Different 4x4 Suspension Systems - Links, Leaves, & A Bar Named TrackPosted in How To: Suspension Brakes on September 1, 2008 Comment (0)
Some Wheelers Like Mud, and some like rocks. Some build trucks, and some build SUVs. Some drop off their 4x4s at a local 4x4 shop, and some roll them into the garage and do it themselves. No matter what type of wheeler you are, we support you because as long as you're getting that truck out and using it, you're helping our sport grow. Suspensions are the same way. There are tons of unique suspensions available and most of them work-and like wheelers, each is different depending on what they are trying to do off-road. We recently returned from a trip where the different suspension styles were mind-boggling. We saw everything from air shocks to quarter-elliptic half leaf springs to coilovers shocks with external bypass shocks for high-speed off-roading. It's a long way from the days when every old Jeep bouncing down the trail had some rough-riding skinny leaf springs and open "C" shackles. Whether you're planning a buildup of your own or just want to understand what your local 4x4 shop is talking about the next time you come in to get some work done, these photos will be enlightening.
The Toyota on page 30 is running heavy 1-ton axles and a rear leaf suspension that has a drop-away bracket that allows the axle to droop to extreme angles. This pivoting rear bracket is commonly known as either a droop shackle, a goofy shackle, or when replaced with a half of a leaf spring, a 3/4-elliptic suspension. Though it offers mad flex, it's not the best-performing suspension in our view because it has a portion of its travel uncontrolled when the shackle mount droops away from the frame. For example, when this truck was trying to climb a steep ledge, the rear suspension would droop out and not propel the truck forward, but rather just wrap out the axle and bang the shackle mount off the frame. Eventually the leaf springs were overextended and de-arched even with the limiting straps that were installed to keep the shackle from inverting. The owner has since replaced the droop shackle with a simple shackle mount welded directly to the frame for a standard leaf-spring suspension.
The second common type of link suspension is when a three- or four-link suspension is aided with a track bar (also known as a Panhard bar). This style of suspension has no angled links to locate the axle from side to side, but rather it uses a track bar that runs from the chassis to the axle and is mounted perpendicular to the other three or four links. These other three or four links still run to the top and bottom of the axle to deal with axlewrap and controlling the fore and aft movement of the axle as well as the propulsion of the vehicle. Running four links adds a certain amount of security should one link fail; however, four parallel links will inherently bind while three links (two lower and one upper as shown) will result in more articulation.
When using a steering box with a drag link and a link-style suspension, it is best to use a track-bar-style layout. The track bar (again, often referred to as a Panhard bar) should run parallel to the drag link and be as close to the same angle and length as possible. Also the more level the drag link and track bar are at ride height, the more stable the vehicle will feel.
This type of leaf-spring suspension is known as a quarter-elliptic. This uses leaf springs that have been cut in half and mounted to the frame with the arch inverted and curving down to the axle. Unlike a standard leaf-spring suspension, this setup requires a system of links to locate the axle fore and aft and side to side. We have seen good and bad results with quarter-elliptic suspensions and feel they are a rugged and reliable suspension design for the budget builder. Though most of the vehicles with these suspensions run the leaves parallel such as in the photo, we have heard that mounting them at an angle so the front ends are pointed slightly towards the opposite front wheel works even better. Notice that the shackles have a swiveling mount so that the leaves don't get twisted out of shape under extreme articulation.
The geometry of a link suspension would take an encyclopedia to properly explain, so here is a condensed version. The four-link uses four separate suspension links to locate the axle fore and aft and side to side while still allowing it to articulate and move up and down over obstacles. This is done with at least two of the links angled for side-to-side axle location, and we have seen every variant of link configuration such that the upper pair, lower pair, or both pairs of links are angled inward. We have also seen the converging ends at both the axle or at a chassis-mounted crossmember. Just remember that if you angle both the upper and lower link pairs, they need to have opposite ends angled inwards. The other job of the link suspension is to push the truck forward as the axle drives over obstacles, and depending on the geometry and lengths of the links and their mounting points, the truck will either do that well or it can jack up or squat the chassis. The links must also be separated vertically at both the axle and frame ends so that they can fight axlewrap. Plus as the 4x4 corners the geometry of the links in relation to the vehicle's center of gravity can determine body roll, which is how much the chassis wants to lean side to side. Many of these issues can also be influenced by sway bars, spring rates, and shock valving, not to mention the ratio of unsprung weight (axles and tires) to sprung weight (chassis, powertrain, and everything above the suspension). If you are considering putting a four-link in the front end of your 4x4, steering will become an issue, unless you go to full hydraulic steering. This is because the steering drag-link geometry is rarely built to follow the same geometry as a four-link and will result in bumpsteer. Another variant of this design uses one giant A-arm style upper link with two parallel frame mounts and a single upper axle mount in conjunction with two lower links. This is what is known as a three-link suspension. A three-link usually works just as well as a four-link of similar design, but the upper A-arm sees higher stress loads as the single joint end.
Link suspensions are a great design for your 4x4, and can result in optimal performance with greater approach and departure angles, freer yet more controlled axle movement, and simpler spring and shock installation, but remember that the vehicle is still supported by the springs whether they be coils, coilover shocks, quarter-elliptic leaves, airbags, or air shocks. These must all be mounted to move in the same arc of movement as the axle, and limiting straps may be necessary to keep the axle from drooping out past the travel of shocks and driveshafts. As with any suspension, all welds, link materials, and brackets need to be up to dealing with the stresses of suspending a heavy-duty 4x4 bouncing over rough terrain. We've seen every link mount imaginable tear off of frames and axles, not to mention the links bending and rod ends peeling open, and when they go they can take expensive shocks with them. Even though trick suspensions can perform amazing acts on the trail, they also need to be built to take the abuse. Those old leaf-sprung Jeeps didn't go that fast, but maybe that's what kept them in one piece.