Roof Design

AIM:-

To develop the essential flanges of roof(front ,rear ,bow ,center roof) and checking draft by performing Draft analysis as well as Calculating Heat distortion and Snow load on the roof

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.

When it is seeing from outside of the car, it is just closing the top portion. But in design point of view, it is hard to create the design. Because of you have to create no of support rails for enough strength to withstand the load. Bow roofs are primarily fixed in flatter region to enhance strength.

Front roof and rear roof rails are fixed to the ditch area using spot weld operation. But front, rear bow roof and center roof rails are fixed to roof using mastic sealants. We don’t want so much gap between roof rails and bow roof area. So, we reducing the NVH issues.

Types of Roofs:

1) Hardtops:

When we talk about hardtops, we are referring to the common rigid forms of automobile roofs which we see around. In modern-day cars, this type of roof is made from metal or plastic.

So, the common Toyota Corolla, Camry, and many of the cars in Nigeria are all Hardtops. However, we should say that there are different types. The one we just described is the fixed hardtop. Other types include:

• The detachable hardtops
• The retractable hardtops

2) Sunroof:

This type of roof is also quite popular. You might have been seeing it around. A sunroof is an operable type of roof that allows for an opening in the roof of an automobile; say a window in a roof. It can allow either light or air or both to enter the vehicle. They are either operated manually or automatically. There are 3 popular types of sunroofs. They include,

A.Pop-up:

The pop-up sunroof usually tilts when you open it. It is the most basic and relatively cheaper type of sunroof. Some can even be removed so that they are kept properly, maybe in the trunk whenever you want to go ‘sunroof mode.’

b. Spoiler:
Wait, this is not spoiling anything o. It just means that it can slide. So, the spoiler is a pop-up that would slide, instead of tilting upwards. The advantage of this is that it can easily be restored to its original position by sliding.

c.Panoramic sunroof:

Please take the pronunciation easy. Panoramic sunroofs are distinct. While other sunroofs basically allow air and light or both into the front seats, the panoramic sunroof would allow for both the front and rear.

3) Convertibles:

convertibles are passenger cars that can be driven with or without the entire roof in place. They were first introduced as open-air vehicles; no roofs, no sides. There are different types of convertibles based on the method of conversion and the materials used.

A.Retractable hardtops:

Made from metal or plastic and is retracted automatically.

B.Detachable hardtops:

This type of hardtop is manually removed from the roof and then stored usually in the trunk. They are also made from metal or plastic.

C.Soft tops:

This type of convertibles have roofs that are not made from metal or plastic. They are made of textile. The earliest versions of the convertible used canvas for the roofs. More recent models use cloth materials instead. Other materials used now include latex, vinyl, polyvinyl chloride.

4) The T-Top roof:

An ‘almost convertible’ car. Actually it was built this way because the car manufacturers were not sure the government of the day would approve convertibles. So they made a ‘T-shaped bar’ from front to rear and made the roof on either side detachable. Of course, this type of roof is no longer in production.

TESTING OF ROOF:

Roof strength evaluations are conducted on a quasi-static test system manufactured by MGA Research Corporation (Figure 3).

The system consists of an upright assembly and an attached loading head that can be fixed at varying heights from the ground as well as at pitch angles ranging from −5 to +5 degrees to accommodate testing on the driver or passenger side.

The roll angle is permanently fixed at 25 degrees. Four hydraulic actuators control the movement of the platen along two linear guides. The entire system is mounted on a T-slot bed plate anchored to the floor of the test facility

ROOF COMPONENT:-

Here we have some master section for each roof , so we follow as per master section data and we make each roof by own style.

Initial styling surface,

1.Front Roof Rail-

The front roof rail is located on the front of the roof where the wind shield joins with the roof. The visibility criteria and the head room clearance of the passenger are taken into consideration for the bottom flange. One end of the rail is joined using spot welding while the other end is joined with the roof using mastic sealants for increased strength.

As per given master section,

Different view front roof,

Draft analysis done for front roof,

2.Bow roof 1-

Bow roof is an additional siffening member for the roof to increase the strength and so that it can satisfy the norms. Both the ends are joined together using mastic sealants to increase strenght and also since this would not affect the styling of the roof.

As per given master section,

Different view bow roof1,

Draft analysis done for bow roof1,

3.Centre Roof Rail-

Centre Roof rail or the reinforcement centre is located at the centre of the roof . The cross section of this is more since it needs to provide more strength to the roof. Here both are ends are joined together using mastic sealants.

As per given master section,

Different view center roof,

Draft analysis done for center roof,

4.Bow roof 2-

This bow roof also have the same purpose as that of the first one. This one is located between the centre bow roof and rear roof rail.

As per given master section,

Different view bow roof2,

Draft analysis done for bow roof2,

5.Rear Roof Rail.

This roof rail joins with the back door and the roof. Inputs from the backdoor are taken into consideration for designing this roof rail.

H ere one end is joined using spot welding while the other end is joined using mastic sealants.

As per given master section,

Different view rear roof,

Draft analysis done for rear roof,

Curvature Study On The Roof

The heat distortion study plays an important role in sheet metal usage. Heat distortion temperature is a temperature limit above which the material cannot be used for structure application. This study is used to predict the heat distortion temperature at where the material is starting to soften when exposed to a fixed load at elevated temperature. To avoid bending or damages on the roof, based on heat distortion temperature, this study will predict the bow roof position on the roof to strengthen the roof.

Heat Distortion Criteria:-

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

Judgement Condition : OK< 2.7> 3.1

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]

The thickness of the roof

Roof Outer Panel, t = 0.75 mm

Front Roof Rail, t = 0.75 mm

Rear Roof Rail, t = 0.75 mm

Bow Roof Rail_1, t = 0.75 mm

Centre Roof Rail, t = 0.75 mm

Bow Roof Rail_2, t = 0.75 mm

Roof Length for styling surface(L):-

For the L we want to take the total length of the styling surface don't include the ditch area as shown below fig,.

Bow Roof Span(l):-

Here we want to find the distance from the front roof rail to bow roof_1 to find the bow roof span as shown in fig, Similarly, we can measure the remaining span distance also want to measure.

Finding Rx & Ry (X & Y Curvature):-

From the 0-Y axis, at -3Y about the distance of 300 mm is drawn parallel on the perpendicular plane as shown in fig. After that 100 mm lines are offset to either side of the -3Y line. Now center lines are drawn between each adjacent roof rail edge respectively. Intersection points are marked on the sketch and exit the sketch. Make all geometry in construction lines except the points.

Project the intersection points in the sketch to roof surface as shown in fig,.

Join the projected curves as follows & put them into the formula to calculate 'W"

Projects the points and make the arc

Finding the Rx curvature and Ry curvature

Rx value

Ry value ,

Apply the values in the formula:-

To find the Heat distortion apply all the necessary values from the roof of the given formula.

HEAT DISTORTIONS CRITIRIA CALCULATIONS:

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

R = 2(Rx*Ry)/(Rx+Ry)

CALCULATIONS,

ROOF RAILS	Rx(mm)	Ry(mm)	Rx*Ry	Rx+Ry	R(mm)	L(mm)	t(mm)	l(mm)	W(< 2.7)	Result
FR-BOW1	3074.48	2472.26	7600913.924	5546.74	2740.67	2032.51	0.75	402.17	1.45	OK
BOW1- CR	4929.53	6365.17	31377296.470	11294.7	5556.10	2032.51	0.75	409.74	2.0	OK
CR- BOW2	2491.87	10631.23	26491643.100	13123.1	4037.40	2032.51	0.75	424.48	1.70	OK
BOW2-RR	4355.90	14536.89	63321239.151	18892.49	6703.32	2032.51	0.75	400.15	2.37	OK

From the above table, it can be concluded that all values of W<2.7 and also not W > 3.1 so thus infer that the current positioning of bow roofs are good in the state as per the design and found OK.

SNOW LOAD CRITERIA :

This test is done to know how is the roof behaving when there is a snow over . normally due to the snow weight the dent will happen. But the roof should be designed in such a way that when the snow is removal the roof should go it its original position . This is the basic requirements for snow load criteria .

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]

Find the Rx Curvature and Ry Curvature for example shown in fig below.

Rx value.

Ry value.

Moment of Inertia for the roof rails:-

Calculation

Roof Rails	Iy(mm)	Lx(mm)	Ly(mm)	L1(mm)	L2(mm)	Rx(mm)	Ry(mm)	t(mm)	Y(mm)	S	My	α	Qr	Result (Qr ≥ 3.1)
FR-BOW_1	1999.23..	1186.04	2032.51	402.10	424.45	14918.63	2537.88.	0.75	402.10	413.275	655587.861	0.92	3.8	OK
BOW_1-CR	5295.44	1131.18	2032.51	424.45	409.95	3772.60	6283.23.	0.75	424.45	417.20	682541.067	0.87	32.46	OK
CR-BOW_2	3105.70	1104.50	2032.51	409.95	340.67	4279.64	11146.85	0.75	409.95	375.31	665168.472	0.81	9.70	ok

CONCLUSION :

Hence the roof , front roof rail, bow roof rails , central roof rail and the rear roof rail is designed by following the master sections and the moment of inertia and draft analysis are carried out to check feasibility manufacturing.