Challenge

Roof challenge

VVishnu S19 Jul 2022

ROOF DESIGN

OBJECTIVES:-

Design the roof from the given styling surface and also create the front, rear and bow roof rails.
Study the roof design and validate the design by doing the heat distortion and snow load criteria.
Perform the draft analysis and find the section modulus.

INTRODUCTION

An automobile roof or car top is the portion of an automobile that placed above the passenger compartment, protecting the vehicle occupants from sun, wind, rain and other external elements. Though it just looks like a styling surface that protects from sun, wind, and other elements, various environmental conditions are taken into account for designing the same. The roof should be provided with proper reinforcements for strengthening. Vehicle roofs can be subjected to high heat conditions or load during snowy conditions. To make the roof withstand these conditions, various calculations are done. This ensures the safety of the passengers in the vehicle.

Some of the common roofs are as follows:

Sun Roofs
Moon Roofs
Panoramic Roofs
Convertible Roofs
Hardtops
Vinyl Roofs

DESIGN SONSIDERATIONS OF ROOF

Visibility criteria
Crashworthiness of Roof

This is a test done on the roof to find the strength-to-weight ratio (SWR) and rate the given car roof. Roof strength evaluations are conducted on a quasi-static test system manufactured by MGA Research Corporation. Once the vehicle is positioned correctly, the roof is crushed to a minimum displacement of 127 mm at a nominal rate of 5 mm/second. The maximum force prior to 127 mm of platen displacement is divided by the measured curb weight to obtain the SWR.

Head room clearance
Curvature study
Hear distortion and snow load criteria

Generally, bodies tend to deform when subjected to change in temperature. So, when we design a roof we need to make sure that the roof is able to withstand the extreme climatic changes. Heat distortion criteria tell us if the roof is passing this test or not.

During extreme snow conditions, the roof tends to bend or deform with a heavy application of the snow load. So, when the load due to the snow is removed the roof should go back to its original shape. To check this condition snow load criteria are used to validate the roof.

Draft analysis.

Draft analysis needs to be done for the part after all the design is completed to know whether the part is producible by deep drawing. There should always be a positive draft for the part to be producible. For sheet metal, the draft angle is taken as 7 degrees and sometimes around 5 degrees is also accepted.

MANUFACTURING ASPECTS

Basically, the roof rails and the roofs are joined with the help of mastic and spot welding.
The front most end is joined by spot weld and the backside will be joined by the mastics.

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 JOINTS

Mastic in automotive sheet metal generally used to bond two different sheet metals by using additives and sealant. it is very helpful to reduce vibration between the sheet metal part.

DESIGNING THE ROOF

Front Roof Rail

The front roof is the part of the roof which connects the windshield and the inner panel of the hood and the bodyside outer.
The front roof panel is created using the master section sketch given below.
Visibility, clearance between driver head and front roof rail backside edge.on the ditch area, there will be some extra components added to fix the windshield. Components which made for absorbing shocks (or) vibration.

Master Section For Front Roof Rail

Rear Roof Rail.

The rear roof rail is the support that joins the back door of the car and the bodyside outer

while designing the rear roof rail we have to consider the backside view clearance so the driver able to see backward through the windshield. which is called rear visibility criteria.
Same as the front roof rail, the rear roof bottom flange is taken considering the headroom clearance.

Master Section For Rear Roof Rail

Bow Roofs.

The bow roofs are given to improve the torsional stiffness and load bearing capacity of the roof structure. The number of bow roofs are given by the roof dimension. Longer the roof area we have to give the more bow roof to the structure.
Positioning of the bow roofs are decided by the curvature study. The Heat distortion and Snow load criteria are checked to analyse the bow roof position by various condition
Same like center rail the bows are joined using the mastic sealants and in the ends with the nuts and bolts or spot welds.

Master Section For Front And Rear Bow Roof Rail

Center roof rail.

Center roof rail is the reinforced support added to the roof to provide structural strength
For the center roof also headroom clearance is also taken into consideration.

Master Section For Center Bow Roof Rail

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

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< 2.7

Measuring L

Measuring Rx

Measuring Ry

Measuring I

BOW	Rx	Ry	R	I	L	T	R^2/t	W	Ok/NG
FRR – 1 st BRR	5735.17	2265.75	3248.24	356.3	2036.9	0.75	14067737.7	1.49	OK
1 st - CBRR	4958.21	6249.87	5529.61	500	2036.9	0.75	40768782.3	2.14	OK
CBRR – 2 nd BRR	11204.4	3856.05	5737.50	500	2036.9	0.75	43891875	2.20	OK
2 nd BRR - RRR	3155.85	15414.8	2619.55	350	2036.9	0.75	9149389.6	1.39	OK

Since heat distortion criteria satisfied all the components no changes is required. But creating embosses on front and rear roof rails increases strength for the respective components.

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

Measuring Lx

Measuring Y

Measuring Rx

Measuring Ry

Measuring Iy

Bow Roof Front

Bow Roof Rear

Center Bow Roof

BOW	Iy	α	Lx	Ly	Y	Rx	Ry	t	Qr	OK/NG
FRR – 1 st BRR	39.62	0.899	1158.42	2036.95	412.72	5008.74	2351	0.75	3.72	OK
1 st - CBRR	47.97	1.025	1100.29	2036.95	580.93	3712.00	6160	0.75	1.86	NG
CBRR – 2 nd BRR	39.11	0.965	1069.17	2036.95	579.85	2738.72	11854.37	0.75	0.96	NG
2 nd BRR - RRR	-	0.678	1055.84	2036.95	363.62	3553.04	17586.81	0.75	-	-

From the above values, we can say that snow load criteria is not valid between Front bow roof and center bow roof and also in between center and rear bow roof rail.

Now to improve the part we have two solutions:

Strengthen the outer panel by creating embosses in the regions where the part is failing
By repositioning the roof rails and increasing the moment of inertia.

As the second step is difficult we prefer to do the embossing part. If the manufacturer does not want embosses on the outer panel second method should be followed.

Draft Analysis

Roof Outer Panel