Rear suspension systems can be tricky little devils, and as usual, the "devil" is in the details.
The roll center of any stick axle suspension depends upon what is controlling the lateral movement of the axle assembly, preventing actually (lateral being side to side, not to be snoot about things). It is a pretty hard thing to describe, it depends entirely upon the given geometry of a particular system. They all work differently, but let's tackle the two in the title.
A parallel four link (or three link, or torque arm setup with parallel links for that matter) depends "entirely" upon the Panhard Bar, or Watt's linkage to resist axle movement in the lateral direction, i.e. under cornering forces. IN this case, the roll center is defined by the intersection of the PHB height (keeping things as simple as possible) with the centerline of the axle, and again, to make things simple, let's assume that the links are parallel in top (plan) view. The roll center is an imaginary point in space that the car will roll about, specifically, the sprung mass of the car, under a given lateral acceleration (cornering force), and it is a very important attribute for certain. More important than where it "is" in a static case (not moving, just sittin' on the curb), at least in my opinion is what it does dynamically. PHB's do a really nice job at controlling the roll center in a dynamic sense, the other "better" solution is a frame mounted Watt's linkage, as Den' mentioned.
A converging four link does things a bit differently. IN this case, the angled arms (usually on the top, as in GM A body, late Mustang, plenty of aftermarket setups), not only control the pinion angle, but they are also responsible for controlling lateral axle movement. To find the roll center of these setups, you need to look at the convergence angle and intersection, and frankly, without a picture, it is really hard to describe. In the case of the GM A body, and also the Mustang (and the derivitives of thes in the aftermarket), they suffer from a really high static roll center, and worse, the roll center moves all over the place. Not only that, but think about it, the upper arms are asked to do two different things at the same time. I am personally not too good at multi-tasking, and these setups aren't in usual practice either. If you have ever driven a later Mustang in anger, there is no question that the back end of the car has come around on you in an abrupt fashion, known as snap oversteer. This occurs when (and because) the upper arms fully compress their suspension bushings, and the system becomes rigid (a trait of ALL four link systems in roll, by the way), and not at the time you'd like.
The Satchell setup is essentially a GM A-Body turned upside down, and it can be made to work pretty darned good. The engineer that designed it is named, get this, Terry "Satchell." It's far better than the other type, because the roll center height is a lot lower, but frankly, it suffers the same binding issues as the other four link systems. It's a bear to package too, the long upper links need to go somewhere, and that means no back seat.
So what do you do? For a street car? Whatever you want, and just do a good job tuning it. They will all work pretty good if you aren't nutso about the last ounce of performance. What's the ultimate solution? There isn't one, it ALL depends upon what you want in terms of packaging, performance, cost, ease of install, tune-ability, appearance, etc.
What would I buy? You're joking, right?
M
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