The vertical force is developed in the tire as a function of the deformation of the tire.  It has an elastic component and a deformation velocity dependent component (typically insignificant compared to the stiffness part).

Where:

The “ucpz” is the road plane normal unit vector, the Rim Velocity is the three component vector of the Wheel Carrier velocity, and Rl is the loaded radius of the tire.

The vertical force can only be positive:  in case of lift off condition of the tire from the ground, the resulting negative reaction force is set to 0.

See Also:

Longitudinal Force

Lateral Force

Rolling Resistance Moment

Aligning Torque