Sarah Clarity Studios

Track Geometry, Bogies, Idlers & Road Wheels

Created by Sarah Choi (prompt writer using ChatGPT)

Track Geometry, Bogies, Idlers & Road Wheels for Vehicle Concept Artists — Land Vehicles: Tracked & Articulated

Tracked vehicles trade tire patches for belts of links supported by road wheels, idlers, bogies, and sprockets. Their geometry determines ground pressure, trench‑crossing, ride quality, and the unmistakable cadence that players read at a glance. Whether you work on combat, utility, or construction platforms—and whether your seat leans concepting or production—understanding how bogies, idlers, and road wheels shape silhouette and motion will help you draw honestly and hand off cleanly.

Tracked geometry begins with a loop: drive sprocket, idler, lower contact run, return path, and the components that carry the load. The shape of that loop is a promise about capability. A long, flat contact patch and wide shoes shout flotation on muskeg or sand. A shorter patch and higher leading idler advertise step‑climb agility. A taut upper run with small sag reads light utility or rubber belts; a catenary droop with return rollers reads heavy steel links under weight. Design this loop first, then decorate—because the loop sets silhouette, particle paths, and rigging anchors.

Road wheels are the primary load interface to the track. Their diameter, spacing, and number determine contact length, ride frequency, and ground pressure. Large‑diameter road wheels reduce rolling resistance and handle obstacles smoothly, but add weight and height; small wheels distribute load and lower hulls but increase bearing count and maintenance. Rim profiles vary: single‑flange wheels ride outside the guide horn, double‑flange straddle it, and center‑grooved wheels capture it; your cutaways should match the link guide geometry you draw on the track. Tire materials differ too—solid rubber bonds, segmented tires for heat, or all‑metal for extreme heat and weight. In silhouette, a tight, evenly spaced wheel train reads torsion‑bar or hydropneumatic suspensions; paired wheels on bogies read Horstmann or roller‑frame families.

Bogies are sprung subassemblies that carry one or more road wheels. On combat vehicles with Horstmann or similar systems, a bogie may carry a pair of wheels via a swing arm and external coil spring. The visual tells are the compact spring housings and the distinct “pair, gap, pair” rhythm. On construction equipment, roller frames act like elongated bogies: clusters of small rollers conform the lower run to terrain, supported by oscillating frames that keep contact even on rocks and lumps. Bogie geometry defines how the lower run flexes; a long bogie frame creates a graceful belly curve for traction; short, stiff bogies keep the lower run flatter for grading precision. Production callouts should fix bogie pivot coordinates, spring/damper stroke, and service access for bushings and pins.

Idlers set tension and the forward geometry of the belt. The idler’s diameter and vertical placement control approach angle and how the track meets obstacles. High idlers pull the belt upward for better step climbing and trench approach; low idlers shorten the contact run but simplify tensioning. Idlers usually ride on carriers with screw or hydraulic adjusters; your callouts should show adjuster stroke, grease port locations, and the clearance envelope at full forward/aft travel. Idler face profiles must mirror road wheel/guide geometry: if your wheels straddle a center guide horn, the idler needs a matching groove; if your links use outer guides, the idler flanges sit accordingly. Because idlers are often the first point of impact in debris, add guards and mud scrapers with believable fastener logic.

Return path support keeps the upper run stable. Heavy steel link systems use return rollers spaced along the upper hull; rubber belts can ride sliders or top rollers. Return roller diameter and spacing create a signature shadow rhythm under side skirts; too few rollers will sag unrealistically, too many will read like a conveyor. Sliders save parts but add heat and wear—good for short belts on compact track loaders, poor for MBTs. Show dust shields and wear strips where sliders contact belts; provide replaceable pad logic on rollers for serviceability.

Drive sprockets are the teeth that pull the loop. Their location (front vs. rear) and tooth form (cast, bolted segments) reveal era and role. Rear‑drive often accompanies MBTs and some IFVs; front‑drive is common in dozers and excavators. Tooth count and pitch must match the links you depict—draw the pitch once, then copy‑fit across sprocket, idler groove, and the link pins. Segmental sprockets (bolt‑on tooth rings) are common for maintenance; showing bolt patterns and segment joints in an exploded view helps rigging and damage states.

Track links and shoes give the belt its contact language. Double‑pin links resist twist and show a pronounced center guide; single‑pin links are lighter with different clamp logic. Bolt‑on rubber pads enable road travel and urban operations; grousers or chevron bars project soft‑ground bite. Shoe width is a flotation dial: snowcats and muskeg carriers carry very wide belts; urban armored kits run narrower shoes to fit streets and save weight. The silhouette must display pad bolt patterns and guide horns at realistic cadence—noisy textures cannot fix an impossible pitch. In callouts, include pad bolt torque ranges, pin types (dry pin, sealed and lubricated), and ice/snow cleat options so variants feel engineered rather than painted.

Suspension families sit beneath the geometry and set ride and firing behavior. Torsion‑bar systems place swing arms at each road wheel station with bars running across the hull; they are compact and common in combat. Hydropneumatic systems link arms to gas‑charged struts that can kneel or trim the hull—useful for artillery or amphibious trim control. Horstmann‑style bogies package springs externally for easy field service. Roller‑frame bogies on dozers and excavators deliver maximum ground conformity for pushing and digging, with oscillation joints between frames and hulls. Rubber‑belt utility suspensions often use torsion beams with small independent bogies; they read light and fast in silhouette. Your cutaways should show spring/damper orientation, arm pivots, bump stops, and travel; your rig notes should include ride‑height bands (transport/combat/kneel) with degrees or millimeters of change.

Combat platforms leverage geometry for mobility and protection. MBTs emphasize wide shoes, long contact runs, and side skirts with modular tiles; IFVs show higher aft hulls with crew ramps and more idler‑to‑sprocket daylight for trench approach. Self‑propelled guns need hydropneumatic kneel to stabilize firing. In concept, stage trench‑crossing and fording scenes to validate proportions; in production, provide metrics for trench length, vertical step, fording depth, neutral‑steer capability, and ride‑height range so physics and animation can match intent. Damage states should interpret thrown tracks, broken bogies, or pad loss along logical fastener lines—not random tears.

Utility carriers and snowcats turn geometry into low ground pressure. They wear very wide belts with shallow grousers, large radiused leading edges to reduce digging, and generous fender daylight for snow ejection. Cabins sit forward for visibility; cargo decks float on subframes; heaters route to belt guides and cab glass. In production packages, include ground pressure targets (kPa), belt thickness, molded guide geometry, heater intake and exhaust paths, and idler/roller service hatches; in VFX/audio notes, provide snow plume references and belt squeak/rattle behavior on ice.

Construction tracks optimize for push, pull, and cycle time. Dozers carry long roller frames and tall grousers; excavators have car bodies with swing bearings atop track frames that house rollers and idlers; compact track loaders use rubber belts on bogies and tensioners behind sealed covers. Your silhouettes should expose blade/arm envelopes and roller‑frame logic; production callouts need blade tilt/angle/pitch ranges, swing clearances, roller diameters, and clean‑out doors. Particle paths for mud and aggregate should follow the lower run and eject at realistic stations—scrapers, sprocket fling, and idler shedding.

Articulated alternatives (ADTs, loaders) keep tires but hinge frames. The articulation joint permits yaw steering and roll oscillation. In silhouette, a narrow waist, large hydraulic steering cylinders, and oscillation pins tell the story. Provide yaw and roll ranges, hose routing with rub strips, and lock‑out pins in callouts. For tracked skid‑steer machines, differential speed turning produces its own wear—design shoes and pads that survive that abuse, and stage VFX for dust and ground scuffing accordingly.

Rigging and implementation succeed when you supply the right anchors. A track path spline annotated with lower run, wraps, upper run, and sag target lets teams choose between link rigs and UV scrolls for belts. Wheel station coordinates, sprocket/idler centers, and bogie pivot points allow clean hierarchies. Tensioner travel and pose states prevent interpenetration at extremes. Ejecta emitters tied to the lower run and sprocket wrap keep dust, snow, and mud believable. LOD gates should reduce visible link frequency while preserving shoe silhouette and guide rhythm.

Camera readability depends on cadence. At far range, the player must read shoe width, contact length, and skirt rhythm. Mid‑range should reveal wheel/bogie spacing and sprocket/idler positions. Near, pad bolts, guide horns, and roller wear become texture anchors. Test under dust, mud, snow, and night lighting. Place emissives at corner landmarks and along ramps/ramps, not as strips that wash detail along the skirt.

Deliverables turn drawings into instructions. A metrics sheet should list track pitch, link count per side, shoe width, contact length, sprocket/idler diameters, wheel station spacing, suspension family, ride‑height bands, neutral‑steer yes/no, trench/step/fording limits, and ground pressure targets. Orthographics need a visible track‑path overlay, measured centers for sprocket/idler and wheels, return roller spacing, skirt panelization, and fender edges with ejecta channels. Cutaways should expose suspension arms, springs/struts, adjusters, and guides; exploded views should break down a link + pad + pin stack, a bogie assembly, a road wheel with hub and bearings, an idler carrier and tensioner, and a segmented sprocket. Callouts must bind pad bolt torques, tensioner stroke, sag targets, shock strokes, hydropneumatic pressures, bogie pivot bushings, and hose routing minima. A rig pack should include a named hierarchy, pivots in project units, recommended bone counts, UV‑scroll speeds for belts, and link‑frequency LOD thresholds.

Indie and AAA cadences differ in density. Indie projects succeed with a single evolving canvas per vehicle: silhouette loop + side ortho with track path + a compact suspension cutaway + rig notes and VFX emitters. AAA pipelines split by gate: mobility metrics lock, suspension family lock, orthos/callouts for modeling kickoff, rigging & FX pass, and camera‑read sign‑off across biomes, backed by a shared undercarriage kit of links, pads, rollers, skirts, and fasteners to protect style and performance.

From the concept seat, truth in bogies, idlers, and road wheels is how you sell purpose before paint. From the production seat, coordinates and ranges are how you keep motion cheap and reliable. When both sides align, tracks look inevitable on rough ground, articulated frames feel alive under load, and your vehicles read capability the instant they hit the screen.