Roof Design

INTRODUCTION

The roof of a car or other vehicle is the top part of it, which protects passengers or goods from the weather. The roof also protects the passenger from any injury when the car gets crashed or is rolled over or something heavy falls on the roof.

 

OBJECTIVE:

 

          1.Main objective of the project is to create the Roof Sheetmetal design in NX cad, and also designing Front roof rail, rear roof rail, bow roof front, bow roof rear, center roof rail and Reinforcement are creating to build the strong roof.

 

          2.Do curvature study on Roof and performance calculations for Heat distortion and Snow load criteria to determine the position of the Bow roofs.

 

          3.Finding the moment of inertia and section modulus for Bow-roofs and center roof rail.

 

          4.And, also finding the 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. When it is seeing from outside of the car, it is just closing the top portion. But in design point of view, it is hard to create the design. Because of you have to create no of support rails for enough strength to withstand the load. Bow roofs are primarily fixed in flatter region to enhance strength. Front roof and rear roof rails are fixed to the ditch area using spot weld operation. But front, rear bow roof and center roof rails are fixed to roof using mastic sealants. We don’t want so much gap between roof rails and bow roof area. So, we reducing the NVH issues.

 

 

SOFTWARE USED: NX 12.0

To design the Sheetmetal ROOF in NX by using some Solid/Surface features.

NX Software is a flexible and powerful integrated solution that helps you deliver the products faster and efficiently.

NX Cad software is widely used automotive, aerospace, thermal power station. Because of it is having modelling, drafting and manufacturing platform.

 

METHODOLOGY

We get the model of the roof of the from the styling team. Then we will have to do all the engineering work. We create the ditch area for the roof and also the roof bows and roof rails for the roof which would we designed by us considering all the safety norms and regulations.

 

 

ROOF CRUSH TEST:

 

OVERVIEW OF THE TEST:

          Roof strength evaluations consist of a quasi-static test conducted on a vehicle’s roof in a manner similar to tests used to judge compliance with Federal Motor Vehicle Safety Standard 216 (Office of the Federal Register, 2009). The main differences between the procedure specified by the Insurance Institute for Highway Safety (IIHS) and that specified by the U.S. federal government are that the IIHS procedure are,

Specifies testing one side of a vehicle’s roof

Does not include a headroom criterion

Specifies testing to a given displacement instead of a given force level

Specifies setting the vehicle’s pitch angle during testing based on the measured on-road pitch angle.

An overall rating is assigned based on the peak strength-to-weight ratio (SWR) measured within 127 mm of plate displacement.

 

CURB WEIGHT MEASUREMENT:

 

          Curb weight values used for calculating the SWR are based on IIHS measurements of a vehicle, not the manufacturer’s specified curb weight.

Vehicle curb weight is measured with full fluid levels using scales manufactured by Longacre Racing Products (Computer scales DX series 72634).

 

TEST VEHICLE SELECTION:

          For information on how we select vehicles for our crash test programs, including how we define typically equipped vehicles, see Test Vehicle Selection (IIHS, 2021). Whenever possible, the vehicle acquired for testing is the same vehicle used for the curb weight measurement.

At times, it is necessary to test a vehicle that does not meet the IIHS definition of a typically equipped vehicle, and the curb weight measurement is applied from another vehicle that does meet the definition. In these cases, the test vehicle does not differ from our typically equipped vehicle definition in any way that might influence the roof strength (e.g., a vehicle with a sunroof would not be tested if the typically equipped vehicle lacks this option).

Once the test vehicle has been acquired, either the driver or passenger side of the vehicle is selected for testing. For most vehicles, the test side is chosen at random. However, for roof designs that appear asymmetrical, engineering judgment is used to select the side of the roof that may result in a lower peak force.

 

TEST VEHICLE PREPARATION:

 

          With the vehicle on a level surface, the on-road pitch angle at the front door sill is measured on both sides of the vehicle. Unless the vehicle manufacturer requests otherwise, roof racks and other nonstructural items that may be contacted during the test are left as installed from the factory.

Any trim or other components are removed if they interfere with supporting the vehicle along its rocker panels. For vehicles with vertical pinch weld flanges on the bottom of the rocker panels, the vehicle support system consists of one I-beam (HR A-36 W4X13) for each rocker panel.

Each I-beam has a clamping system incorporated on the top that is tightened against the pinch weld flange to clamp the system in place (Figures 1 and 2). When the pinch weld flange has a bend that prevents supporting the sill with one I-beam, more than one I-beam can be used on each side of the vehicle.

For vehicles with no pinch weld flange, or with a nonvertical flange angle that precludes clamping, mounting by another method may be necessary, such as that described in the National Highway Traffic Safety Administration’s Laboratory Test Procedure (2006).

Prior to testing, the front-row seat back on the side being tested is reclined to prevent interaction with the crushing roof. Rear seats are latched in their upright position. All windows are closed and doors are locked.

 

 

TESTING OF ROOF:

          Roof strength evaluations are conducted on a quasi-static test system manufactured by MGA Research Corporation (Figure 3).

The system consists of an upright assembly and an attached loading head that can be fixed at varying heights from the ground as well as at pitch angles ranging from −5 to +5 degrees to accommodate testing on the driver or passenger side.

The roll angle is permanently fixed at 25 degrees. Four hydraulic actuators control the movement of the platen along two linear guides. The entire system is mounted on a T-slot bed plate anchored to the floor of the test facility.

Two HR A-36 — W10x88 I-beams are mounted on the bed plate perpendicular to the longitudinal axis of the platen. The vehicle, with attached rocker panel support system, is placed on      these beams. The vehicle is adjusted so that:

 

The longitudinal centerline of platen is within 10 mm of the initial roof contact point.

The yaw angle of the vehicle relative to the longitudinal axis of the platen is 0 ± 0.5 degrees.

The midpoint of the platen’s forward edge is 254 ± 10 mm forward of the most forward point of the roof (including windshield trim if it overlaps the roof) lying on the vehicle’s longitudinal centerline.

The pitch angle of the vehicle matches the on-road angle, while also accounting for any difference between the platen’s pitch angle and the nominal −5 degrees. The maximum combined difference of the vehicle and platen pitch angles from their targets is ± 0.5 degrees. (For example, if the onroad vehicle pitch angle is −0.2 degrees and the platen pitch angle is −5.2 degrees, the target sill angle for the test is −0.4 ± 0.5 degrees.) If necessary to achieve this angle, shims are inserted between the rocker panel supports and the W10x88 I-beams attached to the bed plate.

Once the vehicle is positioned correctly, the rocker panel supports are clamped to the two perpendicular I beams, and the beams are marked to allow confirmation that the vehicle position is maintained during the test.

For body-on-frame vehicles, the frame is supported to prevent the weight of the chassis from stressing the body at the body mounts. The roof is crushed to a minimum displacement of 127 mm at a nominal rate of 5 mm/second.

Some tests are conducted to a greater displacement to collect additional strength data for research purposes. Force data are recorded from five load cells (Interface Inc., model 1220) attached to the loading platen.

 

Displacement data are recorded from four linear variable displacement transducers (LVDTs) (MTS Temposonics model GH) integrated into the hydraulic actuators. Figure 4 shows the locations of the load cells and LVDTs on the loading platen.

Force and displacement data are collected with a National Instruments USB-6210 data acquisition system and reported at 100 Hz. These data are based on a sampling rate of 2,000 Hz, with every 20 points being averaged to produce the output data at 100 Hz.

The displacement-time histories from the LVDTs are compared to verify that the platen’s roll angle and pitch angle (relative to the vehicle’s on-road pitch) were maintained at 25 ± 0.5 degrees and −5 ± 0.5 degrees, respectively.

The precrash and post-crash conditions of each test vehicle are documented with still photographs. The position of the vehicle in the test fixture also is recorded. Motion picture photography is made of the test with real-time video cameras.

 

 

CALCULATING THE STRENGTH-TO-WEIGHT RATIO (SWR) RATING:

Force and displacement data are recorded for 5 seconds prior to each test, while the test system holds the loading platen at initial roof contact.

The data recorded from 1 to 4 seconds of this hold time are averaged for each channel to produce a measurement offset that is subtracted from the data recorded during the crushing of the roof.

After removing the offset for each channel, the force-displacement curve is plotted using the summed output from the five load cells and the average displacement from the four LVDTs. The maximum force prior to 127 mm of platen displacement is divided by the measured curb weight to obtain the SWR.

Both the force and curb weight are rounded to the nearest pound prior to performing the calculation, and the resulting SWR is rounded to the nearest one-hundredth of a unit.

Displacement is rounded to the nearest 0.1 mm. (The maximum displacement where the load can be used for the vehicle rating is 126.9 mm.) The vehicle’s rating is assigned based on the boundaries listed in.

 

 

All trim levels sharing the tested vehicle’s body type and roof structure are assigned the same rating as the typically equipped trim level, provided their curb weights do not exceed 110% of the selected vehicle’s weight.

Based on published curb weights from multiple sources, any trim levels that may exceed this weight are identified and weighed. If the weight does exceed 110% of the weight of the selected vehicle, a unique SWR is calculated for that trim level.

If this SWR results in a lower rating, both ratings are reported for the model, with a split according to trim level. If a trim level weighs more than 110% of the typically equipped model, but the lower SWR does not fall in a lower rating band, only the original SWR and rating are reported for the model.

 

 

DESIGN CONSIDERATION-

Visibility criteria

Head clearance

Curvature study

Hear distortion and snow load criteria

Draft analysis.

 

 

Components of roof-

STYLING SURFACE FOR ROOF:

 

 

1.Front Roof Rail-

The front roof rail is located on the front of the roof where the wind shield joins with the roof. The visibility criteria and the head room clearance of the passenger are taken into consideration for the bottom flange. One end of the rail is joined using spot welding while the other end is joined with the roof using mastic sealants for increased strength.

 

 

2.Bow roof 1-

Bow roof is an additional siffening member for the roof to increase the strength and so that it can satisfy the norms. Both the ends are joined together using mastic sealants to increase strenght and also since this would not affect the styling of the roof.

 

 

3.Centre Roof Rail-

Centre Roof rail or the reinforcement centre is located at the centre of the roof . The cross section of this is more since it needs to provide more strength to the roof. Here both are ends are joined together using mastic sealants.

 

 

4.Bow roof 2-

This bow roof also have the same purpose as that of the first one. This one is located between the centre bow roof and rear roof rail.

 

 

5.Rear Roof Rail.

This roof rail joins with the back door and the roof. Inputs from the backdoor are taken into consideration for designing this roof rail.

 

Here one end is joined using spot welding while the other end is joined using mastic sealants.

 

 

We then also make the ditch area around the roof.

 

Ditch area the extended portion of the main roof concealed by other BIW panels.

 

 

 

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)/2] + [1.10 x 10^(-3) x I]-2.68

          Judgement Condition: OK<2.7>3.1

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]

          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

 

 

Bow position	Rx	Ry	R	l	L	t	R2/2	W	OK/NG
FR and 1st	5687.43	2854.31	3674.63	335.41	2020.02	.75	6751452.81	1.579	OK
1st and CTR	4867.53	6611.71	5521.14	428.29	2020.02	.75	15241493.44	1.567	OK
CTR and 2nd	3977.89	10724.13	5871.67	429.34	2020.02	.75	17238254.29	1.605	OK
2nd and RR	3317.65	14723.54	5438.68	316.98	2020.02	.75	14789620.07	1.436	OK

 

 

 

 

SNOW LOAD:

 

          Snow load criteria study is used to check the durability of the roof to withstand the snow fall on the roof and reflex it is original shape without any permanent deformation after load being removed.

 

SNOW LOAD CRITERIA:

 

Qr = [IY x t2] / [α x s x [(RX + RY)/2]2 x 10-8]

 

Where,

 

α = My x LX2 x10-12, 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 center point between roof rails 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],

 

          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

 

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 [mm4],          

 

          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[mm4],

 

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

 

RX & RY (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.

 

Roof Rails	Iy(mm)	Lx(mm)	Ly(mm)	L1(mm)	L2(mm)	Rx(mm)	Ry(mm)	t(mm)	Y(mm)	S	My	α	Qr	Result
FR-BOW_1	734.94	1179.33	2032.02	413.76	463.64	4343.02	2588.83	0.75	413.076	438.706	669571.25	0.69814	6.67130	OK
BOW_1-CR	1035.86	1132.02	2032.02	463.64	464.73	3578.18	6018.92	0.75	463.64	463.54	718894.74	0.75402	4.97254	OK
CR-BOW_2	756.25	1102.82	2032.02	464.73	440.82	4270.64	11015.21	0.75	464.73	458.72	748052.11	0.79274	1.58848	NG

 

 

DRAFT ANALYSIS

 

 

MOMENT OF INERTIA

 

 

 

 

 

 

 

CONCLUSION

 

I was successfully able to design the roof with all the roof rails for better support and strength and  designed it so as to satisfy the norms and regulations by calculating the heat distortion criteria and the snow load critera. While doing this i was also able to understand about the methodology behind designing the roof of the car and what all we take into consideration while designing so.