This force (Fy) is acting on the tire centre which creates a moment, whose moment arm length is caster offset at ground. See fig. Lateral force depends on steer angle, cornering condition and with positive caster produces a moment attempting to steer the vehicle out of turn causing understeer.
Net moment = ML = (Fyl + Fyr) r tan γ
Fyl, Fyr = Lateral forces at left and right wheels.
r = Tire radius
Tractive force (Fx)
This force (Fx) is acting on the tire centre which creates a moment, whose moment arm length is kingpin axis offset at ground also called scrub radius.
Net moment = MT = (Fxl – Fxr)d
Fxl, Fxr = Tractive force on left and right wheel
Tractive force = power of engine by velocity of vehicle
d = scrub radius
Left and right moments of tractive force is opposite in direction and tends to balance each other. Imbalance can blowout the tire, brake malfunction, or split coefficient surfaces, and will produce steering moment which depend on scrub radius. So try to keep it minimum.