Roof Design

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 another external element.

Under the styling surfaces seen from the outside, there are many other structures under the roof that contribute 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:

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

This test can be conducted on either side of the roof structure, the front left or the front right side but not on the both sides in one single test and still meet the requirement of the test. 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,

The lateral axis or the roll of the plate is such that it forms a 25’ outboard angle with a horizontal surface as viewed from the front of the passenger car as shown in below, fig

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.

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

Roof Outer 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.

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 provide effective support to the roof's flat area 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 roof:

The bow roof are given to improve the roof structure's torsional stiffness and load bearing capacity. The number of roofs presented depends on the overall size of the roof. Presently in this project, two bow roofs are calculated.

Screen shot of Different Views of Complete Roof Design:

DRAFT ANALYSIS

The draft analysis command enables you to detect if the part you drafted will be easily removed. This type of analysis is performed based on color ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.

Minimum draft angle of 7 degree is considered for analysis, green color infers on the parts that all face along the tooling direction has positive draft angle greater than 5 degree and passed in analysis.

Design Calculation of Roof:

To make sure the roof design passes the Analysis test some of the criteria to be considered before the design of bow rails under the roof. They are,

• 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.

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]

Judgment Condition

OK< 2.7

Here we have to find out Rx and Ry values. To find out

• Create a sketch on Top roof and draw a horizontal line at a center.
• Then offset this line below 300mm and called it as _3y line.
• From the _3y line offset 100 mm above and below and convert all these line into reference line.
• Draw the vertical lines at the middle of each reinforcements and offset it at 100 mm both sides.
• Intersection point is created where the lines are met.
• Project there points the outer roof panel.

From the above table, it can be concluded that all values of W<2.7 and also not W > 3.1 so thus infer that the current positioning of bow roofs are good in the state as per the design and found OK.

SNOW LOAD CRITERIA

Snow load criteria study is done to check the durability of the roof to withstand snowfall and reflex back to its original shape without any permanent deformation once the load is removed.

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

Here we have to find out Rx and Ry values. To find that

• Create a sketch on Top roof and draw a horizontal line at athecenter.
• Offset this line to 100 mm on both sides. Convert all these line into reference llines
• Draw the vertical lines at the middle of each reinforcement and offset it at 100 mm on both sides.
• Intersection point is created where the lines are met.
• Project there points the outer roof panel.