Designing and Modeling the roof of a car using CATIA V5

Introduction

Roof of an automobile sits above the passenger compartment. Roof houses the vehicle as a shielding part, which safeguards the occupants from sunlight, heat, rain, wind, dust and other external elements. In case of convertibles, it is called the hood.

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

Roof has the following functions

• Protects from external damage
• Act as a safety gear when a roll-over accident occurs
• Luggage carrier
• Design of the roof also affects in giving aerodynamic shape to vehicle
• Sunroof
• Solar paneled roof in EV

Since the roof has numerous functions. Therefore, the roof has to be stronger enough to withstand the damage experienced by it while roll-over. For that, the roof should have good strength and stiffness. So, the roof is made with embosses and some reinforcements, such as

• Front roof rail
• Rear roof rail
• Center roof rail
• Bow roofs

They are bonded with the roof through the spot welding and mastic sealant.

While designing the roof, few criteria should be followed, they are visibility, headroom clearance, heat distortion, snow load, curvature study.

Ditch area is provided on all the four sides of the roof. Roof ditch areas are provided to mount roof to the main body of the car also to mount windshield glass for the front ditch area, and to mount tailgate to rear ditch area

Front roof rail

Figure 1: Front roof rail

Front roof rail design should consider headroom clearance and visibility criteria and has mounting area for windshield glass. For additional strength, embosses are been provided.

The thickness of front roof rail is 0.65 mm.

Commands Used:

Offset, fill, join, parallel, curve smooth, split, extract, sweep, multi section surface, edge fillet, plane, Sketch, project, extrapolate, symmetry, thicksurface, etc.

Rear roof rail

Figure 2: Rear roof rail

Rear roof rail is similar to the front roof rail. Rear headroom and rear visibility criteria are considered and also tailgate will be mounted on the rear roof rail so tailgate conditions have to be considered while designing the rear roof rail.

The thickness of rear roof rail is 0.65 mm.

Commands Used:

Offset, fill, join, parallel, split, sweep, plane, Boundary, curve smooth, multi section surface, edge fillet, Sketch, project, extrapolate, symmetry, thicksurface, etc.

Center roof rail

Figure 3: Center roof rail

Figure 4: Lamb Bracket

Center roof rail is a reinforcement support for the roof and it has the bracket for cabin light mount.

The thickness of Center roof rail is 1.3 mm.

The thickness of lamp bracket roof rail is 0.65 mm.

Commands Used:

Offset, plane, split, sweep, curve smooth, multi section surface, join, edge fillet, extract, sketch, extrude, line, healing, thicksurface, etc.

Bow roofs

Figure 5: Bow roofs

Bow roof has been provided for additional strength especially against snow conditions and rollover situations. In this model, one bow roof has been provided between front roof rail and Centre roof rail and another bow roof has been provided between Centre roof rail and rear roof rail. But the number of bows on the roof is based on the size of the car like small cars do not require a bow because it increases the weight of the car as well as the other rails are enough to provide the support for the roof. But for large cars 3 or more bows are needed to support the roof.

The thickness of Bow roof rails are 0.65 mm each.

Commands Used:

Offset, plane, split, sweep, extract, boundary, curve smooth, join, edge fillet, sketch, extrude, line, multi section surface, thicksurface, etc.

Main roof

Figure 6: Emboss over the main-roof

Main-roof is modelled using the styling surface as reference, then the ditch area and emboss are created on it.

The thickness of Main roof is 0.75 mm.

Commands Used:

Offset, plane, Sketch, project, split, sweep, extract, fill, join, edge fillet, extrapolate, symmetry, extrude, healing, thicksurface, etc.

Assembly

Front and rear rails are placed and mated in the front and rear of the main-roof respectively. The bow roofs, center rail and the lamp bracket are placed and mated with the main-roof. The rails are mounted with the main-roof through the mastic sealants and spot welds.

Figure 7: Assembly

Position analysis

The position and moment of all the reinforcements are taken and checked, whether it satisfies the heat distortion test and snow load test. Necessary modifications should be taken if it fails to meet the requirements.

Heat distortion criteria

The roof is directly exposed to sunlight. The temperature changes with respect to time and season, it causes deformation in the roof. In order to prevent failure it should be designed in such a way else, it gets failure.

It is checked using the formula,

W = [1.73x10^-3 x L]+[1.85x10^-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]

Judgement Condition: OK < 2.7 mm < NG

Table 1: Heat distortion criteria

	Rx (mm)	Ry (mm)	L (mm)	l (mm)	R (mm)	t (mm)	W
FR-BOW1	5771.602	2739.185	2027.244	380.896	3715.164	0.75	1.586578
BOW1- CR	4964.424	6135.713	2027.244	259.699	5488.271	0.75	1.855789
CR- BOW2	3985.18	11069.37	2027.244	259.903	5860.479	0.75	1.960207
BOW2- RR	3319.538	16169.63	2027.244	369.091	5508.26	0.75	1.981542

FR- Front roof rail

CR- Center roof rail

RR- Rear roof rail

Satisfying condition is W is lesser than 2.7

From the above table it is found that the value of W is lesser than 2.7, therefore the heat distortion condition is satisfied with this model.

Snow load condition

The roof is designed in such a way that the roof would bend slightly when the load is applied and regain its shape once the load is removed. Hereby the roof is tested to avoid plastic deformation.

It is checked using the 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)

Judgement condition : Qr ≥ 3.1

250 ≤ s ≤ 380

t = Roof plate thickness [mm]

Ly = Distance between the Front and rear roof Rails on the Vehicle along 0Y[mm]

· Length of Roof panel with the center point between Roof rail Front /Rear as the reference point of the front and the rear.

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

· Width of the roof panel 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

Figure 8: Moment of inertia for Bow 1

Figure 9: Moment of inertia for Center roof rail

Figure 10: Moment of inertia for Bow 2

Table 2: Snow load condition parameters

	Rx	Ry	Lx(mm)	Ly (mm)	Iy (mm4)	t (mm)	y (mm)
FR-BOW1	5084.687	3929.765	1098.478	1947.06	1.10E+03	0.7	516.549
BOW1- CR	4446.932	8174.768	1065.706	1947.06	9.20E+03	0.7	449.581
CR- BOW2	3848.156	13222.96	1046.817	1947.06	1.16E+03	0.7	449.821
BOW2- RR				1947.06		0.7	530.83

Table 3: Snow load condition (Q_r) calculation

My=Y(Ly-Y)	α= My*Lx^2*10^-12	s=(L1/2)+(L2/2)	Qr
738931.0927	0.891634097	483.065	6.176759
673239.9046	0.764618271	449.701	32.90416
673491.3435	0.738029211	490.3255	2.163353
751779.4942	0	265.415

To satisfy the snowload condition,The Q_r value should be greater than or equal to 3.1

From the above calculations shown in table 3, it is found that the snow load condition (Q_r) value is lower than 3.1 at the rear portion, therefore the Embosses are created.

Thus the roof was modeled.