Triangulation Of Bars
Triangulating the bars is a means of limiting lateral movement. And because it is a means of limiting lateral movement, it is also what sets the roll center. This one is just a little harder to explain. If you find the instant center (IC) of both upper bars and the IC of both lower bars on a three-dimensional plane and then take those two separate theoretical points and draw a line between them, you have the axis around which the rearend pivots. The point on the axis that crosses over or under the axle centerline is the roll center.
To make things easy, let's just assume that the roll center is very near the height of the triangulated bar ends that are nearest each other. For example, if the upper bars are triangulated and are mounted close to each other on top of the third member, the height of those joints is fairly close to the roll center.
So, if a low roll center makes for better handling (via more weight transfer), how would you lower the roll center on a triangulated link system? Triangulate the lower bars. A gentleman by the name of Satchell came up with this design to help trucks with limited space achieve a lower roll center. To properly set up a Satchell link, the lower bars must be triangulated with the bars coming to a point at the chassis, not the rearend housing. By doing this, the roll center stays "coupled" to the chassis and not to the constantly moving rearend housing. By having a roll center that is constantly moving up and down, the chassis can feel twitchy. Again, this isn't too much of a concern on a daily-driven, 'bagged truck, but it's good dinner conversation if you're truck is supposed to handle well.
Designs In Practice
Any time two or more links, bars, arms, or any other types of locating devices are employed, there will be an instant center (IC). The IC is found by extending an imaginary line through the points of each individual locating device until they meet the imaginary line of another related locating device. The point at which they meet is called the instant center or instantaneous center. Take for example a simple four-link. From a side view, the four-link appears to have only two links. If you were to extend an imaginary line through the pivots of each individual bar, at some point those two lines will cross. The placement of the IC is what determines where the forces generated by the tire/rearend relationship will be transmitted to the chassis. By moving the IC forward or backward or up and down, you change what type of driving the suspension excels at. This is the most important theoretical point to understand in the world of suspension design.
A properly set-up three- or four-link will have an instant center that is somewhere in front of the rearend. By moving the IC front to rear, you change the amount of force applied to the chassis. By moving it up and down, you can dial in the desired amount of antisquat. The position of the IC combined with bar length determines what it is that the link system excels at. A longer bar will allow more travel (but at the cost of energy transfer), while a shorter bar has less travel capability but more force transfer (think off-road truck versus a drag car.) Since most of you will probably be building a daily driver with adjustable ride height, the system should be somewhere in the middle. A medium-length bar with the IC around the front bumper at half travel is a very neutral system. It is essentially how Detroit has been doing it for years.
Also note point A point is a percentage of the center-of-gravity line (Figure 7). That percentage is the measure of antisquat. When a truck accelerates, weight is transferred to the rear, making the suspension "squat" from the extra weight. This chassis movement can be minimized or completely eliminated by using geometry to negate the squatting effect. On any rear suspension system (assuming it is a full-forward system, even a two-link), moving the IC higher will produce a higher percentage of antisquat, making the truck not squat as much during acceleration. A lower IC will have a lower percentage, and the truck will squat more. Going overboard and running the bars high in the front in order to negate any squat will induce excessive roll oversteer, making the truck less stable while cornering. It is not so important to calculate antisquat for a daily driver; it's more important you understand that it exists.