Roof Design

Introduction

The car roof refers to the top part of a vehicle that provides protection and coverage to the interior space of the car. It serves several purposes, including:

1. Protection from the elements: The car roof shields the occupants from external weather conditions such as rain, snow, sunlight, and wind.

2. Structural integrity: The roof plays a crucial role in maintaining the structural strength of the vehicle. It helps distribute the weight evenly and contributes to the overall safety of the car during accidents or rollovers.

3. Noise reduction: The roof helps in reducing external noise levels inside the car, making for a quieter and more comfortable driving experience.

4. Mounting points: Many cars have roof rails or crossbars attached to the roof, allowing the installation of accessories such as roof racks, cargo carriers, or sports equipment carriers.

Car roofs are typically made of steel, aluminum, or fiberglass. In some cases, panoramic sunroofs or convertible tops can be installed to provide a larger opening and a more open-air driving experience. It's important to note that car roofs can vary in design and features depending on the make and model of the vehicle.

Car roof contains the different elements

1. Outer roof Panel
2. Front roof rail
3. Rear roof rail
4. Bow roof 1 & 2(bow string roof 1 & 2)
5. Centre roof

Let's go through each of the elements mentioned above

 1. Outer Roof Panel: The outer roof panel is the visible exterior surface of the car roof. It is the part of the roof that is exposed to the outside and is typically made of metal, such as steel or aluminum. The outer roof panel is designed to provide structural integrity, protection from the elements, and contribute to the overall aesthetic appearance of the vehicle.

Side view

Top View

Roof Bottom view

Iso View

2. Front roof rail: The front roof rail is a component that runs along the front edge of the car's roof, connecting the A-pillars (vertical pillars at the front of the vehicle) to the roof structure. It helps to provide additional strength and rigidity to the front section of the roof, contributing to the overall safety of the car. 

Top View

Side view

3. Rear Roof rail: The rear roof rail is similar to the front roof rail but runs along the rear edge of the car's roof, connecting the C-pillars (vertical pillars at the rear of the vehicle) to the roof structure. It provides structural support and stability to the rear section of the roof.

Top View

Side View

Side View

4. Bow roof 1 & 2(bow string roof 1 & 2): Bow roofs refer to a type of roof design commonly found in older vehicles, particularly classic cars. This design features a curved or arched roofline with a central peak, resembling the shape of a bow or bowstring. Bow roofs were popular in the mid-20th century and provided a distinctive and elegant look to the cars of that era.

5 . Centre roof: The center roof refers to the central portion of the car's roof between the front and rear roof rails. It can include various components such as a fixed or operable sunroof, roof-mounted accessories, or interior features like lights or air conditioning vents. The center roof plays a significant role in the overall aesthetics and functionality of the car's interior and exterior design.

Centre roof outer

Centre roof inner

Centre roof assembly

Complete roof with all the elements

TESTING OF ROOF:

          Roof strength evaluations are conducted on a quasi-static test system manufactured by MGA Research Corporation (Figure 3).

The system consists of an upright assembly and an attached loading head that can be fixed at varying heights from the ground as well as at pitch angles ranging from −5 to +5 degrees to accommodate testing on the driver or passenger side.

The roll angle is permanently fixed at 25 degrees. Four hydraulic actuators control the movement of the platen along two linear guides. The entire system is mounted on a T-slot bed plate anchored to the floor of the test facility.

Two HR A-36 — W10x88 I-beams are mounted on the bed plate perpendicular to the longitudinal axis of the platen. The vehicle, with attached rocker panel support system, is placed on      these beams. The vehicle is adjusted so that:

1. The longitudinal centerline of platen is within 10 mm of the initial roof contact point.
2. The yaw angle of the vehicle relative to the longitudinal axis of the platen is 0 ± 0.5 degrees.
3. The midpoint of the platen’s forward edge is 254 ± 10 mm forward of the most forward point of the roof (including windshield trim if it overlaps the roof) lying on the vehicle’s longitudinal centerline.
4. The pitch angle of the vehicle matches the on-road angle, while also accounting for any difference between the platen’s pitch angle and the nominal −5 degrees. The maximum combined difference of the vehicle and platen pitch angles from their targets is ± 0.5 degrees. (For example, if the onroad vehicle pitch angle is −0.2 degrees and the platen pitch angle is −5.2 degrees, the target sill angle for the test is −0.4 ± 0.5 degrees.) If necessary to achieve this angle, shims are inserted between the rocker panel supports and the W10x88 I-beams attached to the bed plate.

Once the vehicle is positioned correctly, the rocker panel supports are clamped to the two perpendicular I beams, and the beams are marked to allow confirmation that the vehicle position is maintained during the test.

For body-on-frame vehicles, the frame is supported to prevent the weight of the chassis from stressing the body at the body mounts. The roof is crushed to a minimum displacement of 127 mm at a nominal rate of 5 mm/second.

Some tests are conducted to a greater displacement to collect additional strength data for research purposes. Force data are recorded from five load cells (Interface Inc., model 1220) attached to the loading platen.

Displacement data are recorded from four linear variable displacement transducers (LVDTs) (MTS Temposonics model GH) integrated into the hydraulic actuators. Figure 4 shows the locations of the load cells and LVDTs on the loading platen.

Force and displacement data are collected with a National Instruments USB-6210 data acquisition system and reported at 100 Hz. These data are based on a sampling rate of 2,000 Hz, with every 20 points being averaged to produce the output data at 100 Hz.

The displacement-time histories from the LVDTs are compared to verify that the platen’s roll angle and pitch angle (relative to the vehicle’s on-road pitch) were maintained at 25 ± 0.5 degrees and −5 ± 0.5 degrees, respectively.

The pre crash and post-crash conditions of each test vehicle are documented with still photographs. The position of the vehicle in the test fixture is also recorded. Motion picture photography is made of the test with real-time video cameras.

If the displacements go above the requirements level, then we have to increase the bow roof thickness or add more mastic points to increase the strength. The stronger the roof, the better it can protect the occupants in the roll-over crashes.

CALCULATIONS:

For our study, we are considering the following conditions for testing the roof.

* Heat Distortion Criteria:

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

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>