Terra Firma - Rear Suspension Guide - Popular Hot Rodding

In a perfect world, all the roads would be perfectly flat, without bumps, and suspensions wouldn't even be needed. But as we all know, that is far from reality. Once you start talking about curves and performance then a properly functioning suspension becomes essential. One area of confusion lies in the numerous choices for your rear suspension. Terms like four-link, three-link, triangulated-four-link, Panhard, Watt's and such get tossed about and if you don't know what they mean, picking the right rear suspension can be difficult at best.

Suspension 101
The suspension on your car has two main functions. Its first job is to smooth the ride of your car. According to Mr. Newton and his famed laws of physics, all forces of motion have both a magnitude and a direction. A bump in the road causes the wheel to move up and down perpendicular to the road surface. The bigger the bump encountered, the bigger the movement. The movement experienced by the wheel is called vertical acceleration.

Without an intervening structure, all of the wheel's vertical energy is transferred to the frame, which tries to move in the same direction. In such a situation the wheels can lose contact with the road completely. Then, under the downward force of gravity, the wheels can slam back into the road surface. What you need is a system that will absorb the energy of the vertically accelerated wheel allowing the frame and body to ride undisturbed while the wheels follow bumps in the road and stay in contact with the asphalt.

The Role Of Shocks
Unless a dampening structure is present, a spring will extend and release the energy it absorbs from a bump at an uncontrolled rate. The spring will continue to bounce at its natural frequency until all of the energy originally put into it is used up. A suspension built on springs alone would make for an extremely bouncy ride and, depending on the terrain, an uncontrollable car. Enter the shock absorber--a device that controls unwanted spring motion through a process known as dampening. Shock absorbers slow down and reduce the magnitude of vibratory motions by turning the kinetic energy of suspension movement into heat energy that can be dissipated. The shock can be air-filled, gas-filled, or oil-filled. In any case, its job is to control the rate of spring and suspension movement.

All rear suspension systems use some sort of shock and spring combination, but there are huge differences in how they are mounted and in the overall design of the systems. Knowing the differences in the choices out there can go a long way to helping you pick the one right for your ride.

Leaf-Spring Rear Suspension
This type of spring consists of several layers of metal, called leaves, bound together to act as a single unit. Leaf springs were first used on horse-drawn carriages and were found on most American automobiles until around 1986. Today, they are still used on many trucks and heavy-duty vehicles. Why? Because they get the job done when used within their design parameters. It's when cars are pushed beyond these factory parameters that leaf-spring suspensions need a little help. There are also mono-leaf springs with just one layer of metal. According to John Hotchkis of Hotchkis Performance, "The mono-leaf is an acceptable spring, but due to its single leaf it does not have the variable spring rate benefit of a multi-leaf." So, for enhanced performance, you will be looking at a multi-leaf spring.

In a solid-axle equipped car, the suspension is simple. The leaf springs clamp directly to the drive axle. The ends of the leaf springs attach directly to the frame and the shock absorber is attached to the clamp that holds the spring to the axle. When you add performance to your car, it can have a serious effect on how your leaf springs react. More horsepower means faster acceleration. This can cause the rear end to rotate and negatively affect the pinion angle. If you turn corners hard, it can cause lateral movement of the rear end that results in unpredictable handling. Whether you drag race or road race, your suspension is just as important as your engine when it comes to hitting your goals.

The main advantages of running a modified leaf-spring rear suspension are cost and ease of installation, since you are just modifying something that your car came equipped with. We have seen some very fast cars that were running well setup leaf springs, including our very own '76 Camaro, Project g/28. If your bank account isn't overflowing with cash, or you just want a simple bolt-in installation, then a company like Hotchkis, Global West, or Detroit Speed can help you wring out the most performance from your leaf-spring suspension.

We asked John Hotchkis about the disadvantages of leaf springs and he said, "Weight and packaging issues are drawbacks to leaf springs. A set of First-Generation Camaro multi-leaf springs with shackles and hangers weighs nearly 100 pounds. Space is another issue. Unlike present day modular SLA, or multi-link suspensions, live axle/leaf-spring suspensions need a significant amount of space for the suspension travel necessary to produce a comfortable ride. With the live axle/leaf-spring combination, the lack of rear suspension adjustability is another disadvantage."

So what can be done to improve the handling of a rear leaf-spring suspension? The answer is, quite a bit. To control the lateral movement of the rear end under hard cornering, Hotchkis recommends a Panhard rod or Watt's link as the best choice, however, you can also spend a lot less and get good results just by swapping better performing bushings into the leaves like the spherical units offered by Global West. As for drag racing, John told PHR: "During hard acceleration, the torque reaction on the axle causes the leaf springs to twist into an 'S' shape. The twisting action of the leaf causes wheel hop as the spring attempts to return to its normal shape." One way to counteract this is to install a set of traction bars to lessen this axle wrap, however, they do add weight and they can increase the rear roll stiffness during cornering and lessen the overall ride quality.

John also points out that matching your components is vital to getting the most from your rear leaf-spring suspension. "Proper spring rate, front/rear roll stiffness and matched suspension dampening are the link from the engine to the tires. All the power in the world won't cut it without a good suspension."

The Watt's Link and Panhard Rod
There are two main types of rearend centering devices. The first is the less common Watt's link. This has been used since the days of Trans Am racing to great effect, and until recently, was mainly seen on track cars. Due to its complexity, you don't see this on any cars from the factory. With a leaf-spring rear suspension there is very little to stop your differential from moving side to side, but the Watt's link changes all that. We contacted Jim Fay of FAY'S2 Suspension for his expertise with this type of centering device. According to Jim, "A Watt's link lowers your car's roll center and helps keep your car's handling response symmetrical in both left and right cornering. Because the Watt's link keeps the axle centered under the car, the rear of the car now 'follows' the front suspension rather than fighting it." Since the leaves are no longer under as much lateral stress, they can now function better under the right geometry. A Watt's link is far more adjustable and allows for better tuning of your rear suspension compared to a Panhard bar. Watt's links are not just seen on leaf-spring applications and they can be found on many types of rear suspensions like three-link and torque arm systems.

The second type of centering device is called a Panhard rod. This is what you most commonly see if you look under the back of any modern solid-axle car. It's best described as a lateral bar that keeps the rear tires centered within the body of the car. It connects to the frame on one side and the rear axle on the other. It's also sometimes referred to as a track bar. Its only function is to keep the rear end centered under the car when under lateral load (i.e. cornering). As inferred earlier, the placement of this bar can greatly affect the way your car handles and performs. According to John Hotchkis, "The Panhard rod should be as long as possible and be mounted horizontal to the axle at static ride height. An angled Panhard rod will cause the axle to move sideways during suspension travel."

The height at which the Panhard bar is mounted helps to determine the height of the rear roll center. The roll center is an imaginary point around which the rear of the car rolls. The height of the rear roll center is critical to handling. When you lower the Panhard bar, the rear roll center drops. A lowered rear roll center promotes side bite at the rear, which tends to improve corner handling. Nevertheless, an extremely low roll center can generate excessive chassis roll, which can cause suspension geometry problems. Also, excessive roll can delay corner exit acceleration. So a happy medium must be found.

The Four-Link: Two Different Choices
One of the more popular replacement rear suspensions is the four-link. Many manufactures offer this type, but that can be both a good and bad thing. It's good if you're able to find the right setup to fit your specific application. It's bad if you can't properly choose from the many different types--which can start to make your head spin if you don't know what to look for, or if you fail to ask the right kind of questions. You are not going to have much fun on the road course if you install a four-link designed for the drag strip.A four-link uses four mounting points on the axle and four on the frame (two on each side). At each mounting point, aircraft-style rod ends provide movement at various angles. The result? The four-link system improves how the suspension works in all sorts of driving conditions.

There are primarily two types of four-links: parallel and triangulated. Both styles of four-link accomplish the same thing, which is to hold the axle in the vehicle. The function of a four-link is to keep the rear axle in its proper place under the vehicle, even under hard acceleration or cornering. The bottom two links keep the axle in place front to back. The upper two links keep the axle from rotating and keeps the pinion angle constant. In a triangulated four-link, the upper bars also keep the rear end centered under the car so a Panhard rod isn't needed. Many companies offer various designs of four-link suspensions. We contacted Craig Morrison of Art Morrison Enterprises, and asked him why they offer a triangulated four-link. According to Craig, "The triangulated links hold the rear end securely in place and square to the chassis without the need for a Panhard bar or Watt's link. This provides more clearance for exhaust, fuel tank, batteries, and generally gives the rear suspension a clean, uncluttered look." He also feels that the design provides for a low roll center and helps plant the car well through the twisties.

On the down side, a triangulated four-link has associated roll bind inherent in its geometry. Roll bind, which is the unintended, non-linear resistance to body roll, happens when side load is placed on the pivot points of all four links under hard cornering. Many factory suspensions, such as the '78-88 GM G-body (Chevy Monte Carlo and Malibu, Buick Regal, Pontiac Grand Prix, and Olds Cutlass) and Fox-body Mustangs, suffer from severe roll bind. This can manifest itself as snap oversteer, or an unexpected transition from understeer to oversteer. The use of Heim joints or urethane bushings goes a long way toward rectifying this trait, and is far more obvious in OEM triangulated four-links than in aftermarket designs with non-deflecting spherical rod joints or urethane bushings.

One of the companies offering a parallel four-link is Detroit Speed and Engineering. They use a patented "swivel-link" rod end that allows each link to rotate freely with none of the bind typically associated with parallel four-links. According to Kyle Tucker of DSE, "The Quadra Link has great anti-squat geometry and a long Panhard rod to positively control the rear axle's lateral movement. This helps keep the rear end correctly located under the car, even under hard cornering." He also points out that their system allows you to run a full three-inch exhaust system. We asked Bret Voelkle of Air Ride Technologies why his company decided to offer a triangulated four-link design with their performance Air-Bar rear suspension. He said it came down to the fact that the triangulated design did not require a Panhard bar to control the lateral movement and this not only saves weight, but cost as well. When we asked Bret why a four-link might outperform a leaf-spring equipped car he said, "In a leaf-spring suspension, the leaves perform two functions. First, they hold the rear axle in the car. Secondly, while they are doing this, they also support the load of the vehicle. With a four-link suspension, the functions of locating the rear axle and supporting the vehicle have been separated. We like the four-link rear suspension because of its ability to 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, which is when the axle tries to twist the leaves out of the vehicle."

Downsides to swapping in a high-tech four-link to replace those leaf springs would be cost and difficulty. Most of the systems on the market require a bit of fabrication, welding, or even cutting into the bottom side of your ride. For those willing to tackle the cost and work involved, they can expect to be rewarded with better, more consistent performance.

The Three-Link Set-Up
Another suspension type common on the racetrack but not seen very much on street cars, is the three-link. Historically, three-link suspensions have not been common with OEMs, but they have recently started showing up, most notably on the new '05 Mustang. Many racecars have won with this design, but there are not many choices for this in the aftermarket short of fabricating your own setup. One company that is working on a solution to this is Lateral Dynamics in Carlsbad, California, where mechanical engineer Katz Tsubai and owner Mark Magers have been hard at work making this design work in our classic cars. We asked Mark to give the quick explanation of what a three-link suspension is. Mark told PHR: "Three-link rigid axle suspension systems derive their name from the fact that the axle is located under the car with three trailing arms, in conjunction with either a Panhard bar or Watt's link to locate the axle assembly laterally. In virtually every case, there are two lower arms that connect under the axle assembly, and one upper link that is attached to the axle assembly in the middle, right above the pumpkin."

One big advantage to using a three-link: there's virtually no bind associated with the design. Each link has one job and one job only. The two lower links working together prevent the axle from skewing relative to the vehicle axis and the upper link is responsible for controlling pinion angle changes in bump and droop. This allows the system to be set up for whatever type of driving you plan on doing, whether it's drag racing, road racing, or a combination of the two. When we asked Mark and Katz why they chose the three-link approach, they said, "Whether we want to accept it or not, all suspension systems are faced with compromise; you simply cannot have everything in one system that you'd like to. You need to make choices as to what is most important. In our opinion, the three-link approach arguably offers the fewest high-performance compromises of any rigid axle suspension system. In addition to being very simple and highly tunable, the three-link also provides the lightest unsprung weight--in a rigid-axle suspension--relative to all other setups. These facts are the reasons that the fastest road race rigid-axle cars predominantly use this setup, such as today's Trans Am and SCCA GT1 cars."

Mark was quick to point out that, like all rear suspensions, the three-link also has a couple of pitfalls. The biggest downfall with a well-executed three-link application on early musclecars is packaging. A good three-link setup will usually require some intrusion between the rear seats in the interior of the car for the upper link, though this can be minimized, and a custom rear seat can be installed that will easily accommodate clearance requirements. A quality three-link system will also set you back a good stack of cash. Lateral Dynamics Mechanical Engineer Katz Tsubai also added this in regards to three-links: "Not all three-links, or any other suspension configuration for that matter, are created equal. These advantages and disadvantages not only show you inherent characteristics of each configuration, but also design possibilities and flexibilities. The suspension is only as good as how you can implement those possibilities into the design. For example, let's say someone decides to build a three-link. His design consists of 10-inch long upper and lower links, all three mounted level to the ground, with a 20-inch Panhard bar mounted above the third member. Technically, it's a three-link and there is no roll bind, but it is hardly a good suspension system. Engineering, design, and execution are just as important as the suspension configuration itself."

Independent Rear Suspension
Independent rear suspension, or IRS, has been around on production cars for decades. It works in very much the same way as your front suspension does but without the steering provision. There are upper and lower control arms, along with a coil spring and shock. The main advantage an IRS has is its ability to react over uneven surfaces. Unlike a solid axle, the movement of the left wheel is not affected by the right wheel hitting a dip or bump, and visa versa. Several companies, including Heidt's and 21st Century Street Machines, offer rear IRS kits. We contacted Ed Bednar of 21st Century for his take on independent rear suspensions. When we asked Ed why he would steer a customer toward an IRS he said, "The real performance of a car and its suspension begins with the performance of the tire. In order to maximize the performance of the tire, as much of the tire contact patch as possible must be maintained with the road. The contact patch is maximized when the tire is vertical to the pavement. IRS geometry is designed to try and maximize the contact patch for overall performance." An IRS allows for greater control of camber settings in an effort to keep the tires as vertical as possible to the road surface. This ability to maximize the contact patch and to react 'independently' to pavement variations is why almost all modern sports cars run an IRS. Overall, the upside to this system is that you can get new-car ride and handling characteristics in your classic musclecar.

Down sides to IRS would be that they are expensive to buy, and in most cases, not very easy to install. They are also at a disadvantage when it comes to drag racing compared to their stronger solid-axle counterparts. According to Bruce Griggs of Griggs Racing, "In many cases people talk of the lower unsprung weight of an IRS system, however, they forget that the overall weight of the system usually negates the unsprung savings." More than a few owners of new Cobra Mustangs have found themselves swapping in solid-axle rear ends in place of the factory IRS since the IRS was hurting them so much at the drag strip. Again, picking the right system for how you will use the car is critical.

Truck Arms--The NASCAR Favorite
Chances are, the only time you have seen a truck-arm style (aka center drive) rear suspension is if you looked under a NASCAR racer. Winston Cup, Busch Grand National, Craftsman Truck, and IROC drivers have been tweaking this technology for the last 30 years. Hot Rods To Hell, in Burbank, California, has been retrofitting classic cars to center drive technology for over 10 years, so we hit them up for their insight on this system. HRTH had this to say, "To start with, a truck-arm suspension cannot bind under any type of condition. The suspension stays supple, the arms converge on an actual instant center and, because of their length, they remain neutral throughout bump, droop, and roll travel. The axle travels straight up and down so the wheelbase also remains constant. Your driveshaft slip yoke barley moves, and torque-induced chassis twist is eliminated. Also, the chassis is no longer being pushed around from the rear outer extremities by short little links, or the front half of leaf springs." HRTH also mentioned that the system eliminates wheel hop and is fully adjustable. These adjustments allow you to set the ride height and still maintain a level Panhard rod.

To find out a bit more about how a truck-arm suspension works, we hit up suspension specialist Kurt Binkley for his take. Kurt works for PPC Racing, which handles the cars for Kenny Wallace, John Andretti, and Craftsman Truck Series driver, Terry Cook. In Kurt's words, "Well, the standing joke is that all NASCAR cars are basically '67 Chevy trucks with headers and an aero-package!" Kurt went on to explain how the truck-arm suspension works, "The truck arm is an I-beam arrangement that pivots on a mounting point just aft of the trans tailshaft and is hard-mounted to the axle. There are various ends used at the forward mounting point depending on racetrack distance. The adjustable Panhard rod allows the opposite sides to be set for proper handling at various tracks. Coil springs rest in pockets on top of the truck arm, just forward of the axle. The upper pockets have a vertically adjustable plate that can compress or decompress the spring to change the ride height for specific tracks and conditions. When a driver comes into the pits and the pit crew adds or removes height, they are adjusting this plate up or down." This design allows for quick fine-tuning of the suspension, including the pinion angle, by the NASCAR mechanics. When we asked Kurt why NASCAR continues to use this technology he said, "It's required that we run this type. NASCAR is still old school." It also can be argued that this setup gets the job done on some of the fastest racecars around.

According to mechanical engineer Katz Tsubai, "The pros of a truck arm suspension are stable IC location, good roll steer characteristics, and good yaw response. This results in consistent behavior under both acceleration and braking. As for downsides, he points out the overall weight of the system, both sprung and unsprung, is quite high. Also, since the trailing links are rigidly mounted on the axle, components must elastically deflect in order for the suspension to roll. Try visualizing the whole suspension system as a giant anti-roll bar, with the axle assembly being the bar and trailing links being arms. You can see why the system restrains the roll." It should also be noted that running a full exhaust system with this type of rear suspension is quite a challenge due to how much real estate the components eat up under the car.

Torque Arm
A torque-arm suspension uses a long arm--rigidly mounted to the rear center section--that runs from the center of the differential to a point near the transmission to absorb rear axle torque reactions. The design specification of a torque-arm setup is similar to that of a three-link style suspension. The most important factor being that the lateral location of the rear end must be kept with a low roll center to avoid unpredictable suspension responses. In this type of suspension, a Panhard bar or Watt's link is required for keeping the rear end centered. Torque-arm suspensions have been used on cars for quite some time, all the way back to the 1930s. When designed properly, this style of suspension can offer good performance characteristics. The most notable performance cars to use this suspension are Camaros and Firebirds made from 1982 to 2002. They employ a torque arm along with trailing arms and a Panhard rod.

One other concept seen when discussing torque-arm rear suspensions is "decoupling." When a torque arm is directly connected to the chassis, the rear end can physically be lifted off the ground under hard deceleration. This is referred to as "brake hop" and is something you want to avoid. By decoupling the torque arm from the chassis, you avoid the braking torque being transferred through the torque arm. This is accomplished by only letting the torque arm contact the chassis under acceleration and by allowing the other links in the system to absorb the braking torque. In this way the acceleration and braking functions are separated and each can be optimized individually.

Getting a properly functioning torque-arm suspension is not an easy task. The length of the links and their placement has to be dead-on. Also, the bushings used, damping rates, spring rates, and preload must be calculated to work together in controlling all the forces.

Luckily for racers (especially if you drive a late-model Mustang), there are companies like Griggs Racing. Bruce Griggs has been racing for decades and has worked extensively with torque-arm suspensions. Bruce feels that you don't have to give up ride quality to get a great handling suspension. As he puts it, "most others throw stiff springs, shocks, and bushings at the car, limiting suspension travel in an attempt to delay the manifestation of the undesirable handling characteristics caused by the stock suspension geometry. This has the added effect of making the car very skittish over uneven road surfaces as well as degrading the overall ride quality. Unfortunately, many people believe that in order to handle well, the car must ride this way, and that this is a natural trade-off." Bruce also points out that where racers can run any type of suspension system they choose, you will find quite a few torque-arm setups. Another point that Bruce mentioned is that a torque arm isn't affected as much by ride height changes like a three-link or four-link would be. Griggs Racing offers its street torque-arm suspension with a non-adjustable Panhard rod since this allows the use of a full exhaust. On their all-out competition system, they employ a Watt's link.

When we asked mechanical engineer Katz Tsubai about what he felt were the pros and cons of torque-arm suspension he said, "One pro is that the system is kinematically free in roll. As with a three-link, the suspension is free to roll when Heim joints are used. As a result, tuning is much easier and the end result is predictable and won't surprise you by causing conditions like snap-oversteer." He also pointed out that it's possible to achieve good roll steer characteristics with a low roll center. Added benefits would be that the system is fairly simple to retrofit to an older car since you do not have to cut up the floor and trunk. Additionally, depending on the layout and centering device used, it's relatively easy to route a full exhaust system.

On the down side, Katz stated that he felt the system had a low anti-squat value, depending on how the trailing links were arranged and that it's difficult to get high anti-squat without causing roll oversteer or severe brake hop. This would be more of a problem in a short-wheelbase car that would use a relatively short torque arm. Kats also related that in some cases the system has limited adjustability. Since the length of the torque arm is fixed, any changes to adjust the anti-squat value will also change the roll steer characteristics. On the subject of decoupled torque-arm setups Katz added, "The idea is to free up the torque arm from reacting to braking torque by adding a telescoping auxiliary link. This allows you to have very high anti-squat value, while keeping brake hop, which is normally associated with high anti-squat, at bay. The system is very sensitive to tuning, particularly preloading."

In practice, we've not noticed degraded anti-squat or brake hop in either factory torque arms, or the Griggs GR-40 system for late-model Mustangs. Under hard track use, the most common torque-arm designs offer adequate, or even superior, anti-squat characteristics. It's important to note that the ill-handling traits regarding torque arms apply only to short torque arms, so if you plan on fabricating one yourself, make sure you've got the real estate necessary for an effective design.

A Word About Tuning
No matter what system you choose, you will have to properly tune your suspension. Race teams spend countless hours tweaking and adjusting their setups to squeeze out every bit of performance. Mark Magers of Lateral Dynamics best stated it when he said, "For any and all suspension systems, it is critical to choose not only the right design for your particular performance emphasis, but to also have the system properly installed, and most importantly, tuned for the given application. You see it all the time on racing coverage: cars are constantly being adjusted to deal with different tracks, weather conditions, etc., and for good reason. Small changes in setup parameters can have a very profound effect on the overall handling characteristics of the car, and the more particular you are about running your car close to the edge, the more important these attributes become. We constantly see examples of an ill-handling car that has all the right pieces, but doesn't get the job done because the last and most important part, tuning, was poorly executed. It's a small population of people who truly understand what is happening with suspension systems or understand how to optimize them."

Getting a great handling car is no easy task. It takes the combination of the right parts, proper installation, and good tuning to make it all work. The reward is a car that consistently goes where you point it and puts a smile on your face.