Roof Design

                                                                                                             REPORT ON ROOF DESIGN

• INTRODUCTION-

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.

IN THE PAST ROOF WAS INTRODUCED AS COVER FOR PASSENGER COMPARTMENT BUT LATER ON, IT WAS ALSO USED AS A LUGGAGE CARRIER, SUNROOF AND IN RECENT PAST YEARS WITH THE INTRODUCTION OF THE SOLAR PANELLED ELECTRIC VEHICLE GOT A MAJOR BOOST IN THE AUTOMOBILE INDUSTRY.

ROOF HAS A FOLLOWING FUNCTIONS-

1.PROTECT FROM EXTERNAL DAMAGE

2.LUGGAGE CARRIER

3.DESIGN OF ROOF ALSO AFFECTS IN GIVING AERODYNAMIC SHAPE TO VEHICLE.

4.PROVIDES HOUSING FOR SOLAR PANNELS.

SINCE THE ROOF HAS NUMEROUS FUNCTIONS SO THE ROOF HAS TO WITHSTAND THE DAMAGES. HENCE TO INCREASE THE STRENGTH OF THE ROOF THE OUTER ROOF PANEL IS MADE UP OF DIFFERENT COMPONENTS NAMELY FRONT ROOF RAIL, REAR ROOF RAIL, BOW ROOF RAIL AND CENTER ROOF RAIL.

1.FRONT ROOF RAIL-

THE FRONT ROOF RAIL IS A COMPONENT THAT RUNS ALONG THE FRONT EDGE OF THE CAR ROOF, WHICH CONNECTS WITH A-PILLER DURING ASSEMBLY OF ROOF STRUCTURE. IT HELPS IN PROVIDING ADDITIONAL STRENGTH TO THE OUTER ROOF PANEL. IT IS DESIGNED IN SUCH A WAY THAT IT SHOULD NOT OBSTRUCT THE VISION OF THE PASSENGER AS WELL AS MINIMUM HEAD ROOM/ SPACE NEED TO BE PROVIDED FOR THE PASSENGERS COMFORT.

2.REAR ROOF RAIL-

REAR ROOF RAIL IS A COMPONENT THAT RUNS ALONG THE REAR EDGE OF THE CAR ROOF, WITCH CONNENTS WITH EITHER C OR D-PILLER DURING ASSEMBLY OF ROOF STRUCTURE. THESE DESIGNS ARE OF TWO TYPES BASED ON THE TYPE OF OPENINGS REQUIRED FOR BACKDOOR. THESE ARE ADDED TO PROVIDE STRENGTH TO THE OUTER ROOF PANEL. 

3.BOW ROOF RAIL-

BOW ROOF REFERS TO A TYPE OF ROOF DESIGN COMMONLY FOUND IN OLDER VEHICLES, PARTICULARLY CLASSIC CARS. THIS DESIGN FEATURES A CURVED OR ARCHED ROOFLINE WITH CENTRAL PEAK, RESEMBLING THE SHAPE OF A BOW. N NUMBER OF BOW ROOF RAILS CAN BE ADDED BASED ON DESIGN REQUIREMENTS. MAINELY THE BOW ROOF RAILS ARE ADDED ON THE ROOF REGION WHERE THE RADIUS OF CURVATURE IS LESS COMPARED TO OTHER AREAS ON OUTER ROOF PANEL (ON FLAT SURFACES) TO INCREASE THE STERNGTH AT THAT PARTICULAR REGION.

4.CENTER ROOF RAIL-

CENTER ROOF RAIL IS A TYPE OF COMPONENT WHICH IS MOSTLY PLACED AT THE CENTER OF THE OUTER ROOF PANEL. IN CASE OF PRESENCE OF SUN ROOF ON THE OUTER ROOF PANEL THE CENTER ROOF RAIL IS SHIFTED TO ACCOMODATE THE SPACES FOR THE SUN ROOF, DUE TO WHICH AN ADDITIONAL REINFORCEMENT IS PROVIDED TO THE CENTER ROOF RAIL TO INCREASE ITS STRENGTH OF THE ROOF.

• MANUFACTURING PROCESS-

OUTER ROOF PANEL CAN BE MANUFACTURED WITH THE FOLLOWING PROCESS, FIRST SELECT APPLICABLE COLD COILING SPECIFICATION, COLD COILING IS OPENED FLAT. THEN ACCORDING TO DESIGN THE MATERIAL IS SELECTED AND CUT INTO SMALLER PLATES OF REQUIRED SIZE. THEN THESE SMALLER SHEETS ARE PUNCHED/BLANKED INTO DESIRED SHAPES AND THE EXCESS MATERIAL IS TRIMMED OFF.

THE REMANING COMPONENTS LIKE BOW ROOF RAIL, CENTER ROOF RAIL, FRONT ROOF RAIL AND REAR ROOF RAIL ARE ASLO MANUFACTURED IN THE FOLLOWING PROCESS, WHERE THE LARGE SHEETS ARE CUT INTO SMALLER SHEET OF REQUIRED DIMENSIONS. SECONDLY THE CUTOUTS ARE MADE BY PUNCHING OPERATIONS. THIRDLY THE SHEETS ARE PLACED BETWEEN THE PUNCH AND DIE TO OBTAIN THE DESIRED SHAPE AND SIZE AND THE EXCESS MATERIAL IS TRIMMED OFF AND RE-STRICKING IS CARRIED ON IF NECESSARY.

THE FRONT ROOF RAIL, REAR ROOF RAIL, BOW ROOF RAIL AND CENTER ROOF RAIL ARE CONNECTED TO OUTER ROOF PANEL THROUGH MASTIC SEALS PRESENT BETWEEN THEM AND THESE ARE ASLO CONNECTED BY SPOT WELDINGS AT THE FRONT, REAR AND DITCH END OF THE OUTER ROOF PANELS. 

• DRAFT ANALYSIS-

IT IS A STUDY WHICH ENABLES US TO CHECK WEATHER THE COMPONENTS CAN BE MANUFACTURED OR NOT. HERE THE RESULTS ARE OBTAINED IN THE FORM OF COLOUR CODES. EACH COLOUR CODES INDICATES THE DRAFT ANGLE RANGING FROM PARTICULAR VALUE AS SHOWN IN BELOW IMAGES.

MINIMUM OF 7 DEGREE DRAFT MUST BE PROVIDED FOR THE COMPONENT AS PER DESIGN RULES.

1.FRONT ROOF RAIL

2.REAR ROOF RAIL

3.BOW ROOF RAIL 1

4.CENTER ROOF RAIL

5.BOW ROOF RAIL 2

• TESTING OF ROOF-

THE ROOF IS TESTED FOR CRASHWORTHINESS. THESE TESTS ARE ROOF CRUSH TEST, HEAT DISTORTION CRITERIA AND SNOW LOAD CRITERIA.

1.ROOF CRUSH TEST

HERE THE CRASH TEST SYSTEM IS AS SHOWN IN BELOW IMAGE WHICH IS MANUFACTURED BY MGA CORPORATION. THESE TESTS ARE CARRIED ON ONLY AFTER THE COMPLETION OF THE ROOF ASSEMBLY. IT HAS A UPRIGHT ASSEMBLY AND ATTACHED LOADING HEAD THAT CAN BE FIXED AT VARIED HEIGHTS. PITCH ANGLE CAN BE VARIED BY +5 TO -5 DEGREES. BUT THE ROLL ANGLE IS FIXED TO 25 DEGREE. HYDRAULIC ACTUATORS ARE USED TO CONTROL THE MOVEMENT OF RECTANGULAR BLOCK. THESE RECTANGULAR BLOCK IS 30 INCHES WIDE AND 72 INCHES LONG, APPLIES A LOAD 1.5 TIMES THE UNLOADED VEHICLE WEIGHT WITH DIFFERENT ANGLE OF INCLINATION TO RECTANGULAR BLOCK. THE MOVING DISTANCE OF ROOF STRUCTURE MUST BE LESS THAN 127MM OR 5 INCHES AS PER FMVSS STANDARDS. 

2.HEAT DISTORTION CRITERIA

THE HEAT DISTORTION TEST EVALUATES THE ABILITY OF CAR ROOF TO MAINTAIN ITS PHYSICAL STRUCTURE WITHOUT DEFORMING WHEN SUBJECTED TO ELEVATED TEMPERATURE CONDITIONS. AS CAR ROOF ARE EXPOSED TO SUNLIGHT DURING DAY TIME, SO THESE CRITERIA SHOULD BE MAINTAINED. THESE TESTS ARE MANUALLY CALCULATED BY DESIGNERS TO VERIFY BEFORE SENDING THE ROOF MODEL FOR FEA ANALYSIS.

CALCULATIONS-

HEAT DISTORTION CRITERIA IS GIVEN BY THE 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>

TO MEASURE THE ROOF CURVATURE WE SHOULD FOLLOW CERTAIN METHODS WHICH ARE SHOWN BELOW.

THE INTERSECTION IS OBTAINED AT THE CENTER OF FRONT ROOF RAIL AND BOW ROOF RAIL 1, SIMILARLY THE INTERSECTION IS OBTAINED BETWEEN THE OTHER ROOF RAIL AND THE ROOF CURVATURE IS DETERMINED BY FINDING THE RADIUS OF CURVATURE ALONG X AND Y DIRECTIONS AS SHOWN IN BELOW IMAGES.

RX1

RX2

RX3

RX4

 RY1,RY2,RY3 AND RY4

ALL THE VALUES ARE DETERMINED AND TABULATED AS SHOWN BELOW.

REGION	t THICKNESS                 (mm)	l  BOW ROOF SPAN           (mm)	L            ROOF LENGTH                   (mm)	RX     (mm)	RY      (mm)	R      (mm)	W	RESULT
FRONT ROOF RAIL TO BOW ROOF RAIL 1	0.75	407.568	2028.405	5778.3	2883.5	3847.174	1.642	OK
BOW ROOF RAIL 1 TO CENTER ROOF RAIL	0.75	435.217	2028.405	4854.9	6676.2	5621.715	2.086	OK
CENTER ROOF RAIL TO BOW ROOF RAIL 2	0.75	418.656	2028.405	3889.2	11764.2	5846.591	2.432	OK
BOW ROOF RAIL 2 TO REAR ROOF RAIL	0.75	302.290	2028.405	3266.5	16799.3	5469.49	1.898	OK

SINCE THE VALUE OF W IS LESS THAN 2.7 HENCE THE HEAT DISTORTION CRITERIA IS ACHIVED

JUDGMENT CONDITION FOR THE ROOF IS OK IN ALL SECTIONS.

3.SNOW LOAD CRITERIA

IN SOME PART OF THE WORLD WHERE SNOW FALL OCCURS, SNOW LOAD CRITERIA SHOULD BE CONSIDERED. DUE TO THE SNOWFALL THE WEIGHT ON THE ROOF GETS INCREASED WHICH CAUSES THE DENTS ON THE ROOF, BUT ROOF ARE DESIGNED IN SUCH A WAY THAT ON REMOVAL OF SNOW, ROOF SHOULD RETURN TO ITS ORIGINAL POSITION, HENCE THESE CRITERIA PLAYS A MAJOR ROLE IN SNOWFALL REGIONS.

CALCULATIONS-

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

TO FIND THE RADIUS OF CURVATURE ALONG X AND Y DIRECTIONS WE HAVE TO FOLLOW CERTAIN METHOD WHICH IS GIVEN BELOW.

AS VALUE OF L2 IS ZERO FOR LAST REGION SO THE VALUES ARE CALCULATED ONLY AT 3 REGIONS.

RX1

RX2

RX3 

RY1, RY2 AND RY3

ALL THE VALUES ARE CALCULATED AND TABULATED AS SHOWN BELOW

REGION	LX & LY	RX & RY	L1 & L2	ly   (mm^4)	S & Y	t (mm)	α & My	Qr	RESULT
FRONT ROOF RAIL TO BOW ROOF RAIL 1	1131.42&2028.405	4438.5&2819.4	502.972&517.037	1.589x10^3	510.004&502.972	0.75	0.982&767250.086	13.546	OK
BOW ROOF RAIL 1 TO CENTER ROOF RAIL	1082.31&2028.405	3802.4&6599.6	517.037&500.41	1.919x10^3	508.723&517.037	0.75	0.915&781433.176	8.573	OK
CENTER ROOF RAIL TO BOW ROOF RAIL 2	1055.72&2028.405	4304.0&11669.8	500.41&428.583	1.605x10^3	464.496&500.41	0.75	0.852&764623.977	3.575	OK

SINCE THE VALUE Qr IS GREATER THAN 3.1, HENCE THE SNOW LOAD CONDITION IS ACHIVED.

JUDGMENT CONDITION FOR THE ROOF IS OK.

•  ORIENTATION VIEWS-