Roof Design

INTRODUCTION :

An automobile roof is considerd as an integral part of BIW (Body In White) design. It ia a part of the car positioned above the passenger comprartment. A car roof is responsible for protecting the passenger from rain, wind, sunlight, accidents etc. It is made curved to increasing the strength, if the roof is designed as flat there are more chances to bend when the load is applied. A car roof has some reinforcement to provide the additional strength to the roof. The roof of car plays major role during the accidents.

DESIGN CONSIDERATIONS OF ROOF :

• Visbility criteria
• Bow roof position
• Heat distortion and snow load criteria
• Head room clearance
• Draft analysis

REINFORCEMENTS OF ROOF :

• Front Roof Rail
• Rear Roof Rail
• Center Roof Rail
• Bow Roof Rails.

1. FRONT ROOF RAIL

Front roof rail is designed at front end of the roof to provide the strength to that area. When we design the front roof rail we should consider the visibility and headroom clearance. It is joined with the outer roof dich area by spotwelding. And the embosses are created to produce the stength of the front roof rail.

MASTERSECTION OF FRONT ROOF RAIL

2. REAR ROOF RAIL

Rear roof rail is designed at the rear side of the roof, it joined withe the outer roof by spotwelding. We should consider the visibility criteria and rear headroom cleaarance while designig this. The embosses are created on the rear roof rail to provide stength.

MASTERSECTION OF REAR ROOF RAIL

3. CENTER ROOF RAIL :

Center roof rail is effectively support to the flat area of the roof. Because center of roof is more chances to failure when pressure is applied. Reinforcement center roof railes are fixed at the center of the roof for connectiong to the B piller.

MASTERSECTION OF CENTER ROOF RAIL

4. BOW ROOF RAILS :

The bow roofs are provided to increase the strength of the roof and load bearing capacity of the roof. The number of bow roofs are depends on the length of the roof structure. If the roof ls longer, we have to provide more bow roofs . The postioning of the bow roofs are decided by the curvature study. The heat distortion and snow load criteria are checked to anallyse the position of the bow roof by various conditions.

MASTERSECTION OF BOW ROOF RAIL

All the roof reinforcements thickness is 0.75 mm.

SECTION MODULUS :

FRONT ROOF RAIL :

S=I/Y

I=846.09mm^4

Y=120.8536/2

=60.42

Therefore S =846.09/60.42

S =14.0034 mm^3

REAR ROOF RAIL:

I =5.216mm^4

Y=190.17/2

=95.085

S=0.055mm^3

CENTER ROOF RAIL:

I =265.20

Y =80.007/2

=40.0035

S = 6.62mm^3

BOW ROOF 1 :

I =221.021

Y =69.896/2

=34.948

S =6.324 mm^3

BOW ROOF 2:

I =232.04

Y =69.967/2

=34.98

S =6.6335mm^3

CURVATURE STUDY

The better curvature of the roof the better is the torsional stiffness of the roof component. If the curvature is not suitable at certain locations we have to provide bow roof to increase the strength at that area. The positions of the bow roofs are governed by certaine conditions. So we are going to perform a curvature study by using the conditions of HEAT DISTORTION and SNOW LOAD CRITERIA to check if the component passes or not.

1 HEAT DISTORTION : Heat distortion is an important factor helps to finde the postions of the roof. It is the temperature at which the materital is no longer suitable for stuctural applications. When the load and temperature is exposed to high the materital will be soften and cannot be used. There is formula for HEAT DISTORTION Iis

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

CALCULATIONS OF HEAT DISTORTION :

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

ROOF RAILS	Rx	Ry	l	L	R	T	W	RESULT
FR - BOW1	5741.19	2051.77	1425.69	2023.1	1511.56	0.75	2.44461	PASS
BOW - RC	4974.98	6186.62	435.07	2023.1	2757.51	0.75	1048613	PASS
RC - BOW	9840.69	10540.10	437.64	2023.1	5089.19	0.75	1.940264	PASS
BOW - RR	2613.70	4818.72	354.44	2023.1	1694.54	0.75	1.280711	PASS

Therefore, from the above table we consider that the model has passed the heat distortioncriteria.

2. SNOW LOAD CRITERIA : The roof must be able to withstand load especially snow which results in heavy load actiong on the roof. If the roof is not able to withstand it, there will be deformation of the roof. To prevent that, snow load criteerai calculations can help us to determine whether the roof desighn will withstand the snow load.

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

250 ≤ s ≤ 380

CALCULATIONS OF SNOW LOAD CRITERIA:

Qr = [Iy x t²] / [α x s x [(Rx + Ry)/2]² x 10^-8]

ROOF SECTION	Iy	α	Lx	Ly	Y	S	Rx	Ry	t	Qr	RESULT
FR - BOW1	221.021	0.8985647764	1112.9	2023.1	465.9	487.85	4618.25	2479.36	0.75	2.2519	NG
BOW-CR	265.20	1.1842786272	1076.5	2023.1	975.6	511.05	3764.03	6094.85	0.75	1.1795	NG
CR - BOW2	232.128	0.8895239471	1056.9	2023.1	1487.9	492.65	4271.89	11031.08	0.75	0.5089	NG

From above calculations for snow load criteria this model of roof has not passed the snow load criteria so we need to creat an embosses on the top of the roof to provide strength for better load resistance capacity.

DRAFT ANALYSIS :

Draft analysis allows to easily analyse a part for proper draft angle. It was performed with minimum 7 degree draft angle.

FRONT ROOF RAIL:

REAR ROOF RAIL :

CENTER ROOF RAIL :

BOW ROOF :

CONCLUSION :

The model of roof and its reinforcements are designed by using NX software by following the provided master sections . After designing the roof i did the curvature study for positions of the bow roofs and strength of the roof. It was failed at snow load condition so we need to provide the additional embosses on the top of the roof for addition strength. And finally i did the draft analysis for proper draft angle wehtere the components will manufacturable or not.