Lateral forces

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.

steering moment caused by lateral force
steering moment caused by lateral force

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.

steering moment cause by tractive force
steering moment cause by tractive force

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.

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