A Look at the Dependent and Independent Suspension Types for Vehicles

  Suspension systems can be broadly classified into two subgroups, dependents and independents. These terms refer to the ability of opposite wheels to move  independently of each other.

  A dependent suspension normally has a beam, a straight axle for example, or driven live axle that holds wheels parallel to each other and perpendicular to the axle. When the camber of one wheel changes, the camber of the opposite wheel changes in the same way, by convention on one side this is a positive change in camber and on the other side this a negative change. Many suspensions are in this category as they rigidly connect the wheels together.

  An independent suspension allows wheels to rise and fall on their own without affecting the opposite wheel. Suspensions with other devices, such as anti roll bars that link the wheels in some way are still classed as independent.

  A third type is a semi dependent suspension. In this case, the motion of one wheel does affect the position of the other but they are not rigidly attached to each other. A twist beam rear suspension is such a system.

  Dependent systems may be differentiated by the system of linkages used to locate them, both longitudinally and transversely. Often both functions are combined in a set of linkages.

   Fully elliptical springs usually need supplementary location links and are no longer in common use. Longitudinal semi elliptical springs used to be common and still are used in heavy duty trucks. They have the advantage that the spring rate can easily be made progressive, non linear. A single transverse leaf spring for both front wheels and or both back wheels, supporting solid axles was used by Ford, before and soon after World War II, even on expensive models. It had the advantages of simplicity and low unsprung weight, compared to other solid axle designs. In a front engine, rear drive vehicle, dependent rear suspension is either live axle or deDion axle, depending on whether or not the differential is carried on the axle. Live axle is simpler but the unsprung weight contributes to wheel bounce.

  Because it assures constant camber, dependent, and semi independent, suspension is most common on vehicles that need to carry large loads as a proportion of the vehicle weight, that have relatively soft springs and that do not, for cost and simplicity reasons, use active suspensions. The use of dependent front  suspension has become limited to heavier commercial vehicles.

  Two transverse leaf springs, or four quarter elliptics on one end of a car are similar to wishbones in geometry, but are more compliant.  Because the wheels are not constrained to remain perpendicular to a flat road surface in turning, braking and varying load conditions, control of the wheel camber is an important issue. Swinging arm was common in small cars that were sprung softly and could carry large loads, because the camber is independent of load. Some active and semi active suspensions maintain the ride height, and therefore the camber, independent of load. In sports cars, optimal camber change when turning is more important.

  Wishbone and multi link allow the engineer more control over the geometry, to arrive at the best compromise, than swing axle, MacPherson strut or swinging  arm do, however the cost and space requirements may be greater. Semi trailing arm is in between, being a variable compromise between the geometries of  swinging arm and swing axle.