Roof Design

ROOF DESIGN

Roof is not a single element, it consists of Front roof rail, Rear Roof Rail, Bow Roof Rail, Center Roof Rail. These Bow Roofs are placed in the most Flat area of the Panel. Rear Roof Rail joins with the Back door and Body side outer. Front Roof Rail joins with the Wind shield glass and body side. Here are some reinforcement used in the Roof.

ROOF CRUSH SYSTEM:

Here Crush test system is shown which is manufactured by MGA Corporation. It has an upright assembly and attached loading head that can be fixed at varied heights. Pitch angle can be varied by +5 to -5 degrees. But the roll angle is fixed to 25 Degree. Hydraulic actuators are used to control the movement of plate.

ROOF CRUSH TEST:

A rectangular block measuring 30 inches wide and 72 inches long is used to apply the load on car roof with 1.5 times the unloaded vehicle weight with different angle inclination to rectangular block. The moving distance of roof structure must be less than 127mm or 5 inches as per FMVSS standards.

FRONT ROOF RAIL:

This will be a supporting room lamp for some cars. We need to consider the Visibility criteria line for designing Front roof rail.

REAR ROOF RAIL:

Here we need to keep Head room clearance and rear view criteria for designing. Back door will be mounted here.

BOW ROOF AND CENTER ROOF RAIL

The bow roofs are given to improve the torsional stiffness and load bearing capacity of the roof structure .The number of bow roof present depends on the overall size of the roof . Presently I have added two bow roofs.

Centre roof rail helps in providing effective support to the flat area of the roof as it is more susceptible to failure under the action of load. generally the center roof rail is placed at the center of the roof and is connected to the B-pillar structure.This helps in adding the strength to the roof during the rool- over test.

Complate the assembly.

After complate assembly next doing thicken operation.

DRAFT ANALYSIS :

Draft analysis is done in NX software. We can change the draft angle and analyze is it clearing the required criteria. It is shown is different color zones. So identifying it will be easier. It will let us know that the part can be removed easily during manufacturing or not. Minimum draft angle of 7 Degree is considered for analysis. green color infer on the parts that all face along the tooling direction has positive draft angle greater than 7 degree and passed in analysis.

Front roof

Rear roof

All Bow roofs

Moment of inertia

Moment of inertia is defined as the quantity expressed by the body resisting angular acceleration which is the sum of the product of the mass of every particle with its square of a distance from the axis of rotation. Section moment of inertia is used to predict the amount of deformation and deflection of a part. Higher the moment of inertia lesser the amount of deflection. Thickness, flanges and embosses will increase the section inertia. Here NX software used to find the moment of inertia using section inertia Analysis.

Here is the value for bow roof 1. Iy(min) = 51.633372040mm^⁴

Here is the value for Center roof. Iy(min) = 243.139652247mm^⁴ .

Here is the value for bow roof 2. Iy(min) = 133.381475452mm^^4.

Here is the value for Front bow roof = 8.247512792+10^3mm^^4.

Here is the value for Rear bow roof = 5.005992296+10^4mm^^4.

HEAT DISTORTION CRITERIA:

The heat distortion test evaluates the ability of a car roof to maintain its physical structure without deformation under elevated temperature conditions. Car roofs are exposed to sunlight at day time. Distortion of the car roof can create more susceptibility to damage.

How to Calculate?

Here we have a formula for calculating it.

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

SL. NO	BOWS	RX	RY	R	I	L	T	R^2/T	W	REMARK
1	FR &1ST BOW	5721.25	2785.01	3746.355	427	2033	0.75	18713567.7	1.65	OK
2	1ST & CTR	4921.51	5127.64	5022.46	424	2033	0.75	33633472.6	1.92	OK
3	CTR& 2ND	3996.97	10482.02	5787.18	391	2033	0.75	44655269.8	2.09	OK
4	2ND&REAR	3349.23	14138.12	5415.55	382	2033	0.75	39104242.4	1.98	OK

SNOW LOAD CRITERIA :

Unlike in India there are countries with heavy snowfall. In Alaska, it was recorded a record-breaking 78" in 24 hours in 1963. So calculating and designing the roof of the car in such a way this snow does not cause serious damage to the car.

We can calculate with the below-given formula.

Snow Load Prediction Formula

Qr = [Iy x t^2] / [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]

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 RoofBow [mm]

Roof panel X curvature radius of length s in Side view

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

SL.NO	BOWS	Iy	a	Lx	Ly	Y	S	Rx	Ry	T	Qr	DETERMINATION
1	FR & 1ST BOW	51.633	0.96	1139.6	1969	510	493	5532.59	2846.21	0.75	3.49	OK
2	1ST & CTR	243.13	0.84	1087.8	1969	476	460	5098.89	15212.75	0.75	3.43	OK
3	CTR & 2ND	133.38	0.76	1062.5	1969	444	491	4028.70	9320.67	0.75	4.51	OK
4	2ND & REAR	-------	0.84	1047.1	1969	538	-------	3773.33	11206.78	0.75	------	-------------