Can you expand on this? IC can vary greatly in length and height without changing AS. Keeping it simple, let's use 100% AS and move the IC along the AS line. How will the Jeep behave with a short,low IC? As the IC moves longer and higher along the AS line how will the suspension act differently?
I can mostly wrap my head around the height of the IC. What I don't really grasp is the full significance of the IC along the x axis. For example, an IC that is near the AS line vs an IC that is infinitely far away. Some distance away from the AS line seems to be desirable. But can too far be too much? Ultimately, I feel like that is an academic question that doesn't concern most of us on a practical level, with one possible exception sort of described below.
When looking at the calculators and for the propose of understanding these points in space, imaginary lines and measurements — 100% antisquat is the same as neutral antisquat or no squat/no rise when the suspension is loaded (I really wish the terminology was different).
Relative to the imaginary 100% antisquat line (as partially defined by the center of gravity) a high instant center can cause the loaded suspension to rise. This can result in the uncontrolled hopping phenomena as the rear stops rising, loses traction, falls, regains traction, rises, falls, over and over again.
Relative to the imaginary 100% antisquat line, a low instant center can causes the rear to sink. This can result in the the Jeep shifting it's weight backwards to the point that the the front wheels lift off the ground and eventually to a backwards roll.
A stock, unlifted TJ has a fairly neutral squat. Meaning it is incredibly stable in a steep climb. But then we start adding larger tires and taller springs to get the axles and frame out of the rocks. This screws up the stability of the stock suspension by raising the instant center higher and higher above the neutral antisquat line. The result is the hop we sometimes see on a lifted short arm.
The significance of screwing up the stock suspension geometry generally doesn't matter until people start pushing the limits of the Jeep. Meaning that the problems of lifted short arm geometry doesn't matter until it matters. It simply doesn't matter for most (me included).
A long arm where the rear upper mounts are close together will both significantly lower the instant center and also pull it in close to the AS line. While this will reduce and even eliminate the hopping, this change to the IC is too much. This is where a long arm can potentially lead to a backwards somersault. But, just like the problems of a lifted short arm, the problems of a lifted long arm don't matter until they matter. In effect, one problem got changed to a different problem. And neither one matters until it does.
Where these hops and squats are significant enough to matter and are worthy of fixing, the goal is to restore the lifted suspension geometry to something closer to what a stock TJ has. This means moving the mounts to reposition the instant center back to a more neutral antisquat. These mount positions also take into consideration other practicalities like strength and clearances. To achieve that, the mounts will end up in a place that looks like what the Savvy mid arm looks like. The mounts are placed in a particular place to position the instant center. The frame and axle mounts are connected together with control arms. And the arms end up being a length that is neither short nor long.
To circle back to the 4 link calculators we see on places like Pirate and JeepForum, they are only useful in learning to understand how changes to the mounts and vehicle weight effect the geometry. They cannot design an ideal suspension, unless that vehicle never moves and never leaves flat ground.