There are many ways to lower a truck, and the options multiply with rear suspension design-especially when you're dealing with an adjustable suspension with airbags or hydraulic cylinders. One only has to walk among the rows of lowered trucks at any show and peek through the holes in the bed floors to see the plethora of ways to make the rear axle of a truck cycle up and down. If you have ever wondered which linked suspension design is right for your truck and the type of driving you intend to do with it, this article will provide not only the answer but also a good definition of how a suspension should work. By the time you're done reading, you'll be able to pick a rear suspension design that will offer a good compromise between ride quality, traction, handling, and of course, getting your truck flat on the ground.
Suspension Designs
Three- and Four-Links
We've grouped these together because they are essentially identical when viewed from the side. The names are indicative of their designs: a three-link has three links, and a four-link has four links (the Panhard bar or Watt's links are not counted.) These link systems can be configured to excel at just about anything you want your truck to do, whether it's road racing, drag racing, off-roading, towing, cruising, or hopping. If properly designed, these systems present few pinion-angle changes or driveshaft "plunge" issues, which can be a big deal when trying to get the most usable travel out of your rear suspension.
A different type of link system that falls into the three- and four-link group is the wishbone three-link. It is basically a triangulated four-link in which the triangulated bars are joined together to form a single point. Technically, however, it is still a three-link. The advantages over a traditional triangulated four-link are that the wishbone can be built much narrower and it can be oddly shaped to fit into seemingly impossible confines, with comparable lateral control.
Two-Links and Ladder Bars
These are grouped together because they are essentially the same thing: A ladder bar is merely a type of two-link suspension. This system is limited in what it can do because it typically does not offer the articulation (one wheel up and one wheel down) that is necessary for most daily-driver or performance applications.
Chevy did build trucks in the '60s and '70s with a two-link that worked (NASCAR rear suspensions are based on this design), but it was engineered/designed to do so. You see, the "truck trailing arm" two-link has the forward frame mounts right next to each other with only enough room for a driveshaft to fit in between. These narrow mounting points allow the rearend to move more with just bushing flex alone, and on top of that the bars are actually engineered to allow some twist. This gives the design limited articulation. So, to properly set up a two-link that offers the articulation desirable in a street application, you'd have to take into consideration the design attributes of the Chevy truck system, which aren't easy to recreate.
Two-links installed on today's trucks are not as capable as GM's version. The traditional two-link also has travel limitations because of pinion-angle issues. Theoretically, the rearend should not rotate as it runs through its travel (it is necessary for it to, in order to have an instant center, but only a small amount), and a two-link forces it to rotate around a single point set by the length of the links themselves. This is not a big issue if the bars are long enough with a limited amount of travel, but it's a weakness just the same. The major issue is its inability to articulate.
For simplicity's sake, we used a ladder bar in the roll-steer diagram on p. 107. Look closely and you'll notice that as the suspension cycles though its travel, the rearend housing rotates around the single front pivot point. As the suspension travels up it rotates forward, and as it travels down it rotates back. You can begin to understand how having one wheel up and one wheel down can be an issue. Essentially, on a two-link/ladder-bar system, the rearend housing acts as an antisway bar-a really big antisway bar-and that is not desirable for good handling or even daily driving.