Multi-Link Suspension Tech
You've probably seen multi-link suspensions on trail-tough 'wheelers featured in OFF-ROAD magazine. Tubular rod links attached by flexible metal connectors make up the axlehousing control system on these wild rigs where maximum suspension travel and articulation are the main goals. Some multi-link systems are an OE suspension design, but many are custom, high-performance setups built by four-wheelers for their personal rides. A crucial design factor associated with custom-built systems ensures that the basic laws of geometry are followed in order to avoid handling nightmares. To fabricate a strong-performing multi-link suspension, a solid understanding of the fundamentals of suspension design is required.
In a solid-axle suspension system, the axle moves primarily vertically, with leaf or coil springs providing support for the vehicle's weight and shocks damping the movement of the axle and the related components. Within the system, there must also be a way to confine other movements of the axle, these being fore, aft, side-to-side, and axle roll.
In a leaf spring system, the leaf spring packs position the axlehousing in the fore, aft, and side-to-side planes. The rigid structure of the spring packs serve these duties. As for axle roll, the basic leaf spring design prevents axle roll (or wrap) based on the springs' thickness and spring rate. Thinner leaf springs do less to control axle roll; some type of supplemental axle constraint may be needed in such a case. Traction shocks, ladder bars, or track bars may be used to control movement with soft leaf packs.
In a coil spring system, the coil springs serve only to support the weight of the vehicle. They offer no directional control of the axlehousing; leaf spring packs, on the other hand, do. With coil springs, it is necessary to use radius arms or suspension links to locate and control the path and the roll of the axle.
A third style of multi-link suspensions are the various leaf spring hybrids such as double shackle systems (a shackle at each end of the spring), and 1/4 or 3/4 elliptic leaf spring setups that are found on exotic, scratch-built systems.
With this in mind, let's look at some of the basic types of multi-link systems used in 4WD suspensions. The simplest is a radius arm system, which has been commonly used on the front of Ford trucks and Broncos for many years. Coil springs are used to support the vehicle and two radius arms rigidly mounted to the axle control fore or aft axle position, axle roll, and also define the arc of travel of the axle as it moves up and down. A third control, a Panhard rod, is added and runs parallel to the axle. One end is attached to the axle and the other end to the frame. This rod controls side-to-side movement, positioning the axle laterally.
Four-link systems have been used by a few automakers and are often a prime choice for home-brewed off-road systems. In this type of system, there are usually two links mounted to the top portion of the axle and two links mounted to the bottom portion. The forward ends of each of the links attach to some portion of the frame. By attaching links above and below the axle, the links can apply leverage to the housing and control axle roll.
Typically the top two links form a triangle and both meet near the center of the axlehousing. In some cases, such as on some Land Rovers, these two links may actually be one single triangular (or wishbone) link with a single connection point on the top center of the axle. The two parallel lower links cannot control side-to-side movement, but the triangulated top links can. The top links provide lateral positioning, and, combined with the lower links, keep the axle positioned perpendicular to the desired direction of travel.
Another variation of the multi-link system is the five-link. It is similar to the four-link except that the four main links are parallel or near parallel. Since there is no triangulation of these links, they provide no lateral positioning or control of the axlehousing. A fifth link, a Panhard rod, is used for this purpose. This system is similar to the radius arm type described above, except the links in this system are not rigidly mounted to the axle, so four links are needed to control all the movement directions. The five-link is the more complicated of the two, but typically offers a smoother ride and more travel.
Multi-link systems come in many variations and allow many different suspension design possibilities. Whether you need concise axle control for a monster truck, smooth movement for street and trail use, or maximum articulation for rockcrawling, one of these could work wonders on your vehicle.