Instant center is the point where the extended centerlines of two suspension links cross — the pivot the wheel is momentarily swinging about. Move the links and the instant center moves with them.
Longacre spells out the front-end construction: “The angles of the upper and lower control arms, meaning the angle of a line extending through the centers of rotation of the ball joints and inner mounts of each arm, intersect at a point we call the Instant Center (IC).” That point is why bump steer exists — Longacre’s rule is that “we need to have the tie rods on each side point toward the IC for its side.” At the rear, the same crossing-lines idea locates the side-view instant center that sets anti-squat, and four-bar, Z-link and swing-arm layouts put it in wildly different places. OptimumG notes that a force acting along the line from contact patch to instant center creates no wheel movement against the sprung mass.
Anti-squat, bump steer and how the rear reacts under power all fall out of where the instant center is, so moving a link without knowing where it went rewrites three things at once.
Read the full post:
Panhard Bar Adjustment and J-Bar Setup: How Height and Angle Change Your Car