$PH, $PHSTEP, $PHASESTEP

This parameter is relevant to vehicles that use multiple phases to depict moving parts (usually wheels and/or valve gear).  The setting modifies the distance which vehicles travel across the screen before the next phase picture is displayed.

The diameter of the driving wheels (input with the +/<Diameter> - parameter in the picture's macro definition) is multiplied by the value of $PHASESTEP and divided by the number of phases in the picture.  The geometrically correct value for this parameter is p (Pi) = 3.14.  However, often the vehicle's motions appear much more fluid when a lower value is selected.  One can therefore use this parameter to set the value to one's own taste.