From the rear the link bars...
From the rear the link bars are of little importance-only if the system chosen is a wishbone-type three-link. If that is the case, then the point at which the wishbone meets is the roll center. If a Panhard bar is chosen (shown), then the point at which the Panhard bar intersects the centerline of the vehicle is the roll center.
The separation of each of these designs is from above. From this view you can see how each bar is run. The triangulated four-link would be the most obvious and is what is most commonly used in the mini-truck industry. The triangulated four-link does not need a Panhard bar or any other lateral limiter. (The triangulation of the bars does the limiting for us.) This would be a good time to touch on the "wishbone"-type three-link. What this system does is bring the close ends of the triangulated into each other, creating the wishbone. It acts exactly like the triangulated four-link, except that the single point of the wishbone is the roll center. Mini-trucks don't really use this system because of space limitations. Early Cadillacs used this system and had a reputation of breaking the ball joint used at the single point. The overhead view is also another addition to the roll steer picture. By pointing the bars towards the front or rear of the vehicle, it can change the roll steer effect.
The three-link and parallel four-link need a lateral limiter of some sort, which would be viewed from the rear (in most cases): a Panhard bar, track locator (viewed from above), Watts link, etc. There are many different ways to limit lateral movement and all have their advantages and disadvantages. But that's for another day
Forward, reverse-what's the difference? A forward-facing four-link transfers energy to the chassis, planting the rearend under power and a small amount of anti-dive under braking; and it can be tuned infinitely for a specific purpose. A reverse four-link, on the other hand, should only be used as a compromise. They will drive fine and turn corners and stop the car without too much upset, but again, as I've stated several times before, "the confines of the mini-truck do not leave much room for dangerous designs." On top of that, there aren't bucket-loads of horsepower pouring out of the average mini-truck and if so, it usually doesn't make it to the ground. Another saving grace of the reverse orientation is that roll steer is almost always roll understeer, making the truck feel stable in corners. Also, the pinion angle, when properly designed, is kept well within working limits. The point is, the reverse four-link works. It holds the truck off of the ground and keeps the rearend under the truck.
You can see from the top view...
You can see from the top view that all of the mounting points are the same distance from front to rear but only differ from each other by bar orientation. The parallel four-link and three-link bars can also run on top of each other, making the top view look like a simple two-link.
The 2-forward, 2-reverse works because it does nothing. It doesn't help, it doesn't hurt...nothing. The biggest issue with the 2-forward,2-reverse is pinion angle. It's a little tough to calculate and doesn't offer much travel because of this. On an odd note, this system doesn't move much front-to-rear as it runs through its travel. We've never seen the reverse four-link or the 2-forward, 2-reverse-type four-link used anywhere else except the mini-truckin' industry. Bob Grant called me the other day and challenged me to figure out how to build a rear suspension that doesn't move front-to-back in the wheelwell. I immediately thought "A-arm." Then he clarified "straight axle." I then promptly said that it could not be done. Then he told me his idea. It would work, but if the rearend does not follow a set point in space (instant center), it will not add or subtract anything to the cause of traction.
Well, we hope you were able to get through this and decipher the cryptic writings about the black magic of link-type suspension. Stay tuned as Max reveals more and more suspension wisdom for our quest of knowledge.