Vehicle physics and driving feel

Driving feel is the game. A cops-and-robbers chase is only tense if the car is enjoyable to control, so this system gets more iteration than anything else.

Model: raycast vehicle, not rigid-body wheels

Use a single rigid body for the chassis plus four downward raycasts for wheels. Do not simulate wheels as constrained rigid bodies — it's slower, less stable, and much harder to tune. Every mainstream driving game uses raycast vehicles.

Per wheel, per tick:

1. Raycast down from the wheel's mount point, up to suspension rest length + travel.
2. No hit → wheel is airborne; no suspension or tyre force, and steering authority drops.
3. Hit → compute suspension force along the contact normal: F = k * compression - c * compressionVelocity, clamped to >= 0 (suspension pushes, never pulls) and clamped to a max to survive big landings.
4. Compute tyre forces in the contact plane: longitudinal (drive/brake) and lateral (grip), each limited by the friction circle (see below).
5. Apply all forces to the chassis at the contact point — that's what produces weight transfer, body roll, and dive under braking for free.

The friction circle

The single most important concept for feel. A tyre has one total grip budget, shared between turning and accelerating/braking:

sqrt(F_long² + F_lat²) <= mu * F_normal

Compute both desired forces, then scale both down if they exceed the limit. This one rule gives you, without special-casing: understeer when accelerating into a corner, oversteer when braking mid-corner, loss of steering under full throttle, and drift.

F_normal comes from the suspension, so weight transfer feeds grip automatically — the front tyres grip more under braking, the rears under acceleration.

Tyre slip

Lateral force is a function of slip angle (angle between wheel heading and actual velocity direction); longitudinal force is a function of slip ratio. Both curves rise steeply to a peak and then fall off — the falloff after the peak is what makes a car break traction and what makes drifting controllable.

Don't implement full Pacejka unless you need it. A curve that rises linearly to a tunable peak and then decays to a tunable sliding-friction value is enough and is far easier to tune. Keep the curve in the tunables module as named points.

Tunables

Every one of these lives in the tunables module with units in the name, and none of them appear as literals in logic:

Group	Parameters
Chassis	mass, centre-of-mass offset (lower and slightly rear = stable), inertia
Suspension	rest length, max travel, spring rate, damping (separate bump/rebound), anti-roll
Tyre	peak grip coefficient, peak slip angle, sliding grip, per-axle grip bias
Drive	torque curve, gear ratios (or simplified direct drive), drive layout, top speed
Brakes	brake torque front/rear, handbrake torque (rear only), ABS on/off
Steering	max angle, speed-sensitive reduction, input smoothing rate, self-centring
Assists	traction control, stability assist, drift assist — strengths, all tunable to 0

Centre of mass is the highest-leverage single value. Too high and the car rolls and flips; too far forward and it won't rotate. Tune it before anything else.

Feel targets for this game

• Arcade, not simulation. The fantasy is a movie car chase. Grip levels above realistic, forgiving recovery from slides, no stalling, no clutch.
• Speed-sensitive steering is essential. Full steering lock at 120km/h is undriveable on a touchscreen.
• Input smoothing matters more than on a wheel or stick, because touch input is binary. Ramp steering toward the target angle at a tunable rate rather than snapping.
• Handbrake turns must work and be reliable — it's the core chase verb. Handbrake cuts rear longitudinal grip and reduces rear lateral grip; the car should rotate predictably, not spin uncontrollably.
• Airtime should be survivable. Cap angular velocity in the air and apply gentle auto-levelling, or every jump ends on the roof.
• Cop cars differ by tuning, not by different code — heavier, more top speed, less agile. One vehicle system, several tunable profiles.

Collisions

• Chassis collides as a simplified convex shape, never the visual mesh.
• Impacts scrub speed and impart rotation, but should not stop the player dead — a full stop from a glancing wall hit ends chases and feels awful. Tune restitution and add glancing-blow deflection.
• If damage exists, it modifies tunables (grip, max speed, steering) so it's felt rather than merely displayed.

Tuning workflow

1. Change values only in the tunables module — never logic.
2. Use a dev-only tuning overlay (sliders bound to the tunables) to iterate live.
3. Test on a flat plane first, then a slope, then the real city.
4. When it feels right, write a golden-replay test capturing that behaviour: a fixed input script, asserting tolerance-bounded position/velocity at checkpoints. This is what stops a later physics change silently ruining the handling.

Diagnosing bad feel

Symptom	Usual cause
Floaty / bouncy	Suspension damping too low, or spring rate too high for the mass
Rolls over easily	Centre of mass too high; add anti-roll
Won't turn (understeer)	Front grip bias too low, or steering angle reduced too aggressively with speed
Spins constantly (oversteer)	Rear grip too low, or centre of mass too far rearward
Twitchy on touch	No input smoothing, or no speed-sensitive steering
Sticks to walls	Restitution too low and no deflection handling
Vibrates at rest	Suspension spring/damper stiff relative to timestep; check resting-contact test