Virtual Scrub Radius and Caster Trail
Request Number: 50000340
Scrub Radius and Caster Trail both depend on the steer axis.
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 kingpin 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 parameters that rely on it. These requests should be used on suspensions that do not have a clearly defined steer axis, such as a 4 link or 5 link independent suspension.
For the computation of the kingpin axis, the steering needs to be free (no motion) and the suspension travel needs to be locked. The lock needs to be defined in the model and will be invoked, when computing the steer axis automatically. Defining the lock in the model involves the definition of an array, containing the position, direction and body to lock and calling a user subroutine to pass this information to the solver. See Locking Steering and Spring Travel for details.
| Component | Unit | Comment |
|---|---|---|
| 1 (X): Left Virtual Scrub Radius | mm | The magnitude is the global lateral distance between the left tire patch point and the left virtual kingpin ground intersection point. The sign is positive when the kingpin intersection point is inboard of the tire patch point. |
| 2 (Y): Right Virtual Scrub Radius | mm | The magnitude is the global lateral distance between the right tire patch point and the right virtual kingpin ground intersection point. The sign is positive when the kingpin intersection point is inboard of the tire patch point. |
| 3 (Z): Average Virtual Scrub Radius | mm | The mean of components 1 and 2 |
| 4 (RX): Left Virtual Caster Trail | mm | The magnitude is the global longitudinal (X) distance between the left tire patch point and the left virtual kingpin ground intersection point. The sign is positive when the kingpin intersection point is forward of the tire patch point. |
| 5 (RY): Right Virtual Caster Trail | mm | The magnitude is the global longitudinal (X) distance between the right tire patch point and the right virtual kingpin ground intersection point. The sign is positive when the kingpin intersection point is forward of the tire patch point. |
| 6 (RZ): Average Virtual Caster Trail | mm | The mean of components 4 and 5. |