Roof Design

ROOF DESIGN CHALLENGE 

AIM:

                To design a roof with all the components like front roof rail, rear roof rail, and bow roofs by following the given master sections and the high-quality surface, considering all the design requirements as per the OEM standards.

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. The roof is not a single panel as seen from the exterior it consists of front roof rail, bow roofs, center roof rail, and rear roof rail with different thicknesses which act as a reinforcement for the top.

Types of Car Roofs:

• Convertible roofs.

• Hard Tops

• Sun roofs.

• T- Tops.

• Vinyl roofs.

Design Consideration of roof:

• Visibility Criteria
• Head Clearance
• Curvature Study
• Heat Distortion and snow load criteria
• Draft Analysis
• Safety Parameters

ROOF CRUSH TEST FOR PASSENGER CAR :

In safety point of view, vehicle roof plays a very crucial role especially roll over kind of accidents, In which vehicle tips over onto its side of the roof Vehicle rollover crashes are the cause of many fatalities and head, neck and spine trauma around the world.

Strength of the roof structure to occupant protection in real world roll over crashes, requiring the maximum moving distance of the roof structures should be less than 127 mm, (5 in), where the induced load is 1.5 times the vehicle weight.

For the test, the passenger ca shall be rigidly placed or positioned on a horizontal surface with the doors locked and the window closed. Furthermore, a flat, rigid block with a lower surface a rectangle measuring 30 inches wide by 72 inches long (762mm x 1829 mm) shall not move more than 5 inches (127mm), measured as the distances between the original location of the lower surface of the test device and its location as the specified force level is reached, when it is used to apply a forces of 1.5 times the unload vehicle weight (UVW) or 5000 lbs. (22224N), whichever is less. To either side or forward edge of the vehicle.

The rigid plate shall not move faster than 0.5 inches/second and the force applied on the plate shall not exceed the lesser of 1.5 times the unloaded vehicle weight (UVW) expressed in kilograms multiplied by 9.8 or 5000 lbs (22240N).

Also, the direction of the force shall be downward and perpendicular to the lower surface of the rigid plate and such that it moves in a straight line without rotation. The duration of the test shall not exceed 120 seconds. The longitudinal axis of the plate is pitched forward at a 5’ angle as viewed from the side of the passenger car as shown

COMPONENTS OF A ROOF :

• roof outer panel
• reinforcements

roof outer panel :

            Roof outer panel is the one which is exposed to exterior of the car,so it demands high quality surface finish and good aesthetic looks.

reinforcements:

            Reinforcements are introduced below roof outer panel to provide strength over the top of car.other wise the roof would be a weak one and it could not withstand snow loads also in a safety perspective, to protect the occupant, roof should posses some strength.

types of reinforcements : 

• front roof rail
• rear roof rail
• central roof rail
• bow roofs

according to the length of vehicle and design of roofs, number of reinforcements will vary.

vehicle with sunroof would have additional bow roof and a limousine would have multiple roof rails.

• 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:

                     Just as the front rail roof, the rear rail roof provides reinforcement to the rear part of the car roof. Similar to the front rail roof, the rear roof rail is to be designed considering the rear visibility criteria and the rear passenger’s headroom clearance.

• 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 center of the roof which is connected to the B-pillar support structure. This central roof rail helps in the strength to the roof during the roll-over test.

• bow roofs:

                      The bow roof are given to improve the torsional stiffness and load bearing capacity of the roof structure. The number of roof presented is depends on the overall size of the roof. the number of bow roofs are governed by the curvature of the roof .

JOINING :

                     Mastics are introduced between the roof outer panel and the reinforcements for joining. Where ever the mastics are introduced there is remarkable strength. Spot welding is also introduced to join the reinforcement and the outer panel.

GIVEN MASTER SECTIONS:

QUALIFYING CRITERIA :

                     for the roof to be qualified by the analysis team it should maintain some strength criteria.so, this criteria tests can be done within design phase.

• Heated Distortion study on the roof
• Snow Load Condition

Heated Distortion study on the roof:

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

Procedure:

• Create a center line at the 0-Y region of the roof and then offset the same to -3Y.
• Create a symmetric line from the -3Y region at a distance of 100mm.
• Draw a vertical line at the midpoint of the line drawn from the center points of the FRR and BR 1 and followed the same till the rear roof rail.
• Offset the vertical line to a distance of 100mm on each side.
• Create intersection points and project them on the roof.
• From these projected points, make a 3-point arc and the Rx and Ry values.
• 

BOW – roof prediction study

          W= [1.73x10 ^ (-3) x L] + [1.85 x 10^ (-8) x (R^2)/t] + [1.10 x 10^ (-3) x l ] – 2.68

Where’

L = Roof strength 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]

I = Bow Roof span [mm]

Judgement Condition: OK< 2>

SL.NO	BOW ROOF	RX	RY	R	t	l	L	R2 / t	W	RESULT
1	front to bow 1	5740.28	2444.70	1714.51	0.75	347.45	2028.40	39,19,392.72	1.28	OK
2	bow 1 to centre	5002.31	6072.09	2742.76	0.75	458.59	2028.40	1,00,30,309.89	1.51	OK
3	centre to bow 2	3885.92	11048.12	2874.78	0.75	543.86	2028.40	1,10,19,146.73	1.63	OK
4	bow 2 to rear rail	3230	14914.35	2655	0.75	214.8	2028.40	93,98,700	1.23	OK

Snow Load Condition:

This test is done to know how the roof is 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 basic requirement for snow load criteria.

Procedure:

• Create a center line at the 0-Y region of the roof and then offset the same to 100mm on each side.
• Draw a vertical line at the midpoint of the line drawn from the center points of the FRR and BR 1 and followed the same till the rear roof rail.
• Offset the vertical line to a distance of 100mm on each side.
• Create intersection points and project them on the roof.
• From these projected points, make a 3-point arc and the Rx and Ry values.
• Project the extreme edges of the styling surface fro measuring the width at the respected sections.
• 

Qr = [Iy x t2]/ [∝ x s x [(Rx + Ry) / 2] 2 x 10^-8]

Where

∝ = My x L x ² 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 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 Roof Rail to Roof BOW [mm]

S = Distance for which roof Bow bears divided load [mm]

  S = L ½ + 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]

JUDGEMENT CONDITION, Qr ≥ 3.1    

SECTION INERTIA ANALYSIS

on front roof

on bow roof 1

on central bow roof

on bow roof 2

on rear bow roof

SL.NO	BOWS	L1	L2	S
1	B1	430	532.5	481.25
2	CRR	532.5	617.83	575.16
3	B2	617.83	338.73	478.28
4	RRR	338.73	0	169.36

SL.NO	BOWS	RX	RY	LX	LY	Y	MY	∝	Qr	RESULT
1	front to bow 1	4598.03	2509.02	1139.64	1919.08	430	6,40,304.4	0.8316	14.02	OK
2	bow 1 to centre	3759.12	5977.18	1087.58	1919.08	962.51	9,20,708.1907	1.0890	5.88	OK
3	centre to bow 2	4311.07	11662.18	1055.69	1919.08	1580.34	5,35,324.37	0.5966	3.9	OK
4	bow 2 to rear rail	3921.88	16858.69	1042.66	1919.08	1919.08	0	0

for snow load calculation, moment of inertia of bow roof 1, centre roof rail and bow roof 2 is considered

JUDGEMENT CONDITION, Qr ≥ 3.1 

DRAFT ANALYSIS:

                      Draft analysis enables us to know weather the component designed is easily removable or not from the die. draft angle provided is enough or not.it indicates different angle of inclinations with colour ranges. the nominal draft angle is 7 degrees.

draft analysis on roof outer panel :

draft analysis on front roof rail :

draft analysis on bow roof 1 :

draft analysis on central roof rail :

draft analysis on bow roof 2 :

draft analysis on rear roof rail :

FINAL ROOF ASSEMBLY:

CLIP SECTIONAL VIEWS OF MATING AREAS OF COMPONENTS :