Link Suspension For 4x4 Trucks & Jeeps - Leading LinksPosted in How To: Suspension Brakes on September 1, 2009 0) (
Any truck with a coil-sprung, coilover shock, air-bag, or even quarter-elliptic front solid-axle suspension requires a link-style suspension. Because the springs cannot locate the axle, links are required to keep it from moving side-to-side or front-to-back while still allowing it to articulate and move up and down.
There are many types of front link suspensions but the three most commonly used versions for a front suspension are radius arms, a three-link with a Panhard bar, and a four-link. Each version of suspension has good and bad points.
When building your own front link suspension there are many factors to take into consideration, and oftentimes the best-laid plans are scratched when you actually try to make them fit. In a perfect world your links would be made out of unbendable lightweight material (not always cheap), mounted as level as possible (not always room to do that), so long that they get a nice gradual arc of travel (but not too long), and would attach to the axle above and below the axletube and at least 6 inches apart (ground and engine clearance problems quickly arise); and all the bracketry would be welded by a certified professional welder (you are certified, aren't you?). But that doesn't always happen. Though not a how-to article, this introduction to front link suspensions should help get some ideas flowing for your next buildup.
The most extreme front suspension is the triangulated four-link. In this style two of the four links must be triangulated either from the frame to the axle or vice versa in order to keep the axle from moving side to side. Some suspension builders make both the upper and lower links triangulated, but you really only need one pair. The four links combined keep the axle from moving frontward or backward or twisting.
Since the axle moves in an arc from where the links attach to the frame, this system usually requires full hydraulic steering because using a steering box with a crossover draglink will cause bumpsteer. We've seen complicated steering systems with either the box or a bell crank back near the frame mount of the upper suspension link, and a draglink running to another bell crank on the axle, but this is added complexity. Even though the multiple links spread the load and make it very strong because of the steering issues, we don't usually recommend a front triangulated four-link on a street-driven vehicle.
Three-Link With Panhard Bar
The best middle-ground front link suspension is a front three-link with a Panhard bar. This is usually built with one upper link inside the framerails and two lower links on the outside or bottoms of the framerails all running parallel to the framerails. The Panhard bar (also known as a track bar) runs perpendicular from one framerail down to the axle to control sideways movement.
Three-Link With Panhard Bar
The three-link with Panhard works best with a bar that is straight, long, and level at ride height, but it should also be as parallel and equal in length to the drag link as possible in order to reduce bumpsteer throughout the range of motion of the suspension. Unlike a triangulated four-link, a Panhard suspension will move the axle side to side as it moves up and down, but oftentimes the Panhard suspension is easier to package than a triangulated four-link up front.
A radius-arm suspension uses two links to a single points on the chassis, but each link attaches to the axle at two points, usually above and below the axletube, or clamps onto the axletube. This suspension uses bushings rather than Heim joints to lessen the binding. In addition a radius arm suspension will also have a Panhard or track bar like a three-link.
Factory Radius-Arm (Ford)
Most fabricators we spoke with admitted that building a radius-arm suspension is the easiest route but their least favorite since it will inherently bind during articulation. However, this might actually be a bonus for a street-driven truck. The binding acts like a sway bar, and that's likely why Ford, Land Rover, and 80 Series Land Cruisers all used some form of radius arms. Another option is mixing a single upper or lower link with one radius arm to alleviate the binding, but it's not our favorite layout.
What About Five-Links?
Jeeps have been running four straight links with a track bar since the first Jeep Cherokee XJ arrived in 1984. This has continued through the TJ, ZJ, MJ, LJ, WJ, and JK as well as late-model solid-axle Dodge trucks. So why don't we recommend this five-link design in this story? It really depends on the link ends you use. If more than four total links (including the track bar) are used, the suspension will bind under articulation, but if your link ends have some flexibility then they can deal with this binding, and every one of these Jeeps has some sort of flexibility in their bushing link ends.
We do appreciate the two upper and two lower links for additional spreading of the loads the axle sees under hard acceleration and braking. The five-link design is also better than a radius arm since it spreads the load across more of the frame. For maximum articulation either a triangulated four-link or a three-link with a Panhard bar is recommended.