Roof Design

******** CAR ROOF ASSEMBLY DESIGN ********

Objective:

• To design 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 moment of inertia and section modulus for Front roof rail, Bow roofs, Central roof rail and Rear roof rail.
• Perform the draft analysis for each reinforcement.

Software Tool used: NX CAD Student Version

Introduction:

Car's Roof: An automobile roof or car is the top portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements.

Procedure:

In this challenge we are creating

• Front Roof Rail
• Bow Roofs (Front and rear)
• Centre Roof Rail
• Rear Roof Rail
• Ditch area

Open the Assembly Navigator in NX-CAD software

Adding outer roof part file in NX-CAD assembly and create Roof

Roof Outer Panel:

First, we will be extracting the top half portion of the roof surface. Since the roof surface is symmetrical in shape, we can easily work only on half of the portion and reflect it at the end to create the whole roof.

Front_Roof_Rail (FRR):

Front roof rail is designed using master section of the front roof rail shown below considering the dimension windshield which is accommodated in front of front roof, Adjacent to windshield. Dimensions shown 32mm and 80mm in the image is based on the factors such as head room clearance and visibility criteria.

Rear_Roof_Rail:

Rear Roof Rail is the member used to give strength for back door joint and body side outer

Bow_roofs (1 - 2) :

The bow roofs are used to improve the torsional stiffness and load bearing capacity of the roof structure. Bow roofs are placed on the flat regions of the roof which provides strength in bigger curvature region and in between Front roof rail,centre roof rail and rear roof rail. The number of bow roof present is depends on the overall size of the roof. Presently in this project two bow roofs are added. Bow roofs are also designed based on the design consideration similar to Front Roof Rail(FRR), refering the master section attached below:

Centre_Roof_Rail:

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

Ditch Area:

Ditch area is the extended portion of the roof styling surface which is used to mount the reinforcements and also ensures sealing between parts of the roof. Opening angle of ditch is 7 degrees used.

Section Analysis for Moment of Inertia (MOI)

A) Front Roof_Moment of Inertia (MOI)

MOI _max: 1.488e+05[mm⁴]

MOI _min: 7.785e+03[mm⁴]

B) Bow-Roof1_Moment of Inertia (MOI)

MOI _max: 2.068e+04[mm⁴]

MOI _min: 1.575e+03[mm⁴]

C) Centre-Roof-Rail_Moment of Inertia (MOI)

MOI _max: 5.837e+04[mm⁴]

MOI _min: 1.641e+03[mm⁴]

D) Bow-Roof2_Moment of Inertia (MOI)

MOI _max: 2.117e+04[mm⁴]

MOI _min: 1.710e+03[mm⁴]

E) Rear-Roof-Rail_Moment of Inertia (MOI)

MOI _max: 5.341e+04[mm⁴]

MOI _min: 3.308e+03[mm⁴]

DRAFT ANALYSIS:

A) Front Roof Rail Draft

B) Bow Roof-1 Draft

C) Centre Roof Rail Draft

D) Bow Roof-2 Draft

E) Rear Roof Rail Draft

JOINING PROCESS :

Mastics are introduced between the roof outer panel and the reinforcements for joining. Wherever the mastic are introduced there is remarkable improve of the strength and also for NVH.

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

DESIGN CONSIDERATIONS FOR A ROOF :

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

ROOF CRUSH TEST :

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 up 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 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 their 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

Roof curvature calculation:

Heat Distortion Criteria:

The heat radiating from the sun causes the roof of the car to change in the metallic regions. The heat distortion study plays a major role in sheet metal usage. heat distortion temperature is a temperature limit above which the material cannot be used for the structural applications. This study is used to predict the heat distortion temperature at where the material 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.

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R²)/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(R_x*R_y) / (R_x + R_y)

R_x = X curvature

R_y = Y curvature

t = Roof plate thickness [mm]

l = Bow Roof Span [mm]

Snow Load Criteria:

This test is done to know how is the roof 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 is the basic requirements for snow load criteria

Q_r = [I_y x t2] / [α x s x [(R_x + R_y)/2]2 x 10-8]

Where

α = My x Lx2 x 10-12,

My = Y(Ly-Y)

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 centre point between Roof rail Front /Rear as the reference point of the front and the rear.

L_x = 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]

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 [mm]

Roof panel curvature Radius of the Length Lx in Front view

R_y = 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

Judgement condition = Q_r ≥ 3.1

250 ≤ s ≤ 380

Conclusion:

1. Designed Roof reinforcements such as Front roof rail, Bow roofs, Centre Roof rail, Rear Roof Rail and Ditch area as per given master sections
2. Performed Draft analysis for all reinforcements
3. Performed Section Inertia Analysis
4. Peformed Curvature study and calculated Heat Distortion Criteria & Snow Load criteria

By performing above actions, I learnt and designed Roof reinforcements.

**** Heat load calculation and snow load criteria calculation completely satisfies the curvature study and the bow roof placements are acceptable.****