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The earliest documented Grand Theft Auto vehicle system was not built around a sophisticated real-world tyre simulator. Pat Kerr describes a compact two-layer design: a general 2D rigid-body simulation that handled movement and rotation, plus an intentionally simple set of tyre forces that made a car grip, slide, steer and spin.
Kerr dates the prototype to one weekend in late August 1996. He first wrote it as a wireframe demo in GFA BASIC on an Atari ST, then ported the work to C for use in the game. His account says this simulation later became the basis of GTA’s vehicle system, although the surviving evidence is a first-person technical history rather than a reconstruction of the original binary.
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The prototype was a 1996 physics experiment, not the 1995 design document
Pat Kerr’s project history separates three things that are often blended together in retrospectives: the planned Race’n’Chase concept, the later wireframe physics prototype, and the released game.
In Kerr’s account, the decisive experiment happened over one weekend in late August 1996. He wrote a small 2D simulation in GFA BASIC on his Atari ST. It began as a wireframe demonstration rather than a finished game feature, and he later ported it to C for use in the actual game. Kerr writes:
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“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 direct source for that chronology and for his description of the recreated model. It does not establish that every line of the original game’s code survived unchanged, nor does it provide a binary dump for independent verification.
The date also matters because the prototype was not the March 22, 1995 Race’n’Chase design document. That document, version 1.05, describes the project’s intended fast racing-and-crashing format, including driving, stealing cars, collisions and stunt-oriented handling. It records design intent before Kerr’s weekend experiment; it is not proof that every listed feature shipped exactly as written.
The vehicle system had two distinct layers
Kerr’s explanation is easiest to understand as a stack. The bottom layer is a reusable 2D rigid body. The upper layer supplies vehicle-specific forces at tyre locations.
| Layer | What it represents | What it contributes |
|---|---|---|
| Rigid-body core | Position, velocity, angle, angular velocity, force and torque | Translation, rotation, momentum and the ability of off-center forces to turn the body |
| Vehicle layer | Engine force, steering and resistance at four tyre positions | Forward motion, sideways grip, sliding and handbrake behavior |
The same underlying physics could be reused with different force rules. Kerr’s recreation includes car, ship and brick modes: the car adds tyre and engine behavior, while the other modes exercise the general body with different forces. That reuse is important. The vehicle was not a completely separate animation system; it was an application of a more general simulation.
What the rigid body stores
A rigid body in this context is a simplified object whose shape does not deform. Its position describes where it is on the 2D plane, and its velocity describes how quickly it is translating. Angle gives its orientation, while angular velocity gives the rate at which it is rotating. Forces change translation; torque changes rotation.
If a force acts through the body’s center, it primarily accelerates the object. If the same force is applied away from the center, it also creates a turning effect. This is the core reason a collision or tyre force can make a car rotate without a separately scripted spin animation. Rotation emerges from where the force is applied.
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Why torque is the key idea
Torque gives the prototype a coherent relationship between impact point and spin. A push near one end of the car can produce a different rotation from an equal push near the center. The simulation therefore carries angular momentum as part of the vehicle’s state instead of faking heading changes frame by frame.
That does not make the model a complete real-world vehicle simulator. It means only that the body-level motion follows a consistent rigid-body abstraction. The realism question belongs mainly to the tyre layer, which Kerr explicitly characterizes as approximate.
How the car layer creates driving behavior
Once the rigid body exists, the car model supplies the forces that make it behave like a car. Kerr describes engine force, steering and resistance applied at four tyre positions.
Four contact points instead of one artificial steering rule
Each tyre position has a local movement that can be compared with the car’s orientation. The recreation resolves the velocity at each tyre into two components: a forward component along the tyre’s rolling direction and a sideways component across it.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesThis decomposition lets the model treat rolling and lateral movement differently. A tyre can be moving mostly forward and receive relatively little rolling resistance, while sideways velocity is damped much more strongly. Steering changes the direction associated with the front tyres, so the resulting forces can alter both the car’s path and its rotation.
Resistance supplies the grip
Kerr says the recreated car applies much stronger resistance to sideways motion than to rolling motion. In practical terms, a car that is sliding broadside is pushed toward alignment, while a car moving in the direction its tyres point is allowed to continue more easily.
That is an economical way to create controllable grip. It gives the player a difference between forward travel and lateral slip without calculating a detailed contact patch, rubber compound, suspension geometry or road surface. The resulting behavior can feel convincing at game scale while remaining mathematically simple.
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The handbrake changes the rear tyres
The handbrake is implemented by changing the rear tyres’ resistance characteristics. Kerr describes increased rear rolling resistance and reduced rear sideways grip when the handbrake is engaged. With less lateral resistance at the rear, the back of the car can break loose; with more rolling resistance, the rear slows relative to the front. Together those changes make a handbrake turn or induced slide possible.
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Why this is not a realistic tyre simulation
Kerr calls his tyre model “simple, approximate, and technically incorrect.” That qualification should remain attached to every explanation of the prototype. The model is useful because it gives a game designer controllable differences between rolling, steering and sliding; it is not an engineering model of tyre friction.
The recreated forces are damping terms proportional to velocity and do not have a force cap. Real tyres have finite friction limits: once available grip is exceeded, additional demand does not produce unlimited corrective force. Kerr’s approach does not attempt to model that limit, nor does it fully represent slip angles, load transfer, suspension, camber, changing surfaces or a tyre’s friction ellipse.
Calling the system “arcade physics” is therefore more accurate than calling it realistic simulation, but “arcade” should not be mistaken for random movement. The rigid body supplies a consistent foundation, and the tyre rules deliberately shape that foundation into a playable driving response.
Why early GTA cars could slide and keep rotating
The combination of layers explains several recognizable behaviors without requiring a large catalogue of special-case animations.
- Momentum persists: the rigid body retains velocity, so releasing the accelerator does not instantly stop the car.
- Heading and travel can diverge: weak enough lateral resistance allows the car’s orientation to differ from its direction of movement.
- Impacts can spin the car: a collision force applied away from the center creates torque as well as translation.
- Steering affects more than the front edge: forces at tyre positions can create a turning moment for the whole body.
- A handbrake can start a slide: reducing rear lateral grip changes the balance of forces and lets the rear rotate around the car.
These effects are consequences of the model’s state and force placement. They do not require a claim that the game reproduced real tyre mechanics. The prototype’s success came from choosing a small set of forces that generated many useful driving outcomes.
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What the 1995 Race’n’Chase document tells us
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 sections discuss driving, stealing, racing, collisions and navigation, and its physics discussion includes handling for stunts such as handbrake turns and spinning wheels.
The document is valuable as evidence of the project’s ambitions. It shows that stunt-friendly driving was part of the design conversation before Kerr’s 1996 prototype. It cannot establish that the final game implemented every proposal, or that the later code followed the document’s wording literally. Design documents describe targets; Kerr’s account describes a working technical approach.
Read together, the sources suggest a progression rather than a single master plan: an early design called for dramatic driving, and a later reusable physics experiment supplied a practical way to produce some of those behaviors.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why development was difficult even when driving was fun
In a 2015 Guardian retrospective, DMA programmer Mike Dailly remembered that the first driving implementation was immediately enjoyable but unstable. He said:
“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.”
Dailly also recalled major disagreement and difficulty over car collisions. That testimony is a retrospective account of early development, not a precise diagnosis of Kerr’s later physics code. It does, however, explain why a compact model could still require substantial engineering. A rigid body can produce plausible motion, but collisions add difficult edge cases: overlapping bodies, extreme angular velocities, repeated contacts and interactions with the city.
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The distinction matters when interpreting the prototype. A physics idea can be elegant on its own and still be hard to integrate into a complete game with many vehicles, walls, pedestrians and gameplay rules.
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What Motion Lab shows in 2026
In 2026 Kerr published a JavaScript recreation called Motion Lab. It offers car, ship and brick modes and exposes controls for the camera dead zone, speed-related zoom, side barriers and vertical gravity. Those controls make the underlying ideas visible: changing the forces or camera rules changes how the same 2D body is perceived and constrained.
Motion Lab is an independent recreation, not the original Atari program. Kerr states that it is not affiliated with Rockstar Games, Take-Two or the Grand Theft Auto franchise. A browser recreation can illustrate the architecture and behavior Kerr describes, but it cannot prove that the released game used identical constants, update order or collision handling.
The most useful way to view it is as an explanatory model. You can observe how a general body behaves before vehicle forces are added, then see how tyre resistance and handbrake changes alter that body. The demonstration makes the separation between reusable physics and vehicle-specific rules easier to grasp than a static code listing would.
What can be stated confidently—and what remains uncertain
The strongest claims are those Kerr makes directly about his own work:
- He wrote the original 2D simulation in late August 1996.
- It began as a GFA BASIC wireframe demo on an Atari ST and was later ported to C for the game.
- The simulation used a 2D rigid body with translational and rotational state, including torque from off-center forces.
- The car layer applied engine force, steering and resistance at four tyre positions.
- The tyre behavior was an approximation rather than a physically correct friction model.
- He says the simulation later became the basis of GTA’s vehicle system.
Several stronger-sounding claims would go beyond the evidence. The available material does not reproduce the original executable, establish exact numerical constants, give a complete collision solver, or prove that every released-game vehicle behaved exactly like the recreated demo. It is also not evidence that the 1995 design document’s proposed handling shipped unchanged.
The technical contrast in one view
| Question | Rigid-body core | Approximate tyre layer |
|---|---|---|
| What state is tracked? | Position, velocity, angle and angular velocity, with forces and torque acting on the body | Velocity at four tyre locations, resolved into forward and sideways components |
| What causes turning? | Torque, especially when a force acts away from the center | Steering direction, tyre resistance differences and the resulting force balance |
| How is sliding represented? | Angular and linear momentum remain part of the body state | Sideways resistance is stronger than rolling resistance; rear grip can be reduced by the handbrake |
| How realistic is it? | A simplified but coherent 2D rigid-body abstraction | Deliberately approximate damping, without a capped, physically accurate tyre-friction model |
| Can it be reused? | Yes; Kerr’s recreation uses the core for car, ship and brick modes | Vehicle-specific rules sit on top of the shared core |
Why the prototype mattered to Grand Theft Auto
The lasting lesson is architectural. Kerr did not need a full automotive simulation to obtain a rich set of game behaviors. A reusable rigid body handled the difficult relationship between force, movement and rotation. A small number of tyre rules then supplied the distinctions players actually feel: acceleration, steering, grip, slide and handbrake rotation.
That division also made experimentation practical. Changing tyre resistance could alter handling without rewriting the body integrator. Reusing the same core for other objects reduced the need for entirely separate movement systems. The result was a physics foundation broad enough for multiple modes but simple enough to prototype quickly on period hardware.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11So the original demo reveals neither a hidden realistic tyre simulator nor a collection of arbitrary tricks. It reveals a pragmatic combination: physically coherent rigid-body motion underneath, deliberately non-realistic tyre forces above it. Kerr’s account supports calling that prototype an important basis for GTA’s vehicle system, while the surviving evidence warrants caution about details that cannot be independently checked.
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