Over the years, drag racers with door cars have gone through all sorts of suspension arrangements. Lift bars, slapper bars, Super Stock springs, leaf links,ladder bars, three-links, four-links, and torque arms (there may be others too, but it’s a start). Of all of the systems, the most common you’ll see today is the equal length four-link, probably followed by the ladder bar.
To complicate things, there has been quite a bit penned over the years regarding suspension setup–some right and some wrong (some very seriously wrong). That has led to all sorts of misinformation, which in turn is bad news for the racer. What we’ve done here is cut through the maze of information (good and bad) and we’ve gone straight to an established expert on the subject–Jerry Bickel.
It should be no surprise to anyone reading this that Bickel has built more than his fair share of championship winning race cars, and he is certainly no stranger to tuning suspension systems. What you’ll see below is Bickel’s personal take on ladder bars and four-link setups.
Let’s start at the beginning.
When a drag car launches, the rear-end wraps up. The purpose of a traction device is to turn that wrap-up into forward motion.
Consider the case of a ladder bar.
This is a simple triangular-shaped device that connects the rear-end housingto the frame. The ladder bar prevents excessive suspension wrap up by pushing up on the frame at the point of forward connection (basically the point where the upper and lower bars of the ladder bar intersect). The front of the car is lifted or “picked up” by this section. As a result, the forward attachment point is called the pickup point. If the car has two ladder bars (as nearly all do), there are really two pickup points–one on each side of the car (see illustration #1 below).
Consider what happens when the ladder bars push up on the chassis at the pickup point. The respective bar on each of the car also pushes down on the tires and wheels. This effectively “plants” thetires, which in turn, makes the car hook (or at least, that’s the hope!).
That all makes sense, but over time racers quickly discovered that changes in that pickup point location can have a considerable influence upon the behavior of the chassis. If, for example, the pickup points were short and high, then the launch would prove violent. This regularly resulted in the slicks wadding up at the launch. Following this initial violent hit, there usually wasn’t sufficient load transfer to actually maintain traction and keep the tires hooked. Between an excessive amount of tire wadding coupled with the reduced traction following the violent hook, elapsed times can suffer (no brainer, we’re sure).
Here’s something else to consider is this: A ladder bar with a short, high pickup point can also create considerable body separation during the launch. It definitely looks wild, but it also can result is horrendous driveshaft alignment–the results of which can prove disastrous.
So instead of a short, high pickup point, what if you had a long, low pickup point? Bickel tells us that this arrangement will tend to hit the tires less violently, but it can also create more total load transfer to the rear tires. Bickel notes that this is generally an acceptable situation, but if you go too far, the chassis may squat excessively or worse, rattle the tires.
The big question with a ladder bar setup is this: How do you make the ladder bar adjustable so that you can tune it from a range of short, high pickup point to long, low pickup point?
It’s not that simple.
It can be accomplished with several different ladder bars along with several different front ladder bar mounting positions. If you begin with a four-link, Bickel notes that it is entirely possible to create both pickup point extremes (short and high versus long and low). Bickel goes on to tell us that the ideal pickup point locations for a race car are dependent upon multiple factors. But there is one truth: All race cars all have a neutral line that determines how the chassis will behave (see illustration 2 below). If the pickup point is located about this line, the body will separate upon acceleration. If the pickup point is located below this neutral line, the body will squat.
In a perfect situation, the front pickup point should be located near the neutral line. This setup will ultimately work well and prove very stable. The car will neither show squat, nor will it encounter excess body separation. The dilemma here is how do you actually figure out the neutral line location?
Typically the line of thought states that the neutral line of a drag car can be determined by extending a line level with the height of the center of gravity (CG) until it crosses a vertical line through the front spindle. The neutral line is then represented as a diagonal line that intersects this location and the center of the rear tire-to-pavement contact point (see illustration 3 below).
So far so good, but Bickel points out that there are several difficulties with this method.
“It is difficult to measure the height (Y) of the CG accurately, Bickel said. Most racers use the camshaft centerline as the CG height, but without an accurate measurement, it is impossible to locate the neutral line with precision.
“Another problem with this traditional neutral line location theory is that many drag race cars wheel stand through low gear. Once the front tires are in the air, I do not believe that they have an effect upon neutral line location. Experience has shown us that the pickup point distance from the rear axles is at least as important as its height.
“In practice, I find that pickup point location must be changed, depending upon race track conditions and vehicle performance. You should rely on conventional neutral line theory only as a starting point for rear suspension setup.”
Chassis Instant Center
That changes things, and we’re not finished with it yet.
What we need is an idea of something called the “instant center.”
What is instant center? The instant center or “IC” is an imaginary point about which the chassis or a suspension member rotates in a given (instant) position. You can find it by simply projecting lines along suspension members to a point of intersection (for example, the respective bars of a four-link). Where they intersect is called the instant center (see Illustration 4 below). If you examine Jerry Bickel’s drawing, you can see that the four-link has an instant center that acts as a pickup point, even though the actual point is invisible.
Because the respective brackets found on something like a four-link are under load during acceleration and braking, they must be stout. When the race car accelerates, the rear-end wraps up, placing the upper bars in tension while the lower bars are held in compression. As you brake, the forces are reversed.
Establishing Four-link Instant Center
When you or your chassis builder installs the rear-end in your car with a four-link setup, that’s the time to decide exactly which bracket holes you will use for the links. The choices made here will determine the length and height of the instant center. These decisions will ultimately impact how the car works.
Bickel tells us that if you drive the tire down too hard by way of the IC location, it tends to fold up the sidewalls, which in turn makes for poor surface contact. Opposite to this, if insufficient force is applied to the tire, it will simply spin without accelerating the race car. If you look closely at Bickels’s illustration (below), it’s easy to see there are many possible IC locations in a four-link. You have to pick the one that works best for your particular car, but it’s not cut and dry.
So where do you begin?
Bickel offers a very simple explanation to choose the IC pickup points:
“Long, low intersect points create traction for the longest time, but react slowly,” he said. “Short high intersect points create traction for the least time, but react fast.”
Figuring The IC Length
The next thing you’ll have to do is to determine the IC length.
The instant center length can actually affect the overall (race car + driver) reaction time. Bickel states that if your reaction times are good, a long IC point ( inches is generally more desirable than a short one (50 inches or less). This will plant the tires smoothly and keep them planted a long time. However, as we noted much earlier in this article, if too much power is applied to a long IC point, tire shake can result.
The other consideration Bickel tells us about is the amount of torque your engine can deliver to the drivetrain coupled with the type of drag slicks you have.
“For example, racers of high-powered Pro Mod cars often use tires that were initially designed for solid suspension Funny Cars and Top Fuel dragsters,” he said. “The sidewalls of these tires are tall and very flexible, acting like a sort of spongy suspension system. They work best when you limit the rear movement to as little as possible with an IC point on or near the neutral line of the car.
“Further to this, I like to run a long, low IC point in an application such as a high rpm, lower horsepower small block clutch car. This combination seems to help overcome the impact from the high rpm launch and the engine usually doesn’t have enough power to shake the tires.”
Remember when we initially talked about location of the bars in the four-link?
That location relative to the rear axle centerline will make a difference in performance, even if the IC point remains the same. The closer to the housing the upper bar is, the less the car tends to wheel stand. Bickel’s experience shows that when the bottom bar is located lower in the car the suspension, it seems to have better control and simultaneously is less apt to experience tire shake (see illustration 6 below).
Figuring The IC Height
The neutral line (examined previously) slopes within the car from front to rear. Bickel says that should you decide to change the length of the IC, you must also change the height in order to maintain the same anti-squat relationship.
When it comes to IC height, Bickel has this to add:
“The farther forward you move the IC, the lower it must be,” he said. The farther back you move the IC, the higher it must be.”
Some other food for thought is this (again, from Bickels’s tuning bag of tricks): Automatic transmission cars along with lower torque stick shift cars work best when the IC point is from 1- 2 inches above the racing surface. Big power stick shift cars typically need to stay inches above the racing surface.
The bottom line here is you have to take your time setting up the four-link for your particular car. Like any other part of the race car, Bickel recommends you follow the above methodology and that you make only one change at a time. It’s very important to keep notes of the changes too.
You can tune your four-link.
There’s absolutely no voodoo or black magic involved.
SOURCE: Jerry Bickel Race Cars
Author: Wayne Scraba Wayne Scraba is a diehard car guy and regular contributor to OnAllCylinders. He’s owned his own speed shop, built race cars, street rods, and custom motorcycles, and restored muscle cars. He’s authored five how-to books and written over 4, tech articles that have appeared in sixty different high performance automotive, motorcycle and aviation magazines worldwide.
Parallel 4 Link Rear Suspension System
The parallel 4 link rear suspension. A newer style that works well as an all-around type of suspension and also does great on the track.
This is a 4 link suspension with all 4 links running parallel to each other. This kind of suspension always has some kind of panhard bar to keep the axle centered.
The parallel 4 link is designed to keep the rear axle centered, and to keep the pinion angle from changing (keep the axle from rotating). It works especially well under hard acceleration at the drags or hard cornering at the track.
Check out Tuning 4 Link Rear Suspensions for the Drag Strip
Advantages: This style of suspension works well for an everyday driver and also works well on the track. This type of suspension can also be made to have adustable anti-squat geometry for the drag strip and the long Panhard rod really controls the rear axle's side to side movement to help in hard cornering.
Disadvantages: 4 links designed for the drag strip don't work well on a road course, and may not ride well on the street. Extreme cornering can lead to some roll bind, due to it's geometry. Changing out the bushings to Heim joints or urethane bushings will usually cure it though. Another disadvantage if you are going to swap in a parallel 4 link is difficulty and cost. You will also need to have some welding and fabrication skills.
Ok, so how do I lower my ride with a parallel 4 link system?
Well, you don't really You can lower it a little bit with lowered springs, but if you lower it too much you will change the angle of the 4 links. This can affect how the suspension handles. If you want to lower it a lot for ride height, you will have to raise the mounting points where the 4 links attach to the frame.
Some people do use this system for an air bag suspension so they can drop the frame on the ground. However, at ride height you still need the 4 links at the correct angles.
Return from Parallel 4 Link to Suspension
Return from Parallel 4 Link to How-To-Build-Hotrods
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The 4-Link Difference
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.
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.
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.
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.
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.
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. 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. A link length change of 1" or a link angle change of 5º can make a noticeable change to handling. 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!
|•||Increasing the upward angle (to the front) of any link will enhance forward bite and increase loose roll steer.|
|•||Decreasing the upward angle (to the front) of any link will decrease forward bite and reduce loose roll steer.|
|•||Suggested angle adjustment parameters:|
|• Upper Links: 12º to 20º (upward) RS 10º to 18º (upward) LS|
|• Lower Links: -5º to +5º|
|•||You may need to reduce link angles when using clamp bracket/s and/or when running on rough race tracks.|
|•||You may need stiffer rear shocks when using clamp brackets (to control wheel hop).|
|•||Angling the links inboard (at the front) tends to increase loose roll steer.|
|•||You can correct roll steer handling problems by leading or trailing the right rear tire (or left rear).|
|•||Shortened bottom links (especially R.S.) tend to reduce loose roll steer.|
4-Link Suspension Guide:
Anti-Squat, Anti-Dive, & Roll Center
One of the most common questions we get about suspension setup and tuning is how 4-link geometry affects the performance and handling of a vehicle. In this article we'll cover what we consider to be the three most important elements: anti-squat, anti-dive, and roll center.
Need help with Anti-Squat, Anti-Dive, and Roll Center? Check out the comments section below!
The Basics of Anti-Squat, Anti-Dive, and Roll Center
When building a 4-link suspension, the lengths of the links, their positioning, and the angles at which they are mounted, will all determine how the suspension affects the vehicle chassis under acceleration, braking, and cornering. While a chassis with too much body roll can easily be improved with the addition of a properly tuned sway bar, undesirable rear squat and front nose dive characteristics can only be fixed be changing the 4-link geometry. To emphasize that point further, and because it is a very common mistake, a vehicle with too much rear squat under acceleration or too much nose dive under braking cannot and should not be fixed by using heavier springs and/or shock valving. These unwanted characteristics are caused by incorrect 4-link geometry and they can only be improved by changing that geometry.
Don't Over-Think It: There are many thick textbooks, expensive software, and hundred-page internet forums out there on the subject of 4-link geometry and it is very easy to get lost and frustrated. You are not building a Formula 1 racecar so don't expect to be plotting the movement of your suspension under every posible situation. The best advice we can give is to not over-think it, as long as you are familiar with the basic concepts of anti-squat, anti-dive, and roll center, and the axles move nice and smooth when you cylce the suspension, you will be in great shape.
Download a 4-LInk Calculator
The first step in either building a 4-link suspension or troubleshooting an existing suspension is to download one of these Triaged calculators created by Dan Barcroft and plug in the dimensions and weights it asks for. While they may look complicated, they are actually very easy to learn by just entering numbers and watching the outputs and graphics change.Triaged 4 Link Suspension Calculator (.xls)
Triaged 3 Link Suspension Calculator (.xls)
Rear: Anti-Squat Explained
Anti-squat in a linked suspension system determines how the rear end of a vehicle moves under acceleration or upon the rear axle contacting an obstacle at speed. The anti-squat value is determined by the vertical angle of the rear links as they relate to the front axle position and the center of gravity of the vehicle.
How to Calculate Anti-Squat:
- Find the horizontal center of gravity height of the vehicle or use the crankshaft. (Yellow)
- Draw a line from the center of the front tire contact point up to the center of gravity line. (Dotted Green)
- Draw a line from that intersection to the center of the rear tires contact point. (Solid Green)
- Draw lines to extend the upper and lower rear links and find the point where they intersect. (Instant Center)
- The vertical distance from the ground to the instant center is the anti-squat value.
How Anti-Squat Affects the Vehicle:
- Anti-squat above % causes the rear end to move up and the suspension to unload under acceleration.
- Anti-squat under % causes the rear end to move down and the suspension to compress under acceleration.
- % anti-squat results in no movement under acceleration.
Rear: Anti-Squat Over %
Suspension systems with anti-squat values over % will cause the rear end of the vehicle to raise up and unload the rear suspension under acceleration or when the rear tires contact an obstacle at speed. These characteristics are desired for drag racing and heavy acceleration applications because the forces that push the rear end up also push the rear tires down for more traction. At speed, however, when the rear tires impact an object, that immediate increase in traction will cause the power applied to the rear axle to raise the chassis up at the same time as the suspension is trying to compress and absorb the impact.
- Anti-squat between % and % works well for drag racing on smooth pavement with heavy rebound valving.
- Anti-squat between % and % works well for hardcore technical rock crawling and some styles of rock bouncing.
- Anti-squat between % and % works well for mud drag racing and some hill-n-hole racing.
Rear: Anti-Squat Under %
Suspension systems with anti-squat values under % will cause the rear end of the vehicle to drop down (squat) and compress the rear suspension under acceleration or when the rear tires contact an obstacle at speed. These characteristics are desired for desert racing to absorb rough terrain at speed because the impact forces are transferred directly to the rear suspension. Under hard acceleration, however, some of the power applied to the rear axle is used to compress the rear suspension which lifts up on the tires and robs traction and power.
- Anti-squat between 10% and 50% works well for high speed desert racing.
- Anti-squat between 20% and 80% works well for open road racing and rally racing.
- Anti-squat between 70% and % works well for rock crawling and trail running.
Rear: % Anti-Squat
Suspension systems with % anti-squat values will have no effect on the chassis under acceleration or when the rear tires contact an obstacle at speed. These characteristics make the vehicle neutral and keep the power and suspension dynamics independent. While it's rare to have a vehicle permanently set up at % anti-squat, many people choose to make their suspension systems adjustable to above and below % anti-squat.
- % Anti-squat is a good universal default starting point for a multi-purpose vehicle.
- Anti-squat between 80% and % works well for almost every off-road application (excluding desert racing).
- Anti-squat between 80% and % works well for almost every street and track application (excluding drag racing).
Need help with Anti-Squat, Anti-Dive, and Roll Center? Check out the comments section below!
Front: Anti-Dive Explained
Anti-dive geometry in a linked suspension system determines how the front end of a vehicle moves under braking and acceleration. The anti-dive value is determined by the vertical angle of the front links as they relate to the rear axle position and the center of gravity of the vehicle.
How to Calculate Anti-Dive:
- Find the horizontal center of gravity height of the vehicle or use the crankshaft. (Yellow)
- Draw a line from the center of the rear tire contact point up to the center of gravity line. (Dotted Green)
- Draw a line from that intersection to the center of the front tire's contact point. (Solid Green)
- Draw lines to extend the upper and lower front links and find the point where they intersect. (Instant Center)
- The vertical distance from the ground to the instant center is the anti-dive value.
How Anti-Dive Affects the Vehicle:
- Anti-dive above % will prevent the front end from compressing under hard braking and stiffens the chassis.
- Anti-dive above % will compressing the front suspension under hard acceleration in a 4x4 application.
- Anti-dive above % will significantly reduce rear weight transfer under hard acceleration.
- Anti-dive under % causes the suspension to compress under braking.
- Anti-dive under % causes the suspension to extend and lift under 4x4 acceleration.
- % anti-dive results in no movement and transfers all energy into the chassis.
Front: Anti-Dive Over %
Suspension systems with anti-dive values over % will cause the front end to stiffen up under hard braking to prevent the suspension from compressing which is ideal for aggressive braking and hard cornering. Under hard acceleration, high anti-dive geometry will cause the suspension to compress, thus keeping the front end down and under tension which is desired for steep hill climbs. As a trade-off, a suspension system with a high anti-dive setup with be less able to absorb rough terrain under hard braking as is required by rally cars and short course trucks.
- High anti-dive suspension geometry work well for aggressive street driving or pavement racing.
- High anti-dive suspension geometry is desired for hill-climb racing as it keeps the front end down under acceleration.
Front: Anti-Dive Under %
Suspension systems with anti-dive values under % will cause the front suspension to compress under breaking, often called nose-dive. Under acceleration, a low anti-dive geometry will cause the front end to lift and the suspension to extend which also shifts weight to the rear of the vehicle. These characteristics are great for aggressive driving on hard packed dirt tracks and hill climb racing. 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.
- Low anti-dive suspension geometry works well for rally racing or short course off-road racing.
- Low anti-dive suspension geometry is used by many rock bouncers to keep the suspension extended during a climb.
- Low anti-dive suspension geometry is desired in mud-drag racing to improve rear traction and absorbs rough terrain.
Front: % Anti-Dive
Suspension systems with % anti-dive values will have no effect on the chassis under braking or acceleration. These characteristics make the vehicle neutral and keep the power and suspension dynamics independent. For many applications, a % anti-dive front end may be a desirable starting point or default setting, especially if the 4-link mounts are fabricated to allow for adjustability above and below %.
- % Anti-dive is a great universal default starting point for many applications.
- Circle track racers often use low anti-dive on the left front and high anti-dive on the right front tire for improved left turns.
Need help with Anti-Squat, Anti-Dive, and Roll Center? Check out the comments section below!
Roll Axis and Roll Center Explained
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.
Body Roll Explained
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.
Need help with Anti-Squat, Anti-Dive, and Roll Center? Check out the comments section below!
Other Suspension Related Resources
How To Measure For Coilovers
Coilover Install and Setup Guide
Coilover Spring Re-Calculation Guide
Hydraulic Bump Stop Guide
Shock Tuning Guide
Shock Valving Guide
Shock Valving Shim Stack Examples
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Street Legal Laws
To find the street legal laws for your state including maximum suspension lift heights and tire size rules visit LiftLaws.com
Link diagram 4 suspension
Four-Link Suspension - Part 1
What Is a Four-link? And Is It For You?
A four-link suspension uses links to locate the axle from moving side to side and front to back, while allowing it to travel up and down and articulate. We must agree with the current majority that a four-link suspension with coils, coilovers, quarter-elliptics, or air springs is definitely cool, and that is the most common argument for building your own setup. The problem arises when you think you know what you are doing and just start putting bars and links under your truck. Just because you saw it on some race truck or rockcrawler doesn't mean you need it for your weekend wheeler, though we have no problem with building one just because you want to try it.
There are some definite benefits to running a four-link, but to do it right takes time, money, and some more time and money. We hope to give you a realistic overview of a simple rear four-link suspension, but first, the pros and cons of building one. The choice is yours, but please consider everything before you get started.The fact remains that a well designed and tested four-link will provide a superior translation of power to the ground and higher ride quality than a leaf-sprung suspension. The secret is really in the testing portion. If you build a four-link on your rig then be prepared to fine-tune it and tear it apart quite a few times before it works right. And during this testing stage we would not recommend driving it to work on the highway at 60 mph. You may get lucky the first time, but if not, remember that tearing your truck apart and re-building it is fun.
The biggest question with building a four-link is how long should the links be and where should they attach to the frame and axle. This alone will determine how the axle pushes the vehicle, if the rear of the vehicle lifts or squats under acceleration, if wheel articulation causes the rear axle to pivot and steer, and how the body rolls in turns and over obstacles. The desired amount the vehicle does each of these things is different depending on what the vehicle is designed to do (go fast, corner, crawl, articulate) and how the driver desires the vehicle to respond on different terrain. There is no one right way to build a four-link the same as there is no one perfect off-road vehicle, but it can be tuned to do certain things better than others.
For most truck owners an all-around four-link is the desire, but that will not necessarily be the best rockcrawler, desert jumper, and mud bogger suspension.In addition to all the geometry of designing a four-link there is also the problem of what will actually fit on the vehicle you are building. Will the frame support the links where you want them? Will the fuel tank, exhaust, crossmembers, and driveshafts all fit with the links and allow for proper articulation? Unless you are building a truck or buggy around the suspension, plan on doing some compromising to get the best setup you can. If you are starting to like the idea of keeping the leaf-spring suspension, we don't blame you. If you are up for the challenge, stay tuned for next month where we start getting into the technical part of the buildup.
Till then you have a bit of homework. You'll need to round up a tape measure, a calculator, graph paper, and a pencil. Now go measure your wheelbase and decide on the height of the tires you want to run on your rig. Follow that by measuring the rear axle width just inboard of the brake-mounting plates and the height of the frame at various points between the axles along the framerails while the truck is on level ground. Next find the height of the top center bolt of your bellhousing to the ground. Plus start doing research of where you can buy the materials we mentioned below. Just remember you will want to wait until you have read the second part of this story next month before you attack the four-link issue under your truck.
The benefits of a four-link over a simple leaf-spring suspension include controlling axlewrap, better departure angles, controlling axle path, and reducing the uncontrolled variables of axle movement down to just spring rate and shock valving. In addition, a four-link can also allow for more travel and articulation that can provide more traction, though we feel that too much of both can cause problems. Weight is also a concern of the modern-day wheeler since excessive weight eats power. Though a coil spring is lighter than a leaf spring, when you consider the weight of the links and mounts and everything else, the gains in weight are minimal.
The major benefit of a leaf-spring suspension over a four-link is cost and maintenance. It will take more time and money to remove leaf springs and design, build, test, and rebuild your four-link than it would to just put on a good leaf suspension, and this is if you do it yourself. With shop rates ranging from $25 to $75 per hour, a professionally fabricated four-link is gonna take a serious bite out of your wallet. Plus we have seen some very impressive leaf-sprung suspensions that allow plenty of travel and articulation.
Material is an important factor and concern for strength and safety. Your lower links could be hitting trail obstacles depending on how low you mount them to the axle, so we would recommend no less than 1 3/4-inch DOM tubing with inch wall thickness. If your truck is a fullsize or extremely heavy, or if you are planning on mounting your shocks on the lower arms like some race trucks, then you will want to go to an even larger tubing size, or better yet, sleave the 1 3/4 x with a slightly larger piece of tube. The upper links are less likely to be hit by rocks and such, but we still do not recommend anything less than wall, 1 3/4-inch DOM tubing.
Four-Link Suspension Tech - Part 2
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4 Link Comparison Triangulated vs Parallel
Lets compare two popular types of 4-link rear suspension systems. A 4-Links is a great way to upgrade your cars performance and ride quality especially when compared to leaf springs. Although there are several theories as to which type of 4-link is better, it comes down to available space and preference. Triangulated 4-links and parallel 4-links accomplish the same thing. They locate the rear axle in the vehicle in its proper place. The bottom 2 links keep the axle in place front to back. The upper 2 links keep the axle from rotating, keeping the pinion angle as constant as possible. Beyond that one basic function, the two designs begin to differ.
On a triangulated 4-link the uppers bars are placed at an angle relative to the vehicle centerline. When connected securely to the axle and the frame, they form a triangle. This is what keeps the rear axle centered under the vehicle. There is no need for a separate lateral locating device.
Conversely, with a parallel style 4-link, all bars are parallel to each other and the vehicle centerline. A Panhard bar or other separate device is required to keep the axle centered. The Panhard bar runs perpendicular to the frame rails and horizontally across the vehicle. It connects the axle to the frame by way of links thereby allowing only up and down movement.
A parallel 4-link fits most trucks better because the fuel tank is usually right in front of the axle, inboard of the frame. Universal parallel 4-links are therefore sometimes easier to install because the main link bars utilize a one-piece frame mount instead of two less time in placing and welding the upper bar mounts. But a parallel 4 link requires a Panhard bar which adds slightly to the expense and can use up valuable space needed for your car or trucks exhaust system. A Panhard bar will also induce a small amount of side to side movement during suspension travel not enough to feel but it may
concern the customer who has an extremely tight tire to fender clearance. With a parallel 4 link you are locked into a side frame link position with a triangulated 4 link the lower links can be placed beside the frame or under the frame for clearance purposes.
Either system is very straight forward to install. You will spend more time with the tape measure than the saw or welder. All else being equal, for the absolute rookie, the parallel may be a bit easier to visualize and understand during installation.
- NO side to side movement at all you can run tighter tire to fender clearance.
- Less hardware to buy and install (no Panhard bar)
- Allows flexibility in bar placement to avoid obstacles
- Angled upper bars can interfere with exhaust
- Angled upper bars can interfere with the fuel tank on late-model trucks
- 4 more attachment points to plot and install (parallel has bar mounts built together)
- Slightly easier to visualize and install (bar mounts are built together)
- Can be installed beside frame-rail, inboard or outboard
- May allow more room for exhaust (no angled upper bars)
- Requires a Panhard bar (extra cost and installation)
- Panhard bar will induce a slight amount of side to side movement during suspension travel requires slightly more tire to fender clearance.
- Panhard bar may interfere with exhaust
Why should I put a 4-link under my car?
What will a 4-link do better than a leaf spring?
In a leaf spring suspension, the leafs themselves must perform 2 functions. First, they hold the rear axle in the car. They prevent both forward and aft movement and minimize pinion angle change during suspension travel. Secondly, while they are doing this, they also support the load of the vehicle. This is a compromise when compared to link type suspension where the two functions are isolated.
Also, an OEM vehicle that has had thousands of hours of development time behind it will operate within a predictable range of suspension travel, leaf springs do a very adequate job. When the operating envelope is changed lower ride height, more horsepower, different weight distribution, maybe a trailer, the leaf springs cannot be expected to perform as intended if the operating parameters are changed. To clarify, picture a de-arched leaf that cannot compress.
With 4-link suspension, the two functions of locating the rear axle and supporting the vehicle, are isolated. Springs take care of supporting the vehicle which leaves the links free to deal with locating and articulation. 4-link rear suspension will properly locate the rear axle no matter how soft we want to make the spring. With a leaf spring rear suspension, softening the spring rate can cause other problems such as side to side flex or axle wrap (when the axle tries to twist the leafs out of the vehicle).
just a few FAQs
I have a straight axle under my ‘32 with a 4-link and am happy with the ride height and quality. Can I use an air suspension on the rear only?
ABSOLUTELY! We have several customers who have installed a 4 link and air springs or ShockWaves® under the rear of their vehicles to improve ride quality. The rear of the vehicle is where you will actually see the most ride quality improvement. This is because you sit closer to the rear end and any load changes (fuel, passengers, luggage, trailer) will be supported by the rear suspension. For those customers who are looking for ride quality start with the rear end.
In setting up my rear 4-link air suspension, should I place the air springs in front or behind the axle? Inboard or outboard of the frame rail?
It really comes down to where there is the most room. A forward position will offer slightly more travel and can sometimes offer better ride quality. A rear position can offer slightly more load capacity. Any spring, coil, leaf, or air, will perform better if placed farther apart under the chassis. Keep in mind these performance differences are quite small and that the real criteria should be available space in your particular vehicle.
At what angle should I place the 4 link bars? The Panhard bar? How critical are the angles?
We typically try to place the lower bars level at ride height. this will minimize “roll steer” (slight wheelbase change caused by the arc of the bars going through their travel). The upper bars should also be level, or slightly down at the front if need be. This configuration will provide stable handling and braking characteristics. It is important to get the bars exactly the same from side to side to avoid unpredictable handling problems. It is also extremely important to make sure the Panhard bar is level at your highway ride height. This will minimize side travel of the rear axle induced by the arc of the Panhard bar going through its travel. Obviously there are precise formulas for placement of the 4 link bars to maximize certain performance criteria, but these performance differences are quite small on a road car. Put the bars in level, or close to it, at ride height, and youll be fine.
What about reverse 4 links? What happens when you run the bars backward?
NO NO NO!!! By the way, did we say NO?! It doesnt matter what the truck magazines say DO NOT run the 4 link bars backward! Heres what happens When the top bars are run backward, the diverging arcs of the upper and lower bars will create such a massive pinion angle change that under extreme amounts of suspension travel, you may actually pull the driveshaft out of the transmission! If you want to see this effect for yourself, get a sheet of pegboard and a couple of yardsticks simulate the scenario for yourself. The second effect of running the upper bars backward is completely screwed up handling dynamics. With a normal 4 link, when you hit the brakes, the suspension geometry wants to lift the rear of the vehicle. therefore trying to plant the rear tires and assisting the braking action. When the upper bars are reversed, this dynamic is eliminated or even reversed when you hit the brakes the suspension actually unloads the tires thereby massively reducing available braking performance. This is not our opinion it is simply physics.
We dont know who thought up this backward 4 link stuff but apparently it was originally used to provide clearance for an air spring sitting on top of the lower bars that pointed to the front. The truck magazines picked it up, the readers took it as gospel, and the rest is history.
For bolt-on and Universal 4-Links, click HERE
For a 4-Link Installation video, click HERE
updated December 6,
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His stick was even thicker than Nikolai Ivanovich's and walked very tightly. I stood on my heels with my legs wide apart for stability. He nagged me for about three minutes, but blew me off with a good stream. For a while he held my ass, waited for everything to merge.