Roof Design
Aim:
Design of the front roof rail, rear roof rail, and bow roof for the roof of the vehicles with the help of the given skin data of the roof according to snow load condition and hear distortion criteria.
Introduction:
An automobile roof is considered as an integral part of BIW (body in white) design. A car roof is responsible for protecting the passenger from rain, wind, sunlight, accidents etc. It also offers aerodynamic support to the car when it is in motion. A roof panel assembly includes a roof outer panel/ditch area and inverted hat like reinforcements rails which underlie the outer side edges of the roof outer panel. Roof rails are provided to give strength to the outer panel and also provide support to other commodities present in a car.
Automobile Roof :
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.
The roof itself may be constructed of soft or rigid material. Soft-tops are made of vinyl, canvas or other textile material; hard-top convertibles have roofs made from steel, aluminium, plastic or carbon fibre composites.
Types Of Roof :
Hardtop :-
A hardtop is a rigid form of automobile roof, which for modern cars is typically constructed from metal. A hardtop roof can be either fixed That is not removeable, detachable for separate storing or retractable within the vehicle itself.
The pillarless hardtop (often referred to as simply "hardtop") is a body style of cars without a B-pillar, which are often styled to give the appearance of a convertible design.
Structure of the Mercedes SL hardtop
- Sun Roof :- A sunroof is a movable panel that opens to uncover a window in an automobile roof, allowing light and fresh air to enter the passenger compartment. Sunroofs can be manually operated or motor driven, and are available in many shapes, sizes and styles.
- Convertibles :- A convertible or cabriolet is a passenger car that can be driven with or without a roof in place. The methods of retracting and storing the roof vary among eras and manufacturers.
- T-tops :- A T-top is an automobile roof with a removable panel on each side of a rigid bar running from the center of one structural bar between pillars to the center of the next structural bar.
- Targa top :- Targa top is a semi-convertible car body style with a removable roof section and a full width roll bar behind the seats.
- Vinyl Roof :- Vinyl Roofcovering was originally designed to give the appearance of a convertible to models with a fixed roof and eventually evolved into a styling statement in its own right. Vinyl roofs were most popular in the American market,
Etc.
Roof:
The roof itself may be constructed of soft or rigid material. Soft-tops are made of vinyl, canvas or other textile material; hard-top convertibles have roofs made from steel, aluminium, plastic or carbon fibre composites.
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 (interior trim). Headroom clearance and visibility are two important criteria for the driver and both of them depends upon the front roof rail dimensions, hence it is important to select appropriate dimensions for the roof rail.
Rear Roof Rail:
This is located at the back of the roof which bears the edge load and strengthens the roof from the rear side.
Center Roof Rail:
Centre Roof rail or the reinforcement center is located at the center of the roof . The cross section of this is more since it needs to provide more strength to the roof. Here both are ends are joined together using mastic sealants.
Bow Roof Rails:
Bow roof is an additional stiffening member for the roof to increase the strength and so that it can satisfy the norms. Both the ends are joined together using mastic sealants to increase strength and also since this would not affect the styling of the roof.
Number of bow roof increases if the car size becomes longer and taller. More taller car means more flat region means more Bow roofs.
The roof bows are secured to the top by way of staple-like fasteners or placement of each bow within a pocket sewn to the lower side of the convertible roof. The soft top roof is commonly secured to the number one or forwardmost roof bow by being sandwiched between the number one bow and a trim panel.
Design Consideration:
- Visibility criteria.
Good all Round Visibility is one of the main requirements of body design
The fig. show eye position of driver, angle of visibility & space between the pillar and seat
Visibility depends upon the window opening and their relative position to the occupants
If occupant is placed close to windscreen the forward visibility is considerably improved but comfortably entry is adversely affected as dimension between pillar & seat is decreased requiring suitable shaping of the door, if possible
The Downward visibility can be improved by positioning the driver as high as possible in Relation to the lower edge of windscreen.
Rearward visibility is achieved by increase in glass area required to maintain rear view vertical angle with back window
- Header clearance.
The roof header is the leading component of the safety critical roof structure, supporting the windshield and protecting the passengers in rollover. Roof header materials must be high in tensile strength, low in weight to lower the vehicle center of gravity, high in toughness, and integration-friendly.
The head clearance in important of vehicle development head not Toch the roof keep some distance. in case head is so closer to roof to disturb the driver and passenger to impact to head and make a disturb for good driving.
An achy back, stiff neck, sore shoulders: The everyday shakes, rattles and rolls of daily commuting can take a toll on your body.
- Curvature study.
The curvature is the improve roof strength & improve the quality of the roof
The act of curving : the state of being curved
A measure or amount of curving
The rate of change of the angle through which the tangent to a curve turns in moving along the curve and which for a circle is equal to the reciprocal of the radius
- Heat distortion Criteria
Thermal boundary layer on hot car roof, seen by distortion of background grid pattern
The roof is directly exposed to sunlight. The temperature changes with respect to time and season, it causes deformation in the roof. In order to prevent failure it should be designed in such a way else, it gets failure.
The heat accumulated inside the vehicle with undesired temperature rise would cause the parts of the car's interior to degrade. Even the passengers are affected with the thermal condition inside the vehicle itself.
In this criteria, we check the heat distortion capacity of the roof at higher temperatures with the fixed load on it. So when the surrounding temperature is high and the roof is loaded with some weight on it then if the roof is deformed it fails, so to calculate whether the roof deform or not, according to the position of the bow we do these calculations as follows
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>
Steps to calculate the Heat distortions as follows:
We take the horizontal line as OY and make another line 3Y at a distance of 300 mm from the OY axis now we make 2 lines at 100 mm distance on either side of the 3Y line. The intersection of these lines will give us the points which will be projected on the Styling surface to determine Rx
Similarly at the OY we take the Ry by taking the center between the bow roof edge and front roof rail edge.
Calculation
Front Roof Rail to Bow Roof 1
Rx=5748.66 mm
Rx=2178.34 mm
L=2028.6 mm
I=261.5 mm
t =0.75 mm
Soln.
R=2(5748.66 * 2178.34) / (5748.66 + 2178.34)
R=3.15*10^ (-3)
W = [1.73 x 10 ^ (-3) x 2028.6] + [1.85 x 10 ^ (-8) x ( (3.15 x 10^(3) )^2 ) / 0.75 ] + [ 1.10 x 10^(-3) x 261.5] - 2.68
W=1.35 mm
The condition for the heat distortion criteria is less than W>2.7 so the above the value of W is less than 2.7 so the position of the bow Roof 1 is correct according to the heat distortion criteria.
Like this, we do the calculation for the remaining Bows positions as shown in the following table,
|
Calculation Table |
||||||||
|
Roof Rails |
Rx (mm) |
Ry (mm) |
R (mm) |
L (mm) |
I (mm) |
t (mm) |
W |
Ok / NG |
|
FR-BR 1 |
5748.66 |
2178.34 |
3.15x10^(3) |
2028.6 |
261.5 |
0.75 |
1.35 |
OK |
|
BR1-CR |
5278.06 |
4860.36 |
5.06x10^(3) |
2028.6 |
319.7 |
0.75 |
1.18 |
OK |
|
CR-BR 2 |
4571.34 |
7845.48 |
5.77x10^(3) |
2028.6 |
219.8 |
0.75 |
1.18 |
OK |
|
BR2-BR3 |
3886.51 |
11037.5 |
5.74x10^(3) |
2028.6 |
378.1 |
0.75 |
1.19 |
OK |
|
BR3-RR |
3234.49 |
14848.3 |
5.31x10^(3) |
2028.6 |
266.2 |
0.75 |
1.12 |
OK |
From the above calculation table value of w it can be less than W>2.7 thus, that infer that current positioning of Bow Roof Rails are good as per design as found ok.
- Snow load Criteria.
The heavy snow load are dangers of the roof snow leading to the collapse or partial collapse of a roof.
The snow load predication is use to calculate the load condition for snow weight acting on a roof, and this calculation is used to improve roof strength of car.
In this criteria we check how is the roof behaving when there is a snow over. The normally due to the snow weight on then 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 is the basic requirements for snow load criteria.
Snow Load Prediction Formula
Qr = [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 10-8]
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
Calculation
Front Roof Rail to Bow Roof 1
Iy=387.58 mm^4
Rx=4789.62mm
Ry=2272.50mm
Lx=1121.7 mm
Ly=2055.5 mm
L1=358.2 mm
L2=339.2 mm
Soln.
My=395.9(2055.5-395.9)
My=657.03x10^3 mm
S=(4789.62/2+2272.50/2)
S=378.7 mm
α=657.03x10^3x(1121.7)^2x10^-13
α=826.68x10^-3
The condition for the Snow load criteria is Greater than Qr≥ 3.1 so the above the value of Qr is greater than 3.1 so the snow load criteria of the bow Roof 1 is Pass.
Like this, we do the calculation for the remaining Bows positions as shown in the following table,
|
Snow Load Calculation Table |
|||||||||||
|
Roof Rails |
I y (mm^4) |
My (mm) |
α (mm) |
Lx |
Y |
s |
Rx |
Ry |
t |
Qr |
OK/NG |
|
FR & 1st |
387.58 |
657.0x10^(3) |
826.6x10^(-3) |
1121.7 |
395.9 |
378.7 |
4789.62 |
2272.5 |
0.75 |
55.85 |
OK |
|
1st&CTR |
452.95 |
1.00x10^(6) |
1.17 |
1083.8 |
795 |
349.4 |
3809.26 |
4778.5 |
0.75 |
33.80 |
OK |
|
CTR&2nd |
396.40 |
1.05x10^(6) |
1.19 |
1066.0 |
1094.7 |
374 |
3926.04 |
7876.7 |
0.75 |
14.38 |
OK |
|
2nd&3rd |
398.15 |
789.75 |
868.6x10^(-6) |
1048.7 |
1544 |
444.5 |
4312.48 |
11648. |
0.75 |
9.10 |
OK |
|
3rd&RR |
4.04 |
143.7x10^(3) |
1983 |
3983.89 |
16516. |
0.75 |
|||||
From the above calculation table value of Qr it can be greater than Qr>3.1thus, It has passed the Snow Load Test .
- Draft angle.
Draft angles are the degree of taper that is incorporated into the side wall and cores of the die cast tool to aid in the removal of a part. Primarily, they function to reduce the friction between the part and the tool as it is ejected. Without draft angles, parts can be stripped, dented, or get stuck in the tool.
It is not common practice to compute the draft angle for each feature on a component, and it is normally generalized with some exceptions
Twice as much draft angle is recommended for inside walls or surfaces as for outside walls or surfaces
Draft Analysis :
The Draft Analysis allows the user to easily analyze a part for proper draft angle. A Draft Angle describes the amount of taper for molded or cast parts perpendicular to the parting line. Find the Draft Analysis in Tools tab. Draft Analysis is applicable for 3D part models.
To create a Draft Analysis for any 3D part file, the user needs to:
1.Select Part/Surface
2.Select the Pull Direction
User can change the pull direction the Part/Surface.
A panel of gradient specifies a range of deviation from the pull direction. There is provision to change the deviation range and color of the result. With the help of mouse hover user can check draft angle on the component.
1) Roof.
2) Front roof Rail.
3) Bow Roof Rail1.
4) Center Roof Rail.
5) Bow roof Rail2.
6)Bow Roof Rail3.
7) Rear Roof Rail.
Section modulus:
SECTION MODULUS is the ratio of the moment of inertia about neutral axis upon the extreme point of the section from the neutral axis.
The section modulus (S) is geometry property of the cross section used for designing beams and flexural members. It does not represent anything physically.
Therefore
S=I/Y___________ (1)
Where,
S= Section Modulus
I= Moment of Inertia
Y= Extreme Point From Neutral Axis (mm).
- Roof
MOImax=6.89x 10^8 mm^4
MOImin=2.19 x 10^6 mm^4
As per the section modulus equation,
S=I/Y
Soln.
I = 2.19 x 10^6 mm^4
Y = Total distance / 2,
Total distance = 2132.56mm
Y= 2132.56/2 = 1066.28 mm
S = 2.19 x 10^6 / 1066.28
S = 2.0538 x 10^3 mm^3
- Front Roof rail
MOImax=1.91 x 10^5 mm^4
MOImin=6.46 x 10^3 mm^4
As per the section modulus equation,
S=I/Y
Soln.
I = 6.46 x 10^3 mm^4
Y = Total distance / 2,
Total distance = 123.11mm
Y= 123.11/2 = 61.55 mm
S = 6.46 x 10^3 / 61.55
S = 104.95 mm^3
- Bow Roof Rail 1
MOImax=2.500 x 10^4 mm^4
MOImin=387.580 mm^4
As per the section modulus equation,
S=I/Y
Soln.
I = 387.580 mm^4
Y = Total distance / 2,
Total distance = 71.46 mm
Y= 71.46/2 = 35.73 mm
S = 387.580 / 35.73
S = 10.84 mm^3
- Center Roof Rail
MOImax=4.4689 x 10^3 mm^4
MOImin=452.959 mm^4
As per the section modulus equation,
S=I/Y
Soln.
I = 452.959 mm^4
Y = Total distance / 2,
Total distance = 87.88 mm
Y= 87.88/2 = 43.94 mm
S = 452.959 / 43.94
S = 10.30 mm^3
- Bow Roof Rail 2
MOImax=2.51 x 10^4 mm^4
MOImin=396.40 mm^4
As per the section modulus equation,
S=I/Y
Soln.
I = 396.40 mm^4
Y = Total distance / 2,
Total distance = 72.10 mm
Y= 72.10/2 = 36.05 mm
S = 396.40 / 36.05
S = 10.99 mm^3
- Bow Roof Rail 3
MOImax=2.510 x 10^4 mm^4
MOImin=398.157mm^4
As per the section modulus equation,
S=I/Y
Soln.
I = 398.157 mm^4
Y = Total distance / 2,
Total distance = 71.83 mm
Y= 71.83/2 = 35.95mm
S = 398.157 / 35.95
S = 11.07 mm^3
- Rear Roof Rail
MOImax=5.62 x 10^5 mm^4
MOImin=4.04 x10^4 mm^4
As per the section modulus equation,
S=I/Y
Soln.
I = 4.04 x10^4 mm^4
Y = Total distance / 2,
Total distance = 174.71 mm
Y= 174.71/2 = 87.35mm
S = 4.04 x10^4 / 87.35
S = 462.50 mm^3
Conclusion:
We were successfully able to design the roof with all the roof rails for better support and strength. We designed it so as to satisfy the norms and regulations by calculating the heat distortion criteria and the snow load criteria. While doing this we also understood about the methodology behind designing the roof of the car and what all we take into consideration while designing so.