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The angle between the direction a tire is pointed and the direction it is actually traveling. Tires must operate at a slip angle to generate cornering force, and peak grip occurs at a specific slip angle for each tire.
Katz explains in Race Car Aerodynamics: Designing for Speed: "When cornering, the side force is created by a sidewise slip of the tire. As a result of this slip, the actual direction of travel (vector V) is at an angle B to the direction of heading. For small angles of B (less than 4 degrees) the slope of the curve is linear, whereas for large side-slip angles, tire slip is large (vehicle may slide) and certain tires will actually create less side force at those larger slip angles. In practice, the tire should operate in the linear range because vehicle response to control inputs is then predictable." He adds the critical insight: "a vehicle in steady-state cornering must maintain a slip angle relative to its heading. For the vehicle body to assume a true tangential orientation relative to the circular path, all four wheels should maintain a side-slip angle." Understanding slip angle is essential because the balance of front versus rear slip angles determines whether a car understeers or oversteers.