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 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 . front roof rail and rear roof rail .
• Perform the draft analysis for each reinforcements.

AUTOMOBILE 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 reinforcements to provide the strength to the overall structure and to with stand the over head forces like downward  force due to  snow on top of the roof.

THE IMPORTANCE OF REINFORCEMENTS IN ROOF DESIGN:-

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 reinforcements 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 joints on body.

REINFORCEMENTS USED IN DESIGN:-

• Front roof raiL

• Bow roof rails (1&2)

• Central roof rail and

• Rear roof rail

ROOF CRUSH TEST :-

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

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

• How fast the car can reforms to its original shape after the roll over and
• 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 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

  SECOND CONDITION :

  Fix the body sill or chassis frame on a hard lever surface horizontally

   THE FORCE APPLIED ON ROOF IN ROOF CRASH TEST :

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

  RESULTS :

  If the displacements goes above the requirements 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 roll over crashes.

  HEAT DISTORTION STUDY ON THE ROOF :

  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 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 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]

 Judgement Condition : OK< 2> 3.1

 CALCULATIONS :

ROOF RAIL	Rx	Ry	Rx*Ry	Rx+Ry	R	L	t	I	w	RESULT
FRONT RR - BOW RR1	2564.55	5792..59	14855386.68	8357.14	3512.66	2031.4	0.75	392.65	1.573421	PASS
BOW RR 1- CENTRE RR	6205.44	4921.34	30539080.09	11126.78	5591.21	2031.4	0.75	439.47	2.094535	PASS
CENTRE RR -BOW RR 2	10615.33	3976.44	42211222.83	14591.77	5792.46	2031.4	0.75	420.53	2.115463	PASS
BOW RR 2 - REAR RR	14612.34	3305.4	48299628.64	17917.74	5389.325	2031.4	0.75	306.54	1.894352	PASS

From the above table it can be concluded that all values of w<2>

SNOW LOAD CRITERIA :

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

Q_r = [I_Y x t²] / [α x s x [(R_X + R_Y)/2]² x 10^-8]

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 reference point of the front and the rear.

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 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]

 CALCULATIONS :

ROOF RAIL	Iy	Lx	Ly	L1	L2	Rx	Ry	t	Y	S	My	((Rx+Ry)/2)^2	a	Qr	RESULT
FRONT RR -BOW RR1	1141.39	1130.92	2029.95	499.98	524.22	3372.89	2169.44	0.75	503.44	513.45	767302.5	7713827.773	0.9832	16.4871	PASS
BOWRR1 -CENTRE RR	1105.65	1079.54	2029.95	522.31	499.32	3769.42	3269.54	0.75	518.43	507.99	786403.94	12473904.55	0.9432	10.40588	PASS
CENTRE RR-BOW RR 2	2340.55	1045.33	2029.95	502.43	504.76	3024.11	11065.44	0.75	515.43	489.65	766035.87	49724532.54	0.8754	6.176997	PASS

As we can see that value of Qr is greater then 3.1 , therefore design passes the snow load criteria . if the value comes less then 3.1 then embose or extra area on roof surface is to be created to pass this criteria

SECTION ANALYSIS OF EVERY REINFORCEMENT PROVIDED TO ROOF:-

FRONT ROOF RAIL

REAR ROOF RAIL

CENTRE ROOF RAIL

BOW ROOF RAIL (1&2)

DRAFT ANALYSIS OF REINFORCEMENTS OF ROOF

FRONT ROOF RAIL

CENTRE  ROOF RAIL

BOW ROOF RAIL (1&2)

 REAR ROOF RAIL

DIFFERENT VIEWS OF ROOF DESIGN WITH REINFORCEMENTS:-

CONCLUSION:-

This design pass all the given criteria(both roof crash test and snow load criteria) as per regulations and rules ,therefore design pass. 

This shows that this design can be manufactured and used in automobiles .