Roof Design
Roof Design Report
INTRODUCTION: Automobile roof is the upper portion of an automobile which protects the passengers from sun, wind, rain. Various environmental conditions are considered for designing of the same. Even though it serves as styling surface roof should be provided with proper reinforcements for safeguarding passengers. Vehicle roofs are vulnerable to high heat conditions & moderate load during snowy conditions. Heat load calculation & snow load calculation are done to get a proper design of roof rails. This ensures the safety of the passengers in the vehicle.
DESIGN PARAMETERS
For designing roof, we must create roof rails for enough strength to withstand the load. Bow roofs are primarily fixed in flatter region to enhance strength. Front roof and rear roof rails are fixed to the ditch area using spot weld operation. And front, rear bow roof and centre roof rails are fixed to roof using mastic sealants. Gap between roof rails and bow roof should be less for Better NVH. Following criteria are considered for roof rail design.
Visibility criteria
Head clearance
Curvature study
Hear distortion and snow load criteria.
Draft analysis
Roof
Front roof Rail
This is the reinforcement provided at front. This part is joined to BIW body and windshield. Visibility criteria is taken in account for joining it with windshield. It is fixed to BIW with spot welds and roof surface is attached with the help of mastic sealants. A draft angle is provided for deep drawing operation. Draft analysis is also done on the section to check its manufacturing ability.
Master section
Rear Roof rail:
It is the reinforcement provided at rear side of the roof. Backdoor and bodyside panels are attached with this part. It is also attached with mastic sealants and spot welds.
Bow roof 1:
It is an additional reinforcement provided to increase the strength of the roof. It is fixed with roof body with mastic sealants.
Centre rail roof:
Centre rail roof can be considered as reinforcement joining the left and right part of the roof. It is fixed to roof by mastic sealants and to BIW by spot welds.
ROOF CRUSH TEST:
Roof crush test is the design given by the IIHS (insurance institute of highway safety) . After analysing the many accidents IIHS has then fixed some regulations, that how to perform the crush test .
IIHS gives the rating based up on this test. For this they consider two points while giving the rating
1.How fast the car can reform to its original shape after the roll over and
2.How deep is the protrusion after the roll over
GOAL OF ROOF CRUSH TEST:
To minimize the protrusion into the occupant compartment, so that the seat belts and air bags can do their job in protecting the occupant.
CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST:
There are two conditions that 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 in 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 crush speed at which the load applied is 13 mm/sec or less than that.
RESULTS:
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 the roll over crashes.
HEAT DISTORTION STUDY ON THE ROOF:
The heat distortion study plays a major role in sheet metal usage. heat distortion temperature is a temperature limit above which the material cannot be used for the structural applications. This study is used to predict the heat distortion temperature at where the material starts 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 .
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]
Judgement Condition: OK< 2.7> 3.1
From the above table it can be concluded that all values of 'W' is less than 2.7,which implies the position of roof rails are as per standard
SNOW LOAD CALCULATION
Snow load criteria study is used to check the durability of the roof to withstand the snow fall on the roof and reflex it is original shape without any permanent deformation after load being removed.
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)
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 centre 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]
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 colour ranges identifying zones on the analysed 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 colours infer on the parts that all face along the tooling direction has positive draft angle greater than 7 degree and passed in analysis.
Draft analysis for front roof rail:
Draft analysis for bow roofs
Draft analysis for centre rail
Draft analysis for rear end
Final assembly
CONCLUSION: --
The results obtained after doing a curvature study on the roof was adequate.
The position of the bow roofs can be arranged better (over the flat areas) which helps in increasing the strength of the roof.
Embossments are added to improve the strength & Aerodynamics of the roof but can be avoided if not necessary as it increases the cost of manufacturing.