Roof challenge

Design of a CAR ROOF

Aim :

For the given roof styling, develop the essesntial flanges and reinforcements, provide appropriate dimensions and check for draft analysis and to perform roof curvature study using NX-CAD.

Objectives :

• For the Given Roof styling, develop the essential flanges and reinforcements.
• Creation of Roof ditch area with tool opening angle of 3 degree.
• Provide appropriate dimensions and validate for Draft analysis.
• Section modulus study on 0-Y section for the Master sections created for Front roof rail, Rear roof rail, and Bow Roof Rails.
• Perform curvature study on the roof and calculations to determine the position of the Bow-roof.

Theory :

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.

Roof with Sunglass :

Front Roof rail joins the roof panel with Windshield glass and body side panel and also this Front roof rail used for the bracket map lamp and room lamp mountings where as Rear Roof rail joins the Backdoor and body side panel of the car.

Bow Roofs provides the additional strength to the Roof panel in order to withstand the forces acts on the Roof, Bow Roofs joins with the Roof outer panel and the body side panels and these Bow Roofs positioned in the most flat areas of the outer roof panel.

The roof is a styling surface part which is fixed with the BIW body through the rails and panels inside the upper roof to provide it strength and stiffness. Ditch area plays an important role here to fix the reinforcement on the component. The rails and reinforcements are joined to the ditch area with a spot weld where it's not the styling part.

Types of Car Roofs :

• Convertible roofs
• Hardtops
• Sunroofs
• T-tops
• Targa tops
• Vinyl roofs

Below mentioned are some of the famous Roof Types in the Market:

1. Hard Tops: A rigid form of Roof, typically constructed from metal.
2. Sunroofs: An operable window in a Roof, which allows light or fresh air into the car compartment.
3. Convertible: Allows an open-air driving experience, with the ability to provide a roof when required.

Accidental and Safty Review :

Safety point of view vehicle roof plays very crucial role specially roll over kind of accidents, in which vehicle tips over onto its side of roof. And vehicle rollover crashes are the causes of many fatalities and several head, neck and spine trauma around the world. Therefore, passenger safety is also very important parameter in front of OEM’s. It is not only a marketing strategy but also it is a obligation stipulated by international standards that are now place in several countries, as well as a governmental requirement.

Rollover crashes, especially in the country, are usually very destructive events. Vehicle damage often includes deformation of the roof and its supporting structures. Head and neck injuries are common, and associated with roof deformation. Strengthening of the roof is suggested as an appropriate countermeasure for such injuries.

• What is a rollover accident?

A rollover is a type of vehicle crash in which a vehicle tips over onto its side or roof. Rollovers have a higher fatality rate than other types of vehicle collisions.

Types of Rollovers :

1. Tripped-NHTSA data show that 95% of single-vehicle rollovers are tripped. This happens when a vehicle leaves the roadway and slides sideways, digging its tires into soft soil or striking an object such as a curb or guardrail. The high tripping force applied to the tires in these situations can cause the vehicle to roll over. E.g. Soft soil, Guardrail, Steep Slope
2. Un-Tripped -Un-tripped rollovers are less common than tripped rollovers, occurring less than 5% of the time, and mostly to top-heavy vehicles. Instead of an object serving as a tripping mechanism, un-tripped rollovers usually occur during high-speed collision avoidance manoeuvres.

For this reason, strength is very important criteria for designing vehicle roof.

FMVSS 216 Roof crush standards :

FMVSS 216, Roof Crush Resistance, establishes strength requirements for the passenger compartment roof of passenger cars, multipurpose passenger vehicles, trucks and buses with a GVWR of 2722 kilograms or less. The purpose of the standard is to reduce deaths and injuries due to the crushing of the roof into the passenger compartment in rollover accidents. The standard does not apply to school buses and passenger cars that conform to the dynamic rollover test requirements of FMVSS 208, Occupant Crash Protection, and S5.3 by means that require no action by passenger car occupants. It also does not apply to convertibles, except for optional compliance with the standard as an alternative to the rollover test requirements in S5.3 of FMVSS.

The prescribed static loading device is a rigid unyielding rectangular block 762 millimetres by 1,829 millimetres. It shall not move more than 127 millimetres to achieve the specified resistive load when applied to the forward edge of a vehicle’s roof – see Figures 1 and 2. The resistive load to be achieved is 1½ times the unloaded vehicle weight (UVW) of the test vehicle or 22,240 Newtons, whichever is less.

The force shall be applied in a downward direction perpendicular to the lower surface of the static loading device at a rate of not more than 13 millimetres per second until reaching the compliant resistive load. The test shall be completed within 120 seconds. The static loading device shall be guided throughout the test so that it moves, without rotation, in a straight line with its lower surface oriented as follows and as shown in Figures,

Components of Roof :

• Front roof rail
• Rear roof rail
• Bow roof 1
• Bow roof 2
• Centre roof rail
• Ditch area

Front Roof Rail :

Front roof rail is used to provide strength to the roof panel front and it also acts as link between windshield glass, roof panel and headliner. Spot welds are applied at wind shield end of the rail and the other end is joined with roof using mastic sealent which also increases the strength.

Front Roof rail is the reinforcement for the roof outer panel and as well as it joins the Roof panel with the Windshield glass and body side panel and also supports the bracket map lamp and room lamp mounting. Front roof rail created based on the master section which is developed with above mentioned functional requirements of tool opening angle, required clearance for to join with the windshield glass, visibility criteria, the deepness based on the head room clearances and the clearance mastic joining. See the below master section used for the Front Roof Rail and the Front roof rail design based on the master section.

Rear Roof Rail :

Rear roof rail is again used to strengthen the roof and it acts as a link between back glass and roof which connected to the backdoor of the car.Spot welds are applied at back doorend of the rail and the other end is joined with roof using mastic sealent.

Rear Roof rail is also the reinforcement for the outer panel and it joins the backdoor and the body side outer. For designing the Rear roof rail also we have developed a master section based on the functional requirements which are Rear visibility criteria, Rear head room clearance, Tool opening and mastic clearance. See the below image of Rear Roof rail design.

Bow Roof and Centre Roof Rail :

Center roof rail will be at the centre of the roof to provide strength and stability to the roof. Here the centre roof rail is joint with the roof through mastic sealent.

Bow Roof Rails are also the reinforcements of outer panel and are used to strengthen the roof panel and these joins to the Roof panel with the help of mastic sealant to the roof panel and connects to the body side using the spot welding technique. These are designed based on the developed master sections developed using the criteria of Visibility and the head room clearance. And the position of these Bow roofs are mostly recommend to place at the flat portions of the Roof outer panel.

Ditch Area :

Ditch area is a extended portion of the main roof cancelled by other BIW pannels.The rails support of the roof are attached to the ditch area using spot weld.

Design considerations of the Roof :

1. Visibility criteria
2. Head clearance
3. Heat distortion and snow load criteria
4. Draft analysis
5. Safety parameters

Thichness & Manufacturing Requirements :

• Thickness of Roof panel: 0.75mm
• Reinforcement Thickness = 0.75mm
• Tool opening angle 3 degree
• Emboss depth should not exceed 6mm.
• Minimum filet radius should be more than the 3+thickness
• No sharp corners inside the component

Functional Requirements :

• Provide adequate clearance for the windshield glass joining for joining with Roof at Front
• Visibility criteria of windshield glass at front Roof rail
• Rear visibility criteria for Rear Roof Rail
• Head impact zone or head room clearance based on the trim parts.
• Mastic clearance to join the reinforcements.

Designing the Reinforcement :

• Front Roof Rail :

• Rear Roof Rail :

• Bow Roof 1 & 2 :

• Reinforcement Centre :

Final Assembly of Roof :

Section Modulus :

Section modulus is a direct measure of strength of any sectional area of an object about an referance axis(neutral axis). Strength is nothing but,capable of supporting greater loads,so higher the strength it has a high capable of supporting greater loads.

Section modulus represented with a symbol of ‘S’

S=I/Y

S-Section modulus

I-Distance from the referance line to one of the extreme end of the object

1. MOI for Front roof rail :

2. MOI for Bow roof rail 1 :

3. MOI for Centre roof rail :

4. MOI for Bow roof rail 2 :

5. MOI for Rear roof rail :

Draft Analysis :

• Roof Assembly

• Front roof rail

• Rear roof rail

• Bow roof 1

• Bow roof 2

• Centre roof rail

Curvature study on the Roof:

After getting the final body it has to be checked and justify that it passes the standards.

Analysis:

It is done to justify the position of the bow roofs.

1.Roof crash test.

2.Snow load test.

3.Heat distortion criteria

Heat distortion criteria:

Heat distortion temperature is a temperature at which the metal softens and deform under a specific load.This study is done to check the ability of the roof to withstand high temperature with respect to the position of the reinforcement which contribute to the increase strength.

• To calculate the heat distortion criteria by the bow roof position formula,
• We need to create a sketch , draw a line on the center of the roof on OY axis (mid horizontal axis)
• Next draw a line from the center line by a 300mm distance
• After that draw a line from 100mm distance on both sides of the 300mm line
• Then draw a line between the center of roof rails and draw the lines from the center line by the distance of 100mm on both sides.
• Atlast make an intersection on these lines from a coincident line to get points.

Heat Distortion calculations

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

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 calculations

Snow load criteria is useful to justify the bow roof rail position in snow fall regions. Due to snow fall snow will get collected on the top roof which will tend to bend the roof. Bow roof will be at such position which will provide more strength to roof against bending.

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]

Roof panel X curvature radius of length s in Side view

Calculations :

Final Design of ROOF :

Top View :

Inner View :

Conclusion :

1. Using all input data we have successfully designed Ditch area flanges, Front roof rail, Bow roof rail 1&2, Rear roof rail as per the requirement
2. As per heat distortion criteria we come to know all bow roof positions are OK
3. As per Snow load criteria we come to know there is improvement required
4. All roof rails draft analysis are found OK