 1

Reinforcement Roof Rail

Bow Roof 2

Bow Roof 3

Rear Roof Rail

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.