FINAL INDEPENDENT PROJECT

INDEPENDENT PROJECT

OBJECTIVE: COMPUTER AIDED 3-D MODELING OF SHELBY AC COBRA 427 (1965 ROADSTER)

SOFTWARE USED:

3D Modeling: DS SolidWorks (2016)

Render tool: SolidWorks PhotoView 360

ABSTRACT

With the widespread popularity and versatility of Computer Aided Design (CAD) practices in various contemporary professional domains like Product Design, Industrial Design, Engineering R&D and even Biomedical research, the skill-set has become an essentiality among the credentials of a Design Engineer.

The idea of the project is to prepare a virtual model of an iconic vehicle, the Shelby Cobra Roadster, by designing and assembling some of the crucial elements of the automobile. The heart of the project in a nut-shell is to create an aesthetically complete & working exhibition model of the concerned vehicle encompassing the externally visible (or tangible) features i.e. to capture the appearance details of the vehicle and some of the constituent components.

Besides insights over different aspects generating a 3D model such as solid/ surface modeling and assembly creation, the project attempts to utilize the relevant engineering design practices - (i) the Product Teardown approach to split up the model into constituent elements (sub-assemblies and components), and (ii) an Iterative Design methodology (formally referred to as the Shighely Design Process) in order to obtain the optimized and desired end result.

INTRODUCTION

Automobile Trivia

One of the most popular and iconic American Sports car, the Shelby Cobra was first unveiled in 1962, under the collaboration of Ford and a British Automaker AC (Auto Carriers Ltd.) and came up as a rival of USA’s Chevrolet Corvette.

The body for the Shelby Cobra was derived from AC’s sports car-Ace, which was modified by Carroll Shelby, an American automotive designer as well as a racing driver. This new body shell sheltered Ford’s FE V8 motor under the hood. The body style of Shelby Cobra can be further traced back to 1948, when the Italian Carmaker Ferrari introduced their 166 MM Touring Barchetta model.

The Shelby Cobra was produced from 1962 to 1967 and it underwent design changes in 1963 (as Mark II) and in 1965 (as Mark III). The specific 427 (427 stands for the engine displacement in cubic inches equivalent to 7.0 liters) Cobra model was produced from 1965 to 1967, and only 348 original copies of this vehicle were made. However, this legendary vehicle has further been through refurbishments, customizations and manufacturing amendments by numerous enthusiasts and restorers, thus still topping in many automotive fans’ favorite list.

Project Motivation

• The bodies of Shelby Cobra were originally hand-laid, hence having smoothly finished and blended surface contours with gradually changing curvatures unlike modern day sports cars which have abrupt surface transitions with sharp or crisp flow lines. Thus, creating the seamless body of Cobra called for admirable surface modeling work in the project.
• As the vehicle has been customized into multiple variants ranging from street cruiser to dragster, the project enabled the flexibility to incorporate any desired design additions to the components e.g. interior upholstery, wheel designs, etc.
• Also, an automobile modeling gave the viability to define certain motion to the components whilst assembly allowing to add more dynamism to the design project.

METHODOLOGY

The project attempts to employ the following engineering design methodologies:

• Product Tear-Down Approach

Simply put, product tear-down refers to the act of dismantling or disassembling a given product with an intention to identify, analyze and categorize the constituent parts involved in the product making. Similar to trick applied while solving aptitude problems wherein the problem statement is broken down into pieces of relevant information to progress with the solution; tear-down approach is used in the project to divide the automobile into project relevant sub-assemblies and further components.

The below depiction shows the different components that make up each of the 3 sub-assemblies into which the entire model has been divided.

Further sub-components have been modeled as separate (or unmerged) bodies within the parent component file to limit the number of part files as well as to simplify the process of design revisions.

• Iterative Design Approach (also Shighely Model of Design Process)

The Shighely Design model mainly aims to analyze an existing product to identify the problems or any upcoming product derived from a previous product with new set of required specifications. This is achieved by making a number of feasible modifications and comparing the results with the preset specifications.

Continuous revisions at any of the design stages namely problem definition, solution synthesis, analysis & optimization and evaluation are made by going back to one of the preceding stages from the current active stage. In other words, the new prototype undergoes suitable changes in an iterative fashion until the requisite final output is reached.

This approach has been employed in the project while creating customized additions to the components of the vehicle by checking for each component’s overall aesthetic appeal, apt dimensions and packaging constraints either in the sub-assembly or final assembly model.

• Available blueprint images of the car are set as reference to help create some curves and estimate boundaries of the vehicle A-surface. The different views of the vehicle are inserted at appropriately placed planes and the images are sized in accordance with original vehicle dimensions. These blueprints aid in generating the ‘Primary Surface’ which not only marks the starting point for the project but also serves as spatial as well as dimensional reference for Chassis & Cabin sub-assemblies. The main reference planes are renamed as – Front, Centre, Rear, Top, Base and Mid-planes.

• Necessary surface blends and extent of curvature of the contours are interpreted from real world images of the vehicle by visualizing the reflections off from the vehicle body.
• Any component with symmetry across respective centre planes such as the Primary Surface and Frame, are generated using the Mid-Plane Mirror method (i.e. one half of the entity is mirrored to create the whole).
• To instill some realistic movement to the components in the final assembly such as tires turning, hood opening, etc., turning pairs (like a hinge) to act as a pivot point for limited rotation are created at the interface of the two mating components.
• Final Assembly is made by bringing together the three sub-assemblies in a new assembly file. The key point here is to solve the inserted sub-assembly files as ‘flexible’ entities to integrate the intended working mates in the resultant model.

3D MODELING TECHNIQUES

• Solid Modeling – The method involves manipulating the bulk of on object via volume addition or subtraction commands. 3-dimensional entities can be generated using 2D sections or curves by drawing the requisite directirx and generatrix. Even Boolean operations like union, intersection and subtraction can be applied on solid bodies to form the sophisticated shapes. Intricate detailing can be carried out in solid modeling much easily, and since the geometry can be made parametric which aids in consequent model rebuild when iterative revisions are carried out.
• Surface Modeling – This modeling deals with the outer shell of the object i.e. the exterior tangible surface. Defining the boundaries and shaping the surfaces through complex curves as well as the geometric relationships (like tangency. normal or curvature) amongst various surface edges form the essence of this type of modeling. It allows free-forming of shapes much flexibly that aid in creating aesthetically appealing product designs. Various surface patches can be knitted with apt tolerance to make a single surface or any excess patches can be trimmed off to obtain desired surface forms.

Commands Used:

• General - Sketch, 3D sketch, Trim/ Extend Entities, Offset Entities, Convert Entities, Project Curve, Composite Curve, Reference Geometry [Plane], Split-Line, Split, Mirror, Combine, Delete Body, Linear Pattern, Circular Pattern, Sketch Picture
• Solid Model - Boss-Extrude, Boss-Extrude [Thin], Swept-Boss, Revolved-Boss, Lofted-Boss, Cut-Extrude, Cut-Extrude [Thin], Dome, Fillet, Chamfer
• Surface Model – Extruded-Surface, Offset-Surface, Swept-Surface, Lofted-Surface, Filled-Surface, Boundary-Surface, Ruled-Surface, Extend-Surface, Trim Surface, Knit Surface, Thicken, Fillet

MODELING THE COMPONENTS

• Primary Surface

Using the reference blueprint file, the outer surface of the vehicle body is created. As the body panels have smooth curvatures with certain converging-diverging geometry and well-blended edges, the tangency between different regions of the body as well as the direction vector for surfaces with their generatrix curve becomes crucial. Lofted surface to create surface transitions from one section to the other turning with respect to a guide, and swept surfaces to generate seamless patches along a definite path, are much extensively used.

For the body regions like the front fascia and rear fender having complex fluidic transitions, the surface is divided into multiple patches and generated individually with well-defined edge relations and then sewed together. This allows better control over the surface forms as well as on the profile gradients. Wheel cut-outs at the fender areas and any excess of the surfaces are trimmed off to match the shape of the original vehicle. The door panel is also split from the body and stored as a separate part file at this point.

Being symmetric, Mid-Plane mirroring method is used to make the whole of the primary surface. After mirroring, the sketches for hood and trunk are drawn on suitable planes and used as templates to split these panels from the primary surface to work upon as separate components.

Splitting is also crucial as movement to these body panels can only be defined when brought in as individual part files during assembly. A body floor is made by connecting the bottom edge of the side panels, and an offset surface is used to create the cabin floor. Walls for the cabin space are made and knitted with the primary surface within which the cockpit sub-assembly sits.

With the main surface created and the necessary panels split, accessories like the headlights, tail-lights, ORVMs, windshield, fuel tank cap, driver side roll hoop, front grille are made as solid bodies along with details like the iconic Cobra emblem on the grille and the engine badge at the side. Connectors for the hinge are made at proper positions for the hood, trunk and the doors.

• Hood

The hood panel is obtained as a split section from the primary surface. Further surface modeling is carried out on this cut-out part for making the hood scoop and other aesthetic details. Hood scoop height is adjusted in accordance with the height of the engine model. The other essential addition is the hood hinge plates that properly mates with the hinge connector modeled on the primary surface.

• Trunk

To complete the trunk lid from the cut-out panel from the primary surface, a trunk handle is made to enhance the appearance and hinge plates are added similar to the hood to facilitate movement.

• Doors (Left & Right)

The doors have more additions than hood and trunk panels. Inner face of the door is embellished with upholstery additions to match the cabin theme. Again, door hinge sockets are placed at proper position to work with the connector on the primary surface.

Instead of mirroring one side door (left) to create the opposite door (right) as ‘Mirror Component’ during the assembly, both the doors are made as separate part files. The reason being when the left door (with moving mates) is mirrored to make the right door, the motion becomes mutual i.e. when any of the doors is turned; the other one also turns accordingly. By making the individual part file for both the doors, the door movement can be made independent and thereby more realistic.

Thus, the right door is made simply by mirroring the fully modeled left door about the mid-plane, deleting the left door and storing it as a separate part file.

• Exhaust

One of the most appealing features of the Cobra is the side body exhaust outlets. The exhaust header tubes are made as swept bodies with each tube having a different swept path. The tubes converge at the header collector which connects to the muffler and finally a bent tail-pipe. Side-pads are made onto the primary surface where the exhaust is to be attached with the body. Finally, a lustrous appearance is given to the exhaust to compliment the semi-matte body paint.

• Frame

The frame composed of structural members serves as the under-body support. Frame created for the project is a simplified ladder-frame like design containing features relevant to this aesthetic project. This includes the peripheral structure with front hoop, cross reinforcement members, engine mounting, control arms (with a vertical stud to mimic king-pin axis), rear-end differential casing and drive-shafts.

Since the frame is meant to sit inside the outer body, the primary surface is used as the spatial as well as dimensional reference while designing the frame. The track-width and wheel-base of the vehicle are also altered by changing the control arm / drive-shaft lengths and by the position of the wheel-arch side structure.

• Engine (Cosmetic)

The 1965 Shelby Cobra was powered by a Ford FE 427 motor. The naturally aspirated 7-litre V8 churned 410 Brake-Horses (@6000 RPM) and an impressive 651 Newton meter of torque (@3700 RPM). The engine was coupled with a 4-speed Borg-Warner manual transmission in a rear-wheel drive layout.

The engine modeled for this project is a cosmetic version i.e. the model appears complete externally but does not contain the internal elements of an engine. The model includes externally evident parts and features inclusive of the oil sump, engine block, valve covers, intake manifold (tunnel-ram type), twin carburetors with cylindrical air-filters, crankshaft / camshaft / idler pulleys with serpentine belt-drive, alternator, exhaust (dynamometer) headers and the transmission bell-housing. The model is furnished with stamped-engravings and apt appearances to make the cosmetic replica of the engine much closer to a real motor.

As no useable blueprints are available for engine models, the dimensional reference for this model is obtained again from the primary surface. The engine bay volume (cuboidal space) is estimated from the outer body after splitting the hood panel. Overall height of the engine is adjusted either by changing the sub-components respective heights or via the engine mounting position during the final assembly, so that the air-filter becomes visible from the hood scoop when assembled. Online images of V8 engines are referred to set realistic apperance to the engine model.

• Wheels (Front & Rear)

The Shelby Cobra could be customized easily just by changing the set of wheels & tires on which the versatile body style rides. Fastening low aspect-ratio radial slick tires would convert the car into a dragster, while a set of high aspect-ratio ribbed tires would make the vehicle cruise ready.

For this design project, the touring (or cruising) type styling theme is adapted. Thus, 16-inch classic spoked-wheels fitted with 23-inch ribbed tires become the choice to align with this vehicle theme. Brake discs and calipers are added to the centre hub.

The front and rear wheels are kept as separate models as the mate conditions to be defined are different. Since the front wheels are to have a turning motion, a C-channel mount is added after the brake system to mimic the presence of an upright/ knuckle which can be used to facilitate limited rotation about the king-pin stud modeled in the frame.

• Instrument Panel & Upholstery

The Dashboard can be extensively customized as per the designer’s choice and any number of desired interior details can be added.

In order to estimate the region where the interior has to fit, the primary surface again comes in as the requisite reference. The cabin boundaries and a reference plane coincident with the cabin floor level as made in the primary surface allow effective sizing of the dash-panel. With the base dash-panel made, accessory details like steering column, glove box, gauges, infotainment section, cup holder and various control levers (gear level, hand-brake, etc.) are further added with iterative corrections till a satisfactory result is achieved.

• Seat

With the cabin space (in the primary surface model) and instrument panel completed, the space left for the seats can be visualized and rough dimensions for the cushion length & width, backrest height & recline can be estimated. To keep a touring vehicle theme, luxury seat look-alike is designed with headrest and some racy add-ons like thigh & shoulder side-supports.

• Steering Wheel

A retro style steering wheel is designed to go along with the 1960s vehicle. A 11-inch wheel with dished centre linking arms is made instead of the regular flat plate type steering wheel for better appeal. The model is finished by adding proper appearance and the iconic Shelby logo over the horn pad.

CREATING THE SUB-ASSEMBLIES

(I) Rolling Chassis

Constituent part files:- Frame, Engine (Cosmetic), Front wheel, Rear Wheel

This sub-assembly completes the structural body having a well-mounted engine and all 4 wheels attached, which goes beneath the outer body of the vehicle. The turning movement to the front wheels is defined within this sub-assembly.

Component	Mate Condition(s)
Frame	- Fixed (As it is modeled having the Primary Surface as spatial reference)
Engine	- Right plane of Engine coincident with Mid-plane of Frame - Front plane of engine at desired distance from Centre plane of Frame (28”) - Bottom face of engine oil-sump at desired distance below the mounting plate face on the frame (3”)
Front Wheels	- Holes of the C-channel mount in the wheel concentric with the kin-pin stud in the frame (control arm) - Bottom face C-Channel mount coincident with the bottom face of the king-pin stud - Right plane of the wheel within desired angle range with respect to mid-plane of frame (Limit Angle: -25˚ to +25˚) [Right planes of both front wheels are made parallel to facilitate mutual turning motion]
Rear wheels	- Periphery of the wheel hub concentric with the drive-shaft periphery in the frame - Inside face of wheel hub coincident with end-face of the drive-shaft - Front plane of wheel parallel to the centre plane of the frame [may be suppressed to rotate the wheel]

(II) Exterior Body

Constituent part files:- Primary Surface, Hood, Trunk, Left Door, Right Door, Exhaust, Wheel [optional]

The sub-assembly combines all the previously split body panels back on to the primary surface with defined working mates to instill realistic panel movements. These component motions are set via appropriate angle limit mates. Being an aesthetic model, the exhaust is also attached within this sub-assembly itself. Wheel model can also be placed inside the trunk space to apper as a spare wheel.

Component	Mate Condition(s)
Primary Surface	- Fixed (All planes coincident with assembly interface’s default planes)
Hood	- Width mate between the side faces of hinge connector on Primary Surface and side faces of the hinge plate on the Hood - Hole of the hinge connector on Primary Surface concentric with hole of hinge plate on the hood - Top plane of the hood within desired angle range with respect to base plane of primary surface (Limit Angle: 0˚ to 60˚)
Trunk	- Width mate between the side faces of hinge connector on Primary Surface and side faces of the hinge plate on the Trunk - Hole of the hinge connector on Primary Surface concentric with hole of hinge plate on the trunk - Top plane of the trunk within desired angle range with respect to base plane of primary surface (Limit Angle: 0˚ to 80˚)
Doors (Left & Right)	- Width mate between the horizontal faces of hinge connector on Primary Surface and parallel faces of the hinge sockets on the doors respectively - Hole of the hinge connector on Primary Surface concentric with hole of hinge socket on the doors respectively - Right plane of the doors within desired angle range with respect to mid plane of primary surface (Limit Angle: 0˚ to 85˚)
Exhaust (Both Sides)	- Face of header tube coincident with outer face of the side-pad on the primary surface - Right plane of the exhaust at a desired distance from sidewall of the side-pads on the primary surface (2.6”) - Top plane of the exhaust at a desired distance from bottom face of the side-pads on the primary surface (2.25”)
Spare Wheel [optional]	- Front plane of the wheel coincident with the Mid-plane of primary surface - Right plane of the wheel at desired distance from the base plane of primary surface (23.5”) - Top plane of wheel at desired distance from rear plane of primary surface (19.5”)

(III) Cockpit / Cabin

Constituent part files:- Instrument Panel, Seat, Steering wheel

The cabin sub-assembly, unlike the previous two sub-assemblies, is essentially static i.e. it does not have mates that define any component motion. Thus, the seat is inserted and positioned in the sub-assembly only once and the adjacent seat is completed using the mirror component command.

Component	Mate Condition(s)
Instrument Panel & Upholstery	- Fixed (As it is modeled having the Primary Surface as spatial reference)
Seat	- Base surface of the seat coincident with the cabin floor reference plane in the instrument panel - Right plane of Seat at desired distance from Mid-plane of instrument panel (13.5”) - Front plane of the seat at desired distance from the centre plane of the instrument panel (18.5”)
Steering Wheel	- Steering rod-end concentric with its respective bore in the steering column on the instrument panel - Bottom face of the steering rod-end coincident with the internal face of its bore in the steering column [Rotation of the steering wheel may be locked with the concentric mate]

CREATING THE FINAL ASSEMBLY

With all sub-assemblies made and stored as separate assembly files, the full vehicle model is created by bringing these sub-assemblies into a new assembly window. Since at least one of the components in each of the sub-assemblies utilize the Primary Surface as spatial reference, these models can be inserted and fixed directly into the new assembly graphics window.

The sub-assemblies when fixed will be solved as ‘Rigid’ entities as default which subsides all the motion-producing mates. Thus, it is vital to set the sub-assemblies as ‘Flexible’ in order to allow the working mates to show up in the final assembly model.

PHOTO-REALISTIC RENDERING

After the assembly, real-world view (or Renderings) of the model can be generated. The rendering process is carried out using PhotoView 360. Suitable scene, lighting and environment are set with desired background. Additional quality refinement settings like Bloom and Direct Caustics which add to the real-world vibe of the model are also adjusted in accordance with some preliminary visualizations via integrated/ preview window option to obtain most captivating output.

The following are the final renderings of the Shelby AC Cobra 1965 Roadster processed for various views.

Isometric View

Front View

Side View

Top View

Additional Renders:

CONCLUSION

The outcome of the design project is a presentable aesthetic virtual model along with photorealistic renders of 1965 Shelby AC Cobra Roadster encompassing utmost externally visual details and certain working components as in an original vehicle, ready to serve as a demonstration prototype.

Approach used in the project in essence resembles the actual vehicle production flow that progresses by producing the Chassis and Body in parallel at separate assembly lines, and marrying them together, followed by the addition of requisite custom accessories and interior upholstery to make a road-ready sports car.

PROJECT SCOPE

• Viable design additions can be made to make the model further realistic and technical, for example- (i) a dynamic shock sub-assembly with a moving control arm to facilitate wheel travel, (ii) a rack & pinion gear assembly with suitable linkages to make a functional steering system i.e. turning the wheels when the steering wheel is moved.
• Such CAD models of products can be exported to any 3D Printer (Slicer) software to generate a tangible scaled version of the product to be used for concept demonstrations or as exhibits.
• Design projects like these can aid in promoting design thinking ability and approach, essential for engineering design practices and reverse engineering.

REFERENCES

Image References: [1] Blueprint

[2] Shelby Cobra Image 1

[3] Shelby Cobra Image 2

[4] Shelby Cobra Image 3

[5] Ford FE 427 Engine image

[6] Shelby Cobra Interior

Decals: [1] Cobra Emblem 1

[2] Cobra Logo

[3] Dashboard Gauges

Quick Sketches: