Roof Design

Report on Roof Design of a car

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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.

Main Parts Of Roof:

• The roof has an outer panel which is the styling part.
• Strengthening parts are present below an outer panel of a roof, which are the front roof rail, Bow roof, Centre roof rail, and Rear roof rail.
• The front roof rail provides provision for the windshield glasses. Visibility criteria and headroom clearance have to be followed while designing the front roof rail. Provision for windshield mounting and inside rear-view mirror is provided on the front roof rail.
• The Rear roof rail has hinge mounting for the back door. It is attached to the portion where the back door meets with the roof.
• Bow roofs are provided in between the front roof rail and center roof rail as well as the Rear roof rail and center roof rail. They are provided to enhance the localized strength of the roof and are attached directly to the bottom of the roof with either spot welding or with the help of Mastics.
• Reinforcement center or center roof rail is used as a cross member between Left hand and right-hand bodysides.
• Both the front roof rail and rear roof rail are provided with the Master section for the design.
• The below image shows the location and Material used for all the strengthening parts.

  

  What material is the roof of a car?

  Most headliners consist of a tricot knit fabric that is napped to provide a soft touch and uniform appearance. The fabric adheres to melted polyurethane foam. This fabric-foam composite is glued to the interior fiberglass roof of the automobile.

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Types of roofs

1. Hardtops

When we talk about hardtops, we refer to the common rigid forms of automobile roofs we see around. In modern-day cars, this type of roof is made from metal or plastic.

So, the common Toyota Corolla, Camry, and many of the cars in Nigeria are all Hardtops. However, we should say that there are different types. The one we just described is the fixed hardtop. Other types include:

• The detachable hardtops
• The retractable hardtops

These two types are designed as convertibles and would be discussed extensively when we get there.

The fixed hardtop is your traditional car roof and is suitable for all climes and weather. It protects car occupants from the sun and wind when driving. It also offers more sense of privacy and noise-free driving. Many cars designed this way also tend to be more stable and solid than other types of roofs.

2. Sunroof:

This type of roof is also quite popular. You might have been seeing it around. A sunroof is an operable type of roof that allows for an opening in the roof of an automobile; say a window in a roof. It can allow either light or air or both to enter the vehicle. They are either operated manually or automatically.

The keyword here is Opening in the roof. It is not the entire roof that is removed – that is a different type of roof.

There are 3 popular types of sunroofs. They include:

1. Pop-up:

The pop-up sunroof usually tilts when you open it. It is the most basic and relatively cheaper type of sunroof. Some can even be removed so that they are kept properly, maybe in the trunk whenever you want to go ‘sunroof mode.’

A pop-up sunroof pops up!

b. Spoiler:

Wait, this is not spoiling anything o. It just means that it can slide. So, the spoiler is a pop-up that would slide, instead of tilting upwards. The advantage of this is that it can easily be restored to its original position by sliding.

Spoiler sunroofs are easy to slide

c. Panoramic sunroof:

Please take the pronunciation easy. Panoramic sunroofs are distinct. While other sunroofs allow air and light or both into the front seats, the panoramic sunroof would allow for both the front and rear. Of course, this means the aperture would be larger than the spoiler or pop-up, but hey, what is the purpose of the window in the roof if it is not big enough?

How the panoramic sunroof appears inside

Some brands also refer to them as moonroofs, which are automatically operated with the push of a button or the turning of a knob.

ROOF CRUSH TEST :

The roof crush test is the design given by the IIHS ( insurance institute of highway safety ). After analyzing the many accidents IIHS has then fixed some regulations, that how to perform the crush test.

IIHS gives the rating based on this test. For this, they consider two points while giving the rating

• How fast the car can reform to its original shape after the rollover and
• How deep is the protrusion after the rollover

GOAL OF ROOF CRUSH TEST :

To minimize the protrusion into the occupant compartment , the seat belts and airbags can do their job of protecting the occupant

CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST :

Two conditions should be followed for the roof crush test, they are

INITIAL CONDITION :

• Close and lock all the doors
• Close all the windows

SECOND CONDITION :

Fix the body sill or chassis frame on a hard lever surface horizontally

METHOD OF APPLYING THE FORCE :

The load is applied in such a way that 5⁰ is tilted from the front end to the rear end and the load will be 1800 mm in length and 750 mm in width. The load is applied at the position of 25⁰ from the flatness of the roof. The load is applied on the roof for almost 120 sec and the crushing speed at which the load is applied is 13 mm/sec or less than that.

RESULT:--

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

Design of Roof and its Reinforcements

For this project, an A-surface along with the roof rails will be provided and the Roof ditch area along with the Front, Rear Centre, and Bow Roof Rails are to be designed

Certain critters are to be followed to design the above parts

Design criteria to be followed:

1. Material Selection

Material selection plays an important part in designing any part. The material for a part is selected keeping in mind the tensile strength, bending, static and dynamic loading that the part would be subjected to. Materials used for the roof and its different reinforcements are mentioned below.

• Roof: SP121AQ T0.7
• Front Roof Rail: SP783AJQ
• Rear Roof Rail: SP783AJQ
• Bow Roofs: SP121BQ
• Centre Roof: SP153-98OPQ

2. Visibility Criteria

The visibility criteria of a driver are one of the most important factors when designing the roof. The roof should be designed such that it does not hinder the visibility range of the driver. The ditch area of the roof is designed keeping this criterion in mind.

3. Head Clearance

This factor is considered when designing the reinforcements of the roof. The reinforcements should be designed in a way that provides ample headroom clearance for the occupants in the car and does not collide with the head in case the car moves over a bump It should not act as a hindrance to the driver’s rear visibility.

4. Position of the reinforcements

The position of the reinforcements is governed by the below criteria.

• Heat distortion

This determines the temperature the roof structure can withstand. The value obtained from the heat distortion prediction formula should be less than 2.7 to be deemed fit for use.

• Snow load criteria

This factor is taken into consideration for areas with high snowfall which then sets on the car roof. This criterion measures the elasticity of the roof wherein the roof should come back to its original shape and form on the removal of the snow, which in this case acts as the load. The value Qr obtained for the snow load prediction formula should be greater than 3.1 for the design to quality for being used.

5. Drawibility

The roof and its parts are manufactured using a deep drawing process. For this reason, the designs should be manufactural and have a positive draft angle for smooth surface build and reduced induced stresses in various parts of the reinforcements.

Design of the Roof and Reinforcements:

1. Roof

The ditch area of the roof is designed using the master sections provided

2. Front Roof Rail

The Front rail is created according to the master section and keeping the visibility criteria in mind

3. Rear Roof Rail

The Rear rail is designed keeping in mind the rear visibility criteria and using the master section provided

4. Centre Roof Rail and its reinforcement

The Centre Roof Rail provides the roof with additional strength in an adverse case of a rollover thus avoiding injury to the occupants of the vehicle. The rail reinforcement is attached to the roof by Mastics and to the rail by spot welds. For the Mastics the reinforcement has a clearance of 5mm from the roof.

5. Bow Roof Rail 1 and 2

The bow roof rails also provide the roof with structural strength and their position has to be justified by the Heat Distortion and Snow Load criteria. Both the Bow Rails are attached to the roof using Mastics thus a clearance of 5mm from the roof is provided for that purpose.

HEAT DISTORTION CRITERIA CALCULATION

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

Judgment Condition: OK< 2.7> 3.1

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]

SNOW LOAD CRITERIA CALCULATION

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)

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 front and rear reference point.

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]

Section Inertia Analysis :

Bow Roof Rail 1

Centre Roof Rail and its Reinforcement

Bow Roof Rail 2

SNOW LOAD CRITERIA CALCULATION:

DRAFT ANALYSIS:

1. Outer Roof:

2. Front Roof:

3. Bow Roof 1:

4. Centre Roof:

5. Bow Roof 2:

6. Rear Roof:

CONCLUSION

• The Roof and its reinforcements were designed successfully according to their master sections and criteria
• The Heat Distortion Criteria were fulfilled
• The Snow Load Criterial was not fulfilled, thus the design needs improvement
• The Draft analysis was also performed and the parts are found to be manufactural