Every link system requires a lateral locating device of some sort to keep the rearend from moving from side to side in relation to the chassis. Some systems have it built in (for example, a triangulated four-link); others need an auxiliary system (like a Panhard bar). The locating device used determines the roll center, and the roll center changes the way that the truck acts and handles. So in a road-racing application, having a system with different mounting points offers a system with a tunable roll center. On a daily driver, the roll center isn't a major concern.
The simplest of all the locating devices is the Panhard bar. It is mounted to the chassis at one end and the rearend housing at the other. The roll center is where the Panhard bar crosses the center of the rearend. So the higher the Panhard bar is mounted in the chassis, the higher the roll center and vise versa. The weak link of the Panhard design is that the single bar pulls and pushes the rearend from side to side as it runs through its travel. The best way to battle the push/pull effect of the Panhard is to run as long of a bar as possible and to be sure that the bar is parallel to the ground at half travel.
The beauty of this system is that if properly set up, the Watt's link has zero lateral movement and the roll center is easy to calculate. The center pivot of the link itself is the roll center.
Figure 4 Center-pivot movement...
Center-pivot movement during suspension compression
Figure 5 Center-pivot movement...
Center-pivot movement during suspension rebound
On a properly set-up Watt's link, both bars will be parallel to the ground at half travel, but the center link itself will be over center. The center link will be straight up and down only at 1/4 and 3/4 travel. Also, because there are two bars limiting movement, the links need only be half as strong as a Panhard bar. It's not unrealistic to be able to use a 7/16 rod end on the connecting links. The center pivot should be as stout as possible because it "sees" the entire lateral load at that one point.
A track locator, or track bar, is typically used on drag cars because of its simplicity. A track bar is a bar that simply mounts to the rear link mount on one bar and the front link mount on the opposite bar, essentially creating a triangle that won't allow any lateral movement (Figure 6). The strange thing about the track bar is calculating the roll center. I have talked to some very knowledgeable suspension guys and cannot find a solid answer to demonstrate the best way to establish the roll center with this design.
By triangulating the upper, lower, or both sets of bars, you can limit lateral movement without the side pull of a Panhard bar. However, the bars must be triangulated as much as possible (up to 90 degrees to each other). It's tough to state a minimum angle because the length of the bars and the compliance of the bushings play a major role.