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Designing and tuning 4-Link Rear Suspensions
This article explains the workings of the 4-link suspension and the tuning methods used to maximize its performance under various track conditions. This information applies only to 4-link rear suspensions having links floated on the rear axle (with birdcages) with all links running forward.
The 4-Link Difference, Upper Links, Lower Links, Indexing, Spring rod and Clamp Brackets
The popularity of the 4-link suspension is due primarily to its ability to let the race car turn freely in the middle of the corner without compromising forward bite. To understand how a 4-link can be made to provide such handling, you must first understand a few basics about rear suspensions.
Realize that you can increase forward bite on any type of rear suspension by angling the trailing arms upward toward the front of the race car. Trailing arms mounted in this manner cause the rear tires to try to drive underneath the chassis as the rear axle pushes the race car forward (See illustration 1). As a result, the loading of the rear tires (during acceleration) is quickened and forward bite is enhanced.
Illustration 1.
There can be a handling trade-off, however, to the forward traction gained by running the trailing arms upward to the front of the race car. During chassis roll, trailing arm/s mounted upwards will cause the right rear tire to move rearward (until the arm/s reach a level position) and the left rear tire to move forward. The condition is referred to as "loose roll steer". (See illustration 2A.)
Loose roll steer causes the rear axle to steer towards the outside of the race track. If excessive, loose roll steer can cause a loose handling condition that negates the benefits of the forward bite gained by running the trailing arms upward towards the front. However, the right amount of loose roll steer can help a race car to turn the corner correctly. At best, any trailing arm arrangement is a compromise between forward bite and roll steer.
The 4-Link Difference
A well designed 4-link provides good forward bite and the proper amount of roll steer. The two most critical factors to the performance of a 4 link suspension are the link lengths designed into the suspension and the angles to which the links are adjusted. The key to correctly designing and tuning a 4 link is to understand the significance of these two factors.
Upper Links
We stated earlier that trailing arms mounted upwards to the front of the race car enhance forward bite by using axle thrust to quicken the loading of the rear tires. We use the upper links on a 4-link suspension to enhance the forward bite. Upper link angles from 15º to 18º on the right and 10º to 15º on the left provide good forward bite. A good starting point for both links is 15º upwards (to the front).
However, keep in mind that chassis roll causes the link angles to change. If the link angles become more upward on the left than on the right, the left rear tire can become loaded more quickly than the right during acceleration (due to the axle thrust effect). This condition may cause a gas pedal push. One fix is to position the links so that the right side link is from 3º to 5º higher than the left when the chassis is at ride height.
Be aware that trailing arms angled uphill too steeply can hold the chassis up during acceleration which can reduce the effectiveness of the shocks and springs. This condition will cause loose handling-especially on rough race tracks. Keep in mind that trailing arm angles can become excessive if the rear of the chassis lifts a lot during acceleration.
The length of the upper links should be at least 17" . We can reduce loose roll steer by making the lower links shorter than the upper links (more on this later). If the upper links are shorter than 17", the lower links have to be extremely short to minimize loose roll steer. But extremely short links change their angles radically whenever the suspension moves. When the rear links are too short forward bite and roll steer are overly affected and handling becomes inconsistent.
Lower Links
We can use the lower links of a 4-link suspension to help offset the loose roll steer tendency caused by the steep angles of the upper links. The following examples and illustrations should help you to understand this important function of the lower links. You should pay close attention to how the lower link adjustments change the paths traveled by the bottom of the birdcages during chassis roll. Keep in mind that any change to the path traveled by any trailing arm will affect roll steer.
For example, in illustration 2A, both the top and the bottom links move the birdcages (and the rear tires) rearward on the right side and forward on the left side during chassis roll. This action will cause loose roll steer.
We can reduce loose roll steer by lowering the bottom links at the chassis. You can see how this adjustment works in illustration 2B. We've lowered the bottom links to a level position and now the bottom of the right side birdcage moves forward during chassis roll instead of rearward as in illustration 2A. On the left, we have reduced the forward movement of the bottom of the birdcage. As a result, loose roll steer is reduced.
Basically, we've position the bottom links to counteract the forward(L.S.) and rearward (R.S.) movements of the birdcages caused by the upper links. As a result, we reduced loose roll steer. We can reduce loose roll steer further by lowering the bottom links further as shown in illustration 2C. Notice how this adjustment, positioning the lower links 5ºdownhill, causes the bottom of the right side birdcage to move forward more during chassis roll than in illustration 2B where the links are level. On the left side, the bottom of the birdcage now moves rearward (until the link reaches a level position) instead of forward as in illustrations 2A and 2B. Consequently, a further reduction in loose roll steer results.
Generally, bottom link angles from 0º to 5º downhill (to the front) are used to help control loose steer. Some forward bite may be lost when the bottom links are lowered but the effect on forward bite is usually minor relative to the overall handling improvement that is realized by reducing loose roll steer.
Another method used to reduce the loose roll steer of a 4-link suspension is to shorten the bottom links. Notice, in illustration 2D, how the shortened bottom link pulls the bottom of the right side birdcage forward during chassis roll more than the longer links in the other illustrations. The bottom of the left side birdcage does lose some of its rearward movement because of the shortened bottom link. But since left side birdcages typically move down much less than right side birdcages move up during chassis roll, the overall effect, when shortening the lower links, is a reduction in loose roll steer. However, if the left rear of your chassis hikes up during cornering, loose roll steer may increase whenever both bottom links are shortened!
We could reduce loose roll steer even further by combining the long bottom link arrangement of illustration 2C on the left side and the short bottom link arrangement of illustration 2D on the right side. The preceding paragraphs should help you understand why.
The length of the bottom links are dependent on the roll steer and traction characteristics desired by the chassis tuner. For most track conditions, bottom links 2æ shorter than the upper links work well. Short links( from 3æ to 4æ shorter than the upper links) generally work best for tight, flat race tracks or on any track where the chassis tends to be loose. Long bottom links (equal in length or no more than 1æ shorter than the upper links) work best for fast tracks or on any track where the chassis tends to push. You should use the information in this article to determine the correct link lengths for your application.
However, a proven 4-link arrangement includes 15 1/2æ bottom links, mounted 5º downwards to the front, coupled with 17 1/2æ top links, mounted 15º upwards to the front.
Indexing
A 4-link birdcage rotates or "indexes" on the axle tube whenever the suspension moves (unless both upper and lower links are equal in length and parallel to each other). Indexing is greatest when there is a lot of length and/or angle difference in the upper and lower links.
Typically, indexing causes the coil-over mounts, if located on the front of the birdcages, to rotate against the shocks and springs during suspension bump (compression) movement. As a result, the springs and shocks are compressed from both ends at once and the suspension becomes very stiff. (Try to bounce the rear of a car with a 4-link rear suspension).
During chassis roll, indexing loads the right rear tire and unloads the left rear tire and wedge is reduced (40 lbs to 80 lbs is typical!).
Indexing can improve driveability by keeping the race car flat in the corners. However, indexing can cause the rear suspension to be too harsh on rough race tracks. When selecting springs for your 4-link, you should keep in mind the effect that indexing has on suspension stiffness.
Springrod
The Spring rod is used to replace the right side upper link when tighter handling during acceleration is desired. The operation of the spring rod is simple. During acceleration the spring rod compresses and allows the right rear tire to move forward. This movement steers the rear axle to the inside of the track and tightens the race car.
When the spring rod compresses it causes the birdcage to rotate away from the right rear spring (if the coil-over unit is mounted ahead of the axle). As a result, the right rear tire loses some of its load (the left rear tire gains load) during acceleration and handling becomes tighter off the corner. The Spring rod can provide a significant handling improvement.
Clamp Brackets
Clamp Brackets are used to mount the coil-over units directly to the axle housing. When clamp brackets are used in front of the axle, axle wrap-up during acceleration causes the rear axle & chassis to separate. The rear axle (and tire) are forced towards the race track.
Clamp brackets are sometimes used on short, slick tracks to improve initial forward bite. Mounting the left coil-over unit ahead of the axle (on a clamp bracket) generally tightens corner handling. Mounting both coil-over units on clamp brackets and ahead of the axle can improve forward bite on stop and go or slick race tracks.
