What do you call the LRA bouncing around when in a turn, which the Watt's link settles down? Just instability?
Oversprung? Blown shocks? Placebo effect? Do the math with the amount of axle deflection with a rod-ended PHB (not one with pencil-eraser bushings) and you'll see it's pretty minimal through the bulk of the normal range of motion.
Lets get some definitions and concepts out on the table.
BUMP-STEER can be defined as change in toe-angle as a direct result of vertical suspension motion. Ideally, you want the toe angle to remain constant as the suspension cycles up and down, which is "zero bump-steer." The leading culprit for bump steer is the different arcs described by the tie rod and the lower control arm as the suspension moves around, but to be honest, with stock components, Ford pretty much got it right for the overwhelming majority of the usable suspension travel on the car. IF, however, you introduce a change in the geometry, either by installing taller ball joints, or relocating the pickup points for the inboard side of the LCA, you introduce a pretty dramatic arc-length difference, and that creates bump-steer.
So, why do we go there in the first place? Because racecar. We want to (at least on this forum, anyway) go faster around the corners than Henry Ford ever intended us to, and in stock form, the Mustang is a wallowing, marshmallow-y, understeering mess. Light years better than previous platforms, but still hardly optimal in stock trim. So what do we do? First thing we do is lower the ride height down from the stock 4x4 spec (lower CG, slight decrease in aero lift at speed). Hopefully, we've also increased the spring rates and have installed dampers that are up to the task of controlling the masses involved. HOWEVER, under the universal law of
TANSTAAFL, that creates almost as many problems as it solves. Of prime importance to us here is the inclination of the LCAs that it induces. By lowering the chassis, we also lower the inboard side of the LCAs, causing them to slope downward. This does two really bad things. First, our front roll center is now somewhere underground, and second, it puts the front suspension in a REALLY bad place in the camber-gain curve. What's that, you ask? Well, Virginia, when you compress the suspension, like you would going into a corner somewhere between light-speed and ludicrous-speed, the outside tire moves vertically upward. The LCA pivot points are fixed relative to the chassis, as is the strut top, at the upper strut mount. Additionally, the knuckle is fixed in angle relative to the strut body. So what else happens when the wheel moves up? The LCA, which started with an upward slope looking from inside to outside, gets even more inclined, which necessarily shortens the distance between the vertical plane passing through the LCA inner pivot points and the ball-joint centerline. That means that the wheel is moving up, AND in. But wait, since you acted now, for no additional cost, we'll make it worse. If the KNUCKLE is moving inward, but the strut top isn't, that means that the whole thing is also pivoting, losing all that beautiful negative camber we dialed in, and we're now abusing the outside edges of the tires... Sigh. Here's the rub. The greater the angle of inclination of the LCA, the greater the
rate of positive camber gain. That means that if the arms are close to flat to start with, "X" amount of vertical motion at the ball joint produces less positive camber change than if the arm started with a heapin' helpin' of inclination. A LOT less.
So, what are we to do? Geometry correction. We want to lower the car for all the goodness that brings, BUT we don't want to pay the price. So, we can install ball joints with a taller stud, which pushes the LCA downward at the outside, getting us close(er) to flat again, OR we can relocate the pickup points on the inboard side of the LCA, raising the inboard end and again getting us close(er) to flat. Or both! The roll center can see daylight, and the camber-gain is minimized. Awesome, right? Not exactly. Remember how Ford had gotten the bump-steer curve pretty much right with stock geometry in the first place? Remember how we had no real problems even after dropping the front end 2" or so? Well, now that we did the roll-center relocation, the arcs described by the tie rod (canted upward at static ride height) and the newly re-angled LCA (flat-ish at static ride height) aren't even in the same zip code, and what does that mean, Virginia? Exactly! Bump-steer. In this particular case, as the suspension moves upward, the toe changes (aggressively) inward. Can you say "darty front end?" I knew you could!!
Simple enough solution, really. Either install tie rods with an extended-length stud (Whiteline, FRPP from the FR500 program), or a bump-steer kit (the rest of the world, like Steeda, MM, Griggs, et al) which does the same thing, but can be fine-tuned. A bit of time, some aggravation, a little swearing, and you're done. Now, you have a properly lowered car, with appropriate spring rates and damper curves, along with minimal impact to the front roll-center and camber gain curves. More grip, crisp turn-in, and it won't eat the tires alive, either!
A bump-steer kit is a supporting mod, used to correct geometry that has been altered from factory-spec. Nothing more. It's not going to "supercharge your suspension" like a Watts link will
! Oh, and in case you missed it, the last sentence was pure sarcasm.