Design, modeling, assembly, and rendering of an American Chopper

OBJECTIVE: The objective of this project is to sketch and build a Harley-Davidson® bike using SOLIDWORKS®

INTRODUCTION: The chopper is a type of motorcycle that emerged in California in the late 1950s. These types of motorcycles tend to be radically modified, featuring lengthened forks and extreme angles. This project details the creation and assembly of the main components of a Harley-Davidson®-style motorcycle, as well as a series of photo-realistic renders in a studio setting.

APPROACH: The basic approach is the same for all parts, although depending on the type of feature, the details on creating the part will vary. In general, all parts start as a sketch, which will then be either extruded, revolved, swept, or lofted, depending on the geometry needed. When the basic shape is done, adding fillets, splits, mirrors, and combines, will ensure the part has no intersecting geometries and no sharp corners. Once all the parts are done, they are assembled and rendered.

DESCRIPTION: Here I will break down the approach used for each of the parts.

• Gas tank: The motorcycle gas tank is often regarded as the "face" of the motorcycle. Therefore, particular emphasis is applied to make it sleek and usually involves making complicated shapes that follow complex curves. The manufacturing process is usually done by welding bent and cut sheet metal parts to accommodate the desired shape.
  
  
  The molding process starts by sweeping a surface across the top "S"-shaped curve. Then, the surface is trimmed with the aid of a top plane sketch that gives it an aggressive look. In order to make the front triangular shape, a composite curve is made out of two project curves, and a surface is then lofted from this curve to the front portion of the top surface. A similar process is followed for the long side surface, creating a composite curve out of two project curves. However, for this operation, a boundary surface is utilized. This allows for finer control of tangency and curvature along each profile, as opposed to the way loft's curvature controls are limited to the start and end of the profile. The surfaces are then mirrored and knitted together. A surface extrude is performed from a sketch following the shape of the seat. The bottom surface is then created out of a boundary surface, connecting both sides together. A cutout for the top chassis support tube is then performed by utilizing the swept cut feature. Finally, the part is filleted to avoid sharp curve transitions and shelled in order to make it hollow.

• Engine: The motorcycle engine is the powerhouse of the motorcycle. The type of engine used here is a V-twin engine, comprising two angled, in-line cylinders, mounted transversely.
  
  The engine starts as a basic set of polygons comprising the gear housing, oil housing, and starter motor housing, all made of basic sketches and revolve features. A cylinder is then added, starting as a coffin-shaped polygon, to which a cut is added and lineraly patterned, to create the cooling fins. The cylinder is then mirrored, and the air intake is added by sweeping a circular profile across a curve. Finally, for each exhaust, a composite curve is created out of two project curves, and a circular profile is then extruded across it.

• Rear and front wheels: Motorcycle wheels are usually filled with air and are attached to a rim. For this model, I have also included a brake pad with a caliper, as well as a sprocket for the chain on the rear wheel model.
  
  The rear wheel starts as a revolved outer rim and hub. Then the webbing on the rim is made, by extruding a profile, removing material from its inside, and making a circular pattern. a curve is then cut from the front and back of the rim to give it it's characteristic look. A hub for the brake rotor and sprocket to sit in is extruded, and then a disk is made. Circular cuts are made to the disk and patterned to allow for better ventilation when the brake pads come into contact with it. The caliper is then extruded out of a sketch profile, and another disk is made, this time for the sprocket to be modeled. A series of cuts are made to it to give it its teeth and reduce its weight. Finally, a revolve is made for the tyre, and an offset sketch is cut out and patterned, in order to model the tyre's tread.
  
  The front wheel follows a very similar process, albeit narrower and omitting the chain sprocket.

• Chassis: The motorcycle chassis forms its skeleton; it lends the motorcycle it's strength, rigidity, and ability to handle well. All the components of the motorcycle are attached to it; therefore, all components must be welded together, creating a strong foundation.
• The chassis starts by sketching a side and front view of the bottom tube, to create a project curve with both angles. At this point, the backplate is also created. We follow this same process to create the back and front tubes. The bottom plates are made by sketching them on the XY Plane and extruding them until they meet the bottom side tubes. The pedal plates are created using an offset extruded sketch. Using a combination of fillets, splits, and mirrors, all the components are fused to create a single part.

• Pedals: The pedals are where the rider's footrests, as well as enabling access to the shifter on some manual motorbikes.
  
  The basic shape of the pedal is created from a sketch on the front plane, which is then extruded. Three circles are then sketched and extruded into 3D geometry on one side, where the rider's foot will be, and two circles are sketched and extruded on the other side, where the pedal will attach to the chassis. A series of ridges are then added to the extrusion for added grip. Finally, everything is filleted to avoid sharp edges.

• Kickstand: The kickstand is usually a simple metallic part that makes contact with the ground and allows the motorcycle to be kept upright. Some touring bikes have kickstands both at the front and the rear. However, for this motorcycle, only one kickstand is needed near the middle.
  
  The part is exceedingly simple, it comprises two profiles lofted together with domes at their ends, as well as a flat part on the bottom and a cutout for attaching to the chassis.

• Transmission belt: American Choppers usually use an open belt primary drive, which allows for less maintenance and easy inspection. It is a flexible belt designed to run over toothed pulleys or sprockets, and are used for allowing the linkage of two rotating shafts.
  
  The transmission belt is only a sketch made of two circles joined together by tangent lines, extruded to the correct length and then filleted to avoid sharp transitions.

• Oil tank: While some motorcycles uses Oil-in-Frame chassis, American Choppers usually have an external oil tank; this is because these motorbikes tend to use dry-sump engines, which allows for a bigger engine to be fit into the chassis.
  
  The oil tank starts from a horseshoe-like sketch which is then combined as a boolean with a wedge shape. The result is a complex geometry created from two simple, basic shapes. The edges are then filleted and the part is given a shell, in order to make it hollow.

• Rear and front fenders: The function of the fenders is to shield both the rider and the motorcycle's internal parts from debris kicked up by the tyres. Therefore, they are almost always made of a very rigid material; although some motorcycles, especially motocross bikes, use fenders made of plastic.
  
  The fender is relatively easy to model and begins as a revolved closed sketch. A split is then performed to curve both edges, and the part is filleted to avoid sharp corners. Finally, a shell is made to give the part curled-in edges that allow it to prevent debris from escaping upwards. The front fender is almost identical, except for different curvature values.

• Chain: The chain is what links the rear wheel to the rotating shaft of the motor, and its function is to transmit mechanical power from the motor to the tyre. The power is conveyed by a roller chain, passing over both the rear and front sprockets, with their teeth meshing with the holes in the links of the chain.
  
  To model the chain, the essential procedure is this: A sketch block of both the roller link plate and the pin link plate is made, and are saved externally. A sketch of the chain profile is then made, and the block is patterned along its edges, following its curvature. Since this curvature varies along the chain's sketch, the procedure must be repeated every time the curvature changes. both the links and the pins are then extruded following an over-under pattern, and are mirrored across the mid plane. This procedure is repeated until the pattern comes full circle. To finish, a final plate is then modeled and mirrored, to bridge the gap between the first and last chain links.

• Front Fork: Forks connect the front wheel and axle to the motorcycle's frame. They usually comprise what are called the triple trees, fixed fork, hydraulic chamber, active fork, and axle for the front wheel. American choppers tend to have unusually long forks at extreme angles.
  
  The model starts by performing a revolved boss on a sketch of one of the fixed forks. A sketch of half of the triple tree is then created, extruded, and patterned in order to create its other counterpart. Half of the axle is extruded, and a revolve is performed in order to create one of the lights. The light is then hollowed out, and additional geometry is created to model the front glass pane. To make the handlebars, a circular sketch profile is swept along a composite curve made out of two project curves. The grips and brakes are then modeled on a plane referenced to the composite curve, so that if the curve were to change, the grips and brakes would change along with it. A similar process is created for the brake lines, albeit with different curvature and smaller radius. Everything created so far is then mirrored, combined, and filleted.

• Assembly and rendering: The assembly starts by fixing the chassis in space. The parts are then added and mated together using different types of mates, ranging from simple coincident and distance mates, to more complex mates, such as "width". Once all parts are added and mated, the bike is then leaned into the ground, and the fork is turned so make it sit as it would in real life.
  
  A "Plain White" scene is added, with a "daytime" environment featuring an outdoor backdrop set in order to obtain more interesting reflections on the reflective parts. Several camera positions with different lenses, positions, and depth of field options are added, and the renders are made using these angles. The render settings include a higher than standard number of reflections and refractions, slightly higher gamma than normal, bloom, and direct caustics, in order to mimic how light would interact with the surfaces in real life.

Light positions:

Camera positions:

Finally, the motorcycle is assembled and rendered using SOLIDWORKS® PhotoView 360.