Looking At Rear Four-Link Tech
Linked suspension setups have become much more popular in recent years and their inherent design can provide significant performance superiority over other suspension types. Tubular rod links and jointed connections make up the control systems on these vehicles and can offer massive travel and articulation. Some of these systems are stock suspension design, but many are custom built by enthusiasts for their own rigs. A basic understanding of the fundamentals of a rear multi-link system is the basis for this article.
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 controlling dampening of the axle and 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 (the tendency of the axle to want to turn in opposition to wheel torque).
In a leaf-spring system, the leaf springs themselves control both the fore/aft and side-to-side movement. The rigid structure of the springs in these two directions serves these duties. The leaf springs inherently prevent axle roll (or torque wrap) based on their thickness and spring rate. Thinner or more flexy packs do less to control axle roll and some type of supplemental axle constraint may be needed in such a case. Added traction shocks, ladder bars, or track bars may be used to control this third type of movement.
In a coil spring system, the coil springs serve only to support the weight of the vehicle. Coilover shocks are similar but add a shock dampening function. However, neither of these can offer any other directional axle control as the leaf springs can. In this case, it's necessary to use radius arms or suspension links to locate and control the travel path and roll of the axle.
With these thoughts in mind, we'll look at what it takes to design a rear four-link suspension setup. For a deep understanding of all the dynamics, far more extensive calculations and physics are involved than will be presented here.
We'll provide a solid introduction and can refer you to these books for even greater technical detail: Chassis Engineering by Herb Adams and Race Car Vehicle Dynamics by Milliken and Milliken.
There are several configurations that can be used, but a very common one for a rear linked suspension is the double-triangulated four-link, which will be discussed here. We'll present the basic design idea and explore some of the performance characteristics.
Using this method, a set of lower links runs from a center point on the chassis back toward each end of the rear axle. The upper links then run from a centered point on the top of the axle housing toward points forward on the frame of the vehicle. In some factory built setups, such as on some Land Rovers, two of these links may actually be one single triangular (or wishbone) link with a single termination point on the top, center of the axle.
Note, however, that wishbone link setups can result in high force loads at the single jointed end of the wishbone. In either case, the "triangulated" links provide lateral positioning and keep the axle positioned perpendicular to the direction of travel.
Before we go further with the technical talk, it's good to know that designing and building a four-link setup is not trivial and if not done with at least some forethought and deliberation, the result may leave you with a vehicle that performs worse than before you made the conversion. You'll need to pull out the tape measure and do some careful planning to design a system that will perform as desired given the amount of expense and work you'll put into it. If done correctly, the end results can be well worth the effort, giving you a suspension that provides excellent traction, travel or articulation, and the ability to fine tune the suspension to your style of wheeling.
Designing The Four-Link
When building a four-link, the end performance will depend on the length and mounting locations of the links and by changing these variables we can significantly change how the rig behaves under acceleration, climbing, and side hilling. Body roll and sway are also affected by the parameters of the design. Additionally, a setup tuned to rock crawl well will typically differ from one tuned to go fast over rough terrain.
A suspension design starts with gathering some critical measurements that will be used for calculations and component placement. We'll need to know wheelbase, width of the axle mount points, width of the frame mount points, and center of gravity. On many vehicles, the height of the center of gravity is often taken to be the height of the top bellhousing bolt on the back of the engine. Also, when building your link setup, it's usually good to design and build it at desired ride height, and then compress and articulate the axle to check clearances.