Roof Design

ROOF DESIGN

AIM: To design the front roof rail, rear roof rail, and bow roof rail for the vehicle with the help of the given master section of the roof using snow load condition and heat distortion criteria.

OBJECTIVE:

• Design an ASSEMBLY by developing the essential flanges for the outer roof and creating the reinforcements.

• Provide appropriate dimensions and check for Draft analysis.
• Do a curvature study on the roof and perform calculations to determine the position of the Bow-roofs.

THEORY:-

ROOF DESIGN

Automobile roof design plays an important role in the overall look, functionality, and safety of a vehicle. The roof design must take into account factors such as aerodynamics, structural integrity, headroom, and visibility, while also being aesthetically pleasing to the eye.

TYPES OF AUTOMOBILE ROOFS:

There are several types of automobile roofs that are commonly used in the automotive industry. Here are some of the most popular types:

METHODOLOGY FOR DESIGNING PARTS OF ROOF:

Designing the parts of an automobile roof involves several steps to ensure that the roof is structurally sound, aerodynamically efficient, and meets the requirements of the automotive industry. Here is a general methodology for designing the parts of an automobile roof:

1. Determine the type of roof: Here we are determining sun roof type of roof which is best suited for the vehicle.

2. Calculate the roof load: The next step is to calculate the snow load condition, heat distortion criteria, roof load which is the amount of weight that the roof will need to support. This includes the weight of the roof itself, as well as any additional weight from roof racks or other accessories.

3. Choose the roofing material: Once the roof load has been determined, the next step is to choose the roofing material. This will depend on several factors such as the desired aesthetic, weight, and cost.

4. Design the roof structure: The roof structure includes the roof rails supports such as front roof rail, rear roof rail, bow roof and center roof railnand any other structural components that support the roof. The structure must be designed to support the roof load and meet the safety standards of the automotive industry.

DESIGN CONSIDERATION:

ROOF

• We are making the hardtop type of roof from the given master section so we are not adding any reinforcement for the central roof rail.
• We are adding the flanges to all sides of the roof for spot welding of the front roof rail, central roof rail, rear roof rail, and bows.
• The length and angle of the flange are decided based on the visibility criteria and according to the draw angle required for manufacturing the sheet metal.
• The thickness of the roof is decided 0.75mm

FRONT ROOF RAIL

• The front roof rail is used as the reinforcement to the roof surface to increase the strength and also used to mount the brackets for some features such as room lamp or map lamp and rear mirror.
• The depth of the front roof rail is decided based on the visibility criteria line decided for the driver visibility.
• The width of the front roof rail is decided in such a way that there should be enough headroom clearance between the head of the driver and the end surface of the roof rail to avoid injuries to the person.
• At the front side we are doing spot welding and on the rear side of the front roof rail, we are using mastic points to join it with the roof surface.
• The minimum radius for the fillet at edges is 3mm

REAR ROOF RAIL:

• The rear roof rail is used as the reinforcement to the roof surface at the rear side.
• The joining method used for this rail is the same as that of the front roof rail.
• The minimum radius for the fillet is 3mm.

BOW ROOF RAIL

• We are making two bows one is in between the front roof rail and central roof rail and another is in between the central roof rail and rear roof rail.
• Both ends of the bow are joined to the roof outer flanges by the spot welding and the inner sides of the bow are joined to the inner surface of the roof with the help of mastic points.
• The minimum radius of the fillet is 3mm.
• The cross-section of the bow is made in such a way that we should get the maximum value of the minimum moment of inertia at which the bow fails.

CENTER ROOF RAIL:

• We have designed centre roof rail at the centre of the roof to give ample strength and roof support.
• Both ends of the centre roof rail are joined to the roof outer flanges by the spot welding and the inner sides of the rail are joined to the inner surface of the roof with the help of mastic points.
• The minimum radius of the fillet is 3mm.

HEAT DISTORTION CRITERIA:

In this criteria, we check the heat distortion capacity of the roof at higher temperatures with the fixed load on it. So when the surrounding temperature is high and the roof is loaded with some weight on it then if the roof is deformed it fails, so to calculate whether the roof deform or not, according to the position of the bow we do these calculations as follows

The formula for the 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

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]

STEPS TO CALCULATE HEAT DISTORTION:

• First we offset the center y-axis line to 300mm distance apart from it represented as (-3Y to 3Y red line).
• After that we offset this -3Y to 3Y to 100mm offset each side represented with the green line.
• We draw the vertical red line at the center of the two consecutive bow rood span. Next, we offset this red line to 100 on each side.
• Now we have three horizontal lines and three 12 vertical lines we take the intersection points of the vertical red line with three horizontal lines and a horizontal red line we take the intersection of the vertical lines with that line.
• After getting all the points we project these points on the roof surface and draw the three points curve along the x-axis point to get the radius of curvature of Ry and along y-axis points to get the radius of curvature of Rx.

For the front roof rail and bow 1

Rx = 5737.3781mm

Ry = 2576.4880mm

R = 2(Rx*Ry)/(Rx+Ry) = 2(5737.3781*2576.4880)/(5737.3781+2576.4880) = 3556.055786

I = 510.3mm

L = 2030mm

t = 0.75mm

Putting all values in the equation,

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

W + [1.73 * 10^(-3) * 2030] + [1.85 x 10^(-8) x (3556.05^2)/0.75] + [ 1.10x10^(-3) x 510.3] - 2.68

W=1.68

The condition for the heat distortion criteria is W>2.7 so the given is less than 2.7 so the position of the bow 1 is correct according to the hear distortion criteria.

We will calculate the rest of the bow roof and centre roof rail by same method.

BOW'S	Rx	Ry	R	I	L	t	R^2/t	L1	L2	L3	W	OK/NOK
FR1-B1	5737.3781	2576.488	3556.056	510.3	2030	0.75	16860710.34	3.4916	0.311923	0.56133	1.68	OK
CNTR-B1	4654.3439	6269.7043	5342.591	445.7	2030	0.75	38057702.61	3.4916	0.70467	0.49067	2.01	OK
CNTR-B2	4121.8562	9860.1278	5813.485	399.5	2030	0.75	45062147.74	3.4916	0.833649	0.43945	2.08	OK
B2-B3	3319.5503	14330.3152	5390.432	630	2030	0.75	38742346.49	3.4916	0.716733	0.693	2.22	OK

SNOW LOAD CONDITION

This snow load condition calculation is done to check the strength of the roof when there is dead weight on the roof. As we know that in some regions we have snowfalls so when snow falls on the roof then this act as dead weight because in some the rate of snowfall is more so the roof should sustain this dead weight for that we do this calculation to check the position of the bow and its sectional area.

The formula for snow load condition:

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

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

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

STEPS TO CALCULATE SNOW LOAD CONDITION:

• Steps is same as heat distortion criteria but the difference is we calculate the Rx and Ry at the centerline of the roof along the y-axis.
• We have to calculate the moment of inertia of each reinforcement(Bow) which can as follows in NX software.
• We use the intersection curve command in NX software to get the sectional area of the bow along the XY plane.
• Now we project these curves on the other plane parallel to XY-plane at some offset distance.
• .Now we use a sectional inertia analysis tool to calculate the MOI as follows;

FOR BOW ROOF - 01:

FOR CENTER ROOF RAIL:

FOR BOW ROOF - 02

CALCULATION

For front roof rail and Bow1:

Ly = 1553.9 mm^4

Y = 510.3mm

My = Y(Ly-Y) =510.3 (1553.9*10^4 - 510.3) = 532549.08

Lx = 1147

α = My x Lx² x 10^{-12 = 532549.08 x 1147^2x 10^-12 = 0.7}

S = L1/2 + L2/2 = 478

Rx = 4510.3897 mm

Ry = 2629.5791 mm

t = 0.75 mm

Qr = [Iy x t²] / [α x s x [(Rx + Ry)/2]² x 10^-8] = [1553.9x 0.75²] / [0.7x 478 x [(4510.387 + 2629.5791)/2]² x 10^-8] = 27.30

Judgemental criteria is Qr > 3.1

so for the front roof rail and Bow1 the Qr is grater than 3.1 so for it passed the snow load condition and the position of Bow1 is correct.

we will calculate the remaing bows and centre roof rail by same method and check wether it passes the judgemental test or not.

Actual test conducted by National Highway Traffic Safety Administration(NHTSA) for rollover of the vehicle to check the roof strength :

In this test, we apply the force at one side at the top surface with the block of dimensions 1829X762mm at 25°. and the strength of the ditch area of the roof and side area of the vehicle.

CONCLUTION

Now we know the types of roofs and reinforcement used to increases the strength of roof and how to design the roof according to the heat distortion criteria and snow load conditions and how to join the front roof rail, central roof rail, rear roof rail and Bows to the roof and also calculated the position of the bows so that it can pass the judgemental conditions of the snow load condition and hear distortion criteria.