unity-physics-tuning

name	unity-physics-tuning
description	Unity PhysX Tuning for RC Racing

Unity PhysX Tuning for RC Racing

Use this skill when configuring Unity's PhysX engine for realistic 1/10-scale RC car simulation. Covers project settings, timestep, collision layers, Rigidbody configuration, raycast vs WheelCollider, scale fixes, determinism, and common physics bugs.

When to Use

• Configuring Project Settings > Physics for an RC racing game
• Diagnosing vehicle jitter, tunneling, flip-on-curb, or suspension oscillation
• Choosing between WheelCollider and raycast-based suspension
• Setting up collision layers and physics materials for track surfaces
• Tuning Rigidbody parameters for 1/10-scale vehicles
• Implementing deterministic physics for replay or ghost systems

When NOT to Use

• General Unity physics concepts (colliders, joints, raycasting basics) -- use unity-physics-3d
• Shader or visual effects work -- use unity-3d-materials or unity-shaders
• Networking/multiplayer physics sync -- use unity-networking
• Non-vehicle physics (ragdolls, destruction, cloth)

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Collision Detection Modes

Set per-Rigidbody via Rigidbody.collisionDetectionMode.

Mode	Cost	When to Use
Discrete	Lowest	Static objects, slow-moving debris, track barriers
Continuous	Medium	Vehicle chassis -- prevents tunneling through thin barriers
ContinuousDynamic	High	Fast projectiles, wheels at high RPM if using collider-based wheels
ContinuousSpeculative	Medium	Good compromise for vehicle body. Uses speculative CCD -- cheaper than full CCD but can allow some ghost collisions

RC racing recommendation: Vehicle chassis = Continuous or ContinuousSpeculative. Track geometry = Discrete (static).

rb.collisionDetectionMode = CollisionDetectionMode.ContinuousSpeculative;

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Physics Materials per Surface

Unity 6: \ was renamed to \ (with an 's') and \ was renamed to . The old names are removed and will cause compile errors.

Create PhysicsMaterial assets for each surface type.

Surface	Dynamic Friction	Static Friction	Bounciness	Combine Mode
Asphalt	0.8	1.0	0.05	Average
Dirt (packed)	0.6	0.7	0.02	Average
Dirt (loose)	0.4	0.5	0.01	Average
Gravel	0.35	0.45	0.03	Average
Grass	0.5	0.6	0.01	Average
Mud	0.25	0.3	0.0	Average

Note: If using raycast suspension (recommended), these PhysicsMaterials are only for chassis-barrier collisions. Tire friction is calculated in your custom tire model using the surface type from the raycast hit.

// Surface lookup from raycast hit
PhysicsMaterial surfaceMat = hit.collider.sharedMaterial;
float gripMultiplier = surfaceConfig.GetGrip(surfaceMat); // Your lookup table

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Manual Physics.Simulate

For substep simulation or replay systems:

// Disable auto-simulation
Physics.simulationMode = SimulationMode.Script;

void FixedUpdate()
{
    int substeps = 4;
    float substepDt = Time.fixedDeltaTime / substeps;
    for (int i = 0; i < substeps; i++)
    {
        ApplyForces(substepDt);
        Physics.Simulate(substepDt);
    }
}

Replay system: Record inputs per-frame, then replay by calling Physics.Simulate with the same inputs and timestep. Requires Enhanced Determinism.

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Conformance Audit

After any physics tuning change, run the conformance checks and record results via ConformanceRecorder:

ConformanceRecorder.BeginRun();
// ... run checks ...
ConformanceRecorder.Record("B", "B6", "Normal force sum at rest", 14.715, measuredForceSum);
ConformanceRecorder.EndRun();

Results are persisted to the local SQLite DB (Logs/physics_audit.db) for trend tracking and regression detection. See the physics-conformance-audit skill for the full catalogue of 93 checks across 12 categories.

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Related Skills

Skill	When to Use
unity-physics-3d	General Unity physics (colliders, joints, raycasting basics) -- this skill builds on top of it
clean-room-qa	Testing methodology for verifying physics tuning changes

Topic Pages

• PhysX Project Settings
• Layer Collision Matrix
• Common Bugs and Fixes
• Contact Modification API
• Scale Issues at 1/10 Scale
• Deterministic Physics