Roof Design

Objective of the Project : To develop the essential Ditch area, Flanges, and Reinforcements for the Roof using the Class-A surface as input and, perform Section Inertia Analysis and Draft Analysis on each of the Roof rails and Reinforcements.

About Roof of a Car : 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.

A Roof is usually made of metal or an alloy but with technological advances, they are starting to be manufactured out of composite material which decreases the overall weight of the vehicle dramatically as the Roof is major component in a vehicle.

The Roof consists of multiple components to hold it and to give it the structural integrity and to withstand failure in most normal conditions. The supporting structures are placed on the Inner panel of the roof, so as to not disturb the aesthetics of the vehicle while at the same time providing the necessary strength to support the roof and keep it from deforming and intruding into the cabin space during a rollover incident.

The Roof Assembly that consists of Roof outer panel, Front Roof rail, Bow Roof rails, Centre Roof rail and Centre Roof rail reinforcement when Roof having Sun Roof, Rear Roof rail and Ditch area.

The above mentioned components has been designed as per given data.

Roof Skin :

Roof Bottom View :

Roof Top View :

Front Roof Rail : The component that sits at the very front of the roof and holds the roof panel, the windshield and connects them to the A-Pillar of the vehicle is known as the Front Roof Rail. The thickness of the front roof rail is 0.75 mm and the front roof rail design is the utmost importance as the change in design directly affects the visibility out of the cabin and also the headroom clearance which is important when it comes to passenger safety requirements.

Bow Roof Rail 1 & 2 : The Bow Roof is the component that is added to provide extra strength to the roof in the required section. A bow roof is not a mandatory structure as the use and placement of it purely depends on the inherent strength of the roof. The number of bow roofs used in a vehicle also vary based on these criteria. In this design study two bow roofs were used, one on either side of the centre roof rail to provide strength in the front and rear sections.

Centre Roof Rail : The centre roof rail itself as the name suggest, a structural component that is placed at the centre of the roof. Unlike the bow roofs, this is not an optional structure and all vehicles tend to have this as it provides strength half way down a long sheet metal component. A gap of 5 mm was provided between the center roof and the roof outer panel as this is the clearance provided for the mastic sealants to attach the roof to the roof rail.

Rear Roof Rail : The Rear Roof rail, is similar to the front roof rail but as the name suggests once again, it sits at the rear most part of the roof and holds the roof panel to the D-Pillar and will also hold the Hinge mechanism for the door of the boot. The rear roof rails design, like the front roof rail will determine the headroom clearance and the rear visibility criteria of the vehicle. A thickness of 0.75 mm was used for the Rear Roof rail too.

Heat Distortion Criteria or Bow–Roof Prediction Formula : The Heat Distortion formula is used to calculate the viability of the roof to withstand the deformation due to change in temperatures which may vary from very high temperature to extremely low temperatures. Under high heat, the metal tends to distort and this calculation can check whether the placement of the reinforcements is correct and if it meets the criteria then the reinforcements help in withstanding the heat distortion of the roof panel.

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]

Calculation for the Heat Distortion Criteria
S.No	Section	Length	Rx	Ry	Roof Curvatre, R	Bow Roof Span, I	Thickness, t	W	Judgment Criteria OK< 2.7	Result
1	FRR - BR1	2033.7048	2575.0678	5736.522	4689.8079	422.8861	0.75	1.84601	OK	Passed
2	BR1 - CNTR	2033.7048	6367.9152	4928.848	5556.721964	380.4416	0.75	2.018432	OK	Passed
3	CNTR - BR2	2033.7048	10237.314	4044.5463	5798.304974	368.7388	0.75	2.073224	OK	Passed
4	BR2 - RRR	2033.7048	14567.297	3282.3461	5357.520027	413.8748	0.75	2.001579	OK	Passed

From, the above table we can conclude that the placement and design of the roof components meets the heat distortion criteria in all the sections.

Snow Load Prediction Formula : The snow load prediction is done especially in countries where there is a lot of snowfall. It is a precautionary calculation done to see if the roof of the vehicle can effectively take the load of the snow that accumulates on top of the roof without causing the roof to deform. There is a formula for this calculation and it is given by,

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

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

Calculation for the Snow Load Criteria
S.No	Section	Rx	Ry	Lx	Ly	Y	Iy	s	Thickness, t	α	Qr	Judgment Criteria Qr ≥ 3.1	Result
1	FRR - BR1	1896.7665	3372.765	1104.6333	1968.5311	510.7303	2295	485.474	0.75	0.9085	42.16289	OK	Passed
2	BR1 - CNTR	3236.0741	3774.1392	1070.3072	1968.5311	970.9479	6988	454.3555	0.75	1.1095	63.46728	OK	Passed
3	CNTR - BR2	10662.294	3217.9725	1051.1	1968.5311	1419.4413	2313	498.7916	0.75	0.861	6.28985	OK	Passed

Section Inertia Analysis of Bow Roof 1 :

Section Inertia Analysis of Centre Roof rail :

Section Inertia Analysis of Bow Roof 2 :

About the Draft Analysis :

Draft analysis is performed on the components where the ejection of materials happens after the material is made inside the tools.

Draft Analysis of Outer Roof rail :

Draft Analysis of Front Roof rail :

Draft Analysis of Bow Roof 1 :

Draft Analysis of Centre Roof rail :

Draft Analysis of Bow Roof 2 :

Draft Analysis of Rear Roof rail :

Conclusion of the Project : The Heat Distortion Criteria Has been satisfied for all the sections of the design. And also, the Snow Load Criteria have been satisfied 3 out of 3 cases as well, so my design for Car Roof can be producible as per criteria. Hence, learned how to design a Roof of Car in the process.