Roof Design
ROOF DESIGN
OBJECTIVE
Design a Roof, Front Roof Rail, Rear Roof Rail, Centre Roof Rail, Bow Roof 1, Bow Roof 2 with given Master-section and also check for the Heat distortation and Snow Load condition.
INTRODUCTION
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. In later years, many variations on the automobile roof developed.
The Roof consists of the following parts:
- Front Roof Rail
- Centre Roof Rail
- Rear Roof Rail
- Bow Roof 1
- Bow Roof 2
DESIGN CONSIDERATIONS FOR A ROOF :
- Visibility criteria
- Head clearance
- Curvature study
- Heat distortion and snow load criteria
- Draft analysis
RUSH TEST :
Roof crush test is the design given by the IIHS ( insurance institute of highway safety ) . After analyzing 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
- How fast the car can reforms to its original shape after the roll over and
- 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 there 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 goes 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.
WHY REINFORCEMENTS :
If the roof is flat it is very weak and cannot able to with stand the forces like snow , so in order to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process
1. Front Roof Rail
The Front Rail Roof is created on the front end of the roof to strengthen it from front.
2. Centre Roof Rail
It is created in the centre of the roof.
3. Rear Rood Rail
It is created on the rear end of the roof and while designing we have to consider the tail gate position as well.
4. Bow Roof 1
It is created in between the Front Rail Roof and Cetre rail roof.
5. Bow Roof 2
It is created in between the the Centre rail roof and Rear rail roof.
The given data for the Roof is as follows:
1. Master Section of Front Roof rail -
2. Master Section of Centre Roof Rail -
3. Master Section of Rear Roof Rail -
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]
Judgment Condition
OK< 2.7
APPLY THE VALUES IN THE FORMULA :
In order to find the heat distortion apply all the necessary values from the roof to the give formula
CALCULATIONS :
From the above table it can be concluded that all values of w<2.7 and also not w>3.1 so thus infers that current positioning of bow row are good in state as per design and found ok
SNOW LOAD CRITERIA :
This test is done to know how is the roof 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 is the basic requirements 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
APPLY THE VALUES IN THE FORMULA :
In order to find the snow load criteria apply all the necessary values from the roof to the give formula
CALCULATIONS :
It is observed from the values that obtained values of QR > 3.1
SECTION ANALYSIS:
Section moment of inertia is used to predict the amount of deformation and deflection of a part. A higher moment of inertia is preferable as it reduces the amount of deflection for a given force. The thickness of the part and addition of flanges and embosses increase the section inertia of the part. Here the master sections of the roof and roof rails and their section analysis are given.
BOW ROOF 1
Moment of inertia maximum,( MAX) I max =1.877× 10^4 ( mm4)
Moment of inertia maximum ,( MIN) I min =629.705 ( mm4)
BOW ROOF 2
Moment of inertia maximum,( MAX) I max =1.873× 10^4( mm4)
Moment of inertia maximum ,( MIN) I min =619.861 ( mm4)
CENTRAL ROOF RAIL :
Moment of inertia maximum,( MAX) I max =2.151 × 10^4( mm4)
Moment of inertia maximum ,( MIN) I min =661.206 ( mm4)
FRONT ROOF RAIL
Moment of inertia maximum,( MAX) I max =1.597× 10^5 ( mm4)
Moment of inertia maximum ,( MIN) I min =8.265× 10^3 ( mm4)
REAR ROOF RAIL
Moment of inertia maximum,( MAX) I max =1.8090x10^6( mm4)
Moment of inertia maximum ,( MIN) I min =3.618× 10^4( mm4)
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 70 is considered for analysis . green colour infer on the parts that all face along the tooling direction has positive draft angle greater than 70 and passed in analysis
DRAFT ANALYSIS FOR FRONT ROOF RAIL :
DRAFT ANALYSIS FOR REAR ROOF RAIL :
DRAFT ANALYSIS FOR BOW ROOF RAIL 1 :
DRAFT ANALYSIS FOR BOW ROOF RAIL 2 :
DRAFT ANALYSIS FOR CENTER BOW ROOF RAIL :
ROOF IMAGES:
FRONT VIEW
TOP VIEW
ISOMERTIC VIEW
BUTTOM VIEW
CONCLUSION :
Hence the roof , front roof rail, bow roof rails , central roof rail and the rear roof rail is designed by following the master sections and the moment of inertia and draft analysis are carried out to check feasibility manufacturing.