Roof Design
Aim:
To a given roof styling create essectial flanges and reinforcement.
Objectives :
1.To design the automotive roof based on the given styling surface and to analyze the prepared model to check the manufacturability and the feasibility of the design.
2. To perform the section modulus study for the master sections and create for front roof rail , Bow roofs and center roof rail.
3.To Create the Ditch area and to perform the draft analysis.
Introduction to Roof :
An automobile 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 other external element.
Under the styling surfaces thats is seen from the outsidethere are many other structures under the roof that contributes 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 enhances the strength .
They are front Roof Rail , Rear Roof Rail , And Bow Roof Design which are given mastics and spot welded to the roof flanges and ditch area 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 clearence
- Curvature study
- Head 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 overkind of accidents in which vehicle tips over onto its side of the roof vehicle roll over 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 127mm , (5 in) , where the induced load is 1.5 times the vehicle weight .
For the test , the passenger car shall be rigidly placed or positioned on a horizontal surface with the doors locked and the window closed. Further more ,a flat rigid block with aa lower surface a rectangle measuring 30 inches wide by 72 inches long (762mm x 1829mm) shall not move more than 5 inches (127mm) measured as the distance between th 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 wei(UVW) or 500lbs (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 stil meet the requiremnts 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 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 degree angel 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 degree outboard angle with a horizontal surface as viewed from the front of the passenger car as shown in below.
Reinforcements and its types :
If the roof is flat It is weak and cannot able to with stand and the external load like snow load , roll-over adn etc .So in order to increase the strength of the roof ,reinforcement like front roof rail , center 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
- Rear Roof Rail
- Center Roof Rail
- Bow Roof Rail
Given data of mastersections:
Roof outer panel:
Front Roof Rail:
ront 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 onthe master section given
Rear Roof Rail:
Rear Roof Rail provides the reinforcement to the rear part of the car roof. The rear rail roof is to be designed considering the rear visibility criteria adn the rear passenger visibilty criteria and the rear passenger's head room clearence.
Center Roof Rail:
Central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load.Generally central roof rail is placed at thecenter of the roof which is connected ro the B -pillar support structure .This central roof rail helps in the strength to the roof during the roll over test.
Bow Roof Rail:
The bow roof are given to improve the torsional stiffness adn load bearing capacity of the roof structure .The number of roof presented is depends on the overall size of the roof presently in this protect two bow roofs are calculated
Different views of Roof assembly:
Heat distortion:
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.
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< 2.7This test is done to know how the roof is behaving when there is a snow over. Normally due to the snow weight the dent will happen. But the roof should be designed in such a way that when the snow is removal the roof should go it its original position. This basic requirement for snow load criteria.
| S.no | Between rail roof | Rx | Ry | R | t | L | I | W | Result |
| 1 | Front roof rail to Bow roof 1 | 859.1 | 745.3 | 798.16409 | 0.75 | 2034.4 | 386.8 | 1.2807063 | Pass |
| 2 | Bow roof 1 rail to Center roof Rail | 847.6 | 6281.4 | 1493.6498 | 0.75 | 2034.4 | 405.3 | 1.3403731 | Pass |
| 3 | Center roof Rail to Bow roof 2 | 818.1 | 10294.9 | 1515.7487 | 0.75 | 2034.4 | 367.4 | 1.3003235 | Pass |
| 4 | Bow roof 2 to Rare roof rail | 785.5 | 14362.6 | 1489.5363 | 0.75 | 2034.4 | 391.9 | 1.3253304 | Pass |
Process:
- create a sketch with 0-Y and offset the line into 300mm and then offset the line into 100mm in symmetry.
- project the edges of front roof rail ,bow roof 1, Central roof rail, bow roof 2, Rear roof rail.
- Create the line between front roof rail and bow roof 1 in vertically than create a midpoint of the line and offset the line of 100mm of symmetry .
- intersect the line of horizontal and vertical values to get Rx and Ry values and intersect points appeared.
- Project the lines into the roof and then use of arc command to connect and use local radius to calculate the values.
- Same process can be done for remain parts to get Rx anD Ry values.
- 'L ' value can find through start and end point of outer roof in 0-Y direction excluding ditch area.
- I values can get from distance between the part front roof rail &bow roof 1 is 386.8mm like this remain are to measure the image of values are in the below.
Rx:
Ry:
L value:
I values:
Snow Distortion:
This test is done to know how the roof is behaving when there is a snow over. Normally due to the snow weight the dent will happen. But the roof should be designed in such a way that when the snow is removal the roof should go it its original position. This basic requirement for snow load criteria.
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
| S.no | Roof Rails | Rx | Ry | Lx | Ly | t | Y | Iy | L1 | L2 | S | My | α | Qr | Result |
| 1 | Front Roof Rail to Bow Roof 1 | 4578.52 | 2533.3 | 1151.1 | 1961.2 | 0.75 | 486.98 | 2025.29 | 486.98 | 491.46 | 489.22 | 717915.6556 | 0.95126065 | 19.35996149 | ok |
| 2 | Bow roof 1 to Centre Roof Rail | 3768.83 | 6180.98 | 1097.93 | 1961.2 | 0.75 | 978.12 | 2044.5 | 491.46 | 452.42 | 471.94 | 961570.2096 | 1.159125083 | 8.494209451 | ok |
| 3 | Centre Roof Rail to Bow Roof 2 | 4246.96 | 10720.62 | 1071.42 | 1961.2 | 0.75 | 1430.5 | 2051.4 | 452.46 | 530.7 | 491.58 | 759166.35 | 0.87147804 | 4.809268675 | ok |
| 4 | Bow roof 2 to Rear Roof Rail | 4014.82 | 16345.8 | 1055.93 | 1961.2 | 0.75 | 1961.3 | 530.7 | 265.35 | -196.13 | -0.000218683 | 0 | N-ok |
Note:
Here Bow roof 2 to rear roof rail L2 value is not appeared so this case we cannot find it.
Hence we arrived to decision after result from calculation one of the Qr result is not acceptable .So this model will pass over to the styling team to change to improve the strength.
Process:
- create a sketch with 0-Y and offset the line into 100mm in symmetry.
- project the sides of each part and create a midpoint with the help of point command then form a midpoint for them than use of midpoint of parts create a line between ex:front roof rail and bow roof 1 than use of that create vertical line than offset of 100mm in symmetry and use of point command intersect the lines of horizontal and vertical to get Rx and Ry value .Points are to projected onthe roof .
- Use arc command to connect the points than use of local radius command to get values.
- 'Lx 'values are get through projec the sides of outer roof than intersect with the vertical lines of each part than points are occured measure them.
- 'Ly' is the distance between the outer roof of 0-Y direction excluding ditch area.
- 'Y' values are take midpoint Front roof rail to each midpoint of every part than get the different values of 'Y'.
- 'Iy' values are taken of section inertia analysis of Bow roof 1, Central roof rail ,Bow roof 2.
- Case -1: L1 value is between front roof rail and bow roof 1 L2 is bow roof 1 and central roof rail.
- Case-2:L1 value is between Bow roof 1 and Centre roof Rail L2 is central roof rail and Bow roof 2.
- Case-3:L1 value is between Centre roof Rail and Bow roof 2 L2 is Bow roof 2 and Rear roof rail .
Rx & Ry values:
Lx values:
Lx1:
Lx2:
Lx3:
Lx4:
Ly value:
Y1:
Y2:
Y3:
Y4:
Section modulus:
It is a geometric property for a given cross-section used in the design of beams or flexural members .Other geometric properties used in design include area for tension , radius of gyration for compression and moment of inertia for stiffness .Any relationship between these properties is highly dependent onthe shape in question. Equations for the section module
1.The Elastic section modulus
2.The Plastic section modulus
The elastic section modulus is defined as S = I / y,
where I is the Second moment of area
y is the distance from the neutral axis to any given fibre. It is often reported using y = c, where c is the distance from the neutral axis to the most extreme fibre.
Section modulus of Bow roof 1:
Section modulus of Central rail roof :
Section modulus of Bow roof 2:
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.
Outer Roof:
Front Roof Rail :
Bow Roof 1:
Center Roof Rail:
BowRoof 2 :
Rear Roof Rail :
Conclusion:
Thus the required reinforcement and the ditch area for the roof were designed and developed from the given styling. The curvature study was done after performing calculation for the Heat Distortion and Snow load conditions with the position criteria met for all reinforcements.
Further the section modulus study was conducted and Draft analysis were done for each reinforcement successfully.