Challenge

Roof Design

MMohana Gowtham4 Apr 2023

ROOF DESIGN

INTRODUCTION

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 glas and body side. Here are some reinforcement used in the Roof.

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.

MANUFACTURING PROCESS:

Outside plate can be manufactured with the following process, first the open straight snips plate: first select applicable cold coiling specification, cold coiling is opened flat. Then according to production material specification it will be opened after flat and be cut into the iron plate of corresponding size. Finally the iron plate after shear is cleaned into iron filings. Then its trimmed and punched. Then its flanged and punched. Finally, after punching completed it is carried out to the operation of side folding.

Bar shaped assembly and ring-shaped component are processed by blanking, stamps out iron plate the outline blank of many roof cant rails, a ring-shaped component and a bar shaped assembly.

ROOF MASTIC :

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 provides strength locally around 80mm of its point of application. So we need order the points in a way that the 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 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 eachother and pressure is applied on specified points. So heat is generated from the resistance and 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:

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.

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.

POSITIONING OF BOW ROOF:

Bow roofs are designed to give strength to the roof panel. It avoids the roof to bend due to snow load. Number of bow roofs needed are estimated with the design of the roof panel. if the length of the roof is large we need more bow roof and if the roof panel is lesser in length we need ideal number of bow roofs.

The positioning of Bow Roofs depends on the curvature of the roof panel. We add bow roof on flat region as bending is more possible here than the curved portion of the roof. Along with this we have Heat distortion criteria and Snow load Criteria for postioning the Bow roof.

HEAT DISTORTION CRITERIA:

The heat distortion test evaluates the ability of 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 susceptible 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

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) and I(C-4).

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

Step 6: Draw two lines on both the sides of the I(C-1), I(C-2), I(C-3) and I(C-4) 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.

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

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 t2] / [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 crosssection on RoofBow [mm]

Roof panel X curvature radius of length s in Side view

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

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.

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

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 center of Front roof rail to the center 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 center roof rail, Y will be the distance between front roof rail to center 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 center roof rail. If you are calculating S value between Bow roof 1 and center roof rail, L1 will be the distance between Bow roof 1 and center roof rail and L2 will be the distance between center roof rail to bow roof rail 2.

Step 14: Iy (moment of inertia) will be the value generated from NX for bow roof 1, bow roof 2 and center 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.

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

This test is done to know about the roof behavior when there is snow over the car roof. This snow weight will cause a dent on the roof. 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 requirements for snow load criteria.

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) = 1994.5261 mm4

Here is the value for Center roof. Iy(min) = 4732.094782 mm4

Here is the value for bow roof 2. Iy(min) = 4159.336964 mm4

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.

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