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The early Grand Theft Auto driving model was not built from a sophisticated real-world tyre simulator. Pat Kerr says it began as a 2D rigid-body physics demo that he wrote over one weekend in late August 1996, using GFA BASIC on an Atari ST. He later ported the code to C for use in the game. A simple, deliberately approximate tyre layer was then placed on top of the rigid body, giving cars enough force, grip, resistance and torque to drive, slide, spin and recover in a readable game world.
That distinction explains why the cars could feel physically coherent while still behaving unlike real vehicles: translation and rotation came from a general physics core, while tyre grip was an intentionally inexpensive approximation.
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Which “original GTA demo” is being discussed?
The demo in this account is Pat Kerr’s wireframe physics prototype, not DMA Design’s 1995 Race’n’Chase design document and not the released 1997 Grand Theft Auto. Kerr dates his prototype to one weekend in late August 1996. In his project history, he writes: “One weekend in late August 1996, I wrote a 2D physics simulation which later became the basis of the vehicle system in the eventually popular game ‘Grand Theft Auto’ aka ‘GTA’.” Kerr’s project history is the source for that first-person account.
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- Late August 1996: Kerr writes the 2D simulation as a weekend project.
- Initial platform: the experiment runs as a wireframe program in GFA BASIC on an Atari ST.
- Game integration: the physics is ported to C for use in Grand Theft Auto.
The available evidence supports Kerr’s account of the prototype’s origin and influence. It does not independently reproduce the original Atari binary or prove every implementation detail of the final game, so claims about the shipped system should be attributed to Kerr rather than presented as a complete source-code reconstruction.
The physics had two deliberately different layers
Kerr describes a general-purpose 2D rigid body first, followed by a vehicle-specific force model. Thinking in those layers is the clearest way to understand the demo.
1. A rigid body that translates and rotates
The core body stores the state needed for movement in a flat 2D world: position, velocity, angle, angular velocity, force and torque. A force applied through the body’s centre changes its linear movement. Apply that force away from the centre and it also produces torque, changing the body’s rotation.
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| Rigid-body element | What it controls | Why it matters in the demo |
|---|---|---|
| Position | Where the body is in the 2D scene | Moves the vehicle through the map |
| Velocity | Linear movement and momentum | Lets the body keep moving between force updates |
| Angle | Current orientation | Defines which direction is forward and where the tyres sit |
| Angular velocity | Rate of rotation | Allows sustained spins and changing heading |
| Force | Linear acceleration input | Drives the body forward or resists motion |
| Torque | Rotational acceleration input | Turns the body when forces act away from its centre |
This is a compact but powerful choice for a top-down driving game. The simulation can represent movement and rotation with one consistent body instead of scripting each visual response independently.
2. A car layer built from tyre forces
On top of the rigid body, Kerr added a vehicle layer. It supplies engine force, steering and resistance at four tyre positions. For each tyre, the model determines how the local velocity is split into a forward component and a sideways component. The resulting resistance forces are then applied to the body, so each contact point can influence both translation and rotation.
Kerr characterizes this as “a simple, approximate, and technically incorrect model of how tyres behave.” The purpose was not to calculate real rubber friction, suspension geometry or a tyre’s physical slip curve. It was to produce controllable game behaviour from a small set of forces.
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| Vehicle input or force | Effect in the model | Interpretation for the player |
|---|---|---|
| Engine force | Pushes the car along its forward direction | Acceleration |
| Steering | Changes the direction associated with the tyre forces | Cornering input |
| Forward (rolling) resistance | Damps movement along a tyre’s rolling direction | Slows the car without making it instantly stop |
| Sideways resistance | Damps lateral movement at each tyre | Provides grip and limits uncontrolled sideways sliding |
| Off-centre application | Creates torque as well as a linear force | Changes the car’s rotation while it moves |
Why the early cars could slide and keep rotating
Sideways damping was stronger than rolling damping
In Kerr’s recreated car model, resistance to sideways movement is much stronger than resistance while a tyre rolls forward. That single relationship gives the car a usable notion of grip: forward motion is allowed to continue, while lateral velocity is actively damped.
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When the balance is disturbed by steering, a collision or a handbrake, the lateral component can become large enough to produce a visible slide. Because the resistance is applied at tyre positions rather than only at the centre of the body, those forces can also change angular velocity. The car may therefore translate in one direction while its nose points elsewhere, then rotate back as the tyre forces act on it.
The resistance is damping, not a capped friction law
Kerr stresses that the recreated model applies damping proportional to velocity without a force cap. In practical terms, the faster a tyre moves sideways, the larger the opposing resistance becomes. There is no documented maximum friction force that would represent a tyre’s finite grip budget, and no claim that the model calculates a physically correct slip angle, contact patch or transition between static and kinetic friction.
That limitation is why the model should not be described as realistic tyre simulation. It is an educated approximation designed to make the rigid body controllable. The car can look convincing in a top-down game because the forces are applied consistently, even though the underlying tyre behaviour is technically incorrect.
What the handbrake changes
The handbrake is implemented by changing the rear tyres’ resistance. Kerr’s description of the recreation says it increases rear rolling resistance and reduces rear sideways grip. The rear tyres therefore contribute less lateral damping while their forward movement is resisted more strongly. With the rear no longer tracking the front as firmly, steering can provoke a slide and the body’s angular motion becomes easier to sustain.
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This is a game-design control, not a detailed model of a mechanical handbrake. It changes selected force coefficients at the rear axle, giving the player a reliable way to initiate a turn or spin. The rigid-body core then carries the resulting rotation and momentum.
How this relates to the 1995 Race’n’Chase plan
DMA Design’s archived Race’n’Chase: Game Design, Version 1.05, dated March 22, 1995, describes a fast multiplayer car-racing and crashing game. Its design goals include driving, stealing, racing, collisions and navigation, and its physics discussion mentions handling for stunts such as handbrake turns and spinning wheels. You can read the document in the archived design-document PDF.
The document establishes that expressive driving and crashes were part of the project’s design intent before Kerr’s 1996 prototype. It does not show that every proposed feature shipped unchanged, nor does it specify Kerr’s later equations. The useful historical connection is one of direction: the project wanted fast, stunt-friendly driving, and the prototype supplied a compact physical foundation capable of producing those behaviours.
Early implementation was exciting but unstable
Mike Dailly’s retrospective account adds the development context around the driving system. In a 2015 interview with The Guardian, he recalled: “When we first got the driving part done, everyone huddled around to see it working. It was brilliant, just bombing around the city, but it was still bug-ridden and the demo was always crashing.” The interview also describes major disagreement and difficulty over car collisions. Read the full recollection in The Guardian’s “How we made … Grand Theft Auto”.
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Dailly’s comments are a retrospective description of early development problems, not a precise technical diagnosis of Kerr’s later physics code. They do, however, explain why a compact, understandable model mattered. A system could be fun to drive before it was robust, and collisions were a separate engineering challenge from making a body respond plausibly to forces.
What Motion Lab demonstrates today
In 2026, Kerr published Motion Lab, a JavaScript recreation of the concept. It includes car, ship and brick modes. The car mode illustrates the tyre-force layer; the ship and brick modes reuse the general physics core with different force behaviour. Controls expose a camera dead zone, speed-related zoom, side barriers and vertical gravity, making it possible to see how presentation and environmental forces affect the same underlying body simulation.
Motion Lab is useful as a visual explanation, but it is a later recreation, not the original Atari program. Kerr states that it is independent and not affiliated with Rockstar Games, Take-Two or the Grand Theft Auto franchise. Treat it as an educational demonstration of the described ideas rather than a preserved build of the 1996 prototype or a guaranteed copy of the released game’s complete implementation.
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Rotation was a first-class part of movement
The most consequential choice was to make the vehicle a rigid body with angular state. A car’s heading, spin and momentum were not merely visual effects layered on a path. Off-centre forces could change the body’s rotation, so steering, tyre resistance and collisions interacted through the same mechanism.
Game feel came from force relationships
The model did not need a full automotive simulation to create recognizable driving cues. Stronger lateral damping than rolling damping gave the car directional stability. Reducing rear lateral grip made a handbrake turn possible. Engine force supplied acceleration, and the rigid body preserved the resulting momentum. These relationships are more important to the game feel than whether the tyre equations match real-world measurements.
Simplicity made the system adaptable
Because the physics core was general, it could support other bodies and force arrangements. Kerr’s ship and brick modes show the same foundation being reused rather than a separate bespoke simulator for every object. That kind of separation—general body mechanics underneath, gameplay-specific forces above—also explains how the prototype could be ported from a home-computer experiment into a larger game codebase.
What cannot be concluded from the demo account
- It does not establish that the released 1997 game used exactly the same coefficients, update order or collision code as the recreation.
- It does not prove that every feature described in the 1995 Race’n’Chase document was implemented or shipped.
- It does not justify calling the tyre model realistic, physically accurate or equivalent to a modern vehicle simulator.
- It does not provide measured frame rates, tyre-force values or a numerical performance benchmark.
- It does not replace testing of the original game; the historical and technical claims come from Kerr’s project history, the archived design document and Dailly’s retrospective interview.
The strongest defensible conclusion is narrower and more useful: Kerr says a 1996 2D rigid-body experiment became the basis of GTA’s vehicle system, and that the driving behaviour was produced by layering simple tyre resistance and steering forces onto that core. The result was physically coherent enough to support momentum, rotation, slides and handbrake turns without pretending to model real tyres in detail.
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