Roof challenge

What is roof?

>The roof is an important part of an automobile as it protects the occupants from various environmental condition like Heat, Snow, rain and wind.

>There are many types of the roof which differs based on the variants of the car and region. The roof must be designed in such a way that it would able to withstand any load or impact due to accidents or objects falling on it and other environmental impacts.

>So, the design is done based on the standards and the design intent which makes the roof to withstand the above said conditions. Also other factors like headroom clearance, aesthetic look play a vital role in design.

>The roof is an assembly of many parts like front roof rail, bow roof, bow roof centre and reinforcement centre. The number of bow roof is depended on length of the roof.

>The roof is attached to the car by mastic sealants provided between bow roofs and front and rear rails. Roof is connected to A-Pillar, B-pillar and C-pillar to cover the body front and rear door. Roof is supported by reinforcement bow-roof with front and back roof rail.

Types of roof reinforcement are as follow

1. Front roof rail reinforcement (FR)
2. Centre roof rail reinforcement (CR)
3. Bow roof-1 rail reinforcement (BR1)
4. Bow roof-2 rail reinforcement (BR2)
5. Rear rail reinforcement (RR)

1. FRONT ROOF RAIL (FR)

> It is the support that is joined with the windshield glass, body side outer and the inner panels. It is usually made from the body side outer scrap and therefor the thickness of the rail is 0.75mm.

> A draft angle of 3-5 degree is provided for deep drawing operation. The visibility criteria and the headroom clearance of the passenger are taken into consideration for the bottommost flange. Windshield end and roof end are spot welded and the other end of the front roof rail is joined with the mastic sealant for increased strength.

2. REAR ROOF RAIL:-

> It is the support that is joined to the backdoor, the body side outer and the inner panels. Input from the back door design is also taken to accommodate require clearance along with the headroom clearance and rear visibility criteria while designing the rear roof rail. Spot welds are applied at the back door end of the rail and the other end is joined with roof using mastic sealants.

3. CENTRE ROOF RAIL AND OTHER ROOFS:-

>Similar to other rail designs, we have to take into consideration headroom clearance and visibility criteria while designing the centre roof rails and bow roof. Here, we don’t use the spot weld for joining instead of that we use the mastic sealants at the both the ends.

BOW ROOF 1

CENTRE BOW ROOF

BOW ROOF-2

BOW ROOF-3

DITCH AREA:-

The roof area is connected with the car body by the ditch area

Calculation to determine bow roof position:-

1. Heat distortion criteria:-

>Heat distortion temperature is the temperature at which the metal softens and deforms under a specific load. This study is done to check the ability of the roof to withstand high temperature with respect to positioning of the reinforcements which contribute to increase strength.

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

Judgement Condition: OK< 2.7> 3.1

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]

	L	Rx	Ry	R	t	l	W	Result
FR to BR1	2078	5730	2428	3411	1	454	1.62	O.K.
BR 1 to BR_C	2078	5122	5552	5328	1	369	1.82	O.K.
BR C TO BR_2	2078	4408	8533	5813	1	339	1.81	O.K.
BR_2 TO BR_3	2078	3800	11460	5707	1	345	1.92	O.K.
Br_3 to RR	2078	3203	15075	5283	1	446	1.95	O.K.

Note:- All dimensions are in mm

2. Snow load criteria:-

>Snow load criteria study is done to check the ability of the roof to withstand snowfall and bounce back to its original shape without any permanent deformation once the load is removed.

Snow load Prediction formulaA

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)

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 center 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]

Roof panel X curvature radius of length s in Side view

	t	Ly	Lx	Rx	Ry	L1	L2	S	Y	My	α	Iy	Qr
FR to BR1	1	1954	1040	4621	2501	454	370	412	412	635304	0.68	111.02	3.12
BR 1 to BR_C	1	1954	1142	3767	5465	370	339	354.5	370	586080	0.76	573.17	9.98
BR C TO BR_2	1	1954	1112	4028	8643	339	345	342	339	547485	0.68	373.61	4
BR_2 TO BR_3	1	1954	1094	4335	12219	345	446	395.5	345	555105	0.66	7715	43.14
Br_3 to RR	1	1954	1083	3908	16877	446	0	223	446	672568	0.79	0	_

SECTION_MODULUS_IMAGES

SOME IMAGES OF ROOF ASSEMBLY

Google Drive Link

https://drive.google.com/file/d/1DEeraNkVhMiXHzOzOz14b08cJDrPydSl/view?usp=sharing