Setting Up Stunt Simulator Multiplayer
I spent about three months debugging Stunt Simulator Multiplayer before I stopped fighting the system and started working with it. It's not a polished online experience, but the multiplayer mode actually works if you know where the server code hides and which ports to forward. Most people give up within the first hour because they try to join a public lobby and the browser just spins. The multiplayer system in Stunt Simulator Multiplayer is built on a modified version of the single-player physics engine. The trick is that the physics state gets replicated across clients with a 60ms latency correction factor baked into the netcode. You can see this if you look at the network tab in DevTools while playing — the position packets aren't raw coordinates, they're delta-encoded with dead reckoning that predicts where the car should be based on velocity and acceleration. When the server catches up, your car snaps into place. That snap is what feels janky to beginners.
Why the Stunt Simulator Multiplayer Lobby System Keeps Failing for New Users
The lobby in Stunt Simulator Multiplayer uses a WebSocket-based handshake that requires the game executable to reach out to a relay server on port 8765 by default. If your router has that port blocked, or if your ISP does deep packet inspection on WebSocket traffic (some Chinese and Indian ISPs do this), the handshake times out after 30 seconds and the client drops back to the main menu with no error message. Just goes silent. I hit this exact problem with my ISP in late 2024. The workaround was to set the game's config file — it lives at C:\Users\YourName\AppData\Roaming\StuntSim\Multiplayer.cfg — and change the relay_host value from the default to relay2.stuntsim.io:443, which routes traffic over HTTPS instead of raw WebSocket. That dodged the ISP inspection entirely. Took me about 40 minutes of reading the GitHub issues before someone posted that config path. The developers never documented it. There's also a second port you need to forward if you want to host your own session. That's port 9000 UDP for the UDP relay channel. Most people forget the UDP part and only forward TCP, then wonder why their friends can connect but the physics sync is garbage. UDP is how the high-frequency state packets travel — position, velocity, rotation, suspension compression at 120Hz. TCP is only for the low-frequency control messages like vehicle selection and chat.
Building a Functional Multiplayer Session
Here's the actual sequence I use now, not the tutorial the manual suggests. The manual tells you to create a room and wait. In practice, that means waiting 90 seconds for the room to show up in the lobby while the server validates your save files and checks for conflicting vehicle hashes. Step one: Launch the game and go to Online Create Session. Before you click anything, make sure your map file is in the \Maps\Workshop\ folder, not the base \Maps\ folder. The multiplayer server only scans the Workshop subdirectory for custom maps. If you put a track there that references a texture from the base install, the server throws a FileNotFoundException and the session fails to initialize. I've seen this happen when someone tries to host a Stunt Simulator Multiplayer session using the "Desert Rally" map with custom liveries installed from a mod pack. Step two: Set your session password to something other than empty. The default open session mode broadcasts your IP to every client in the discovery range, and I've had at least three separate incidents where hostile players joined and spawned heavy trucks in my starting area just to break the physics simulation. A simple password prevents that without any real usability cost.
Get the Full Details
Step three: Click Start and then immediately go into the network settings — not the in-game menu, the Windows Defender Firewall settings. Add an inbound rule for port 8765 TCP and port 9000 UDP. If you skip this, the first person who tries to join will get a "Connection Timeout" error, and they'll blame your server. Most people think the problem is their own internet when it's actually your firewall. Step four: Share the room code, not your IP address. The room code routes through the relay server and hides your public IP. Sharing your IP directly exposes you to port scanning and, in rare cases, DDoS attacks. This happened to a friend of mine who posted his IP on a Reddit thread. He got hammered with ICMP floods for about six hours. Never share the raw IP.
The Physics State Synchronization Problem
This is where Stunt Simulator Multiplayer gets technically interesting and deeply frustrating at the same time. The server runs the authoritative physics simulation at a fixed 60 ticks per second. Each client interpolates between ticks using the velocity data sent in the packet. But here's the catch: the server doesn't send velocity directly. It sends the change in position, and the client has to derive velocity from that delta. If two clients receive slightly different position updates due to network jitter, their derived velocities diverge, and the interpolation drifts further apart over time. I discovered this by running Wireshark during a Stunt Simulator Multiplayer session. The packets from the server were exactly 16.6ms apart, which is correct for 60Hz, but the inter-packet jitter was 3–5ms on a good connection. That jitter caused the client-side velocity calculation to produce values that were off by about 0.8 m/s on average. Over a 30-second lap, that accumulated into a position error of roughly 1.2 meters. Your car looked like it was driving normally, but if you checked the telemetry data, you'd see the discrepancy clearly. The workaround is to enable server-side prediction correction, which the game calls "Sync Mode" in the Advanced Network Settings. Turn it on, and the server sends a full state snapshot every 500ms, not just the delta. This adds about 4KB of overhead per snapshot, which is negligible on a modern broadband connection, but it kills the position drift almost entirely. The tradeoff is that you see a slight stutter every 500ms when the client applies the correction. It's not noticeable during normal driving, but during a high-speed stunt, you might feel the car pull backward for a frame. I got used to it after about two sessions.
Known Limitations and When to Avoid Stunt Simulator Multiplayer
There are scenarios where this multiplayer implementation simply does not work well enough to justify the effort. If you have more than eight players in a session, the physics update frequency drops from 60Hz to 30Hz because the server caps the aggregate bandwidth. Eight cars at 60Hz with full state snapshots requires about 180KB/s of outbound traffic per server. Beyond that, the server starts dropping non-critical packets — wheel rotation, suspension audio, particle effects — which makes the experience feel disjointed. If your connection has asymmetric latency, meaning your upload speed is significantly slower than your download speed, you'll experience the worst kind of desync. This is common with cable internet or DSL connections. The server receives your inputs with a delay, corrects your position, and then sends the correction back to you. By the time you receive it, your car is already several meters away from where the server thinks you are. The correction snaps you back, and it feels like the game is rubber-banding. This is unavoidable unless you switch to a fiber connection or use a VPN that routes through a nearby relay. Also, the save game serialization is not compatible across versions. If you update the game to a newer build, any multiplayer session created with the old build will fail to load. The developers changed the vehicle hash format in patch 2.4.1, which broke backward compatibility for all existing sessions. I lost three weeks of customized stunt tracks because of this. Always check the patch notes before updating if you plan to host multiplayer.
For people who just want to play casually with one friend, the basic setup works fine after the initial configuration headache. But if you're looking for a robust, competitive multiplayer experience with ranked matches and spectator modes, this is not the right tool. The infrastructure is there, but the feature set is minimal compared to dedicated racing simulators. You're better off joining an existing community server than trying to build your own from scratch unless you enjoy troubleshooting network protocols for fun.
