Wheel Friction Curves

According to https://docs.unity3d.com/Manual/class-WheelCollider.html , no force is applied when there is no slip. Is that correct behavior and I somehow misunderstood the graph, or is the graph incorrect?

 

The graph is correct and you've understood it correctly. Surprisingly, you'll find the same thing whenever you look at some information about tire forces. I also questioned the same fact, so I ended designing my own tire friction model that produces the correct tire forces even when there's no slip at all (example video).

PhysX (the physics engine used in Unity) fakes this by forcing the the wheel to stay "stuck" to the ground whenever the slip is smaller than some amount and there's no motor torque applied. Otherwise, the wheel will have to slide to produce some force.