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.