Roof Design

Roof Design:-

Aim:

Design a Car Roof with its components by following the master section of the front roof rail (Front roof rail, bow roofs, centre roof rail and rear roof rail) with all the design parameters.

OBJECTIVE-

• The objective of this project is to design a car roof with essential flanges and reinforcements like ditch area, front roof rail, bow roofs, central roof rail, and rear roof rail for the given styling surface
• Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct positions.
• Finding a moment of inertia and section modulus for Bow roofs, central roof rail. front roof rail and rear roof rail.
• Perform the draft analysis for each reinforcement.

SOFTWARE USED-

NX CAD

CAR ROOF:

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. The roof is the structure above the windshield glass having the reinforcements to provide strength to the overall structure and to withstand overhead forces like snow.

WHY REINFORCEMENT:

If the roof is flat it is very weak and cannot able to withstand 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 bow roofs are introduced and joined to the flat surface of the roof with the different joining process.

TYPES OF REINFORCEMENT:

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

FRONT ROOF RAIL:

The front roof rail is the one that joins the windshield glass, the body side outer, and the inner panel also. Here the front roof rail is designed based on the master section given

REAR ROOF RAIL:

The rear roof rail is the one that joins the back door and the body side outer

CENTER ROOF RAIL:

central 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 central roof rail is placed at the centre of the roof which is connected to the B-pillar support structure. This central roof rail helps in adding strength to the roof during the roll-over test. The thickness of the central roof rail is 0.75mm and it is wider than the other two bow roofs because it withstands more load like snow, a sudden drop of rocks and some accidents

JOINING PROCESS:

Mastics are introduced between the roof's outer panel and the reinforcements for joining. where ever the mastic is introduced there is a remarkable improvement in the strength

Spot welding is also introduced to join the reinforcements and the outer panel.

DESIGN CONSIDERATION

• Visibility criteria
• Head clearance
• Curvature study
• Heat distortion and snow load criteria
• Draft analysis

ROOF CRUSH TEST;

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

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

• How fast the car can reform to its original shape after the rollover and
• How deep is the protrusion after the rollover?

THEME OF ROOF TEST

To minimize the protrusion into the occupant compartment, the seat belts and airbags can do their job in protecting the occupant.

CONDITIONS TO FOLLOW FOR THE 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 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 crushing speed at which the load is applied is 13 mm/sec or less than that.

RESULTS:

If the displacements go 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 rollover crashes.

Section analysis:

REAR ROOF RAIL

Create an intersection curve with the help of the plane and select the bow roof 1

BOW ROOF 1

CENTER ROOF RAIL

BOW ROOF 2

FRONT ROOF RAIL

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 colour ranges identifying zones on the analysed element where the deviation from the draft direction at any point, corresponds to specified values.

A minimum draft angle of 7⁰ is considered for analysis. green colour infers on the parts that all face along the tooling direction has a positive draft angle greater than 7⁰ and passed in analysis.

HEAT DISTORTION STUDY ON THE ROOF:

Predicting Heat Distortion for the roof is important since it is exposed to heat for a long time and high temperatures will affect the roof structure if the design does not

conform with the heat distortion criteria. This study helps to determine the positions of Bow roofs to provide strength to the roof to avoid bending or deformation of the

roof due to heat. A bow roof curvature study using the below formula will help us to check whether the Bow roof positions impart the required strength to the roof.

W = [1.73 x 10^ (-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

Judgement Condition: OK< 2.7> 3.1

Were

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]

APPLY THE VALUES IN THE FORMULA:

In order to find the heat distortion, apply all the necessary values from the roof to the give formula

In case -1 (from front roof rail to roof rail 1)

i)Ry = 6975.229mm

ii)Rx = 2007.072mm

iii)L = 2123.8mm

iv)I = 425.8 mm

v)R = 5643.6 mm

thickness(t)=0.75mm

by substituting all the values in the W we get,

W = 1.462 PASSED

In the same way, do it for all four cases,

Case 2 (from bow roof rail 1 to centre roof rail)

W =1.42 PASSED

CASE 3 (From centre roof rail to bow roof rail 2)

W = 1.50 PASSED

CASE 4 (From bow roof rail 2 to rear roof rail)

W = 1.35 PASSED

From the above table, it can be concluded that all values of w<2.7 so thus infer that the current positioning of the bow row is good in the state as per the design and found ok.

Snow load Prediction formula:

This evaluation is performed in cold regions where snow occurs. Roof design must pass through these calculations to ensure that it has sufficient stiffness to withstand the load due to the accumulation of snow over the roof.

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)

Judgement condition = Qr ≥ 3.1

250 ≤ s ≤ 380

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 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

APPLY THE VALUES IN THE FORMULA:

In order to find the snow load criteria apply all the necessary values from the roof to the given formula,

IN CASE 1 (From front roof rail to bow roof rail 1)

Qr = 18.05 PASSED

IN CASE 2 (From bow roof rail 1 to centre roof rail)

Qr = 20.62 PASSED

IN CASE 3 (From centre roof rail to bow roof rail 2)

Qr = 7.846 PASSED.

As per standards, the Qr value is supposed to be greater than 3.1. In the Above Design, we have found the Qr value is greater than 3.1 in all Components. Hence the Snow load criteria are satisfying.

ROOF IMAGES:

SIDE VIEW:

FRONT VIEW:

TOP VIEW:

Conclusion:

Thus, the Roof is designed and it is provided with Front and rear rails, a Bow roof and Reinforcement for the strength of the roof and also calculated the heat distortion and Snow load criteria. Both the Heat distortion and the snow load criteria are satisfying the Condition.

Draft Analysis has been carried out provided 7 degrees in order to check the manufacturing feasibility of the components.