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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.
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