Roof Design

AIM:

1. To create the Roof Sheetmetal design in NX cad, and also designing Front roof rail, rear roof rail, front bow roof , rear bow roof , center roof rail and Reinforcement are creating to build the strong roof.
2. Curvature study on Roof and performance calculations for Heat distortion and Snow load criteria to determine the position of the Bow row.
3. Finding the moment of inertia and section modulus for Bow-roofs and center roof rail.
4. Draft Analysis of each part.

INTRODUCTION:

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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.

Commonly, the roofs have reinforcements to provide strength to the overall structure and withstand the overhead forces like snow, or roll over etc.

DESIGN CONSIDERATION OF ROOF:

• Visibility criteria
• Head room clearance
• Bow roof position
• Heat distortion and Snow load criteria
• Draft analysis

REINFORCEMENTS :

If the roof is flat it is very weak and cannot able to with stand the forces like snow , so in order to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process

Different types of reinforcements that the car has are :

• Front roof rail
• Bow roofs
• Central roof rail and
• Rear roof rail

1. Front Roof Rail: Is one which joins the windshield glass, body side outer and the inner panel.

Sheetmetal thickness t = 0.75mm

Front Roof Rail Master section

2. Rear Roof rail: It helps in joining the rear door. The design of the rear roof rail differs by the opening method of the rear door.

Head room clearance = 120mm

Sheetmetal thickness = 0.75mm

Master section of the Rear Roof Rail

3. Center roof: It helps in providing support to the flat area of the roof. As the name goes, it is placed at the center of the roof between the front and rear roof rail.

Master section of Center Reinforcement Rail

4. Bow roof 1 & 2: These additional roofs called bow roof are introduced to improve the torsional stiffness and load bearing capacity of the roof structure. The number of bow roof used varies by many factors, one to mention is the roof length.

Master section of Bow Roof

GIVE STYLING SURFACE FOR ROOF:

ROOF CRUSH TEST :

Roof crush test is the design given by the IIHS ( insurance institute of highway safety ) . After analyzing the many accidents IIHS has then fixed some regulations , that how to perform the crush test .

IIHS gives the rating based up on this test . For this they consider two points while giving the rating

• How fast the car can reforms to its original shape after the roll over and
• How deep is the protrusion after the roll over

GOAL OF ROOF CRUSH TEST :

To minimize the protrusion into the occupant compartment , so that the seat belts and air bags can do there job in protecting the occupant

CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST :

There are two conditions that should be followed for the roof crush test , they are

INITIAL CONDITION :

• Close and lock all the doors
• Close all the windows

SECOND CONDITION :

Fix the body sill or chassis frame on a hard lever surface horizontally

METHOD OF APPLYING THE FORCE :

The load is applied in such a way that 5⁰ is tilted from the front end to the rear end and the load will be in 1800 mm in length and 750 mm in width . The load is applied at the position of 25⁰ from the flatness of the roof . The load is applied on the roof for almost 120 sec and the crush speed at which the load applied is 13 mm/sec or less than that

RESULTS :

If the displacements goes above the requirements level then we have to increase the bow roof thickness or add more mastics points to increase the strength . the stronger the roof the better it can protect the occupants in the roll over crashes.

CURVATURE STUDY ON THE ROOF:

• Heat Distortion
• Snow Load

HEAT DISTORTION:

The heat distortion study the plays an important role in Sheetmetal application. Heat distortion is temperature limit above which material cannot be used for structural application. This study is useful to fin the where the material starts to soften when exposed to a fixed load at elevated temperature. In order to avoid the bending, damage to the roof based on the heat distortion temperature. This study will provide the which position of the bow roof give enough strengthen to roof.

Bow Roof Prediction Formula:

HEAT DISTORTION CRITERIA:

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

Judgement Condition: OK<2.7

Where,

L= Roof Length in X- direction [mm](Roof Dimension in 0-Y)

R = Roof curvature

R = 2(R_X*R_Y)/(R_X+R_Y)

R_X = X Curvature

R_Y = Y Curvature

t = Roof plate thickness [mm]

I = Bow roof span [mm]

Thickness for the Roof(t):

Roof Outer Panel, t= 0.75mm

Front Roof Rail, t = 0.75mm

Rear Roof Rail, t = 0.75mm

Bow Roof Rail Front, t = 0.75mm

Bow Roof Rail Rear, t = 0.75mm

Reinforcement roof rail, t = 0.75mm

ROOF LENGTH (L):

BOW ROOF SPAN (I):

Here we want to take the distance from front roof rail to bow roof front for finding the bow roof span(I), we want to measure another bow roof span also :

BOW ROOF SPAN (I): Front roof - bow roof front = 332.0731 mm

BOW ROOF SPAN (I): Bow roof front - Centre roof = 390.1942 mm

BOW ROOF SPAN (I): Centre roof - Bow roof rear = 409.9886 mm

BOW ROOF SPAN (I): Bow roof rear - Rear roof= 445.3757 mm

R_X & R_Y (X & Y Curvature):

• From 0-Y axis, at -3Y about distance 300mm is drawn parallel on the perpendicular plane. After that 100mm lines are offset to either side of -3Y line. Now center lines are drawn between each adjacent roof rail edges respectively. Intersection points are marked on the sketch. Make all geometry lines are construction lines except points.
• Project the intersection points on sketch to the Roof surface.
• Join the intersection points in roof surface using the arc curve an measure the radius using the minimum radius comment.

FOR Rx

FOR Ry

FINAL RESULTS OF HEAT DISTORTION:

We have the necessary values for finding the heat distortion value(W):

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

R = Roof curvature

R = 2(R_X*R_Y)/(R_X+R_Y)

R_X = X Curvature

R_Y = Y Curvature

t = Roof plate thickness [mm]

I = Bow roof span [mm]

Apply all the values,

HEAT DISTORTION CRITERIA CALCULATIONS
ROOF RAILS	RX(MM)	RY(MM)	RX*RY	RX+RY	R(MM)	L(MM)	t(MM)	I(MM)	w	RESULT
Front roof- bow front	5741.05	2468.03	28338278.39	8209.09	6904.12	2029.64	0.75	332.07	2.37	ok
Bow front- Centre roof	5096.02	5595.82	28516410.63	10691.84	5334.24	2029.64	0.75	390.19	1.96	ok
Centre roof- Bow rear	4125.37	10187.17	42025845.50	14312.54	5872.59	2029.64	0.75	409.98	2.13	ok
Bow rear- Rear roof	3346.87	15788.23	52841153.34	19135.10	5522.95	2029.64	0.75	445.37	2.07	ok

From the above table we, conclude that all the values are of W< 2.7

also W> 3.1. So, that all bow roofs areas are placed at the right place.

SNOW LOAD CONDITION:

This test is done to know how the roof is behaving when there is a snow over. Normally due to the snow weight the dent will happen. But the roof should be designed in such a way that when the snow is removal the roof should go it its original position. This basic requirement for snow load criteria.

SNOW LOAD CRITERIA:

Q_r = [I_Y x t²] / [α x s x [(R_X + R_Y)/2]² x 10^-8]

Where,

α = M_y x L_X² x10^-12, M_{y =} Y(L_y–Y)

Judgement condition = Qr ≥ 3.1

250 < s < 380

t = Roof plate thickness[mm]

L_Y = 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 rails front/ rear as the front and rear reference point.

L_X= Distance between the left and right end of the roof on the roof bow[mm],

With of the roof panel exposed on the surface.

Y = Distance between Front roof rails to roof bow [mm],

s = Distance for which roof bow bears divided load [mm],

s = L₁/2 + L₂/2

I_y = Geometrical moment of inertia of roof bow (Y cross – section) [mm4],

R_X = Lateral direction curvature radius of roof panel Y cross section on roof bow [mm4],

Roof panel curvature radius of the length L_X in front view.

R_Y = Longitudinal direction curvature radius of the roof panel X cross section on roof bow[mm4],

Roof panel X curvature radius of length s in side view.

R_X & R_Y (X & Y Curvature):

• At 0-Y axis on perpendicular plane to roof. Draw 100mm lines offset to either side of the of 0Y axis line. Now center lines are drawn between each adjacent roof rail respective. Intersection points are marked on the sketch. Make all the geometry in construction lines except points.
• Project the intersection points in the sketch are projected to roof surface.
• Join the project points using arc curve and measure the radius value using minimum radius comment.

MOMENT OF INERTIA:

BOW ROOF 1 FRONT:

Therefore, we take the Minimum moment of inertia value is I_Y1 = 266.13 mm⁴

CENTER REINFORCEMENT RAIL:

We are taking the Hollow type for finding moment of inertia of the part. Therefore, reason is other two bow roofs are having solid structure. But center reinforcement roofs having the two solid part. So, that why we are joining to them and check for moment of inertia.

Minimum moment of inertia for center reinforcement rail is I_Y2 = 350.95mm⁴.

BOW ROOF 2 REAR:

We take the Minimum moment of inertia value is I_Y3 = 257.09mm⁴.

Minimum moment of inertia for center reinforcement rail is I_Y3 = 350.95mm⁴.

FINAL RESULTS OF SNOW LOAD CRITERIA:

We have the necessary values for finding the Snow load value(W):

S.NO	ROOFS	L	t	Rx	Ry	{(Rx+Ry)/2}2	Lx	Ly	Y
1	FRONT ROOF TO BOW ROOF 1	2029.64	0.75	4626.73	2466.57	12578726.22	1141.40	1952.38	430.71
2	BOW ROOF 1 TO CENTRE ROOF	2029.64	0.75	3753.05	5594.84	21845761.86	1091.87	1952.38	899.13
3	CENTRE ROOF TO BOW ROOF2	2029.64	0.75	4196.43	10186.64	51718175.65	1062.20	1952.38	1386.62
4	BOW ROOF 2 TO REAR ROOF	2029.64	0.75	4103.67	15788.09	98920528.97	1045.52	1952.38	1952.38

S.NO	L1	L1/2	L2	L2/2	s =L1/2+L2/2	My= Y(Ly-Y)	α= {My * Lx2 * 10-12 }	Iy	Qr	CONDITION
1	430.83	215.415	470.24	235.12	450.53	655398.48	0.8538	266.13	3.1	OK
2	470.24	235.12	489.97	244.98	480.10	947008.67	1.1290	350.95	1.66	NG
3	489.97	244.98	568.79	284.39	529.37	784494.13	0.8851	257.09	0.59	NG
4	568.79	284.39	NA	NA	NA	NA	NA	NA	NA	NA

WHAT IF NG:

STEP 1:Try to provide embosses on top surface of the roof.so here i have provided the embosses even though I got two values that are not satisfying the Qr condition.But one values satisfied the Qr conditions

Qr ≥ 3.1

WHAT IF AGAIN NG:

STEP2:If we are getting again NG we have to make trial and error methods for the radius of curvature values.One important point is curvature value should be either gradually increasing or decreasing.But it should not be like sudeen high or drop values.

(OR)

we should interact with styling team about the concern we can share our value.They will check if it is possible or they will come up with their own suitable values.

DRAFT ANALYSIS:

The Draft Analysis command enables you to detect if the part you drafted will be easily removed. This type of analysis is performed based on color ranges identifying zones on the analyzed element where the deviation from the draft direction, at any point, corresponds to specified values.

Minimum draft angle of 7⁰ is considered for analysis. Green color infer on the parts that all face along the tooling direction has positive draft angle greater than 7⁰ and passed in analysis

Front roof rail:

Rear roof rail:

Center roof:

Bow roof 1:

Bow roof 2:

VIEWS OF ROOF ASSEMBLY:

CONCLUSION:

I got outer roof skin data as input.By using it I created the Roof assembly consist of Front roof, Rear roof, Bow roof 1, Bow roof 2, Centre roof with support rail.Also we have founded the Heat distortion criteria and Snow load criteria.Draft Analysis also made whether the roof is meeting minimum draft value of 7degree.Also I have attached Zip file for the reference.