Challenge

Roof Design

PPrajwal Kushwaha18 Jan 2023

Aim:- For the Given Roof styling, create an ASSEMBLY by developing the essential flanges for the outer roof and creating the reinforcements

Theory:-

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. It is design to withstand the external forces & impacts and safe gaurd the passengers.

Components of the Roof:-

Front Roof Rail
Bow Roofs
Center Roof rail
Rear Roof Rail

DESIGN CONSIDERATIONS FOR A ROOF:-

Visibility criteria
Head clearance
Curvature study
Snow load criteria
Draft analysis

Manufacturing Techniques:-

Stamping Process:- The process of turning sheets of metal into a useful part or component is called sheet metal stamping. The metal is fed into a press, where the stamping tool, also known as a die, creates the desired shape
Joining Technique
- Spot welding:- Spot welding (or resistance spot welding) is a type of electric resistance welding used to weld various sheet metal products, through a process in which contacting metal surface points are joined by the heat obtained from resistance to electric current.
- Mastic Glue:- Mastic Glue is used to stick two metal surfaces, by using mastic glue we can avoid the weld marks & used to reduce the NVH between the two surfaces. One mastic glue can influence the radius of 40~80mm(Mastic Influence).

Calculation:-

Bow–Roof Prediction:-

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 If W<2.7 Or 2.7>W>3.4

Heat Distortion Criteria
S.No	L	Rx	Ry	R	t	l	W	Result
Fr & 1st	2022	5622.6788	3400.2649	4237.7738	0.7500	1117.2819	2.490052	OK
1st & Ctr	2022	4858.3192	6662.1241	5619.0069	0.7500	1081.6660	2.786699	OK
Ctr & 2nd	2022	4070.1676	9476.5189	5694.5320	0.7500	1056.9873	2.780629	OK
2nd & Rr	2022	3431.1354	12340.4526	5369.3723	0.7500	1046.8945	2.680788	OK

Snow Load Prediction Formula

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

Snow Load Calculation
S.No	Rx	Ry	Lx	Ly	Y	My	L1	L2	s	α	Iy	t	Qr	Result
Fr & 1st	4572.135	2559.005	1135.3948	2022.226	470.75	730357.4	470.75	510.62	490.685	0.941519	721.83	0.75	6.913014	OK
1st & Ctr	3739.539	6063.405	1083.7325	2022.226	981.37	1021465	510.62	511.09	510.855	1.199686	928.45	0.75	3.547012	OK
Ctr & 2nd	4276.174	11094.53	1054.6716	2022.226	1492.46	790654.9	511.09	478.34	494.715	0.879471	726.8	0.75	1.590864	NG

Result :- The roof has failed the snow load test, so we have used emboss to improved the strength of the roof.

Draft Analysis:- Minimum draft angle is 7 degree is considered for analysis. Green colour show positive draft angle of the part more than 7 degree.The draft analysis is done on the roof parts and the results are as follows:-

In Top roof we have 2 flanges with draft angle between 0~5 i.e. positive draft and 1 flange with negative draft, that surface can be cleared with another direction of the draft analysis.

All the other component have cleared the draft analysis with minimum of 7 Degree of clearance.

Top Roof:-