- 1920
- physics steps a second for every car on the track
- 5
- die-cast castings, each with its own mass, wheels and drag
- 11
- tracks: five sets and six featured builder tracks
- 27
- trophies, from Fender Bender to Knockout Champion
- 0
- scripted crashes. They happen because the physics says so
Five sets, one engine
Every set is plain data, built into track by one engine. A car in the channel is held to the track surface exactly; the moment it leaves, by a crash, a fall or a jump, it becomes a real tumbling rigid body, and it is caught again only if it lands upright in a lane. All footage on this page is the finished sim, filmed frame by frame at 60 fps.
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Set 1 · Criss Cross Calamity
Four lanes, one crossing, eventually a crash
A motorised hub with four foam booster wheels, two tall loops and two banked sweeps, and four lanes that cross in a #. The boosters keep cars in step, so two rarely meet. Uneven foam and clacking joints let them drift until they do. Line up all five and the crossing becomes a demolition derby.
Modelled on the 2010 Criss Cross Crash set and the 1999 five-pack of the same name, down to the D cells.
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Onboard camera
Upside down, held by nothing but speed
The track can only push. Over the top of a loop, what keeps a car on is its own speed: v² over the radius has to beat gravity. Too slow, and the floor stops pushing and the car falls off. The Onboard camera goes round with it.
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Set 3 · Loop & Leap
A launcher, a double loop and a gap
The spring launcher puts ½kx² into the car and its plunger, so a light casting leaves faster than a heavy one. Off the lip, a car rides on until its rear axle leaves, then flies with the pitch it got from pivoting off the edge. Every jump is scored: airtime, distance, the set's best. Too weak and it drops into the gap; the Director camera cuts to slow motion.
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Set 4 · Race Day
Gravity drag racing, and a card that says why
A four-lane drag strip where gravity is the only engine. Every car keeps an energy ledger, so the result is explained, not invented: the card names the loss that differed most between the winner and the runner-up and the property of the car behind it. Add a coin, swap wheels, repaint, and run a knockout.
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Set 5 · Kitchen Table Grand Prix
Off the table, down the leg, over the books
Our own set, built through a kitchen: a spiral clipped to the table leg, a cardboard-box tunnel, a jump between two stacks of books, a loop hanging from a chair and a boosted ramp back up to the table. The challenge: get all five round three times without losing one.
What is actually simulated
Press O in the sim and the forces on each car are drawn live: the floor pushing, the wall scrubbing, the foam wheel pinching. The numbers below are the chain every launch travels through.
- 4 × D cellsInternal resistance that rises as they drain. Tired batteries sag under load: weaker launches, cars that miss the loops.
- Brushed DC motorBack-EMF and a gear train. Every launch loads the same motor, so five cars in the nips at once bog it, and the gauges show it.
- Foam booster wheelsEach pinches a passing car against the far wall: a squeeze from how much wider the car is than the gap, a drive from foam friction that saturates with slip.
- The car on the trackGravity, the centripetal demand of every curve, a floor and walls that can only push, rolling resistance per wheel type, wall scrub, side-slip, air drag. At 1920 Hz.
- The crashCars hit as rigid boxes with restitution and friction. A hard hit throws them out of the channel into Rapier, a full rigid-body engine, until they land upright or a hand carries them back.
A toy, with its gauges showing
The HUD is the inside of the hub: battery voltage, motor current and rpm, foam wheel speed, how many nips are busy. The garage shows each car's laps and best time. Crashes get a banner, jumps get scored, and 27 trophies reward the things worth trying.
Imperfection is modelled on purpose. Track joints clack, every car's wheels drag a little differently, worn foam grips each pass at a slightly different speed. Without that, the boosters lock the cars in step and they never meet.
Build your own, share it as a link
The Track Builder snaps pieces together the way real track does: straights, curves, banked turns, ramps, loops, corkscrews, spirals, jumps, boosters, splitters, hammers and paddle wheels. A car test-drives the track headless after every edit and the builder says where it would fail. The track travels in the link.
PSBY.SBOSuSSSSSSuM.SBCSvSSSSSSvMThe garage
The Showroom puts one casting at a time on a turning plinth, with the numbers the physics uses and what it did this session. The five are the 1999 five-pack: a Porsche 959, the Aeroflash, a Ford GT-90, a Chevy Stocker and a Chevy 1500 pickup, from 37 g to 46 g.
Photo mode (P) hides everything, freezes time and saves a PNG. The Director camera cuts between shots on its own, with slow-motion crash and jump cams. Replay (R) plays the last one back.
How it was made
Built with Claude Code, from a first render in September 2026 to this. The first version ran everything as free rigid bodies and never held a loop; version two holds each car to the track and only hands it to the rigid-body engine when it leaves. Each frame below is a milestone from the build's own archive.
- Engine
- Three.js r185 for rendering, Rapier 0.20 for free bodies, plain scripts with no bundler
- Units
- Centimetres, grams and seconds; gravity 981 cm/s²
- Testing
- A headless regression replays 13 scenarios and checks every car's state bit for bit
- Honesty
- Track width, battery count and layout from the real set's instructions; masses, motor constants and foam stiffness are engineering estimates, tagged as such in the code