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Instant Center Relocation and Why It’s Needed

The basic information regarding instant center is the same for all cars, but for this discussion, we’ll focus on the factory-designed four-link system. The instant center (IC) is an imaginary point defined by extending the line of the upper and lower control arms forward until the two lines intersect. By changing the locating points of either the upper or lower control arms, the IC can be moved longitudinally (fore-aft) as well as vertically. Moving the instant center closer to the rear of the car reduces the leverage on the rear axle, reducing and eventually eliminating the car’s tendency to squat. There are two basic ways to change a Chevelle’s IC position. The OLD SCHOOL “No Hop” bars relocate the rear-axle mounting point of the upper control arms higher, which shortens the IC. The popular approach is to drop the lower rear mounting point of the bottom trailing arm. This is the approach taken by SOUTHSIDE MACHINE PERFORMANCE WITH OUR 4 HOLE RELOCATION BRACKETS.

As the instant center is moved toward the rear by altering the upper or lower control-arm mounting points, this has an effect on tire load, or the “hit” on the rear tire. If you are using a sticky rear tire like a wrinkle-wall M/T E.T. Street or a pair of slicks, moving the instant center rearward will apply more leverage to the rear suspension, reduce the squat, and take maximum advantage of the wrinkle-wall tires. If a set of stiffer-sidewall drag radials are used, positioning the instant center back from its stock location–but not as far back as for a wrinkle-wall tire–would be beneficial.

The height of the instant center will also move as you reposition the control arms, and this affects average tire loading on the rear tire. If you draw an imaginary line between the tire contact patch to the car’s center of gravity (CG), this is the 100 percent antisquat line. If the instant center is located above this line, antisquat will be more than 100 percent, while an instant center below the line is less than 100 percent antisquat. Theories abound on the proper location of the instant center, and this will change based on power, tire condition, track conditions, and perhaps a dozen other variables. This is just a hint of what you can learn about instant centers and traction. If you’re into maximum traction, there’s a ton of material to learn about putting the power to the ground.