Slip angle

A non-zero slip angle arises because of deformation in the tire carcass and tread.

While it is equally valid to frame this as the tire/wheel being deflected away from the stationary tread element, convention is for the co-ordinate system to be fixed around the wheel mid-plane.

While the tread element moves through the contact patch it is deflected further from the wheel mid-plane.

[2] Actual instantaneous slip angles depend on many factors, including the condition of the road surface, but a vehicle's suspension can be designed to promote specific dynamic characteristics.

A principal means of adjusting developed slip angles is to alter the relative roll couple (the rate at which weight transfers from the inside to the outside wheel in a turn) front to rear by varying the relative amount of front and rear lateral load transfer.

Because of asymmetries in the side-slip along the length of the contact patch, the resultant force of this side-slip occurs away from the geometric center of the contact patch, a distance described as the pneumatic trail, and so creates a torque on the tire, the so-called self aligning torque.

There are two main ways to measure slip angle of a tire: on a vehicle as it moves, or on a dedicated testing device.

Various test machines have been developed to measure slip angle in a controlled environment.

That uses a 3-meter diameter disk that rotates under a tire held at a fixed steer and camber angle, up to 54 degrees.

Sensors measure the force and moment generated, and a correction is made to account for the curvature of the track.

'Deflected' tread path, sideslip velocity and slip angle
Graph of cornering force vs slip angle
A coordinate system used for tire analysis by Pacejka and Cossalter. The origin is at the intersection of three planes: the wheel midplane, the ground plane, and a vertical plane aligned with the axle (not pictured). The x-axis is in the ground plane and the midplane and is oriented forward, approximately in the direction of travel; the y-axis is also in the ground plane and rotated 90º clockwise from the x-axis when viewed from above; and the z-axis is normal to the ground plane and downward from the origin. Slip angle and camber angle are also shown.