Roof Design

Aim: Design of the front roof rail, rear roof rail, and bow roof for the roof of the vehicles with the help of the given skin data of the roof according to snow load condition and hear distortion criteria.

Objective:

• Study different type of the roof available in the market.
• Study about the heat distortion criteria to adjust the position of bows and center roof rail.
• Study about snow load conditions to check the strength of the roof according to bows positions.

Theory:

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.

Type of roofs:

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

In this test, e 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.

Methodology for designing the parts of the Roof:

1.Selecting the type of roof from the above list for the given skin data of roof surface.

2.Deciding the number of bows using to support the roof surface according to conditions.

3.Selecting the joining method used to join the roof surface with the front roof rail. rear roof rail, central roof rail, and for bows.

3.Designing the front roof rail and rear roof rail according to the given master sketch.

4.Designing the central roof rail and bows(as required) according to the given master sketch.

5.Calculating the parameters required for heat distortion criteria and checking the values of distortion in the roof are within range or not.

6.If the value of distortion in the roof body is not within the range, we need to change the position of bows and central rail roof so that the distortion in the roof should be within range.

7.Next we have to do iteration for the snow load condition to check the strength of the roof again if the value from the snow load condition is not in the range we have to do modifications in the bows section area and their position on the roof body.

GIVEN DATA :

A SURFACE:

OUTER PANEL:

 EMBOSSES ARE GIVEN TO GIVE STRENGTH

FRONT ROOF

OUTCOME:-

BOW ROOF:

OUTCOME:

HERE W PATTERN IS GIVEN FOR MORE STRENTH COMPARE TO OTHER PATTERN

CENTER ROOF:

OUTCOME:

CENRAL RAIL REINFORCEMENT:

REAR ROOF:

OUTCOME:

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]

JUDGMENT CONDITION:-

OK<2.7

STEPS TO CALCULATE HEAT DISTORTION:

1.First we offset the center y-axis line to 300mm distance apart from it represented as (-3Y to 3Y red line).

2.After that we offset this -3Y to 3Y to 100mm offset each side represented with the green line.

3.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.

4. 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.

5.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.

Hence after calculation you get all the values form a table and find the ''W'' value as below,

HEAT DISTORTION CALCULATION
SR NO	BOWS	Rx	Ry	R	l	L	t	R^2/t	W	OK/N
1	FR & 1ST	5793.797	2213.665	3203.39	373.878	2029.061	0.75	13682276.66	1.49	OK
2	1ST & CR	5026.064	6182.141	5544.48	431.546	2029.061	0.75	40988344.63	2.06	OK
3	CR & 2ND	4025.093	10579.631	5831.53	431.729	2029.061	0.75	45342322.85	2.144	OK
4	2ND & RR	3237.797	15005.933	5326.34	347.153	2029.061	0.75	37826530.39	1.91	OK

SNOW LOAD CONDITIONS:

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.

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

JUDGMENT CRITERIA:Qr>3.1

Steps to calculate snow load condition:

1.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

2.We have to calculate the moment of inertia of each reinforcement(Bow) which can as follows in CATIA V5

a.We use the intersection curve command in CATIA to get the sectional area of the bow along the XY plane.

b.Now we project these curves on the other plane parallel to XY-plane at some offset distance.

c.Now we use a sectional inertia analysis  tool to calculate the MOI as follows;

1.BOW 1:

2.CENTRAL RAIL:

WITH REINFORCEMENT:

2ND BOW:

CALCULATION RESULT IN FORM OF TABEL AS BELOW,

HEAT DISTORTION CALCULATION
SR NO	BOWS	Iy	My	α	Lx	Ly	Y	S	Rx	Ry	t	Qr	OK/NG
1	FR & 1ST	1038	702151.334	0.91	1140.26	1976.751	464.224	486.58	2712.898	2491.762	0.75	19.40	OK
2	1ST & CR	4472	747026.7715	0.88	1086.72	1976.751	508.939	508.99	3763.795	6102.83	0.75	23.01	OK
3	CR & 2ND	1104	747129.3594	0.83	1058.335	1976.751	509.046	501.79	4270.359	11019.837	0.75	2.53	NG
4	2ND & RR	0	733014.6033	0.79	1043.076	1976.751	494.542	247.27	3928.916	16821.021	0.75	0

Since here the snow load between central rail & 2nd Bow roof does not meet the criteria hence measure have to be taken by changing the location of the bow and central rail accordingly to satisfy the condition

Draft analysis to check the manufacturability of roof and its parts:

We are doing draft analysis for the 7°  draw angle in upward direction as follows:

for ROOF OUTER PANEL:

FRONT RAIL:

1ST BOW:

CENTRAL RAIL WITH REINFORCEMENT:

2ND BOW:

REAR RAIL:

FINAL :

Conclusion:

So from this project, we learned types of roofs and reinforcement used to increases the strength of roof and 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.Learned to position the bows so that we can pass the judgemental conditions of the snow load condition and hear distortion criteria and if not than how to overcome it.