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The theoretical point about which a suspension link or assembly rotates at any given moment. The location of instant centers determines how the suspension moves and how forces are transmitted between the tire and chassis.
Haney explains in The Racing and High-Performance Tire: "The concept of instant centers in the kinematic design of linkages can be very complicated. We're going to use it here as a way to simplify the analysis of tire and suspension forces." Jazar's Vehicle Dynamics Theory and Application shows how instant centers are constructed for real suspensions: "A McPherson suspension is an inverted slider crank mechanism. The point I12 is the suspension roll center, which is the instant center of rotation for the wheel link number 2 with respect to the chassis link number 1." For double A-arm suspensions, the instant center is found at the intersection of lines drawn through the upper and lower control arm pivot points — this point moves as the suspension travels, which is why suspension geometry is always described as a dynamic system rather than a static one.