This Easy Tire Model Changed My Unity Car Controller: Slip, Grip & Friction Circle Explained

In this dev vlog, I break down a custom Unity car controller with a focus on vehicle physics and tire behavior.
We start by looking at a simple raycast-based suspension system and how it generates normal force / load force in Newtons for each individual wheel.

From there, we dive into longitudinal slip, slip ratio, and how wheel angular velocity differs from ground velocity to produce traction and braking forces.
Slip ratios are remapped using curves to control grip behavior and tire response.

Next, we cover lateral forces, slip angle, and how sideways motion generates lateral grip during cornering and drifting.
To visualize this, a lateral impulse is applied, making drift behavior, debug lines, and force directions clearly visible.

We then combine longitudinal and lateral forces in a naïve implementation and explain why simply adding both forces works, but does not feel fully realistic.
This leads to an introduction of the friction circle, which limits combined tire slip and forces for more believable traction behavior.

Finally, we touch on edge cases like why the vehicle does not slide down a slope, highlighting missing force balancing and areas for future optimization.
The video closes with a look at what needs to be improved next to achieve more realistic grip, friction, and vehicle handling in Unity.

❤️ If you have any questions, don’t hesitate to leave a comment or upvote the ones you’re interested in.
And if you enjoy this kind of content, please like and subscribe — it helps me know I should make more videos like this.