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Multi-Link Suspension System Technology - Link Basics-Part 1

Posted in How To on August 1, 2009 Comment (0)
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Multi-Link Suspension System Technology - Link Basics-Part 1
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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.

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Building Links & Mounts
Links for a suspension such as this can be built in several ways. Typically the link material is either DOM thick wall steel tubing or solid 7071 grade solid aluminum rod. The ends of the links may use female threads to accept rod ends. For greater detail on rod ends and their qualities you can refer back to the July 2008 issue of OFF-ROAD Magazine.

Despite all the perfected calculations you'll get on paper or from the spreadsheet, the physical constraints of your rig will still have a lot to say as to where you locate the various components. You'll probably need to work with the existing frame rails, gas tank, and chassis crossmembers. This can be an iterative process until you get all the pieces to fit in place and get some performance numbers that you think will work for your application.

There are several specific characteristics that we'll want to focus on that can affect the behavior of the designed suspension. These are anti-squat, instant center, roll center height and roll axis.

Anti-Squat
There is weight transfer from the front of the vehicle to the rear under acceleration. This is often seen on a vehicle when the springs compress and the rear of the vehicle "squats". Anti-Squat is a characteristic that can be designed into a suspension to counteract the natural squat forces. This is determined by the link locations and angles, the wheelbase, and the center of gravity. You can design in sufficient anti-squat that will cause the tail to raise under acceleration. This action can affect tire loading and traction. A 100-percent number means the suspension will fully counteract the forces of weight transfer. Greater than 100 percent means the rear will rise and less than 100 percent means the rear will drop under acceleration. The perfect anti-squat number for your vehicle and terrain cannot always be predicted so it may be helpful to provide some link mount adjustability in your setup to tweak the setting once you try it out.

Instant Center
If you draw a line through the upper and lower links (viewed from the side) and extended them forward, the point at which they would intersect in space is the instant center. This is the point about which the suspension linkage will act. Imagine you draw a line from the rear tire contact patch to a point where the height of the COG and the front axle centerline meet. If the instant center lies below this drawn line, you have less than 100 percent anti-squat. If it lies above the line, you have greater than 100 percent anti-squat. The instant center should often end up near the vehicle center of gravity.

Roll Center Height
Whenever you turn a corner, centrifugal forces cause the body/chassis to move as the suspension allows them to "sway". The roll center height is essentially the height at which the body/chassis "pivots" with respect to the axle. If the roll center height is the same as the COG height, there would be no body lean as you go around a corner. However, almost all vehicles we drive will have a COG that sits higher than the roll center height so our vehicle leans to the outside of the curve when we make a turn or leans on a side slope. Link dimensions and mounting locations can affect the resulting roll center height, and by playing with the locations you can try to reduce the tendency for the body to lean.

Roll Axis
The roll axis is similar to the roll center in that it somewhat defines body roll. You'll have a different roll center front and rear, and a line drawn between the two is the roll axis. This is the axis along which the axles pivot with respect to the chassis and is dependent on where the triangulated links converge. You typically want the forward point of the roll axis lower than the rear point (roll understeer) to transfer weight to the rear when cornering. This provides more predictable handling and helps minimize oversteer in the rear. (As the rear axles moves through its range of travel and articulation it will encounter roll or bump steer which defines how much the axle steers left to right. Your link configuration will determine the extent to which you have this roll steer.) A vehicle with a low roll axis will tend to have greater body lean when side-hilling.

So, let's cut to the chase if you want to make doing a double-triangulated four-link rear fairly simple. Good points to typically shoot for are:
(1) Make the rear links as reasonably long as you can (frame constraints and ground clearance will dictate here)
(2) Make the link angles as reasonably flat as you can
(3) Make the lower links about 70 to 85 percent of the upper link length (stabilizes anti-squat)
(3) Keep your triangulation angles (viewed from the top) fairly high (40-45 degrees) to provide good lateral axle positioning
(4) Try to maximize vertical link separation at the rear axle (within reason; 8 to 12 inches is a good target)
(5) Keep frame end vertical separation approximately 50 to 75 percent of the rear axle separation (work with the calculator here)

In any case, there is no one setup that will work perfectly for all conditions. Variables such as wheelbase, COG, engine power, tire size, and type of terrain all play into making decisions for building a four-link. It can be helpful to talk to other drivers that have a vehicle similar to yours and play on the same type of terrain. Designing a competent link setup is not trivial, but when done right you stand to gain a suspension of high performance that handles well and puts the power and traction where you need it. It also allows you to separate the spring function of the suspension from the axle location function, further allowing you to more finely tune the performance effect of each. Choice with a linked suspension often falls to using coilover shocks with springs or nitrogen "air" shocks, though it can also be done with coil springs and separate shocks.

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