Roof Challenge

Roof Challenge

Roof: Roof of an automobile is that commodity which sits above the passenger compartment. Roof houses the vehicle as shielding part which safeguards the occupants from sunlight, heat, rain, wind, dust, etc. In the past roof was introduced as cover for passenger compartment but later on, it also used as a luggage carrier, sunroof and in recent past years with the introduction of the solar panelled electric vehicle got a major boost in the automotive industry.

Roof has the following functions:

• protects from environmental damage.
• prevents passenger and vehicle from accidents
• luggage carrier
• design of the roof also affects in giving aerodynamic shape to vehicle
• sunroof
• solar panelled roof in EV

Since the roof has numerous functions so the roof has to be stronger enough to withstand the damage experienced by it. To have good strength and stiffness, the roof is made with different components such as front roof rail, rear roof rail, bow roofs, etc. Each component in the roof has a major role. The front and rear roof rails are fixed at the front and rear end respectively whereas position any number of bow roofs changes depending upon the shape, length and curvature of the roof. The number of bow roof increases with the increase in the length of the roof.

Bow roofs are fixed more at flatter portion to increase its strength. Below the roof, the load has to be carried away by the bow roof only so its position and design plays a major role in bearing roof\'s load and it\'s collision to external damages. Since roof also comes under styling surface so for joining the roof with its components mastic sealants are used.

• Objectives:

For the give Roof styling, develop the essential flanges and reinforcements, Provide appropriate dimension and check for Draft analysis and also submit your section modulus report on 0-Y section.

1. Start with the creation of roof ditch area the tool opening angle is only 3 degree.
2. Do a curvature study on the roof and perform calculations to determine the position of the Bow-roof.
3. Do a section modulus study for the Master sections you create for the Front roof rail, Rear roof rail, Bow Roof Rail and Rear Roof Rail.

There are the following factors to judge curvature of the roof:

• • Heat Distortion(theoretical)
  • Snow load condition(theoretical)
  • Roof Crush(practical)

Out of above factors during design, only Heat distortion and Snow load condition can be judged before manufacturing of roof.

• • Heat distortion condition: Heat distortion is done before the manufacturing process. It is a way of predicting how a roof will behave under heavy exposure to the heat under sunlight. So the prediction formula as mentioned below to know the curvature study of the roof and know whether the position of bow roof are correct or not.

W = (1.73*10^-3*L)+(1.85*10^(-8)*(R^2)/t) + (1.10*10^(-3)*l)-(2.68)

Judgement Condition: OK<2.7mm<NG

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 condition: Snowed condition criteria is judged to find out how much load of snow a car roof can bear. This load is directly get exerted on the bow roofs and hence its position on the roof affects the bearing capacity and strength of the roof.

Qr= (Iy*t^2)*10^8/(a*s*([Rx+Ry]/2)^2

judgement condition: Qr>=3.1

a=My*Lx^2*10^-12, My=y(Ly-y)

t= roof plate thickness

Ly= Distance between front and rear roof rail on vehicle.

Lx= Distance between left and right end of the roof.

Y=Distance between bow roof centre

s= (L1/L2)+L2/2

Iy= Geometrical moment of inertia of bow roof

Rx= Roof panel curvature radius of length Lx, fr view

Ry= Roof panel curvature radius of length S, side view

• Roof with ditch area

With the thickness t=0.7mm and flange length = 21.5 mm, creating a roof ditch area with an opening angle of 3 degrees. This 3 degree is very important as more than 3 degrees the flange may move forward to the windshield and restricts visibility criteria and if below 3 degrees is maintained the part may experience failure in manufacturing also it could decrease the headroom clearance for the driver.

figure 1 figure 2

• Roof with all necessary components

Positions of front rail roof and rear rail roof is fixed at the front and rear end respectively by maintaining 2.7mm gap between roof surface panel for mastic sealants which are used for joining the roof rails and roof surface. In the given roof surface figure 3, this is clear that the roof is more curved at the front section and it gradually becomes flatter from the centre to the rear end.

With this observation, In figure 4, tentative positioning of bow roof centre and remaining bow roofs are done at flatter section of roof as strength over here is less as compared to the curved portion.

figure 3 figure 4

• FRONT ROOF RAIL:

It is the front section of the part where the windshield is mounted using rubber paste and also this is mounted to the A-pillar of the frame. The maximum 2.7mm gap is allowed between front roof rail and the roof. By observing the draft angle, fixed flange length, windshield glass (thickness 3.75mm to 4.75mm), rubber paste to join the windshield glass (minimum gap 4 to 5mm), the depth of flange is estimated as 12mm.

The front roof rail gets joined to the ditch area by spot welds and the front roof rail rear section is joined at curved roof surface using mastic sealants and hence the gap of 2.7 mm is provided for the same. Apart from the design specification given, in figure 5 this can be seen that are some embossed made symmetrically throughout its span to increase strength and stiffness. Also, there are some cut-outs to remove the material which reduces material usage and weight of the component. The thickness of the material is 0.65mm.

Figure 5 Figure 6

• REAR ROOF RAIL:

It is the rear section of the part where the back door of the car is mounted and also this is mounted to the C-pillar of the frame. The maximum of 2.7mm gap is allowed between rear roof rail and the roof. By observing the rear ditch area, the same ditch area of roof has been offset for rear roof rail contact surface and then the remaining portion is drafted back to curved roof surface maintaining 34mm clearance from the top surface.

The rear roof rail gets joined to the ditch area by spot welds and the rear roof rail rear section is joined at curved roof surface using mastic sealants and hence the gap of 2.7 mm is provided for the same. Apart from the design specification given, in figure 7 and 8, this can be seen that are some embossed made symmetrically throughout its span to increase strength and stiffness. Also, there are some cut-outs to remove the material which actually reduces material usage and weight of the component. The thickness of the material is 0.65mm.

figure 7 figure 8

• CENTRAL BOW ROOF WITH ROOF REINFORCEMENT CENTRE:

Central bow roof it is fixed most probably at the central region or mid-span of the roof. It supports the roofs more effectively and is mounted to the central b-pillar. unlike front and rear roof rail, bow roofs can be placed anywhere under the roof depending upon the curvature study and strength of the roof. Hence it was observed that the position of central bow roof should be at the centre which quite close to the sunroof as in given in the figure.

Additionally to improve its strength to further extent embossed are provided with some material removal area which helps in reducing the weight of the commodity.

This is also to be noted that the bow roofs support the entire roof till the ditch area on either side, unlike roof rails which support to a certain section this enables the roof to improve its ditch area section also. In the given figure there is additional reinforcement of thickness 1.5 mm mounted placed over the bow roof centre of thickness 0.65mm by spot welds. The combination of reinforcement and bow roof together comes as bow roof with reinforcement centre which is made to fix on roof surface by mastic sealants again for joining 2.7mm gap is provided.

Since this is the epicentre of the roof so here bow roof has to be stronger enough to take up the load and hence we use a combination of bow roof centre with reinforcement.

figure 9 figure 10

• BOW ROOF 1 & 2:

Both the design of bow roof 1 and bow roof 2 is the same given in future 11 & 12, and depending upon the requirement the positioning of the bow roofs are done tentatively at certain section under the roof. here in designing bow roofs, there is a huge section of material has been removed to reduce to weight and also for the best use of the design. The position for mastic sealants are located at every 80mm diameter as up to this mastic sealants improves the strength locally.

With a maximum number of cuts outs and removal material, there is a significant loss in the weight of the bow and hence overall weight of the roof is decreased but with less material, it is lacking some strength so to compensate that strength position of mastic sealants with embosses at every 80mm distance are designed to improve the strength of the bow locally. During positioning of embosses, it is ensured that no other emboss should come in the range of another sealant which gives the best use of the material.

figure 11 figure 12

figure 13

• DRAFT ANALYSIS:

Draft analysis is performed on each component, this was observed that each component so designed can be easily drafted as the draft analysis shows a positive draft indicating easy to manufacture without any failure. In the given figure 14, 15, 16, 17, 18, 19, 20, all components drafts are in the range of 0 to 5 (green) colour.

figure 14 figure 15 figure 16

figure 17 figure 18

CURVATURE STUDY:

• Heat Distortion condition

• W = (1.73*10^-3*L)+(1.85*10^(-8)*(R^2)/t) + (1.10*10^(-3)*l)-(2.68)

  Judgement Condition: OK<2.7mm<NG

• Snow load condition

• Qr= (Iy*t^2)*10^8/(a*s*([Rx+Ry]/2)^2

  judgement condition: Qr>=3.1

• In the above observation, it is quite clear that W value in heat distortion criteria is OK for the first three sections that is FR-CTR, CTR-1ST BOW, 1ST-2ND BOW but for the last section 2nd BOW-RR is NG Value. This shows the position of 2nd BOW ROOF is not correct or the position might not be good for heat distortion condition but since as a designer master section so provided has a finite dimension and curvature.
• But in case if design freedom is permitted the roof shape can be changed by changing the value of Rx and Ry value at this section.
• Since it is an NG condition and now design freedom to change the curvature of the roof is allowed so embosses are introduced to compensate the NG value so obtained in heat distortion condition.
• Positioning of bow roof is ensured at flatter region as in there strength is to be increased and in the given figure this is clear that roof is more curvature a the beginning and becomes flatter as it starts from centre to rear end of the roof. Hence bow roofs are placed after centre bow roof to the rear end and as between the 2nd bow roof and rear roof rail, there is an NG condition so embosses are made which ensures to rectify NG condition.
• The snow load criteria is OK for all section hence it passes this test for bow positions.

• SECTION MODULUS STUDY: It is a geometric property for a given cross-section used in the design of beams or flexural members. It may also be defined as the ratio of total moment resisted by the section to the stress in the extreme fibre which is equal to yield stress.
• The ratio of total moment resisted by the section to the stress in the extreme fibre. It gives us an idea about how much of load the given structure can take. If the section modulus increases the moment of resistance increases and hence its load-bearing capacity.
• We have to consider minimum MOI value given in inertia analysis of the hood section as from this value only the structure starts breaking and at maximum MOI value the structure gets broken down completely.

• Using the formula S = I/y

  S = Section Modulus

  I = Moment of Inertia

  Y = distance between the neutral axis and the extreme end of the object

• Inertia analysis of Center bow roof
• Minimum I= 6.31*10^3 mm^4, Y= 65 mm
• Therefore S=(I/Y), S= 9.70 mm^3

• Inertia Analysis of Bow roof 1
• Minimum I= 949.82 mm^4, Y= 50 mm
• Therefore S=(I/Y), S= 18.99mm^3

• Inertia Analysis of Bow roof 2
• Minimum I= 9334.50 mm^4, Y= 50 mm
• Therefore S=(I/Y), S= 18.69 mm^3
• The study of section modulus on this section was very important to improve its strength locally as it is a geometric property so it does not depend upon the property of the material.

• CONCLUSION:
• Finally, the Roof with all its necessary front rail roof, rear rail roof and bow roofs are assembled together on the roof surface. Roof with additional roof sunroof and embosses are introduced to giving improve NG condition value also by doing this there will be a significant increase in aerodynamic shape and strength of the roof.
• Due to sunroof, there is a huge cutout which gives access to the outside air if needed also improves aesthetic look and ergonomics.
• Embosses introduced on every front rail, rear rail, bow roofs for better strength and stiffness.
• To make the whole commodity lighter, every component of the assembly has cutouts which enables the removal of material this also helps in less usage of the material on the commodity also reduces the material cost.
• Hence Finally design is completed by performing curvature study, section modulus and draft analysis.

Link for the project is given here:

https://1drv.ms/u/s!AtYskr9pUZyAgnR4iPeFUGEbEd1_?e=g2ti9S