Vehicle Damage MP Fix - A Project Zomboid B42.16.x fix

Presented by APOCALIPSE [BR] - A Brazilian Project Zomboid private server.

Fixes a vanilla multiplayer bug where vehicle collision damage is massively multiplied, destroying cars far faster than intended.

THIS IS A SERVER-SIDE JAVA MOD, SUBSCRIBING WON'T DO ANYTHING, YOU MUST FOLLOW THE INSTRUCTIONS BELOW

The Problem

In vanilla multiplayer, when a vehicle hits a zombie, animal, or player, the game applies vehicle damage through two independent paths:

Path 1 — Lua command (damageFromHitChr): Every connected client that has the vehicle loaded independently runs Bullet physics. Each client detects the collision and sends a damageFromHitChr Lua command to the server. With N players nearby, the server receives N copies of the same damage. However, the game has a built-in client-side throttle (VehiclePedestrianContactTracking, 3.5s cooldown) that limits how often this command fires per zombie — so this path, while duplicated across clients, has some built-in protection.

Path 2 — Java network packet (VehicleHitField): The driver's client also sends a VehicleHitField packet directly through the Java networking layer. This packet bypasses Lua entirely and applies damage via addDamageFrontHitAChr / addDamageRearHitAChr in the server-side VehicleHitField.process() method. The critical problem is: the driver's physics loop fires this packet every single frame while the vehicle's hitbox overlaps with the target. At 60 FPS, a single zombie hit produces ~3-5 packets (one per frame of overlap), and there is zero deduplication or cooldown on the server side. Every packet applies full damage.

The combined effect:

• Path 1 sends N×damage (one per connected client) — mitigated by client-side cooldown
• Path 2 sends 3-5×damage per hit (one per frame of overlap) — no protection at all
• Both paths stack on top of each other

This is why vehicles feel like paper in multiplayer — a single zombie hit that should deal 20-30 damage to the vehicle ends up dealing 100-200+.

How we found this

Earlier versions of this mod tried to fix the problem purely through Lua — blocking duplicate damageFromHitChr commands, adding driver authorization, throttling, and scaling. None of it worked. Even when the Lua command was sending zero damage, vehicles were still being destroyed at the same rate. This proved that the real damage was coming from somewhere Lua couldn't touch.

Tracing the Java source revealed the second path: VehicleHitField packets sent directly from the driver's physics loop, processed entirely in Java, with no cooldown or deduplication — unlike the Lua path which has VehiclePedestrianContactTracking. The Lua path already had a throttle; the Java path simply didn't. The fix was to give it one.

The Fix

This mod patches the vanilla VehicleHitField.class with a server-side classpath override. The patched class adds a deduplication cooldown to VehicleHitField.process() — the same kind of throttle that the Lua damage path already has, but was missing from the Java packet path.

What the patch does:

• When the server receives a VehicleHitField packet, it checks whether vehicle damage was already applied for the same (vehicle, target) pair within a 3.5-second window (matching the vanilla VehiclePedestrianContactTracking cooldown). If so, the vehicle damage is suppressed. If not, it applies normally and records the timestamp.
• Only vehicle HP damage is throttled. Character effects (zombie death, knockdown, stagger) are not affected — they pass through on every packet so zombies still die and ragdoll correctly.
• The dedup uses a static HashMap<Long, Long> keyed by a combination of the vehicle's network ID and the target's identity hash. Stale entries are cleaned up periodically (every 10 seconds).
• The cooldown is configurable at runtime via JVM property: -Dpz.vehicle.hit.dedup.cooldown=3500 (milliseconds, default 3500). Set to 0 to disable dedup entirely.

The result: a vehicle takes exactly 1x damage per collision, matching singleplayer behavior. No Lua files are modified.

Installation

This mod is a Java classpath override — it requires placing a patched .class file on your server. No Lua mod subscription is needed (though the Workshop page hosts the files and documentation).

The patched VehicleHitField.class must be placed in the correct folder relative to your projectzomboid.jar. Project Zomboid's classpath loads loose .class files before the JAR, so placing the patched file in the right directory overrides the vanilla class without modifying the JAR.

Option A — Use the included build scripts

Inside this mod's folders, there's a tools/ folder that contains two self-contained scripts that compile the patched source and deploy it automatically:

• Windows: patchVehicleHitField.ps1 — Run in PowerShell. Will auto-download JDK 25 if needed. Default PZ path: Z:\SteamLibrary\steamapps\common\ProjectZomboid. Use -PZDir "your\path" to override.
• Linux: patchVehicleHitField.sh — Run with bash. Requires JDK 25 installed (apt install openjdk-25-jdk-headless). Default PZ path: /opt/pzserver. Edit PZ_DIR at the top to change.
• To revert: run the same script with -Revert (Windows) or --revert (Linux).

YOU MUST CONFIGURE YOUR ProjectZomboid folder path correctly for Windows or for Linux.

Option B — Manual deployment

If you don't have knowledge about scripts, you can get the VehicleHitField.class from tools/out/classes/zombie/network/fields/hit/VehicleHitField.class and place it manually:

• Windows dedicated server: Copy VehicleHitField.class to <ProjectZomboid>/zombie/network/fields/hit/ (create the folders if they don't exist). This is the same directory that contains projectzomboid.jar, with the package path appended.
• Linux dedicated server: Copy to <PZServer>/java/zombie/network/fields/hit/ (Linux servers use the java/ subdirectory as classpath root).
• To revert: simply delete the VehicleHitField.class file and restart the server. The original class from the JAR will be used.

Restart the server after deploying.

Technical Details

• Server-side only — no client-side files are modified, no Lua is changed
• Zero performance overhead — the dedup is a single HashMap lookup per incoming VehicleHitField packet
• Safe — all other network packets and game logic are left identical to vanilla
• Original JAR untouched — the classpath override is a standard JVM mechanism; deleting the .class file fully reverts to vanilla
• No mod conflicts — since no Lua files are overridden, this is compatible with any other mod

Compatibility

• Build 42.16.x
• Multiplayer only (does nothing in singleplayer)
• Safe to add or remove mid-save

With love from Brazil <3