PhysX Vehicle core concepts

This chapter covers the ideas every PhysX Vehicle workflow shares: how contact forces are produced, when the solver runs, and where mass lives.

Raycast suspension model

Each wheel (or tracked road wheel) is a configuration entry, not a PhysX rigid body:

Local anchor ──► ray along suspension dir ──► ground hit
 │
 ├─ spring–damper → normal (suspension) force
 └─ tire / track model → longitudinal + lateral forces
 │
 ▼
 ApplyLinearImpulseAtWorldPoint (force × sub-step dt)

The chassis collider handles body collision with the world. Wheels contribute forces only through rays; they do not add wheel colliders.

Chassis is the only rigid body

Do not parent separate dynamic wheel rigid bodies expecting the vehicle solver to drive them. Visual wheel entities are pose-synced only when Wheel Entity Id is set.

Solve on physics sub-steps

PhysX uses a fixed simulation step. One render frame may run zero or several sub-steps. The vehicle registers:

• OnSceneSimulationStart / related scene simulation hooks for the solve pass
• Debug draw on TickBus (visualization only; forces still land on sub-steps)

Putting drive forces on TickBus alone would desync from PhysX and feel unstable. Prefer VehicleControlRequestBus (or the Drive component) and let the chassis apply impulses each sub-step.

Configuration is Prefab data

Chassis, wheel, drive, and track parameters are serialized on components inside Prefabs. There is no separate .vehicle asset type. Templates (MakeChassisTemplate / MakeTrackTemplate) fill those fields in the editor; they do not create external assets.

Wheeled vs tracked

Both share the same chassis host and control / state buses. Tracked mode ignores ordinary car-wheel lists; the Track component pushes geometry into the chassis at activate.

Tire model note

Tire forces use longitudinal / lateral stiffness scaled by load and clamped by friction. Use Magic Formula on the tire config is reserved and unimplemented in the shipped Gem—leave it off.

Mass and gravity

• Set total mass on the Rigid Body to match the template intent (Sedan ≈ 1400 kg, Off-road ≈ 2000 kg, Truck ≈ 8000 kg, Tank ≈ 35000 kg).
• Chassis Gravity (default 9.81) feeds sprung-mass / auto-stiffness estimation; it does not replace scene gravity.
• Lower CoM (Rigid Body and/or chassis offset Z) improves roll resistance; raise Anti-roll stiffness for remaining body roll.

Glossary

Next steps

• Wheeled vehicles and Tracked vehicles — authoring and tuning.
• Components — full field tables.