WHATS THE DEAL WITH INSTANT CENTER?

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Macguyver

SOUTHSIDE MACHINE PERFORMANCE
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Mar 21, 2017
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We have seen this question asked many times on here and just as many different answers. We thought we would chime in on the discussion with another quick geometry 101 lesson. We have done all the hard work for you in our kits.



Setting Up For Instant Center Relocation


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 factory GM 4 link IC position.​


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 re position 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.​


When using a Mustang housing it is best to use a rear axle truss for the proper angle and length of the upper control arm.

Note: You need at least 45% of the car's weight on the rear wheels to make a 4 link suspension work properly.

Also, get some graph paper. It helps to draw out what we are doing here so you can measure and be able to set up your 4 link the easy way. You don't have to be an artist, just measure your car, height and length of the 4 links, etc. and transfer them to the paper.





Confused yet?

ANTI SQUAT IS BETTER THAN REAR SQUAT IN MOST CASES.​

Always feel free to call the shop. It’s not overly complicated.

 

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