Roof Design

 Aim:

To a given roof styling, create essential flanges and reinforcement.

OBJECTIVE:

We need to design the required reinforcment and flanges for the given Class A surface of Roof. Required reinforcments are Front Roof Rail,Rear Roof Rail,Bow Roof 1,Bow Roof

 Introduction to Roof:

An automotive roof or car top is the portion of the automobile that sits above the passenger compartment, protecting the vehicle occupants from Sun, Wind, Rain and other external element.

Under the styling surfaces that’s is seen from the outside, there are many other structures under the roof that contributes to its strength and stability. The number of such reinforcement structures/supports differ with respect to the size of the vehicle. They are primarily installed on the flatter areas of the roof to enhance the strength.

They are the Front Roof Rail, Rear Roof Rail, Center Rail and Bow Roof Design which are given Mastics and spot welded to the roof flanges & ditch are and improve NVH value by increasing the strength of the roof of using mastics sealants.

 

Types of Car Roofs:

1. Convertible roofs.
2. Hard Tops
3. Sun roofs.
4. T- Tops.
5. Target tops.
6. Vinyl roofs.

Design Consideration of roof:

1. Visibility Criteria
2. Head Clearance
3. Curvature Study
4. Heat Distortion and snow load criteria
5. Draft Analysis
6. Safety Parameters

Roof Crush test for passenger car:

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.

Master Section and its Parameters:

• 

Joining Process:

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.

Heat Distortion study on the Roof :

The heat distortion study plays a major role in sheet metal urges. Heat distortion temperature is a temperature limit above which the material cannot be used for structural applications. This study is used to predict the heat distortions temperature at where the material starts to soften when exposed to a fixed load at an elevated temperature. To avoid bending or damage to 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 ] + [ 1.10 X 10^(-3) X1] -2.68
Where

 L = Roof length in X -Direction (mm) (Roof dimension in 0-Y)

 R = Roof Curvature

Where R = 2 ( Rx*Ry)/ (Rx+Ry)

 Rx = X curvature

 Ry = Y curvature

 t = roof plate thickness (mm)

 I = Bow roof span (mm)

 Reinforcement and its types:

If the roof is flat, it is weak and cannot able to with stand the external load like snow load, roll over and etc. so in order to increase the strength of the roof, reinforcement like front roof rail, central roof rail, rear roof rail and the bow roof are introduced and joined in the flat surface of the roof with different joining process. In general, there are four types of reinforcement are used in the hard roof top they are,

• Front roof rail
• Central Roof rail
• Rear roof rail
• Bow roof rail

Outer roof panel:

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.

Inputs

Bow roof:

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.

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.

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.

While designing the rear roof rail we need to take care of the head clearance of the passenger & the Rear Visibility Criteria.

Snow Load Criteria:

This test is done to know-how is the roof is behaving when there is 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 removed the roof should go to its original position. These are basic requirements for the snow load criteria.

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

  Where

 α= My x L x 2 x 10-12,

 My = Y (Ly - Y )

 Judgement Condition = Qr ≥ 3.1

 250 ≤ s ≤ 380

 t = roof thickness [mm]

 Ly = Distance between the front and rear roof rails on the vehicle along the OY [mm]

 Length of Roof panel with the center point between roof rails 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]

  The width of the roof panel is exposed on the surface.

 Y = Distance of 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]

Conclusion:

The design of the roof, front roof rail, rear roof rail, bow roof_1, bow roof_2, center roof rails by following the master section with the design parameters for the given class A surface is done.