Roof challenge

Roof Design :-

Objective :

• The objective of this project is to design car roof with essential flanges reinforcements like ditch area, front roof rail, bow roofs, central roof rail and roof rail for the given styling surface
• Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct positions.
• Finding moment of inertia and section modulus for bow roofs, central roof rail, central roof rail, front roof rail and rear roof rail.
• Perform the draft analysis for each reinforcements

Introduction :

Design of any products starts from an requirement. The design engineer sketches the product based on the requirement. Then the 2D sketch with the proper dimensions is converted into the 3D model using the advanced CAD software’s. Another important thing in the design is that very product that is designed should be able to manufacture. So the role of the design engineer is to design the product that can be easily manufactured and with less cost effective. This is the place where the design for manufacturing (DFM) and design assembly (DFA) takes a role while developing any product. The process of development of design carries on until the design satisfies the customer needs.

Here in this project the product that we are going to develop is a car roof

CAR 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. Roof is the structure above the wind shield glass having the reinforcement to provide the strength to the overall structure and to with stand the over head forces like snow.

Why Reinforcements:

If the roof is flat it is very weak and cannot able to with stand the forces like snow so in order to increase the strength of the roof these reinforcement like front roof rail central roof rail rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process.

Different types of reinforcements that the car has are :

Front roof rail

Bow roofs

Central roof rail

Rear roof rail

Front roof rail :

Front roof rail is the one which joins the wind shield glass, body side outer and the inner panel also here the front roof rail is designed based up on the master section given

Rear Roof rail :

Rear roof rails Is the one which joins the back door and the body side outer

Central Roof Rail :

Central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load. Generally central roof rail is placed at the centre of the roof which is connected to the B-pillar support structure. This central roof rail helps in adding the strength to the roof during the roll over test. The thickness of the central roof rail is 0.75mm.

Bow Roofs :

The bow roofs are given to improve the torsional stiffness and load bearing capacity of the roof structure. The number of bow roof present is depends on the overall size of the roof. Presently in this project two bow roofs are added.

Roof Bow 1 and 2 are same geometry

Joining Process :

Mastic are introduced between the roof outer panel and the reinforcements for joining where ever the mastic are introduced there is remarkable improve of the strength spot welding is also introduced to join the reinforcement and the outer panel.

Design consideration for a roof :

• Visibility criteria
• Head clearance
• Curvature study
• Heat distortion and snow load criteria
• Draft analysis

Roof crush test :

• Roof crush test is the design given by IIHS (insurance institute of highway safety). After analysing the many accidents IIHS has then fixed some regulations that how to perform the crush test.
• How fast the car reforms to its original shape after the roll over
• How deep is the protrusion after the roll over

Goal Of Roof crush Test :

To minimize the protrusion into the occupant, so that the seat belts and air bags can do there job in protecting the occupant

Conditions To Follow For Roof Crush Test :

There are two conditions that should be followed for the roof crush test, they are

Initial Condition :

• Close and lock all the doors
• Close all the windows

Method of applying the force :

The load is applied in such a way that 5 degree is tilled from the front end to the rear end and the load will be in 1800 mm in length and 750 mm in width. The load is applied at the position of 25 degree from the flatness at which the load applied is 13 mm/sec or less than that

Results :

If the displacement goes above the requirement level then we have to increase the bow roof thickness or add more mastics points to increase the strength. The stronger the roof the better it can protect the occupants in the rollover crashes.

Heat distortion study on the roof top :

The heat distortion study plays a major role in sheet metal usage. Heat distortion temperature is a temperature limit above which the material cannot be used for the structural applications. This study is used to predict the heat distortion temperature at where the material starts 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 which helps 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

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

Apply The values in the formula :

In order to find the heat distortion apply all the necessary values from the roof to the given fornula

Calculations :

Heat Distortion Criteria Calculation
Roof Rails	Rx (mm)	Ry (mm)	Rx*Ry (mm)	Rx+Ry (mm)	R (mm)	L(mm)	t(mm)	l(mm)	W	Results
FR - BOW1	2842.0104	5646.3927	16047106.78	8488.4031	3780.948	2033.0323	0.75	408.63	1.639263	Pass
BOW1 - CR	4756.9856	4692.8793	22323959.25	9449.8649	4724.715	2033.0323	0.75	404.007	1.832186	Pass
CR - BOW2	10081.9157	11059.2243	111498167.1	21141.14	10547.98	2033.0323	0.75	402.1194	4.023888	Fail
BOW2 - RR	3394.9268	15332.5665	52052940.92	18727.4933	5558.986	2033.0323	0.75	423.2736	2.065004	Pass

From the above table it can be concluded that all values of W<2.7 and also not w>3.1 so thus infers that current of positioning bow row are not ok need to adjust bow roof 2 distance from centre Roof bow.

Snow Load Criteria:

This test is done to know is the roof behaving when there is a snow over. Normaly 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 to its original position. This is the basic requirements for snow load criteria.

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 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 condition

Qr ≥ 3.1

250 ≤ s ≤ 380

Apply the values in the formula:

In order to find the snow load criteria apply all the necessary values from the roof to the given formula

Calculations:

Snow Load Criteria
Roof Rails	Iy	Lx	Ly	L1	L2	Rx (mm)	Ry (mm)	t (mm)	Y (mm)	S	My	Alpha	Qr	Result
FR - BOW1	1957.06	1076.3525	2032.25	445.52	419.55	4357.86	2869.36	0.75	1434.38	432.535	857572.8	0.993528	19.61746	Pass
BOW1 - CR	4659.1	1068.96	2032.25	419.55	319.8	3794.6	6608.31	0.75	514.16	369.675	780541.2	0.891905	29.37894	Pass
CR - BOW2	2006.9	1055.38	2032.25	319.8	299.92	4300.106	11449.92	0.75	414.15	309.86	670136.1	0.746416	7.870448	Pass

It is observed from the values that obtained of Qr >3.1

Draft Analysis For Front Roof Rail :-

Draft Analysis of Central Roof Rail :

Draft Analysis For roof bow 1 &2

Draft Analysis for rear roof rail :

Roof Images :-

Conclusion :

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