Deformation Physics

Deformation Physics turns traffic cars from scripted props into physically reactive objects. Instead of bouncing off a collision like a rigid shell, a traffic car reads the speed and direction of the impact, deforms its body accordingly, and — at high enough speeds — can catch fire or explode. It is the difference between “a collision happened” and “a real crash is playing out in front of you”.

Where to find it

Main tab → Quick Settings → Crash Physics master switch. Enabling it automatically turns on the related sub-features. With Show Advanced Settings on, the two sub-toggles (Enable Explosions, Enable Deformable Cars) become individually switchable under Advanced Traffic Settings.

How impacts are evaluated

There are no predefined tiers of collision. Every impact is evaluated continuously from the speed difference between the cars involved and the direction of the contact. A small speed difference produces a small effect; a large one produces a large one. A glancing angle dissipates energy sideways and deforms less; a perpendicular angle drives it straight into the panel and deforms more. Two collisions that look superficially similar can end very differently because the underlying vectors were different.

That is why pile-ups feel unpredictable in a good way — the outcome follows from physics, not from a lookup table.

Material-aware deformation

Deformation uses advanced physics calculations that read the material of the panel being hit and apply a different reaction to each. Metal behaves differently from plastic; plastic differently from glass. A metal fender crumples under sustained load; a plastic bumper cracks at a lower threshold; a glass panel shatters almost immediately.

On top of the base deformation — the push in the direction of impact — specific materials also receive additional crumble effects. Plastic and glass, for example, do not just dent in the impact direction; they also gain wrinkled and crumbled detail on top, as if they were genuinely crushed. That is what gives crashed cars their ragged, un-uniform look instead of a generic “pushed in” shape.

Damage is localised to where the car was actually hit. You see uneven results — one corner of the hood crumples while the other side looks barely touched — depending on where the energy went. And damage is cumulative across the car’s life: earlier hits stay visible under later ones until the wreck is despawned.

Fire and explosions are speed-gated

Fire and explosions do not come from every crash. They are gated by speed: only impacts above a certain velocity have any chance of igniting. Slow urban bumps, no matter how many times you stack them, will not start a fire — the energy is not there. Highway-speed crashes and head-ons are the region where ignition becomes possible, and from there the chance scales with how much speed was actually involved.

When fire does ignite, it starts as a small flame at the impact area and grows and spreads across the car over the following seconds. At the highest speeds the event can escalate further, into a full explosion with shockwave and debris. Both outcomes are rare even at high speed — the threshold gates them.

Chain reactions between cars

When a car explodes, the explosion is not contained to that one car. Nearby traffic can be caught up in it: they get deformed by the blast, thrown away from the shockwave, and — if the impact on them lands above the ignition threshold — they can start burning and potentially explode in turn.

This is how pile-ups with multiple fires and secondary explosions happen naturally. You do not get them from a single rear-end; you get them when a high-speed chain crash delivers enough energy to each follow-on car in quick succession.

The two sub-toggles and when to change them

Enable Deformable Cars

Controls only the mesh deformation side of the pipeline. Turning it off keeps the impact evaluation, the AI “this car has crashed” behaviour, the smoke, and even fires and explosions — but cars do not visibly crumple any more. Use this as the first lever if your GPU struggles during pile-ups; you keep most of the drama and save the mesh-math cost.

Enable Explosions

Blocks only the explosion outcome. Fires still play, crumple still happens, the speed-gated ignition logic still fires — the car just will not escalate to a shockwave-and-debris explosion. Useful for serious driving sessions where the occasional fireball would pull you out of the flow, but you still want crashes to look real.

Performance cost in order

Mesh deformation — the most expensive element during chaos. Scales with how many cars are being hit at once.
Explosion effects — medium cost but short-lived. Only plays during events.
Fire particles — persistent but cheap. Negligible unless dozens of fires are active.

If FPS dips during heavy crashes but is fine otherwise, turn off Deformable Cars and keep the rest. That single change recovers most of the lost frame time.

Interaction with other features

Police Chases. Deformation is essentially required for PIT maneuvers to look right. Without crumple the spin-out looks like a sudden teleport rather than a controlled impact.
Race Cars. Racing AI crashes more often and at higher speeds, so the drama shows up more often with Race Cars on.
Damage (Police sub-toggle). When both Crash Physics and the police Damage option are on, successful busts damage your car too, not only the AI.
Replay Mode. Crashes and their deformation are recorded. You can rewatch a pile-up later.

Online mode

On multiplayer servers Crash Physics runs locally on each client. That means a pile-up may look slightly different on two screens — one player sees an explosion, another sees a smouldering car. This is why the feature is labelled experimental online, and why servers may disable it in their config.

Troubleshooting

Cars pass through each other without any damage: Crash Physics is off. Enable the master switch.
FPS tanks during multi-car crashes: Turn off Enable Deformable Cars. You keep the impact model, fire and explosions — only the live mesh deformation stops.
I never see fire or explosions: They are speed-gated. Low-speed collisions never ignite. Try Race Cars on or an active police chase — both generate the highway-speed impacts needed to cross the threshold.
My car takes damage during chases: You have the Police Damage sub-toggle on. Turn it off if you want chases without personal cost.