The distance in side elevation between the point where the steering axis intersects the ground, and the center of tire contact. The trail (or offset) is considered positive when the intersection point is forward of the tire contact center and negative when it is rearward.

SVCCTR

Left and right kingpin marker identifiers (idksil, idksir)

Left and right front wheel center marker identifiers (idwcfl, idwcfr)

Global origin marker identifier (gblm)

Ground height (ght)

SVC uses the SYSARY utility subroutine to retrieve the positions of the wheel center and kingpin markers with respect to the ground plane. SVC next computes the intersection point of the kingpin axis and the ground plane, and the wheel/ground intersection line. The caster trail is the distance along the wheel/ground intersection line from the kingpin axis/ground intersection point to the tire contact point to the ground. SVC assumes the z-axis of the kingpin marker points up the kingpin axis and y-axis of the wheel center marker is parallel to the spindle axis. SVC also assumes the global origin marker lies somewhere inboard of both the wheel/ground and kingpin-axis/ground intersections.