Interesting and important article regarding geometry directly impacting your performance.
FOUR-LINK REAR SUSPENSION OPERATING CHARACTERISTICS "SQUAT"Of primary importance is that the rear axle be centered in the chassis and perpendicular to the frame rails; that the locating points of the suspension bars be symmetrically mounted on the chassis and axle housing; and that the right and left bars be adjusted to equal lengths when installed. There’s much more to it than this – including setting the pinion angle, aligning the front suspension, determining the center of gravity, plotting four-link intersecting points (instant center) for various bar settings, and so on. The point is that the chassis, as a whole, must be correctly assembled and "baselined" before you can begin to "tune" it – the same as for tuning an engine.
The second feature to understand about chassis tuning is that the four-link does not operate, and should never be adjusted, by it self. In any suspended car, at least four potentially-adustable elements work in conjunction with each other: the springs, the shocks, the suspension locating bars or other attached geometry, and the relationship between front and rear suspension. This does not take into account other contributing factors such as tire pressure and traction, torque converter/gear ratio, ballast (for shifting of the center of gravity), and so on.
A line drawn (see above illustration) from the rear tire contact patch through the instant center of the rear suspension represents the "line of force" through which tire motion is transferred to car motion. The instant center is the point about which the rear end pivots as it moves up and down with the suspension. The instant center of a ladder bar remains constant; that of a four-link change as the rear end moves. The shorter or more angled the bars, the greater the change.
Most confusion surrounding the rear suspension linkage (ladder bar/four-link) is in the action/reaction torque around the rear axle and its housing or (birdcages in a FIRA car); the fact that axle wants to rotate in the opposite direction of the rear wheels. With a ladder bar solidly attached to the rear end housing, and pivot the front end in a bracket somewhere on the chassis, the "counter-rotating" rear end housing appears to be imparting and upward force on the chassis at the ladder bar’s pivot point – "lifting" the chassis vertically at that point on acceleration. However, that would only happen if you tied the rear wheels solidly to the ground, and with enough engine torque the front of the car would lift and be rotated backwards. But that’s not really what is happening when a race car is under acceleration. The tire is trying to push the car forward. It is pushing from the point where it contacts the ground. The center of gravity of the car, however, is higher than this point. Inertia wants to keep the car where it is, rather than letting it move forward.
If the pushing force of the rear tire, at the ground, is strong enough and rapid enough, it tends to "tip the car over' because it is pushing below the center of gravity. Somebody used the analogy of pushing a refrigerator to explain this (see illustration next page). If you try to push a refrigerator across the floor, and you push it vigorously down near the bottom, it will tip over on top of you. This is partially what happens when you launch a car. Of course, the rotational torque about the rear axle helps tilt it, too.
The fact that a car is suspended (unlike a refrigerator) complicates things. For example, it allows the body to shift or tilt more easily (depending on spring and shock rates) as the inertia force (equal to the sprung weight of the car times the acceleration force) acts at the center of gravity, in a direction opposite of acceleration (ie, to the rear). When the body shifts, this alters the location of the center of gravity in relation to the rear tire patch, changing things again.
But consider just the rear suspension for a moment. The axle is not mounted solidly to the frame. It is held in place by some sort of linkages and simply "floats" on the springs (leaf springs are linkages in themselves). These linkages determine a certain arc about which the rear axle swings as it moves up and down in the chassis. With ladder bars, the point about which it swings is obviously the front end of the bar, where it pivots in the frame bracket. This point about which the rear end pivots as it moves up and down is called the "instant center" of the suspension.
Now here's the big point. Not only is the instant center of the rear suspension the point about which it pivots, but it is also the point through which the rear tire pushes the car. That is, the rear tire pushes at the ground, but it also must push on the chassis, in order to move the car. The tire is attached to the rear axle, and the rear axle is attached to the frame by the links or bars. The force that pushes the car forward is transmitted to the chassis by the rear axle connecting links. The rear end isn't pushing the front of the ladder bar up; the "line of force" that moves the car forward pushes from the tire contact patch through the pivot point of the ladder bar. Thus it is pushing mostly forward, and partially up, at the same time. The angled line of force can be considered as horizontal (forward) and vertical (up) vectors. This is oversimplifying quite a bit, but it helps explain the dynamics of the launch.
No mention of anything about "tuning" the rear suspension has been said, right? It all has to do with where the links that position the rear-end attach (or pivot) on the chassis. In the case of a ladder bar, it is the actual pivot point at the front of the bar. With a four-link, it is the imaginary intersect point of the two bars that determines the actual instant center about which the rear end pivots – and through which the pushing force from the tire is transmitted to the chassis.
When you're building the chassis, you could attach the front of the bars most anywhere. You could mount them high or low, or make them long or short. In doing so, you change the relationship between the line of force that pushes the car, and the car's center of gravity. Consider exaggerated examples. Let's say you have long ladder bars mounted low on the frame near the engine's bellhousing. The center of gravity of the car is above this point, and slightly behind it. When the car launches, the inertia force (weight transfer) acts toward the rear at the center of gravity. But the body/chassis is "hinged" (in a manner of speaking) at the ladder bar pivot point; so, as the tire and ladder bar push the car forward, the inertia force tries to push it back, but actually swings the rear of the body down. This does transfer weight to the real wheels, which helps traction, and it is why drag cars in the Sixties were jacked up in the front and used long ladder bars.
However, this arrangement – especially with the shorter bars common today – also tends to force the rear wheels and axle up, compressing the springs, which allows the rear of the body to drop. This condition is known as rear squat. Although it looks like you are transferring more weight to the rear tires and increasing traction, you are actually lifting the rear tires, and decreasing traction.
Chassis engineers talk about "anti-squat" at the rear on acceleration and use a traditional diagram to calculate "percentage of anti-squat" (see illustration next page). Because the weight (center of gravity) of a typical road car is supported by the front and rear wheels, engineers draw a "100-percent anti-squat" force line from the rear tire contact patch to the front wheel vertical center-line, at the height of the center of gravity. If the actual "line of force" for the car – the line from the rear tire contact patch through the instant center of the rear suspension – coincides with the 100-percent anti-squat line, then theoretically the rear suspension will neither lift nor drop, and the rear of the car will not rise or squat, as the car accelerates. That is, if the instant center of the rear suspension lies anywhere on this line, the car will have 100-percent anti-squat. If the instant center is anywhere below the line, the rear will squat (or, have a certain percentage – a fraction – of anti-squat). If the instant center is above this line, it will have more than 100-percent anti-squat, which means the rear of the car will be pushed upwards by the suspension links as the car accelerates.
Look at this another way. If the suspension is trying to push the back of the car up, that's the same thing as saying it's trying to push the rear wheels and tires down, against the track surface, which obviously increases bite. That's what this rear suspension geometry is all about.
Chassis builders and tuners call this situation (over 100-percent anti-squat) "separation." That is, the rear axle "separates" from the chassis as it swings down and/or the body lifts up, and the springs and shocks extend. The point, of course, is to have the axle swing down (or at least try to as much as possible), thus "planting" or "shocking" the rear tires on the ground. This is a dynamic situation, with many variables acting at once.
Of primary importance is that there be enough weight in the back of the car to make it work. If the center of gravity in the car is too far forward, a four-link or ladder bar adjusted to plant the rear tires will simply lift the rear of the body instead. Most builders say you need at least 45-percent of the car's weight on the rear wheels to make a four-link work properly. Spring and shock rates will also influence the effects of the suspension "tune." Particularly important is the extension stiffness of the shocks. Increasing this stiffness decreases the effect of the separation, and vice versa. The same is true of spring rates. The main focus here is to understand that where a builder locates the instant center of the rear suspension can have a positive or negative effect on how the car launches. A fully-adjustable four-link allows a chassis tuner to move the instant center not only up or down, but also forward or back (by increasing or decreasing the angle between the bars) at the track.
The obvious question is "Where is the right point to set it?" I don't think there is any way to accurately calculate such a point in a FIRA car. There are too many dynamic variables. For one thing, when a high-powered race car accelerates, its front wheels are usually unloaded and light or nearly off the ground, so the standard engineering diagram for 100-percent anti-squat doesn't apply. Trying to calculate the "correct" instant center for the four-link in your car is not the point. In fact, it's probably impossible, because as the chassis lifts in the front, the location of the center of gravity changes; and, more significantly, as the rear suspension moves up or down, the instant center of the four-link also changes. The shorter the bars, or the more angled they are, the more dramatic this change will be (unequal length bars also give the same effect), and thus the harsher the shock on the rear tires will (usually) be.
The way most builders/tuners use the four-link is to set it at an adjustment they know from experience is in the ballpark for the given car; then they try some launches and observe (either by eye, or video camera) what the chassis and rear tires do. The beauty of the four-link is that once you see what happens, you can adjust it several different ways. The chassis tuner has two primary concerns: the rear tires should hook up without slipping, and the car should start moving forward as quickly as possible, rather than lifting, squatting, wheel standing, or other monkey business. If the rear of the car squats, he can raise the instant center of the bars; if the rear lifts too much, he can lower the instant center. If the car tends to wheel stand too much, he can angle the bars closer together at the front to move the instant center further to the rear of the car; if the tires are being shocked too violently (as in a trans-brake car), he can do the opposite. This is oversimplifying, of course.
Considering shock and spring adjustments plus changes to the front suspension that can affect the rear, the four-link might seem like a complicated nightmare. It's not. It helps you to get a car dialed-in for the best possible launches. Once dialed-in, it lets you adjust the car for varying track or other conditions.
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