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.
 This top view of a double-triangulated...  This top view of a double-triangulated four-link system for the rear axle shows how both the two upper links and two lower links are triangulated to control lateral movement. Proper geometry design ensures the axle fully articulates and travels without binding or rolling the axle more than slightly. |  When viewed from the side,...  When viewed from the side, attaching links at two vertical points on the axle allow them to apply leverage to the housing and control axle roll. The amount of vertical separation between the links is important. Also, note the point where the two links would converge in space, which is called the instant center. We'll discuss this more as we proceed. |  We like using this online...  We like using this online link suspension program written by Dan Barcroft and Greg Blanchette that can be found at http://mysite.verizon.net/triaged/files/4BarLinkV3.1d.zip. This Excel spreadsheet-based program allows you to enter all your dimensions and it will spit out various design numbers for review. It not only calculates important performance characteristics, but also lets you see expected load forces on the link components which can be important when choosing parts for the build. |
 Rod ends can be added to suspension...  Rod ends can be added to suspension links with the use of weld-in threaded inserts. The inserts are sized to fit the rod end thread and the outer diameter chosen to closely fit the inside of the tubing with your required wall thickness. |  An example of an alternative...  An example of an alternative to a conventional rod end is this Rubicon Express Super-Flex joint. It is considerably larger than a comparable rod end as shown here, so requires greater space within the link brackets. Also shown here is their tool used to disassemble or tighten the side nut. These joints are rebuildable. |  This suspension link is mounted...  This suspension link is mounted in double-shear, which is far superior to single-shear mounting. When a link sits between two plates or mounting surfaces and the bolt spans both plates and the link, this is referred to as double shear mounting. Remember when building link mounts for rod ends or joints, angle the plates so they are parallel to the links when in a sitting ride height position. This will help ensure they have some angular movement room to keep from binding as the links move as the suspension cycles. |
 |  Roll center and roll axis...  Roll center and roll axis numbers will determine how much body lean you experience.However, just placing the roll center higher to combat body lean can have other affects on the performance. As one tire moves up over a bump, the body must move sideways as part of the linked behavior. At high speeds this can be disruptive as the tire moves up quickly, yet the body cannot move sideways rapidly. This can result in the vehicle bucking as the energy kicks the tire up rather than compressing the shock. The final result is a tradeoff between minimizing lean and having a suspension that handles rough terrain at speed well. There's no one setup that can do all types of wheeling optimally. |  On a vehicle with a stock...  On a vehicle with a stock frame you might use a lower link crossmember such as this to span the factory frame rails. You'd also want to add some additional support tubes from the center of the crossmember forward out to the frame to keep it from bending due to rearward pulling forces. |
 The lower links are mounted...  The lower links are mounted on the axle tubes out near the ends of the axle. You can mount these below, at, or above the tube centerline depending on your needs for ground clearance, wanting to keep the link angle reasonably flat, and keeping a respectable vertical separation between the upper and lower links. In any case, the dimensional positioning of these components will be an iterative process as you move parts a bit here and there to get the numbers and clearances to all play well. The greater the vertical link separation at the axle, the better ability the suspension will have to control any tendency of axle wrap or roll under power or braking. Vertical separation of the links at the frame end can be considerably less, typically about half to three quarters of the separation used at the axle end. |  It's not uncommon to make...  It's not uncommon to make the frame end location of the upper links adjustable in approximately 2-inch increments. With some variability in the upper link angle, you can make changes to the instant center which will change the degree of anti-squat. The lower links are typically placed based on axle tube height and the bottom of the chassis so are usually not easily moved. Note that these mount holes are placed on an arc so that the link can move between them without a need to change the link length. |  On a leaf spring rear, down-travel...  On a leaf spring rear, down-travel is usually limited by the amount the leaf packs can droop. However, on a linked setup using long travel shocks, letting the axle fully droop to the shock limit will often cause the driveshaft joints to bind or cause other problems. On rock crawlers, it's common to place a droop limit strap at the center of the axle. This limits straight down travel but still allows lots of axle articulation. On a go-fast setup, you might place a limit strap out at each axle end to limit extreme down-travel, where there's not a need for extreme articulation. |