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The following is for a rear wheel drive car with beam axle.

There exists a great deal of misunderstanding and confusion regarding the significance of the location of the instant center of a 4link rear suspension and of the performance effects of link angles, link separation, etc. The spreadsheet below is intended to clarify some of these matters.

The user has the freedom to place the instant center anywhere he likes. The distance forward is the horizontal distance measured from the axle centerline. He can also change the link lengths by changing the value for "distance forward to front pivots." Obviously, he can also change the link spacing. The rear pivots are assumed to be on a vertical line passing through the axle centerline.

It is assumed that the 4link is adjusted symmetrically. This means that "link A," for instance, is actually representing both the right and left side links. It follows, then, that the link loads in the "ANSWERS" column are the sums for right and left side links.

A horizontal forward thrust of 10000 pounds is used for the total of the 2 rear tire patches. Using force and moment balances, the link loads are calculated and, from this, the vertical component of the tire patch force vector is determined. The ratio of vertical to horizontal force components determines the "force vector slope," which appears in the "ANSWERS" column.This slope defines the line of action of the tire patch force and passes through the rear tire patch.

The "slope to IC" is the slope of a line passing through both the rear tire patch and the instant center. As you work with the spreadsheet, you'll notice that the "slope to IC" and "force vector slope" will ALWAYS be identical. In other words, the line of action of the tire patch force will ALWAYS pass through the instant center.

If the user had picked any point that fell on the line, the results would have been the same.Since an infinite number of points are contained within that single line, it follows that the line is of much greater importance than the location of a single point. The line is defined as "a line of constant percent antisquat." Essentially all 4link software packages (including the one on this site) provide the instant center location AND the percent antisquat. Unfortunately, many users fail to recognize the significance of the antisquat percentage. If the antisquat is over 100%, the rear of the car will rise on launch; if below, squat. It is the antisquat value, then, which is important, for, by comparison, the exact location of the instant center is almost meaningless.

IC distance forward =

IC height =

link A rear height =

link B rear height =

distance forward to front pivots =


slope to IC

force vector slope

load in link A

load in link B