Challenge

Roof challenge

DDinesh Kesavan24 Apr 2022

Roof Design:

An automobile roof or car is the top portion of an automobile that sits above the passenger compartment, protecting the

Vehicle occupants from sun, wind, rain and other external elements.

The roof of a car was designed with supporting rails & embosses. The distance between the rails / Bow supports was calculated

using Heat distortion & snowed condition criteria.

Roof Assembly consist of:

Roof outer panel
Front Roof Rail
Bow Roofs
Center reinforcement Roof Rail
Rear Roof rail.

Front roof Rail:

Rear Roof Rail:

Bow Roof Rail 1:

Center Rail forcement:

Bow Roof Rail 2:

Design calculation of roof

To make sure the roof passes the Analysis test some of the criteria to be considered before the design of bow rails under the roof.

They are,

Heat distortion Criteria
Snowed Criteria

Heat Distortion criteria:

The heat distortion study plays a major role in sheet metal usage. Heat distortion temperature is a temperature is a temperature

Limit above the material cannot be used for the structural applications.

The heat radiating from the sun causes the roof of the car to change in the metallic regions. Heat distortion criteria

Indicate what temperature material start to “soften” when exposed to a fixed load at elevated temperatures.

BOW – Roof prediction formula

W= [1.73 x 10^(-3) X L] + [1.85 X 10^(-8) X (R^2)/t] + [1.10 X 10^(-3) X I] – 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]

I = Bow Roof Span [mm]

Judgement Condition : OK< 2.7 < NG

Heat Distortion Criteria
S.no	BOW Position	Rx	Ry	R	I	L	t	(R^2)/t	W	OK/NG
1	FR to 1st	5711	2781	3,740.5	473.5	2111.5	0.75	1,86,55,120.33	2.11	OK
2	1st to Center	4814	6832	5,648	364	2111.5	0.75	4,25,33,205.33	2.15	OK
3	Center to 2nd	3914	10903	5,760	413.5	2111.5	0.75	4,42,36,800	2.24	OK
4	2nd to RR	3204	14380	5,240	318.5	2111.5	0.75	3,66,10,133.33	1.99	OK

Snow load criteria:

This test is done to know how is the roof behaving when there is a snow over.

Normally due to snow weight the dent will happen. But the roof should be designed in such a way that when

The snow is removed the roof should go to its original position without permanent dent mark.

This is basic requirement for snow load criteria.

Qr = [Ly 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)

Judgement condition = Qr ≥ 3.1

250 ≤ s ≥ 380

t = Roof plate thickness

Ly = Distance between the front and rear roof rails on the vehicle along 0Y.

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) (mm^4)

Rx = Lateral direction curvature radius of roof BOW ( Y cross-section) (mm^4)

Ry = Longitudinal direction curvature radius of the roof panel X cross-section on roof BOW(mm)

Snow Load Criteria
S.no	BOW Position	Rx	Ry	Iy	My	L1	L2	Lx	Ly	t	α	Y	S	Qr	OK/NG
1	FRR TO 1st	5711	2781	23629	878313	570.6	457	1134	2111.5	0.75	0.00199	570.6	484	7.36	OK
2	1st to Center	4814	6832	16446	756106.5	457	494	1082	2111.5	0.75	0.0016	457	475	7.2	OK
3	Center to 2nd	3914	10903	23961	799045	494	470	1057	2111.5	0.75	0.00168	494	482	5.38	OK