Virtual Steer Axis Point

Request Number: 80000310

The kingpin axis or steer axis on an SLA or Macpherson strut suspension is defined by two points (the upper ball joint and the lower ball joint in the SLA suspension, the upper strut mount location and the lower ball joint in the case of the Macpherson strut suspension). When the steer axis is not defined by two distinct points, it must be calculated virtually, using the motion of the spindle to calculate the instant axis of rotation of the knuckle. The virtual requests (numbers 50000310-50000350) use this method to calculate the steer axis location, and the parameters that rely on it. These requests should be used on suspensions that do not have a clearly defined kingpin axis, such as a 4 link or 5 link front independent suspension. Since this is a virtual axis, the location of this point is not unique. Any point on the virtual steer axis would be valid and many points can be a correct solution for the same steer axis.

To compute the steer axis, the steering needs to be free (no motion) and the suspension travel needs to be locked. The locking of the spring travel is done numerically on the compliance matrix, rather than mechanically in the model. The information about the suspension spring needed for locking is encoded in the solver IC Arrays. Wizard build models include the arrays. For more information about defining the lock see Locking Steering and Spring Travel for details.