I hear you but I don't think that's right.
Seems to me on a WL setup what is helping prevent (or a better word might be control, or damp) one wheel from flopping up and down on a one wheel bump is the fact that the other wheel remains in contact with the ground. Thus as one wheel hits a bump, the center pivot point on the WL moves slightly to one side and the chassis of the car needs to move with it. There's no such limitation on a PHB car, where all the vertical movement of the axle needs to be controlled solely with the spring and damper.
It also seems to me that putting the pivot point in the center of the axle like with a WL design, rather than on one end like a PHB, should measurably reduce the unsprung weight the spring and shock needs to control when one wheel hits a bump. Basically, the spring and shock are controlling the weight of half an axle instead of the whole thing.
Okay, a little suspension explaination is in order, here, I think...
SPRINGS: The only thing they do, is keep the body up above the axle. Period. Full stop.
DAMPERS: Are there to control the
rate of the spring compression/expansion, and to damp oscillations from spring movement.
CONTROL ARMS: Are there to locate the axle fore/aft (lower control arms), and prevent it from rolling in side-view (upper control arm) or yawing in plane view. They do nothing (disregarding bushing compliance) to keep the axle centered under the longitudinal centerline of the car. Without these three links, the axle is free to move on any axle, be it pitch, roll, or yaw.
Panhard bar or Watts link (or DeDion tube, or....) has only one function, and that is to positively locate the axle relative to the centerline of the car (top or rear view). They work on different principles, but the effect is pretty much the same.
PANHARD BAR: On the S197, is a long (~48" from memory) bar connecting a chassis pivot point (passenger side) to an axle pivot point (driver's side). With a two-wheel bump (like a speed bump), the axle will move vertically, which WILL cause the distance between the two pivot points (as measured on the ground) to change, forcing the axle centerline to move relative to the chassis centerline. Depending on the angle of the bar when the suspension is static, it may move towards the left or the right. To figure out the net change in centerline position, let's make some assumptions. First, that when static, the axle centerline and chassis centerline are in vertical alignment. Second, that we're discounting bushing compliance completely, to keep the math simple. So, we're going to look at some triangles. Assume that static, the chassis side is higher than the axle side pivot point. And let's say that it's by 2". So, we can define the three sides of a right-triangle: (1) the length of the bar itself, which will remain constant, connecting the two pivot points (“C”). (2) The vertical height difference between the pivot points (“A”), and (3) the "unknown" side, defined by the lateral distance between the two pivot points (“B”). So, let's plug in some numbers. (1) is 48", (2) is 2", and (3) is "X" that we need to solve for. Pythagorean Theorum says that A squared plus B squared equals C squared. Cool! That means that B squared is equal to C squared minus A squared. Plugging in the numbers, B squared equals 2304 (C squared) minus 4 (A squared). Or, B squred is 2300 even. The square root of that is 47.958”.
Now, what happens with our two-wheel, 1" bump? (1) remains at 48", since the length of the tubing hasn't changed, (2) changes to 1 instead of 2, but what happens with (3)?? Well, 48" squared is still 2304. 1 squared is simply 1. That makes B squared 2303 instead of 2304. The square root of 2303 is 47.990. Thus, comparing the two, for a 1" change in bump, the axle will shift 0.032" to the driver's side (47.958-47.990= -0.032).
How about for a 1" droop, like dropping the tires in a big wide pothole? Again, 48 squared is 2304. -2 squared changes to -3 squared, which is -9. Sum is 2295. Square root is now 47.906. Difference of 47.906 and 47.958? 0.052", this time shifted to the passenger side.
Now, for a one-wheel bump, the math becomes a lot more complex, but it should still go to show that we're not talking about very huge lateral movements here. If you think about it, even if you were to DOUBLE the lateral motion from 0.052" to 0.104", it's still spit in a bucket compared to tread squirm or carcass shift in the tires during lateral loading. Even with Hoosier R6, you can see a full INCH of carcass shift in hard cornering... That's a full order of magnitude higher than the axle centerline shift.
Now, if you want to compare that to a Watts link, though, you've got a whole different mechanism going on. There are essentially two styles of Watts available for our cars; differential mounted or chassis mounted, and they each have their own sets of pluses and minuses. With either style, you now have a propeller bolt fixing a vertically oriented "football" in place. This football is now connected to the chassis (for a diff-mount Watts) or the axle (for a chassis-mount Watts) by two links, attaching to the top and bottom "points" of the football. Let's assume that you have the chassis-mount style, which has a nice crossmember spanning the frame rails that the prop bolt and football attach to, and has the two arms attaching to brackets mounted on the axle tubes. You can think of this a TWO Panhard bars, one on the driver's side attaching to a mount connected to but above the axle, and the other on the passenger's side attaching to a mount connected to but below the axle. Instead of one triangle (same legs as above: Fixed-length arm between the football and the axle, height difference, and height difference), you now have two, and they are mirror images both vertically and horizontally, offset by the height of the football.
As you hit a bump, the axle housing itself moves upwards, relative to the propeller bolt on the crossmember. This obviously changes the height differences of the pivot points, causing the "C" value to change as well. But, since the one point is moving away from the prop, and the other is moving towards it, the prop simply spins in place, and as a result the axle goes straight up and down. Continuing the double-PHB analogy, the two arcs described cancel out, and there is no lateral shift.
Now, roll centers... On a PHB, the roll center is described by where the PHB intersects the vertical centerline of the axle. Under bump, it shifts up and down, but only about half of the travel distance of the chassis. It also shifts laterally, slightly, with the mid-point of the PHB describing an arc. Again, very small movements. But, in roll, things change. since the body pivots around the (duh!) roll center, it can do all kinds of "weird" things with the roll center placement in right and left turns, and whether you're trail-braking or on-throttle. With a Watts, the lateral roll-center is constrained, which is good. However, the vertical roll-center is actually freer to move than with a Panhard. With a PHB, under squat in the rear, the roll center drops, but only by about half the distance that the chassis squats over the axle. We’ve already ascertained that the lateral motion (with proper bushings) is actually quite minimal, so again the RC migration, while there, isn’t all that huge. With a chassis-mounted Watts, that roll center is fixed relative to the chassis, and thus drops at the same rate as chassis squat. With a diff-mounted Watts, the roll center is fixed vertically relative to the ground, but is now variable to all the chassis points, and at full rate relative to the motion of the chassis.
What does this all mean? They do the same job, lateral axle location, but they do it in different manners. Each has advantages, and each has disadvantages. With proper bushings, though, neither will allow that “floaty, disconnected” feeling. Which one to choose really comes down to the design goals for the car, and other contraints. If you’re looking for an ultra-leightweight build, PHB. If you’re stuffing all the rubber Akron ever made under the wheelwells, then Watts. Autocross? Watts. Road-course? Well, either, but I would go with the PHB for lack of complexity.
[FONT="]From a subjective standpoint, and this is ONLY my opinion, the Watts just doesn’t bring enough to the table over a rod-ended Panhard bar to make the conversion worthwhile. I’ve driven (and raced) a LOT of S197 Mustangs, with all kinds of power levels, and all kinds of suspension packages under them. When I drove a Watts-equipped (well built) S197, I was frankly underwhelmed. I had honestly expected some SERIOUS handling differences out back compared to mine, and they just weren’t there. For the kilobuck and the extra pounds, I’d stick with a rod-ended PHB. That being said, I don’t autocross, so my experience was all open-track[/FONT]