Roof Design

Ojective :

To Design Car Roof with essential flanges and reinforcements like Ditch area , Front roof rail , Bow roofs, Central roof rail and Rear roof rail for the the given styling surface and Calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct positions.

Concept :

Car 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. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure and to with stand the over head forces like snow.

Car roof 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.

• Front roof rail
• Bow roofs
• Central roof rail and
• Rear roof rail

Front Roof Rail :

Front roof rail is the part which joins the wind shield glass , body side outer and the inner panel also. Here the front roof rail is designed based up on the master saection given

Master section given :

Front Roof Rail Designed :

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 the center of the roof which is connected to the B-pillar support structure . This central roof rail helps in adding the strength to the roof during the rool- over test.

Master section given :

Center Roof Rail Designed :

Rear Roof Rail :

Rear roof rail is the one which joins the back door and side body of car .

Master section given :

Rear Roof Rail Designed :

Bow Roof Rails :

The bow roofs are give to improve the torsional stiffness and load bearing capacity of the roof structure . The number of bow roof present is depends on the overall size of the roof . Presently in this project two bow roofs are added.

Master section given :

Front Bow Roof Rail Designed :

Rear Bow Roof Rail Designed :

Assembly :

Mastics are introduced between the roof outer panel and the reinforcements for joining . where ever the mastic are introduced there is remarkable improve of the strength

Spot welding is also introduced to join the reinforcements and the outer panel.

DESIGN CONSIDERATIONS FOR A ROOF :

• Visibility criteria
• Head clearance
• Curvature study
• Heat distortion and snow load criteria
• Draft analysis

ROOF CRUSH 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

Calculation of Heat Distortion and Snow load on the Roof :

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.

Heat Distortion 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 q ]

"Judgment Condition" - OK< 2.7.

Case 1 : Between Rear Roof Rail and Rear Bow Roof Rail

Case 2 : Between Rear Bow Roof Rail and center roof rail

Case 3 : Between Center roof rail and Front Bow roof rail

case 4 : BetweenFront Bow Roof rail and Front roof rail

Results :

Heat Destortion calculation
S.case	Roof length L	X curvature (RX)	Y curvature (RY)	Rx+Ry	Rx*Ry	Roof curvature (R)	Thickness (T)	Bow Roof span (I)	Heat destortion (W)	Results < 2.7
case 1	2029.1232	14574.1212	3282.3429	17856.5	47837263	0.00074655	0.75	352.7966	1.22	Pass
case 2	2029.1232	10576.8286	3978.3326	14555.2	42078142	0.000691816	0.75	429.4816	1.30	Pass
case 3	2029.1232	6577.8436	4877.6187	11455.5	32084213	0.000714087	0.75	327.4288	1.19	Pass
case 4	2029.1232	2894.7664	5708.8245	8603.59	16525713	0.001041237	0.75	467.8165	1.34	Pass

Snow load on the Roof :

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 .

Snow Load Prediction Formula :

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 and 250 ≤ s ≤ 380.

Case 1 : Front roof rail and Front Bow roof rail

LY

LX

L1

L2

RY

RX

Iy

Case 2 : Front Bow roof rail to center Roof rail

Ly

Lx

Y

Rx

Ry

L1

L2

Iy

Case 3 : Center Roof Rail to Rear Bow Roof rail

Ly

Lx

Y

Rx

Ry

L1

L2

Iy

Results :

Snow Load calculation
s.case	LY	LX	Y	α	Thickness(T)	L1	L2	(S)	RX	RY	Bow (Iy)	Snow Load (QR)	Results
Case 1	1952.3317	1132.2017	568.0557	1.008001589	0.75	568.0557	407.3978	487.72675	2824.7639	4384.352	1949.462606	17.16704792	Pass
Case 2	1952.3317	1083.1951	973.6706	1.118041068	0.75	407.3978	509.4799	458.43885	6547.2774	3790.0944	2641.598541	10.85150288	Pass
Case 3	1952.3317	1058.5266	1480.5133	0.782691719	0.75	509.4799	474.3683	491.9241	11022.4466	4271.2522	1972.438961	4.928016878	Pass

Draft Analysis :

Front Roof Rail :

Center Roof Rail:

Rear Roof Rail :

Front Bow Roof Rail :

Rear Bow Roof Rail :

Roof Images :

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 Heat distortion , Snow load Calculations and draft analysis are carried out to check feasibility manufacturing.