Building a Sonic Game in Roblox Is Harder Than It Looks
I spent three months trying to get basic loop-the-loop physics working in a Roblox Sonic game, and I still can't say it's done right. The community around Sonic-style platformers on Roblox has grown a lot over the past few years, and the quality varies from absolute joke to genuinely impressive. I want to walk through what actually works, what doesn't, and how to avoid the mistakes I made. The core challenge with any Sonic-type game is that traditional Roblox character controllers are built for walking, jumping, and gravity — not momentum-based high-speed movement with variable acceleration and deceleration. When I first tried to adapt the default Humanoid to behave like Sonic, it just didn't work. The Humanoid has a hard speed cap, it snaps to surfaces in ways that break the flow state, and its jump system is one-size-f-all. What actually works is building your own velocity system from scratch using BodyVelocity or, preferably now, using AssemblyLinearVelocity on the character's RootPart. You track horizontal and vertical speed independently, apply friction that changes based on whether the player is on the ground, rolling, or in the air, and then set the character's actual position every frame. This gives you control over things like the initial burst of speed when you start moving, the gradual acceleration as you pick up momentum, and the feeling of sliding to a stop rather than instantly halting.
I've seen a lot of Roblox Sonic experiences where the movement feels floaty or disconnected, and 90% of the time it's because the developer is relying on the Humanoid's MoveDirection combined with a simple walk speed boost. That approach breaks the second you try to implement spin dashes or loops. You need to think about this in terms of vectors and acceleration curves instead of just setting a speed value.
Getting Loop Physics Right
This is where most projects fail. A proper loop requires the character to maintain enough velocity to complete the full circle without falling off, which means you need to calculate centripetal force and compare it against gravity at every point along the loop's path. The naive approach — just rotating the character and hoping — results in the player either falling out of the loop at the top or being crushed into the ground at the bottom. The method I ended up using involves defining the loop as a series of arc segments, each with their own normal vector. As the player enters a loop, you switch from normal gravity to a centripetal system where the "down" direction is always pointing toward the center of the loop. You check the player's velocity against a minimum threshold at the top of the loop. If they're going too slow, they fall. If they're going fast enough, they stay pinned to the track. This whole system runs server-side with client prediction to avoid the lag that makes high-speed games unplayable. Here's the thing nobody mentions: Roblox's physics engine is fundamentally designed for slow, chunky collisions. At Sonic speeds — even the simplified Roblox versions that cap out around 150-200 studs per second — the engine starts missing collision detections. I had a level where a perfectly fair loop was impossible because the character would clip through the track geometry at high velocity. The fix was implementing a swept collision system that does continuous collision detection instead of discrete checks, basically casting a ray from the previous frame's position to the current one to catch any geometry the fast-moving character might skip over entirely. This added maybe 30ms of overhead per player but made the game actually playable at high speeds.
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Level Design Considerations That Matter
Most Roblox Sonic games treat level design as an afterthought. They dump you in a map that looks cool but is functionally broken for the kind of movement the game expects. Green Hill Zone-style levels need long running straights to build momentum, proper ramp angles, and clear visual feedback about where to go. I've seen maps where the intended route is visually obscured by overly detailed textures and lighting that make it hard to see the path ahead at speed. Camera follow systems are another area where I see consistent problems. A camera that just locks behind the player at a fixed distance looks fine at walking speed but becomes nauseating and disorienting once you're moving at 80+ studs per second. The camera needs to predict where the player is heading, not just lag behind their current position. I implemented a lookahead system where the camera interpolates toward a target point roughly two seconds into the player's movement direction. It takes more compute but the difference in playability is night and day.
Common Pitfalls and What I'd Do Differently
If I were starting over, I wouldn't build on top of the default Roblox character rig. The proportions and bone structure of the standard rig are awkward for Sonic-style movement. I'd use a custom rig with a lower center of gravity and simplified collision shapes. The default rig also introduces unnecessary rotational noise when the character's limbs collide with geometry at high speed, which makes your velocity calculations go haywire. Another mistake I made was putting too much logic on the server. High-speed platformers need tight input response, and server authoritativeness introduces latency that makes precision movements nearly impossible. I moved the core movement loop entirely client-side with the server only validating gross position checks and preventing obvious cheating. This cut my input lag from around 150ms down to about 30ms, which is the difference between a level being fun and being frustrating. The honest downsides to building a Sonic game in Roblox: the platform isn't designed for this kind of gameplay. You're fighting the engine the entire time. The networking model introduces latency that affects high-speed precision. Collision detection at velocity breaks down. And the avatar system makes it difficult to create characters that actually feel like Sonic rather than a generic Roblox figure running fast. If you're patient and willing to spend months on the technical side, you can produce something decent. If you expect a weekend project to feel like the actual games, you'll be disappointed. There are alternatives — Unreal Engine 5 with its physics system is far better suited for this, and Godot is free and lightweight — but if you're committed to Roblox because of its audience and distribution, the tradeoff is real.
The community resources available are mixed. Some developers have shared their momentum systems publicly and those are genuinely helpful starting points. Others gate their code behind Discord servers or paid groups. The worst advice I encountered came from a popular YouTube tutorial that recommended using constraint-based physics for the spin dash, which produces wildly inconsistent results depending on the player's orientation and the terrain angle. Avoid that approach entirely. I've updated my project files with better documentation after the third or fourth rebuild of the velocity system. I don't link them directly because they're incomplete and I don't want people building on broken foundations, but if you're working on something similar and run into specific issues with loop clearance calculations or swept collision implementation, I've found that posting concrete code snippets in the Roblox developer forums gets you actual answers from people who've dealt with these problems rather than vague suggestions.
