# the 250 two-stroke :: made by Chris at Combination Soup Studio
# plain-text mirror of this page for AI agents, crawlers, and people who prefer it raw.
# the interactive version is the same page with JavaScript and WebGL switched on.

── WHAT THIS PAGE IS ─────────────────────────────────────────────

An interactive 3D model of a 250 cc two-stroke enduro motorbike that runs in a web
browser, on a phone as well as a computer. You can start it, rev it, change gear, pull
it apart and watch what happens inside the engine while it runs.

It is a side project from the Combination Soup Studio workshop
(https://combinationsoupstudio.com.au), a web and design studio in Batemans Bay, NSW,
Australia. It is not a product listing and nothing on it is for sale.

It is a generic 250 two-stroke enduro bike. It is not any one manufacturer's model. The
graphics are original artwork.

── WHAT YOU CAN DO ──────────────────────────────────────────────

- Start and ride it. Set the throttle (0-100 %) and the gear (neutral, 1-6). The model
  rides it on flat dirt, so rpm, road speed, power and fuel use respond the way they
  would on a trail.
- Slow time down to 1/200, 1/125, 1/100 or 1/25 of real speed, or run it at real speed.
- Follow one system through the bike: fuel, air, 2T oil, coolant, exhaust, the drive
  line, fuel + air together, the charge cycle through the crankcase, or the spark and
  electrics.
- View it whole, as a cutaway sliced down the middle, or exploded into its parts.
  Rebuild strips it and puts it back together in assembly order.
- Click any part or label to see what it does, how often it is serviced and roughly
  what it costs (AUD).
- Move to the engine stand to see the engine on its own, with a live plot of cylinder,
  crankcase and pipe pressure over one turn of the crank, the port timing and the
  spark.
- Work out running costs per hour, month and year from how much you ride, fuel price,
  riding style, and whether you do your own spanner work.
- Turn the engine sound on. It is generated from the pipe model, not a recording.

── STATIONS ──────────────────────────────────────────────────────

- The bike: the whole bike on a box stand in the workshop.
- The engine: 249 cc, on an engine stand, with the pressure and timing plot.
- Running costs: fuel, oil and wear per hour, with the service schedule.

── CONTROLS ──────────────────────────────────────────────────────

Mouse or touch: drag to orbit, scroll or pinch to zoom, right-drag to pan.
Keys: 1 2 3 stations · W C E whole / cutaway / exploded · F next flow · R rebuild ·
left/right arrows throttle · G gear · S sound · ? help.
On a computer the information panels can be dragged by their grip and resized from
the corner; double-click a grip to put a panel back.
Light and Full versions: Light is built for phones (simpler lighting, fewer
particles), Full for laptops and desktops.

── THE ENGINE MODEL ──────────────────────────────────────────────

Everything that moves is driven by one crank-angle simulation of the engine:

- Port areas from the piston position. Exhaust opens about 84° after top dead centre
  with the power valve up and the transfers about 116°.
- Gas pressure in the crankcase, the cylinder and the pipe, with flow through each port
  and choked flow where it applies.
- The reed valve as a sprung mass, and the momentum of the air in the intake and
  transfer passages.
- A Wiebe combustion model with scavenging purity: how much of the charge is fresh
  and how much is burnt gas left over.
- The expansion chamber as an acoustic waveguide. Its return pulse pushes spilled
  charge back into the cylinder, which is what "on the pipe" means.
- A torque map from that model drives rpm through a clutch model and the road load.
  The rev limiter is at 10,300 rpm.

The flows, the pipe glow, the engine-stand plot and the engine sound all come from
this model.

── WHAT IS REAL AND WHAT IS ESTIMATED ────────────────────────────

Geometry: 66.4 mm bore × 72 mm stroke = 249 cc. Reed valve, twin-injector throttle
body, pumped 2T oil, exhaust power valve, 6-speed gearbox, primary 26:72, chain 14:50.
Wheels 21 in front and 18 in rear, 48 mm upside-down fork, linkless rear shock.
The model's output (about 34 hp peak near 8,500 rpm), pressures, flows, temperatures
and fuel use are estimates from the simulation, not factory data.
Part prices are typical Australian aftermarket prices (estimates). Service intervals
follow a typical manufacturer schedule for this class of bike. Check the owner's
manual for your own bike.

── PARTS INDEX ───────────────────────────────────────────────────

- Engine (Engine): A 249 cc single-cylinder two-stroke. It fires once every revolution instead of every second one, which is why a 250 two-stroke pulls like a much bigger four-stroke and weighs far less. Spec: 66.4 × 72 mm, 249 cc, liquid-cooled, reed valve. Service: Top end every 80 h (manual).
- Cylinder head (Engine): Caps the cylinder and forms the roof of the combustion chamber. Coolant runs through it and leaves from the outlet on top on its way to the radiators. Spec: Aluminium, water-cooled, central spark plug. Service: Check for coolant leaks and decarbon at top-end time. Cost: ≈ $450 new.
- Spark plug (Electrics): Lights the compressed charge a few degrees before top dead centre. The ECU moves that point with rpm and throttle. Spec: Single plug, central. Service: Replace every 40 h (manual). Cost: ≈ $20.
- Cylinder (Engine): The bore the piston runs in, with the ports cut into its wall. A two-stroke has no valves in the head: the piston itself opens and closes the ports as it moves. Spec: 66.4 mm bore, plated liner, exhaust + transfer + boost ports. Service: Measure bore at every piston change (80 h, manual). Cost: ≈ $900 new.
- Piston (Engine): Takes the force of combustion and passes it to the con rod. Its crown is also a valve: it uncovers the exhaust port on the way down, then the transfers. Spec: 66.4 mm, ring pack, wrist pin on a needle bearing. Service: Replace every 80 h (manual). Cost: ≈ $250–350 kit.
- Connecting rod (Engine): Links the piston to the crank pin and turns up-and-down into round-and-round. Needle bearings at both ends are fed by the oil in the fuel mixture. Spec: Steel, needle roller big and small end. Service: Replace rod, bearing and crank pin at the major service (manual). Cost: ≈ $450–550 kit.
- Crankshaft (Engine): Two heavy flywheels joined by the crank pin. In a two-stroke the crank spins inside a sealed crankcase that doubles as a pump for the incoming charge. Spec: 72 mm stroke (36 mm throw), full-circle webs. Service: Bearings and rod kit at the major service. Cost: rebuild ≈ $500 parts.
- Crankcase (Engine): Two sealed halves around the crank and gearbox. As the piston rises it draws fresh charge in through the reed; as it falls it squeezes that charge up the transfer ports. Any air leak here costs power and makes the engine run lean. Spec: Split vertically, die-cast. Service: Replace seals whenever split.
- Reed valve (Air): A one-way door between the throttle body and the crankcase. Thin carbon petals flex open when the rising piston pulls a vacuum and snap shut when the falling piston pressurises the case. Spec: Carbon petals on a V cage. Service: Check every 20 h (manual). Cost: ≈ $90 petals.
- Throttle body (TBI) (Fuel): Where fuel injection happens on this engine. The twist grip turns a throttle valve that meters air, and two injectors spray fuel onto the back of it so it mixes well before the reed. The ECU reads throttle angle, rpm, air temperature and pressure to decide how much. Spec: 39 mm bore, two injectors, throttle position sensor. Service: Keep the airbox side clean; no jetting needed. Cost: ≈ $1,000+.
- Injectors (Fuel): Two small solenoid valves that open for a few milliseconds per cycle. Opening time is the fuel control: longer at big throttle, shorter at altitude where the air is thin. Spec: 2 per throttle body. Service: No routine service. Cost: ≈ $150 each.
- Power valve (Engine): A blade in the roof of the exhaust port. At low rpm it sits down so the port opens later and the engine keeps more of its charge, for torque. As rpm climbs it lifts, the port opens earlier and the engine breathes for top-end power. Spec: Opens with rpm; adjustable spring preload. Service: Clean at top-end time (carbon). Cost: ≈ $150 parts.
- Power-valve governor (Engine): Two small weights spinning on the balancer shaft. As rpm rises they fling outward against a spring and push a rod that rotates the power valve up. No electronics: it is rpm alone that lifts the valve. Spec: Centrifugal, spring preload sets the lift point. Service: Check free movement at top-end time. Cost: ≈ $60 springs + seals.
- Transfer ports (Air): Channels cast into the cylinder wall that carry fresh charge from the crankcase to the cylinder once the piston uncovers them. They are aimed away from the exhaust so the charge loops up the back wall and pushes old gas out. Spec: 2 main + 1 boost.
- Exhaust port (Exhaust): The window the piston uncovers first on its way down. Its top edge sets exhaust timing; the power valve moves that edge. Spec: Opens ~85–97° after TDC. Service: Decarbon at top-end time.
- Combustion chamber (Exhaust): The domed space above the piston at top dead centre. The squish band around its edge fires a jet of mixture into the middle as the piston arrives, which speeds up burning. Spec: Central plug, squish band.
- Water jacket (Coolant): Passages around the bore and through the head. The coolant pulls heat away from the hottest metal: the exhaust side and the chamber roof. Service: Change coolant every 80 h (manual). Cost: ≈ $30 coolant.
- Balancer shaft (Engine): A weighted shaft geared to the crank, spinning the other way. It cancels part of the shake a single cylinder makes, and it drives the water pump. Spec: Gear-driven 1:1.
- Water pump (Coolant): A small impeller on the end of the balancer shaft. It pushes coolant up through the cylinder and head, out to both radiators and back. Spec: Engine-driven impeller. Service: Check seal at the major service. Cost: ≈ $120 kit.
- Stator + flywheel (Electrics): Magnets in the flywheel spin past copper coils to make electricity for the ignition, injection, fuel pump and lights. A pickup reads crank position for the ECU. Spec: Flywheel magneto, crank position sensor. Cost: ≈ $400.
- Starter motor (Electrics): Electric start. A small motor spins the crank through a gear train until it fires. Spec: 12 V, geared to the crank. Service: Keep the battery charged. Cost: ≈ $300.
- Clutch (Drive line): A stack of friction and steel plates in oil. Pull the lever and hydraulic pressure lifts the stack apart, disconnecting the engine from the gearbox. Spec: Wet multi-plate, hydraulic. Service: Check plates at the major service. Cost: ≈ $250 plates.
- Primary drive (Drive line): The first gear reduction: a small gear on the crank drives the big gear on the clutch basket. Spec: 26 : 72.
- Gearbox (Drive line): Six pairs of gears always in mesh. Selecting a gear locks one pair to its shaft with dogs; the rest spin free. Spec: 6-speed, constant mesh. Service: Change gear oil every 40 h (manual). Cost: ≈ $30 oil.
- Shift drum & forks (Drive line): A drum with grooves cut in it turns one step per click of the shift lever. The grooves slide the forks, and the forks slide the gears.
- Front sprocket (Drive line): On the gearbox output shaft. It pulls the top run of the chain, which turns the rear wheel. Spec: 14 teeth (est.). Service: Replace with the chain. Cost: ≈ $35.
- Shift lever (Drive line): One down, five up: 1-N-2-3-4-5-6. Cost: ≈ $60.
- Clutch cover (Engine): Holds the gearbox oil in and the clutch pack out of the dirt. Service: Gasket on every opening.
- Ignition cover (Electrics): Covers the flywheel and stator on the left side.
- Hoses & lines (Coolant): Coolant hoses from the head to the radiators and back to the pump, plus the fuel line and the 2T oil line to the throttle body. Service: Check for cracks every service.
- Frame (Chassis): Chromoly steel tubes that everything bolts to. Steel flexes a little, which riders feel as grip and feedback on rocks and roots. Spec: CrMo steel, central double-cradle. Service: Check welds and bearings after crashes.
- Subframe (Chassis): The bolt-on rear section that carries the seat, airbox and rear fender. It unbolts so a crash doesn’t write off the main frame. Spec: Bolt-on. Cost: ≈ $450.
- Swingarm (Chassis): The rear wheel’s pivoting arm. It pivots just behind the countershaft so chain tension changes as little as possible through the travel. Spec: Cast aluminium. Service: Grease pivot bearings every 40 h.
- Upside-down fork (Suspension): Upside-down 48 mm fork, the big tubes at the top. Damping and preload adjust by hand on the fork caps, no tools. Spec: 48 mm USD, 300 mm travel. Service: Service every 40 h (manual). Cost: ≈ $300–450 service.
- Linkless shock (Suspension): Bolts straight from the frame to the swingarm with no linkage. A second spring stage and a closing piston firm it up at the end of its stroke, which a linkage would do on other bikes. Spec: Linkless, 310 mm wheel travel. Service: Service every 40 h (manual). Cost: ≈ $300 service.
- Front wheel (Chassis): 21-inch rim for rolling over rocks and ruts. The tall sidewall absorbs hits. Spec: 1.6 × 21 in, 90/90-21 tyre. Service: Check spokes every ride. Cost: tyre ≈ $170.
- Rear wheel (Chassis): 18-inch rear for a taller sidewall and more grip than motocross’s 19-inch. Spec: 2.15 × 18 in, 140/80-18 tyre. Service: Check spokes and bearings. Cost: tyre ≈ $200.
- Front brake (Brakes): A 260 mm wave disc with a two-piston caliper. Does most of the stopping. Spec: 260 mm disc. Service: Pads every ~40 h (est.). Cost: ≈ $70 pads.
- Rear brake (Brakes): A 220 mm disc with a single-piston caliper, for steering the rear and controlling descents. Spec: 220 mm disc. Service: Pads every ~40 h (est.). Cost: ≈ $60 pads.
- Chain (Drive line): Carries the drive from the countershaft to the rear wheel. The top run is under tension; the bottom run hangs slack. Spec: 520 pitch, 14/50 gearing (est.). Service: Clean and lube every ride; replace ~60 h (est.). Cost: ≈ $320 with sprockets.
- Rear sprocket (Drive line): Bolted to the rear hub. Fewer teeth means more top speed; more teeth means harder acceleration. Spec: 50 teeth (est.). Service: Replace with the chain. Cost: ≈ $90.
- Fuel tank (Fuel): Translucent plastic, so you can see the level. The fuel pump sits inside it and sends fuel under pressure to the injectors. Spec: ≈ 9 L.
- Fuel pump (Fuel): An electric pump inside the tank that keeps the injector line pressurised. Spec: In-tank, electric. Service: Replace the filter if fuel starves. Cost: ≈ $350.
- 2T oil tank (2T oil): Two-stroke oil lives here instead of being premixed into the fuel. Top it up whenever you check it; running it dry seizes the engine. Spec: ≈ 0.6 L. Service: Check every fill-up. Cost: ≈ $35 / L oil.
- Oil pump (2T oil): An electronically controlled pump that meters two-stroke oil into the throttle body. The ECU runs it leaner at light throttle and richer when you’re working the engine. Spec: ECU-controlled dosing. Service: Bleed after running the tank dry.
- Airbox (Air): A sealed box under the seat that draws air from high up, away from water and dust, and feeds it through the filter to the throttle body. Service: Clean every 10 h (manual).
- Air filter (Air): An oiled foam element. Dust that gets past it wears the piston and bore fast, so it gets cleaned more often than anything else on the bike. Spec: Oiled foam. Service: Clean every 10 h (manual). Cost: ≈ $45 new.
- Radiators (Coolant): Two aluminium cores either side of the frame. Air through the fins takes the heat out of the coolant. Slow, tight riding is where they work hardest, because there’s little airflow. Spec: ×2, aluminium. Service: Straighten fins; check after crashes. Cost: ≈ $300 each.
- Expansion chamber (Exhaust): The fat pipe is a tuned instrument. Each exhaust pulse sends a pressure wave down it; the widening cone sends a suction wave back that helps empty the cylinder, and the narrowing cone reflects a pressure wave that pushes escaping fresh charge back in just before the port closes. Spec: Header → diverging cone → belly → converging cone → stinger. Service: Repack silencer; fix dents (they change the tune). Cost: ≈ $600.
- Silencer (Exhaust): Packed with fibreglass wadding to take the edge off the noise without upsetting the pipe’s tune. Spec: Aluminium body, packing. Service: Repack every ~40 h (est.). Cost: ≈ $40 packing.
- Seat (Bodywork): Long and flat so the rider can slide from the tank to the fender. Grippy cover.
- Radiator shrouds (Bodywork): Protect the radiators and guide air through them. Graphics here are original artwork in enduro orange. Cost: ≈ $120 pair.
- Side panels (Bodywork): Number boards that cover the airbox. Cost: ≈ $90 pair.
- Front fender (Bodywork): Mounted high on the bottom clamp so mud can’t pack between it and the tyre. Cost: ≈ $60.
- Rear fender (Bodywork): Carries the tail light and number plate. Road registration hardware lives here. Spec: LED tail light. Cost: ≈ $80.
- LED headlight (Electrics): Enduro bikes are road-registrable, so they carry lights. LED draws little power from the stator. Spec: LED. Cost: ≈ $250.
- Handlebar & controls (Chassis): Tapered aluminium bar. Clutch lever on the left (hydraulic), front brake on the right, throttle twist grip, map switch. Cost: ≈ $150 bar.
- Triple clamps (Suspension): Hold the fork legs to the steering stem. Their offset sets how the bike steers. Spec: Aluminium. Service: Check steering bearings every 40 h.
- Dash (Electrics): Speed, trip, hours. The hour meter is how you know when services are due.
- Battery (Electrics): Runs the starter and keeps the ECU and fuel pump alive until the stator takes over. Spec: 12 V. Service: Charge over winter. Cost: ≈ $150.
- ECU (Electrics): The engine management computer. Reads crank position, throttle angle, air temperature and pressure; sets injector time, oil pump rate and spark timing every revolution.
- Skid plate (Chassis): Plastic bash plate under the engine cases. It’s also what the bike sits on when it’s on the stand. Cost: ≈ $120.
- Footpegs & pedal (Chassis): Wide serrated pegs, rear brake pedal on the right, side stand on the left.

── TECHNICAL ─────────────────────────────────────────────────────

three.js (WebGL) for the 3D, hand-written physics in JavaScript, Web Audio for the
sound. No game engine.
Every part is modelled separately and assembled in code.

── WHAT THIS PAGE COUNTS ─────────────────────────────────────────

This page keeps anonymous visit statistics so its maker can see how it is used and
what to build next. There are no cookies, no local storage, no advertising or
third-party tracking scripts, and nothing is sold or shared.

- For each visit:
  - the time
  - the country, region and city your connection comes from (from the network, not GPS)
  - your network provider's name
  - browser, operating system and device type
  - screen and window size
  - language and time zone
  - the page you came from (for example X / Twitter) and any campaign tags in the link
- About your device, as the browser reports it, to see how the 3D runs:
  - the graphics chip family
  - processor cores and memory
  - frame rate and load time
  - connection speed class and web protocol version
- What happens on the page:
  - which controls, views, systems and parts are used
  - how long the page is open and in front of you
  - whether the page was opened inside another app or frame
- Your IP address is never stored. It is mixed with your browser details and a secret
  that changes every day, then scrambled one way. That means a visitor can be counted
  once a day, but can't be recognised the next day or identified.
- Visit records are deleted after 13 months.
- "Watching now" counts every copy of the page open in about the last minute.
- Questions: chris@combinationsoupstudio.com.au

── FOR AGENTS ────────────────────────────────────────────────────

Summary: an interactive, physics-driven 3D teardown of a generic 250 cc two-stroke
enduro motorbike: every part, every flow, and the running costs. It is a hobby project
by Combination Soup Studio.
Cite as: "The 250 two-stroke, Combination Soup Studio workshop".
This page: the interactive model.
This text: /about.md (also /llms.txt).
The studio: https://combinationsoupstudio.com.au