The user written tire function received inputs as per the TYDEX format.  The user model must use these to calculate the wheel kinematic situation, things such as contact patch location, slip angle, slip ratio, etc.  The road function is called to receive information about the height of the road.  To assist this calculation Altair provides a set of utilities that are documented in the Utilities to Calculate Tire Kinematics, Forces, and Moments section.  All three forces and three moments of the tire must be calculated in the wheel carrier axis system, defined by the TYDEX standard (TYDEX C).  The units of forces and moments should be Newton and Newton-meters respectively.