Many mud-drag racing vehicles use low anti-dive geometry to shift weight to the rear axle under acceleration and extend the front suspension to better absorb the terrain. Unfortunately, vehicle's with low anti-dive suspension geometry may experience excessive nose-dive under hard braking.
The roll axis and roll center of a vehicle's suspension system determin how much body roll or sway the vehicle will experience when cornering. The roll axis is the imaginary line drawn between the two points made where the lower links would eventually connect and where the upper links would eventually connect. In the case of a suspension with parallel links the roll axis line is simply going to be parallel with the parallel links. The point along the roll axis that is directly above the axle center line is the roll center.
The distance between the roll center and vehicle's center of gravity becomes the leverage factor for body roll. In other words, the further above the roll center the vehicle's center of gravity is, the more body roll the vehicle will experience during a turn. The closer the roll center is to the center of gravity, the less body roll the vehicle will experience and, in theory, if the roll center is on the center of gravity line, the vehicle would have no body roll.
One additional note about the roll center is that while the center of gravity of the vehicle always remains the same in relation to the chassis, the roll center and roll axis will move as the suspension cycles.
For most applications this change in roll axis is not worth considering and the calculations should be done at ride height. For racing applications, however, watching the roll axis as the suspension compresses entering a hard turn may be worthwhile. Finally, unlike anti-squat and anti-dive that can only be tuned by adjusting the 4-link geometry, a vehicle with too much body roll can easily be improved by installing a properly tuned sway bar without negatively impacting ride quality or suspension performance.
Crawlpedia is proudly supported by Filthy Motorsports , a specialty off-road racing and 4x4 parts shop in Boulder, Colorado. If your 4 link bars are adjustable for length, you can do it there. The first thing you want to do is make sure the upper bars are the same length and that the lower bars are the same length. This is critical. After you get the bars the same length you need to put them back in and get the car square front to back and corner to corner.
Now the right upper bar sets any type of preload you want. It is the only bar that you adjust! Leave the other three alone. Shortening the upper right bar adds weight to the right rear tire. A little does a lot. Try one 'flat' on the adustment nut at a time and keep track of how much you adjusted and the results.
One-quarter turn can add as much as 50 pounds. If you want to check your 4 link setup, go to a chassis shop or find someone with portable race scales. Have them put the car on the scales weigh the car with the 4 link disconnected. Then connect the 4 link, preload it and weigh again. Then you will be able to tell if each side is preloaded the same.
Alot of tuning can be done this way. Please let other hotrodders know! Here's how Would you prefer to share this page with others by linking to it? Tuning 4 Link Rear Suspensions for the Drag Strip Tuning 4 link rear suspensions can make a car launch quicker and use less horsepower to do it. So, let's get into it and find out how to do it Ok, first we need to know what an 'Instant Center' is.
Next we will figure out where to place the 'Instant Center'. Now then, let's figure out what all this means 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. You should use the information in this article to determine the correct link lengths for your application. 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.
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. Clamp Brackets are used to mount the coil-over units directly to the axle housing. 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. On extremely slick race tracks, you can tighten overall corner handling by using clamp brackets to mount the left coil-over unit ahead of the axle and the right coil-over unit behind the axle. Suspension movement usually increases when the coil-over units are taken off birdcages and mounted to clamp brackets since there's no longer any indexing of the springs.
Consequently, it may be necessary to increase rear spring rate when making this adjustment. You should keep in mind that any loading of the rear tires caused by clamp brackets during acceleration will be accompanied by an unloading of the rear tires during deceleration This unloading can upset the race car upon corner entry -especially when both coil-over units are positioned ahead of the axle and attached to clamp brackets. You may be required to make chassis adjustments to correct any corner entry handling problems caused by clamp brackets.
The 4-link is a relatively complex rear suspension that is very sensitive to adjustments. When designing or tuning a 4-link, it is important to understand the relationship between link lengths and angles and how the relationship affects roll steer and tire loadings.
We highly recommend that you build a full-scale working model of your 4-link, or use the design parameters mentioned in this article, to help you to better understand the 4-link suspension.
You can use cardboard, wood, aluminum strips, etc. The idea is to trace the paths actually traveled by the centers of the birdcages during chassis roll. You should draw the paths to include at least 3" of rebound movement for the left birdcage path and at least 3" of compression movement for the right birdcage path.
You can evaluate the roll steer characteristics of different set-ups by comparing the different paths drawn on your model. You can also check the indexing and the link angle changes during roll or bump. In short you will speed up your learning process by working with a model.
As we stated earlier, the 4-link is a fairly complicated rear suspension. We hope the information in this article, combined with your efforts, will provide you with an advantage! Designing and tuning the 4-Link Rear Suspensions.
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