Thecircle of forces,traction circle,friction circle,[1] orfriction ellipse[2][3][4] is a useful way to think about the dynamic interaction between a vehicle'stire and theroad surface. The diagram below shows the tire from above, so that the road surface lies in thexy-plane. The vehicle to which the tire is attached is moving in the positivey direction.
In this example, the vehicle would be cornering to the right (i.e. the positivex direction points to the center of the corner). Note that theplane of rotation of the tire is at an angle to the actual direction that the tire is moving (the positivey direction). Put differently, rather than being allowed to simply "roll" in the direction that it is "pointing" (in this case, rightwards from the positivey direction), the tire instead must "slip" in a different direction from that which it is pointing in order to maintain its "forward" motion in the positivey direction. This difference between the direction the tire "points" (its plane of rotation) and the tire's actual direction of travel is theslip angle.
A tire can generate horizontalforce where it meets the road surface by the mechanism ofslip. That force is represented in the diagram by thevectorF. Note that in this example,F is perpendicular to the plane of the tire. That is because the tire is rolling freely, with notorque applied to it by the vehicle'sbrakes ordrive train. However, that is not always the case.
The magnitude ofF is limited by the dashed circle, but it can be any combination of the componentsFx andFy that does not extend beyond the dashed circle.[3] (For a real-world tire, the circle is likely to be closer to an ellipse, with they axis slightly longer than thex axis.)
In the example, the tire is generating a component of force in thex direction (Fx) which, when transferred to the vehicle'schassis via thesuspension system in combination with similar forces from the other tires, will cause the vehicle to turn to the right. Note that there is also a small component of force in the negativey direction (Fy). This representsdrag 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 generateslateral 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.
The total horizontal frictional forceF cannot exceed the maximum value (radius of the 'friction circle').
A much used graphical aid to understanding how corning and traction (braking and driving) forces combine is the so called "friction ellipse".
