Roof challenge

CAR ROOF DESIGN

OBJECTIVE :

• The objective of this project is to design car roof with essential flanges and reinforcements like ditch area, front roof rail, bow roofs, central roof rail and rear roof rail for the given styling surface.
• Perform a curvatute 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 postions.
• Finding moment of inertia and section modulus for bow roofs, central roof rail, front roof rail and rear roof rail.
• Perform the draft analysis for each reinforcements.

INTRODUCTION :

Design of any products starts from a requirement. The design engineer sketches the product based on the requirement. Then the 2D sketch with the proper dimension is converted in to the 3D model using the advanced CAD Softwares. The another important thing in the design is that, every product that is designed should be manufactured. So the role of 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 for assembly [DFA] takes a role while developing any product. The process of development of design carries on until the design satisfies the customer needs.

CAR ROOF :

An automobile roof or car top is the position 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 reinforcements to provide the strength to the overall structure and to with stand the over head forces like snow.

REINFORCEMENTS :

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

Different types of Reinforcements of car roof :

• 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 ROF RAIL :

Rear roof rail is one which joins the back door and the body side outer.

CENTRAL ROOF RAIL :

Central roof rail helps in providing effective 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 strength to the roof during the roll-over test. The thickness of the central roof is 1.25 mm.

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 required depends up on the overall size and curvatute of the roof. Presently in this project two bow roofs are required.

JOINING PROCESSES :

Mastics are introduced between the roof outer panel and the reinforcements for joining. Where ever the mastics are introduced there is remarkable improvement in strength.

Spot welding is also introduced to join the reinforcements and the outer panel of roof.

ROOF CRUSH TEST :

Roof crush test is the test given by the IIHS(insurance institute of highway safety). After analyzing many accidents IIHS has then fixed some regulations to perform the crush test.

IIHS gives the rating based up on this test. For thus they consider two points while giving the rating.

• How fast the car can 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 compartment, so that the seat belts and air bags can do their job in protecting the occupant.

CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST :

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

Intial condition

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

Second condition

• Fix the body sill or chassis frame on a hard level surface horizontally.

METHOD OF APPLYING THE FORCE :

The load applied in such a way that 5⁰ is tilted from 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 25⁰ from the flatness of the roof. The load is applied on the roof for almost 120 seconds and the crush speed at which the load applied is 13mm/sec or less than that speed.

RESULTS :

If the displacements goes above the requirements level then we have to increase the bow roof thickness or add more mastic points to increase the strength. The stronger the roof the better it can protect the occupants in the roll over crashes.

HEAT DISTORTION STUDY ON THE ROOF :

The heat distortion study play a major role in sheet metal usage. Heat distortion temperature is a temperature limit above which the material cannot be used for tye structural applications. This study is used ro 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 which helps to strengthen the roof.

Bow-roof prediction formula

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

Where

L = Roof length in x- direction [mm](Roof dimension in 0-Y)

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

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness [mm]

l = Bow roof span [mm]

Judgement condition: OK < 2.7 > 3.1

Apply the values in the formula :

In order to find the heat distortion apply yhe necessary values from the roof to the given formula

CALCULATIONS: HEAT DISTORTION CRITERIA

SL.No	ROOF RAILS	Rx (mm)	Ry (mm)	Rx*Ry (mm^2)	Rx+Ry (mm)	R (mm)	L (mm)	t(mm)	l (mm)	W (mm)	Result
1.	RR-BR2	3209.369	15060.045	48333241.56	18269.414	5291.165	2029.2	0.75	330.714819	1.88	PASS
2.	BR2-CR	3895.986	11001.106	42860154.96	14897.092	5754.164	2029.2	0.75	408.814	2.09	PASS
3.	CR-BR1	4837.0656	6740.8606	32605984.92	11577.9262	5632.44	2029.2	0.75	435.696	2.09	PASS
4.	BR1-FR	5733.3175	2645.2215	15165894.72	8378.539	3620.18	2029.2	0.75	440.6102	1.64	PASS

From the above data it can be concluded that all the values of W<2.7 and also not W>3.1 so this infers that current positioning of bow roofs are good in state as per design.

Section inertia analysis

Bow roof rail 1

centre roof rail and its reinforcement

Bow roof rail 2

SNOW LOAD CRITERIA:

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

Qr = [ly × t^(2)] / [a×s×[(Rx+Ry)/2]^(2) × 10^(-8)

Where

a = My × Lx² × 10^(-12), My= Y(Ly-Y)

Judgment 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 OY [mm]

Length of roof panel with the center point between Roof rail front / Rear as the reference point of the front and rear.

Lx = Distance between the left and right end of the roof on the Bow roof [mm] width of the roof panel exposed to the surface.

Y = Distance from front roof rail to bow roof rail [mm]

s = Distance for which Roof bow bears the divided load [mm] s = L1/2 + L2/2

ly = Geometrical moment of inertia of Bow roof (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 Bow roof [mm]

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

SL.No	ROOF RAILS	t(mm)	Iy(mm^4)	Lx(mm)	Ly(mm)	L1(mm)	L2(mm)	Rx(mm)	Ry(mm)	Y (mm)	My(mm^2)	a(mm^4)	s (mm)	Qr	Result
1.	RR-BW2	0.75	84.742068	1043.118	2029.2	449.3728	485.40375	3883.438	17066.508	449.3728	709931.37	0.772473	467.38825	0.120326	NOT PASS
2.	BW2-CR	0.75	141.4248	1058.039	2029.2	485.40375	509.877	4309.306	11602.7	485.40375	749364.489	0.8388735	497.640365	0.3010552	NOT PASS
3.	CR-BW1	0.75	74.05099	1080.55	2029.2	509.877	530.6394	3799.87	6675.764	509.877	774667.85	0.904493	520.2582	0.3226477	NOT PASS
4.	BW1-FR	0.75	0	1136.35	2029.2	530.6394	0	4468.442	2691.3485	530.6394	795195.29	1.026828	265.3197	0	NOT PASS

Draft Analysis

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

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

1. Roof

2. Front Roof Rail

3. Bow Roof Rail 1

4. Centre Roof Rail

5. Centre Roof Reinforcement

6. Bow Roof Rail 2

7. Rear Roof Rail

views of Roof:

CONCLUSION :

• The Roof and its reinforcements are designed according to their master sections and criterias.
• Heat distortion criteria is fulfilled and snow load criteria is not fullfilled so I provided embosses to improve the strength of the roof.
• The draft analysis is also performed and the parts are found to be manufacturable.