Roof Design

Aim: For the given Roof styling, create an Assembly by developing the essential flanges for the outer roof and creating the reinforcements.

Objective:

For the given Car Roof styling, design and develop the essential flanges and

reinforcements, namely the Ditch Area, Front Roof Rail, Rear Roof Rail, Centre Roof Rail, and

Bow-Roofs with the given master section.

Do a curvature study on the roof and perform calculations for Heat Distortion and Snow

load criterion to determine and justify the position of the Bow-Roofs.

Do a Section inertia analysis for the Master sections created for the Front Roof Rail, Rear

Roof Rail, Bow Roof Rails, and Centre Roof Rail.

Perform a Draft Analysis of each reinforcement.

Theory:

1. Introduction:

An automobile roof is the portion of an automobile that sits above the passenger

compartment, protecting the vehicle occupants from sun, wind, rain, and other external

elements. Under the styling surface that is seen from the outside, many other structures

under the roof contribute to its strength and stability. The number of such reinforcement

structures/supports differs concerning the size of the vehicle. They are primarily installed o

flatter areas of the roof to enhance strength.

These are the Front Roof Rail, Rear Roof Rail, Centre Roof Rail, and Bow Roof Design which

are given Mastics and spot-welded to roof flanges & ditch areas and improve NVH value b

increasing the strength of the roof using mastic sealants.

2. Types of Automobile Roofs:

Convertible: A convertible, cabriolet, or spyder is a passenger car that can be driven with

or without a roof in place.

Hardtop: A hardtop is a rigid form of automobile roof, which for modern cars is typically

constructed from metal. A hardtop roof can be either fixed, detachable for separate

storage, or retractable within the vehicle itself.

Sunroof: An Automotive sunroof is a movable panel that is operable to uncover a window

in an automobile roof.which allows light and fresh air into the passenger compartment.

T-top: A T-top is an automobile roof with a removable panel on each side of a rigid bar

running from the center of one structural bar between pillars to the center of the next

structural bar. The panels of a traditional T-top are usually made of auto-grade safety

glass.

Targa Top: Itis a semi-convertible car body style with a removable roof section and a full-

width roll bar behind the seats.

Vinyl roof: Vinyl roof refers to a vinyl conversion for an automobile’s top. This conversation

was originally designed to give the appearance of a convertible to models with a fixed roo

but eventually, it evolved into a styling statement in its own right.

3. Design consideration of Roof:

• Visibility Criteria.
• Head Clearance.
• Curvature Study.
• Heat Distortion and Snow Load Criteria.
• Draft Analysis.
• Safety Parameters.

4. Roof Crush Test:

Safety point of view vehicle roof plays a very crucial role, especially in rollover kind of

accidents, in which the vehicle tips over onto its side of the roof.

Therefore, passenger safety is also a very important parameter in front of OEMs. It is not

only a marketing strategy but also it is an obligation stipulated by international standards

that are now placed in several countries, as well as a governmental requirement. For this

reason, strength is a very important criterion for designing a vehicle roof.

The strength of the roof structure for occupant protection in real-world rollover crashes,

requiring the maximum moving distance of the roof structure should be less than 127 mm

(5 in.) when the induced load is 1.5 times the vehicle weight.

For the test, the passenger car 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 1829mm) shall

not move more than 5 inches(127mm), measured as the distance between the original

location of the lower surface of the test device and its location as the specified force level i

reached, when it is used to apply a force of 1.5 times the unloaded vehicle weight (UVW) or

5000 lbs(22224N), whichever is less, to either side or forward edge of the vehicle.

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 5000lbs (22240N). Also, the direction of the force shall be

downward and perpendicular to the lower surface of the rigid plate 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 º angle as viewed from the side of th

passenger car as shown in the below figure:

The lateral axis or the roll of the plate is such that it forms a 25 º outboard angle with a

horizontal surface as viewed from the front of the passenger car as shown in below figure:

5. Car Rollover Test:

Vehicle rollovers are divided into two categories: tripped and untripped. Tripped rollovers

are caused by forces from an external object, such as a curb or a collision with another

vehicle. Untripped crashes are the result of steering input, speed, and friction with the

ground.

Vehicles which have a higher center of gravity like SUVs or Small Trucks are more prone to

rollover. Rollover accidents typically involve a singular vehicle that loses control due to icy

or snowy road conditions, soft road shoulders, uneven ground, collision with curbs or

guardrails after a vehicle has lost traction, or hitting an obstacle while driving.

During a rollover accident, the regional body distribution of injuries normally affects five

main areas: head and neck, thorax, abdomen, upper limbs, and lower limbs. The majority o

rollover crash victims sustain multiple injuries throughout these regions with the head and

neck being the most vulnerable area. Some of the most common rollover injuries include

traumatic brain injury, spinal cord injury, broken bones, cuts, scrapes, and bruises. These

injuries are caused by slamming into another vehicle or obstacle, impact with unsecured

cargo, contact with broken glass or torn metal, or being tossed within the vehicle or ejected

from the vehicle.

6. Manufacturing Techniques:

By outer board machining process, bar-shaped assembly, ring-shaped component

processing, and roof cant rail processing technology formed respectively.

Outside plate process: the first step, open the straight snips plate: first select the applicable

cold coiling specification, and cold coiling is opened flat; Then according to the

production material specification, will open cold coiling after the flat and is cut into the iron

plate of the corresponding size; Finally the iron plate after shear is cleaned to iron filings;

The first step, oil pressure, punch press: the first iron plate is extended; Secondly to its

trimming and punching; Then to its flanging and punching; Finally, after punching

completes, it is carried out to the operation of side folding; Completed the making of the

outside plate.

Bar-shaped assembly and ring-shaped component process: the first step, blanking,

stamps out an iron plate with the outline blank of many roof cant rails, a ring-shaped

component, and a bar-shaped assembly; the Second step, carries out annular punching t

ring-shaped component; Bar shaped assembly and roof cant rail are carried out to bar

shaped punching.

Car roof anabolic process: the first step, is by a plurality of M6 nuts of a side spot welding o

the ring-shaped component; By two M6 nuts of the bar-shaped assembly two ends spot

welding; the Second step, spots weld bar shaped assembly the opposite side of the

corresponding nut of the ring-shaped component; And circular groups and the punching

of bar-shaped assembly are aimed at; The 3rd step, is welded to ring-shaped component

the inner side of the outside plate; The 4th step, spots weld roof cant rail the inner side of

the outside plate; Finally car roof integral body is carried out to sanding thorn and ending.

Roof –

When it is seeing from outside of the car, it is just closing the top portion. But from design

point of view, it is hard to create the design. Because, you have to create a number of

support rails, ie bow roofs for enough strength to withstand the load. Bow roofs are

primarily fixed in flatter region to enhance strength.

We get the model of roof from the styling team. Then we will have to do all the engineering

work. We create the ditch area for the roof and also the roof bows and roof rails for the roof

which would be designed by us considering all the safety norms and regulations.

Front roof and rear roof rails are fixed to the ditch area using spot welding operation.

But front, rear bow roof and center roof rails are fixed to roof using mastic sealants. We

don’t want so much gap between roof rails and bow roof area. So, we reducing the

NVH(Noise, Vibration, & Harshness) issues.

Car Roof and it’s Components:

1. Roof Outer Panel
2. Bow Roof
3. Center Roof Rail
4. Front Rail
5. Rear Rail

1.Front Roof Rail-

The front roof rail is located on the front of the roof where the wind shield joins with the roof

The visibility criteria and the head room clearance of the passenger are taken into

consideration for the bottom flange. One end of the rail is joined using spot welding while

the other end is joined with the roof using mastic sealants for increased streng

2.Bow roof 1-

Bow roof is an additional stiffening member for the roof to increase the strength and so

that it can satisfy the norms. Both the ends are joined together using mastic sealants to

increase strength and also since this would not affect the styling and aesthetic looks of the

roof.

3.Centre Roof Rail-

Centre Roof rail or the reinforcement centre is lo

cated at the centre of the roof . The cross

section of this is more since it needs to provide more strength to the roof. Here both the

ends are joined together using mastic sealants.

4.Bow roof 2-

This bow roof also have the same purpose as that of the first one. This one is located

between the centre bow roof and rear roof rail.

5.Rear Roof Rail -

This roof rail joins with the back door and the roof. Inputs from the backdoor are taken into

consideration for designing this roof rail.

Here one end is joined using spot welding while

the other end is joined using mastic

sealants.

CALCULATIONS :

In roof design, we have to observe calculations for 2 processes which are -

1. Heat Distortion
2. Snow Load Criteria

So, In order to calculate both of the factors we use sketch feature to draw sketches on roof

outer panel.

Following are the calculations of both terms :-

Heat Distortion Curvature Study-

This study is used to find out and predict the heat distortion temperature at where the

material starts to soften when exposed to a fixed load. So in order to avoid bending or

damage of the roof based on this calculation we will decide the position on the roof bows

on the roof.

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

Judgement Condition : OK< 2.7> 3.1

Where,

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]

Bow Result	Fr & 1St	1st & CRR	CRR & 2nd	2nd & RR
Rx	2466.18	4830.09	3998.70	3324.94
Ry	5733.58	6747.41	10453.10	14267.69
R	3448.89	5629.98	5784.58	5393.07
I	471.26	420.93	272.25	468.70
L	2038.01	2038.01	2038.01	2038.01
T	0.75	0.75	0.75	0.75
R^2/t	15859789.64	42262233.0672	44615154.3685	3878027.0332
W	1.65	2.09	1.97	2.07
OK/NG	OK	OK	OK	OK

Snow Load Criteria-

We predict the snow load in this of the roof bows and then place and change the design of the load accordingly to satisfy the condition.

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)

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]

Bow Result	FR & 1st	1st & CRR	CRR & 2nd	2nd & RR
Rx	2478.56	6686.50	5071.64	15959.53
Ry	4352.50	3742.05	4189.84	4003.81
S	370.475	346.59	446.095	NA
Y	468.70	810.87	1319.21	NA
Ly	2038.01	2038.01	2038.01	2038.01
Lx	1097.08	1126.12	1160.16	1246.64
t	0.75	0.75	0.75	0.75
Iy	95.76	248.62	197.33	NA

L1	468.70	272.25	420.93	471.26
L2	272.25	420.93	471.26	NA

Qr	14.17	1.17	0.91	NA
OK/NG	OK	NG	NG	NA

Since the Last Roof have not passed the snow load criteria we can increase the strength of the sheet by creating the embosses on the outer roof and it also increases the drag forces too.

Draft Analysis for each part: -

Roof Outer Panel:-

Bow Roof Rail 1:-

Bow Roof Rail 2:-

Centre Roof Rail:-

Rear Roof Rail:-

Results:-

• All the parts have successfully cleared the draft analysis.
• The proposed roof design has successfully cleared the heat distortion analysis.
• The portion between bow roof 1 and Rear roof rail was failing in snowload analysis. So, we provided embosses on roof outer panel.
• Embosses on roof outer panel increases the strength of roof.

Roof Assembly Snaps:

Conclusion:-

The roof has been successfully designed considering all the design rules, Master Sections, Heat distortion Criteria, Draft Analysis and Snowload Criteria.