Longitudinal arm independent suspension refers to the suspension structure of the wheel swinging in the longitudinal plane of the car, and is divided into two types: single longitudinal arm and double longitudinal arm. A single trailing arm suspension will cause a greater change in the caster angle when the wheel bounces up and down, so a single trailing arm suspension is not used on the steering wheel. The two swing arms of the double-longitudinal suspension are generally of equal length and form a parallel four-bar structure so that when the wheels run up and down, the caster's back rake remains constant. Double-arm suspensions are often used on steering wheels.