ROOF DESIGN 

AIM

To a given roof styling with master sections, create essential flanges and reinforcement.

 

OBJECTIVE

1. To design the automotive roof based on the given styling surface and to analyze the prepared model to check the manufacturability and the feasibility of the design.

2. To perform the section modulus study for the master sections and create for front roof rail, Bow roofs and centre roof rail.

3. To create the Ditch area and to perform the draft analysis.

INTRODUCTION TO ROOF

An automotive roof or car top is the portion of the automobile that sits above the passenger compartment, protecting the vehicle occupants from Sun, Wind, Rain and other external elements.

Under the styling surfaces that’s is seen from the outside, there are many other structures under the roof that contributes to its strength and stability. The number of such reinforcement structures/supports differ with respect to the size of the vehicle. They are primarily installed on the flatter areas of the roof to enhance the strength.

They are the Front Roof Rail, Rear Roof Rail, Center Rail and Bow Roof Design which are given Mastics and spot welded to the roof flanges & ditch are and improve NVH value by increasing the strength of the roof of using mastics sealants.

TYPES OF CAR ROOFS

 

1. Convertible roofs.

2. Hard Tops

3. Sun roofs.

4. T- Tops.

5. Target tops.

6. Vinyl roofs.

DESIGN CONSIDERATION OF ROOF

• Visibility Criteria

• Head Clearance

• Curvature Study

• Heat Distortion and snow load criteria

• Draft Analysis

• Safety Parameters

ROOF MASTICS

Mastic points are given on the various points of the roof to join the inner and outer panel of the roof. It also develops Good NVH for the car. Mastic sealants provide strength locally around 80mm of its point of application. So we need to order the points in a way that it is evenly spread over the roof so that we can increase the strength of the roof. Mastic sealants applied on the roof will undergo a hot bath where it expands and fills the 80mm of its surrounding. 

SPOT WELD

Spot welding is a type of electric resistance welding. It is also known as resistance spot welding. The process of spot welding is sheet metals are made into contact with each other and pressure is applied on specified points. So heat is generated from the resistance and a weld is formed. Here below you can see the points to be spot welded in the roof panel.

 

Spot welding prevents the original intended design of the component. Here below you can look at a real time view of spot welding on the roof.

ROOF CRUSH TEST FOR PASSENGER CAR

Safety point of view vehicle roof plays a very crucial role especially roll over kind of accidents, In which vehicle tips over onto its side of the roof Vehicle rollover crashes are the cause of many fatalities and head, neck and spine trauma around the world.

Strength of the roof structure to occupant protection in real world rollover crashes, requiring the maximum moving distance of the roof structures should be less than 127 mm, (5 in), where the induced load is 1.5 times the vehicle weight.

 

For the test, the passenger car shall be rigidly placed or positioned on a horizontal surface with the doors locked and the window closed. Furthermore, a flat, rigid block with a lower surface a rectangle measuring 30 inches wide by 72 inches long (762mm x 1829 mm) shall not move more than 5 inches (127mm), measured as the distances between the original location of the lower surface of the test device and its location as the specified force level is reached, when it is used to apply a forces of 1.5 times the unload vehicle weight (UVW) or 5000 lbs. (22224N), whichever is less. To either side or forward edge of the vehicle.

This test can be conducted on either side of the roof structure, the front left or the front right side but not on both sides in one single test and still meet the requirement of the test. The rigid plate shall not move faster than 0.5 inches/second and the force applied on the plate shall not exceed the lesser of 1.5 times the unloaded vehicle weight (UVW) expressed in kilograms multiplied by 9.8 or 5000 lbs (22240N).

Also, the direction of the force shall be downward and perpendicular to the lower surface of the rigid plate and such that it moves in a straight line without rotation. The duration of the test shall not exceed 120 seconds. The longitudinal axis of the plate is pitched forward at a 5’ angle as viewed from the side of the passenger car as shown,

The lateral axis or the roll of the plate is such that it forms a 25’ outboard angle with a horizontal surface as viewed from the front of the passenger car as shown in below, fig

 

JOINING PROCESS

Mastics are introduced between the roof outer panel and the reinforcements for joining. Wherever the mastics are introduced there is remarkable strength. Spot welding is also introduced to join the reinforcement and the outer panel.

 

REINFORCEMENT AND ITS TYPES

If the roof is flat, it is weak and cannot able to withstand the external load like snow load, roll over and etc. so in order to increase the strength of the roof, reinforcement like front roof rail, central roof rail, rear roof rail and the bow roof are introduced and joined in the flat surface of the roof with different joining process. In general, there are four types of reinforcement are used in the hard roof top they are,

 

• Front roof rail

• Rear roof rail

• Centre roof rail

• Bow roof rails

* 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.

* CENTRE ROOF RAIL

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 centre roof rail is placed at the centre 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.

* BOW ROOF RAILS 

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 three bow roofs.

Different Views of Complete Roof Design

DESIGN CALCULATION OF ROOF

 

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

• Heated Distortion study on the roof

• Snow Load Condition

Heated Distortion Criteria study on the roof :

The heat distortion study plays a major role in sheet metal usage. Heat distortion temperature is the temperature limit above which the material cannot be used for structural applications. This study is used to predict the heat distortion temperature at the material where it starts to soften when exposed to a fixed load at elevated temperature. In order to avoid bending or damages on the roof, based on the heat distortion temperature, this study will predict the bow roof position which helps to strengthen the roof.

Here we have a formula for calculating it.

      W= [ 1.73 * (10^-3) * L ] + [ 1.85 * (10^-8) * ((R^2)/t) ] + [ 1.10 * 10^(-3) * I ] – 2.68

Where,

L = Roof strength in X- Direction [mm] (Roof dimension in 0 – Y)

R = Roof curvature = 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>

Here is the detailed procedure to measure and calculate.

Step 1: Open sketch in NX and Draw a centerline which bisects the roof into two halves. This is the 0-Y line Shown in the picture.

Step 2: Draw a Line offset to the 0-Y line for 300mm. This is the 3-Y line shown in the above picture. 

Step 3: Draw two lines at both the sides of the 3-Y line with an offset value of 100mm.

Step 4: Draw a mindpoint with Front roof rail end point on 0-Y line to start point of Bow Roof 1 on 0-Y line. This is the I(C-1) section in the above picture. We need to do it for I(C-2), I(C-3), I(C-4) and I(C-5)

Step 5: Draw a Vertical line with the midpoint of I(C-1). [Similarly do it for I(C-2), I(C-3), I(C-4)and I(C-5)].

Step 6: Draw two lines on both the sides of the I(C-1), I(C-2), I(C-3), I(C-4) and I(C-5) with 100mm as offset value.

Step 7: Draw intersection points on the created 100mm offset horizontal and vertical lines. As shown in the above picture.

Step 8: Project the points on the surface of the roof and measure the radius of Rx and Ry. You can see the Rx (vertical) and Ry (Horizontal) in the picture given.

Heat Distrotion Criteria Table

S. NO	Bow Roof	Rx	Ry	R	t	l	L	R^2/t	W	Result
1	Front to Bow 1	5541.69	1350.91	2172.28	0.75	196.01	2033.7048	6291733.865	1.1703	OK
2	Bow 1 to Centre	5313.27	3341.73	4102.95	0.75	244.4	2033.7048	22445598.27	1.5223	OK
3	Centre to Bow 2	4633.2	6154.08	5286.43	0.75	248.1	2033.7048	37261789.53	1.8006	OK
4	Bow 2 to Bow 3	3840.23	9695.66	5501.46	0.75	429.01	2033.7048	40354749.51	2.0567	OK
5	Bow 3 to Rear	3123.63	13871.73	5099.06	0.75	375.8	2033.7048	34667217.18	1.8929	OK

By Following the Procedure and using the formula, We got the W value of below 2.7. And thus our Result is OK. You can look at the table for more details.

This study on Heat distortion criteria is used to predict the heat distortion temperature at where the material starts to soften when exposed to a fixed load at elevated temperature .By this way we can avoid bending or damages on the roof, based on the heat distortion temperature. Here we will predict the bow roof position which helps to strengthen the roof.

Snow Load Criteria study on Roof:

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

 

Qr = [ Iy * (t^2)] / [ α * s * {(Rx + Ry) / 2}^2 * (10^-8)]

Where,

My = Y (Ly- Y) and α = My x L x 2 x 10-12, 

Judgment Condition = Qr > 3.1

 

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 centre 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 Roof Rail to Roof BOW [mm]

s = Distance for which roof Bow bears divided load [mm] = L ½ + 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]

Here is the detailed procedure to measure and calculate.

Step 1: Open sketch in NX and Draw a centerline which bisects the roof into two halves. This is the 0-Y line Shown in the picture.

Step 2: Draw a 100mm offset line at both sides of the 0-Y line. 

Step 3: Draw a midpoint in the Front roof rail, Bow roof 1, Center roof rail, Bow roof rail 2, Bow roof 3 and Rear roof rail.

Step 4: Draw a midpoint between all the sections. Like between front roof rail and bow roof 1, between bow roof 1 to centre roof rail, centre roof rail to bow roof  2, bow roof 2 and bow roof 3, bow roof 3 and Rear roof rail.

Step 5: Draw a vertical line on all the midpoints between sections we previously created.

Step 6: Draw an offset line with 100mm at both the sides of the midpoint vertical line drawn.

Step 7: Draw intersection points on the 0-Y, Midpoint vertical line and 100mm offset lines. As shown in the picture.

Step 8:  Project the points on the roof and the radius measured between these intersection points are Rx and Ry. Rx is the X curvature radius and Ry is the Y curvature radius.

Step 9: Draw intersection points of Vertical midpoint lines and the outer roof edge.

Step 10: Project the points on the roof and the distance between these two points is the Lx value mentioned in the formula.

Step 11: Ly is the distance between the centre of Front roof rail to the centre of rear roof rail along the 0-Y line shown in the picture.

Step 12: measuring Y differs with sections. Say If you are calculating My between Front roof rail and bow roof 1. Y will be the distance between Front roof rail to the bow roof 1. If you are calculating between bow roof 1 and centre roof rail, Y will be the distance between front roof rail to centre roof rail. So every Y value will be measured from the front roof rail.

Step 13: L1 and L2 also differ with sections. Say if you are calculating S value between Front roof rail and Bow roof, L1 will be the distance between Front roof rail and Bow roof 1 and L2 will be the distance between Bow roof rail 1 and centre roof rail. If you are calculating S value between Bow roof 1 and centre roof rail, L1 will be the distance between Bow roof 1 and centre roof rail and L2 will be the distance between centre roof rail to bow roof rail 2 and vice versa.

Step 14: Iy (moment of inertia) will be the value generated from NX for bow roof 1, bow roof 2, bow roof 3 and centre roof rail . More Details on Moment of Inertia is mentioned later in this report.

Step 15: measure the required lengths and radius and fill in the formula to attain the result.

Snow Load Criteria Table

S. NO	Bow Roof	Rx	Ry	Lx	Ly	t	Y	S	My	α	Iy	Qr	Result
1	Front to Bow 1	3470.1292	1500.2542	1158.8269	2033.7048	0.75	289.1	313.8	504365.247	0.677301	7501.625	321.40619	OK
2	Bow 1 to Centre	2622.2481	3371.3551	1113.1339	2033.7048	0.75	627.7	340.4	882549.213	1.093537	9833.883	165.46592	OK
3	Centre to Bow 2	3284.2904	5998.2342	1084.1185	2033.7048	0.75	969.9	425.5	1031784.276	1.212669	7501.769	37.96402	OK
4	Bow 2 to Bow 3	4214.6698	10011.3004	1061.6331	2033.7048	0.75	1478.7	507.3	820685.597	0.924965	6628.933	15.70624	OK
5	Bow 3 to Rear	3339.4847	15704.3427	1046.2207	2033.7048	0.75	1984.5	-	97646.9256	0.106882	-	-	#DIV/0!

By Following the Procedure and using the formula, We got the Qr value of Above 3.1. And thus our Result is OK. You can look at the table for more details.

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) =  7501.6255 mm^4

• Here is the value for Center roof. Iy(min) =  9833.8832 mm^4

• Here is the value for bow roof 2. Iy(min) =  7501.7699 mm^4

• Here is the value for bow roof 3. Iy(min) =  6628.9326 mm^4

DRAFT ANALYSIS :  

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

 

Here are the parts that are made as per the master section and also clearing the draft analysis.

• Draft analysis for Front roof rail

• Draft analysis for centre reinforcement roof rail

• Draft analysis for Rear roof rail

• Draft analysis for all rails

• Draft analysis for Outer Roof structure

CONCLUSION

Thus the required reinforcement and the ditch area for the roof were designed and developed from the given styling. The curvature study was done after performing calculation for the Heat Distortion and Snow load conditions with the position criteria met for all reinforcements.

 

Further the section modulus study was conducted and Draft analysis was done for each reinforcement successfully.