Roof challenge

TITLE: Design of a Car Roof and Analysis of Reinforcement.

OBJECTIVE:

• To design a car roof by extracting the styling surface from styling team with essential flanges and reinforcement namely ditch Area, front roof Rail, Rear roof rail, bow roofs, center roof rail.
• Do a curvature study on the roof and perform calculations for heat distortion and snow load criterion to determine and justify the position of the bow-roofs.
• Finding moment of inertia and perform draft analysis for Bow roofs, center roof rail, front roof rail, rear roof rail.

INTRODUCTION:

An automobile roof is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. It also acts as a cross member between body sides of the car body and provide provisions for front windshields, backdoor (in case of hatchback vehicles). Under the styling surface that is seen from the outside, there are many other structures under the roof which contribute to its strength and stability. The number of reinforcement structures/supports differ with respect to the size of the vehicle. They are primarily installed on flatter areas of the roof to enhance the strength. These are front roof rail, rear roof rail, center roof rail and bow roof design which are spot welded to roof flanges and by mastic sealants to improve NVH value.

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

In later years, many variations on the automobile roof developed. These include:

• Convertibleroofs
• Hardtops
• Sunroofs
• T-tops
• Targa tops
• Vinyl roofs

In this project I am going to design a conventional/traditional roof of a car.

Design Methodology:

Here we are creating the roof, front and Rear roof rails along with the three reinforcement, by taking pre-designed styling surface as the reference.

The front roof rail and the rear roof rail are the components, where the front and rear sides of the roof are attached, while the other three reinforcement gives enough strength and stiffness to the roof. This assembly includes Roof Outer Panel, Front Roof rail, Rear Roof Rail, Center Reinforcement, Bow Roof 1 and Bow Roof 2. while Front roof rail and rear roof rail is designed with the help of given master section.

fig. front roof rail master section.

fig. Rear roof rail master section.

For designing a car roof there are several design considerations and standard a design engineer must consider in his designs. Like

1. Visibility criteria
2. Head clearance
3. Head distortion and snow load criteria
4. Draft analysis

Front Roof Rail:

It is the support that is joined with the windshield glass, bodyside outer and the inner panels. The rails are furnished with embosses to improve the strength over length. It is usually made with thickness of 0.75 mm. a positive draft angle of more than 7 degree has been provided for deep drawing operation. The visibility criteria and the head room clearance of min 80 mm for the passenger are taken into consideration for the bottom most flange. Windshield end and the roof end are spot welded and joined and the other end of the front roof rail is joined with the roof using mastic sealant for increased strength. Front roof rail is designed with the help of given master section.

Rear Roof Rail:

It is the support which is joined to the back door and the body side outer and the inner panels. It is also usually made from body side outer scrap and therefore thickness of the rail is 0.75 mm. similar to the front roof design, we have to take into consideration the rear headroom clearance and the rear visibility criteria while designing the rear roof rail. Spot welds are applied at the backdoor end of the rail and the other end is joined with the roof using mastic sealants.

Bow Roofs

The bow roofs are given to improve the torsional stiffness and load bearing capacity of the roof structure. The number of bow roofs provided will depend upon the overall dimensions of the roof. Longer the roof more the bow roofs are integrated to BIW frame they are attached to the outer panel using mastic sealants. Each mastic point will have up to 80mm circular effective area. Special notches on the flanges are provided for the application of mastic sealant. Both Bow roof 1 and Bow roof 2 are similar in dimension and pattern only curvature profile is different at respective position. The positioning of bow roofs is done based on curvature study. The heat distortion criteria and snow load criteria are checked to analyse the position of the bows in various conditions.

Center Roof Rail (Reinforcement center):

Centre roof rail is provided to effectively support the flat area of the roof as it is more susceptible to failure under the action of the load. usually, the center reinforcement roof rail will be placed at the center of the roof connecting the B pillar support structure which result in added support in the roll over test.

positioning of the Rails:

1. Heat Distortion Curvature Study:

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

Bow–roof prediction 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

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]

steps followed:

1. From O-Y axis, at -3y about distance 300mm is drawn parallel on the perpendicular plane as shown in fig. after that 100 mm lines are offset to either side of -3Y line. Now center lines are drawn between each adjacent roof rail edges respectively. Intersection points are marked on the sketch and exit the sketch. Make all geometry in construction lines except the points.
2. Project the intersection points in the sketch to roof surface as shown in fig.
3. Join the projected curves using arc curve and measure the minimum radius and linear dimension as follow and put into the formula to calculate W.

fig. showing -nY axis and intersection points

fig. projecting poits on roof for Rx & Ry

fig. L distance of styling surface

Roof rail spans from	L(mm)	Rx(mm)	Ry(mm)	Rx*Ry	Rx+Ry	R	t(mm)	l (mm)	W	Result
FR-BOW1	2033.1157	5676.90	2884.44	16374677.44	8561.34	3825.26	0.75	447.04	1.69	ok
BOW1-CR	2033.1157	4869.18	4869.18	23450311.72	9685.25	4842.47	0.75	420.51	1.87	ok
CR-BOW2	2033.1157	3965.01	10274.69	40739248.6	14239.7	5721.92	0.75	433.22	2.12	ok
BOW2-RR	2033.1157	3340.36	10826.99	36166044.32	14167.35	5105.54	0.75	368.93	1.88	ok

From the above table it can be concluded that all values of W<2.7 so thus infers that current positioning of Bow roof are optimum as per design and found ok.

1. Snow Load Calculation:

In the winter season, in some countries there will be a huge load of snow on the roof top of the car. Snow load criteria study is done to check the ability of the roof to withstand snowfall and bounce back to its original shape without a permanent deformation once the load is removed.

Snow load Prediction formula

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 type error in the formula given in the video.

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 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] Width of the roof panel exposed on the surface.

Y = Distance from 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 Followed:

1. At O-Y axis on perpendicular plane to roof as shown in fig, draw 100 mm lines offset to either side of OY axis line. Now center lines are drawn between each adjacent roof rail edges respectively. Intersection points are marked on the sketch and exit the sketch. Make all geometry in construction lines except the points.
2. Project the intersection points in the sketch to roof surface as shown in fig.
3. Join the projected curves using arc curves and measure the minimum radius and linear dimension.
4. Project the intersection plane bow roof and center roof rail on the horizontal plane to find the moment of inertia of the respective section to find ly values.
5. Put all the linear and radial dimension in following formula to calculate Qr value.

fig. plotting Rx an RY points on plane

fig. projecting poits for Rx & Ry

fig. section inertia of BOW roof 1

fig. section inertia of center roof

fig. section inertia of BOW roof 2

Roof rail spans from	ly(mm)	L1(mm)	L2(mm)	LY (mm)	Lx(mm)	Rx(mm)	Ry(mm)	t	Y(mm)	s	My	α	Qr
FR-BOW1	52.4605	446.95	420.49	2033.70	1135.14	4324.45	2911.73	0.75	446.95	433.72	709197.91	0.91	0.57
BOW1-CR	248.88	420.49	433.20	2033.70	1083.48	3660.98	6491.41	0.75	420.49	426.84	678338.67	0.79	1.61
CR-BOW2	55.1637	433.20	368.91	2033.70	1059.13	4281.30	10216.33	0.75	433.20	401.05	693336.6	0.77	0.19

As per snow load criteria the values of Qr ≥ 3.1 but mine values of Qr are less than 3.1. thus, my design is failed under snow load condition. Thus, to overcome this failure I improved my design by adding embosses on the upper face of roof to improve the strength of the roof.

Draft Analysis:

The draft analysis command enables you to detect if the part you drafted will be easily removed and manufacturable. This type of analysis is performed based on color ranges identifying zones on the analysed element where the deviation from the draft direction at any point, corresponds to specified values.

Minimum draft angle of 7 degree is considered for analysis. Green color infers on the parts that all face along the tooling direction has positive draft angle greater than 7 degree and passed the analysis.

Front roof rail

Roof

Center roof Rail

Rear Roof

\

Bow roof

Assembly View:

Isometric View:

Top View:

Side View:

Conclusion:

• Roof assembly has been made successfully by following engineering standard, master section and styling surface given by styling team.
• For roof assembly heat distortion criteria has been successfully passed and snow load criteria has failed to overcome this embosses are added to the roof outer panel for more strength.
• Draft analysis has been carried out to check manufacturing feasibility.
• Designed part will be submitted to CAE team to carry out further analysis to improve the design.