The circle of forces, traction circle, friction circle, or friction ellipse is an illustration representing the dynamic interaction between a vehicle's tire and the road-plane direction.

direction points to the center of the corner). The plane of rotation direction. This difference between the direction the tire "points" (its plane of rotation) and the tire's actual direction of travel is the slip angle.

A tire can generate horizontal force where it meets the road surface by the mechanism of slip. That force is represented in the diagram by the vector is perpendicular to the plane of the tire. That is because the tire is rolling freely, with no torque applied to it by the vehicle's brakes or drive train. However, that is not always the case.

that does not extend beyond the dashed circle. For a real-world tire, the circle is likely to be closer to an ellipse, with the y-axis.

) which, when transferred to the vehicle's chassis via the suspension). This represents drag that will, if not countered by some other force, cause the vehicle to decelerate. Drag of this kind is an unavoidable consequence of the mechanism of slip, by which the tire generates lateral force.

The diameter of the circle of forces, and therefore the maximum horizontal force that the tire can generate, depends upon many factors, including the design of the tire and its condition (age and temperature, for example), the qualities of the road surface, and the vertical load on the tire.

See also

• Cornering force

• Racetrack (game)

• Skidpad

• Slip (vehicle dynamics)

• Vehicle dynamics

References