Roof challenge

Introduction:

Design a roof using the provided skin data and also position bow roof rails in accordance with heat distortion and snow weight formula also find section modulus of the roof and roof rails and check the draft of the designed parts.

ROOF:

An Automobile roof is the portion of the automobile that sits above the passenger compartment and protects the occupants from sun wind, rain and other external elements. While designing a car roof forces due to impact, falling objects, vehicle rollover, temperature, roof loading aesthetics etc. must also be taken into account. There are different types of roofs, the most prominent are as follows

• Hardtops
• Soft tops
• Targa tops
• Glass roofs
• T-tops

Some roofs have a window on them and are called sunroofs, while convertibles have special mechanisms that allow the roof to be retracted and be stowed away through a wide array of methods depending on the design.

The roof of the car is made up of multiple parts that provide support and strength to the roof component. The actual roof sheet and the roof rails which support the roof sheet. There are 3 primary roof rails in a car the front, the centre and the rear, while additional roof rails are called Bow roof rails. The number of bow roof rails depends on the size of the roof. Headroom, visibility criteria and target region also play a key role in roof design.

Provided skin of the roof top:

Roof sheet

The roof is made from a sheet of 0.75mm. This protects the cabin from external conditions and objects. The ditch areas are spot welded to the pillars of the car and certain areas of the roof rails, welding on the upper surface is avoided in order to maintain a good aesthetic

Front Roof Rail:

The front roof rail is made from a sheet of at least 1mm thickness. This part acts as a support for the roof sheet on the front it also provides a mounting area for the windshield. It is designed with support requirements, visibility criteria and headroom space in mind. The roof is fixed to the roof rails with the use of mastics while the ends are spot welded.

Rear Roof Rail:

The rear roof rail is made of a sheet of at least 1.2mm thickness. The rear roof rail acts as a support for the rear end of the roof and also supports the rear window in some cases also supports the rear door. It's designed in accordance with the support requirements of roof, window and rear door, and also rear visibility and headroom criteria. The roof is fixed to the roof rails with the use of mastics while the ends are spot welded.

Bow Roof Rail:

The bow roof rails provide additional support to the roof and are at least 1mm thick, they are positioned in such a way that they provide maximum support to the roof, Care must be taken so that they do not intrude into the headroom requirements of the car. Bow roof rails are an optional feature provided as required by the design. Small cars may have none while large ones may have 3 or more, they may also be integrated into the centre roof rail in some cases. The roof is fixed to the roof rails with the use of mastics while the ends are spot welded.

Curvature study on the roof and perform calculations to determine the position of the Bow-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

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]

	SPAN 1	SPAN 2	SPAN3
L(mm)	1923.11	1923.11	1923.11
Rx(mm)	4369	3756.94	3331.32
Ry(mm)	13754.13	11727.57	8693.79
t(mm)	0.75	0.75	0.75
R(mm)	6631.50	5690.81	4816.88
R^2/t(mm)	58626881.33	43180424.60	30936443.91
l(mm4)	333.40	264.11	222.41
W	2.09	1.72	1.45

From the values of "W", we can see that All values are within distortion limits, therefore, the roof design is cleared to the next stage.

Snow Load condition:

The roof of a vehicle is normally not designed to hold up weight but for regions with snowfall, it must be properly reinforced to hold up the weight of accumulated snow on the roof, if done improperly it may cause the roof to bend and intrude into the passenger compartment which is dangerous. This calculation is done in order to check if the location and strength of the roof rails are enough to avoid plastic deformation of the roof.

The formula for the snow load analysis is as follows

Qr = [Iy * t²] / [α *s* [(Rx + Ry)/2]2* 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 * Lx² * 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 front and rear reference point.

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 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

	SPAN 1	SPAN 2	SPAN 3
Ly(mm)	1923.11	1923.11	1923.11
t(mm)	0.75	0.75	0.75
Iy(mm4)	661.59	87.1
Y(mm)	383.63	308.90	392.91
My(mm2)	590590.7124	4,98,629.469	6,01,230.882
Lx(mm)	1068.4	1053.3	1044.9
α(mm4)	0.6741	0.5531	0.6564
L1(mm)	383.63	308.90	392.91
L2(mm)	308.90	392.91	-------
S(mm)	346.25	350.90	196.45
Rx(mm)	4042.58	4331.90	4086
Ry(mm)	8780.84	12466.86	15928.08
Qr	3.88	0.362

In the span 2 we can see that the value of the Qr is less than 3.1 and NG is achieved. To improve the NG condition we do the emboss on this section of the roof.

Designing of Roof Top after emboss used to satisfy the snowed conditions of the bow roof rails.