Aftermarket Suspension Components - How It Works
Fatman Fabrications reveals the science, technology, and engineering behind aftermarket suspension components.
From the January, 2013 issue of Chevy High Performance
By Stephen Kim
Hot rodders have got this airflow thing figured out. With today’s incredible selection of high-flow cylinder heads, even home-built small-blocks can embarrass Rat motors from just a few decades ago. Interestingly, that same universal knowledge doesn’t apply to suspension design, as many DIY engine-building studs are clueless when it comes to upgrading the cornering performance of their rides. What makes the situation even worse is that by putting two to three times more horsepower through a car than it was originally designed to take, any inherent handling deficiencies become exponentially worse. The good news is that there’s no shortage of quality aftermarket suspension components to help tame today’s wicked engine combinations. Even so, haphazardly bolting random suspension bits onto a car can yield a mismatched combination that not only rides terribly, but doesn’t optimize handling, either. To get down to the nitty-gritty of how to properly upgrade an old-school suspension, you first have to understand how they work and where their designs fall short. To make sense of it all, we engaged in a conversation with Brent VanDervort of Fatman Fabrications. His shop has been building cutting-edge suspension components for decades, and by combining that real-world experience with a mechanical engineering degree, he has a knack for explaining highly complicated topics in a way that everyone can understand.
Different Era, Different Standards
As people have grown accustomed to driving late-model vehicles, the handling deficiencies of the typical ’64-72 muscle car becomes more obvious. When discussing all the cars of this era, we have to keep in mind that we are asking these cars to do things they were never designed for. Given that cars are generally designed two to three years before going into production, most of our favorite muscle cars were designed in the late-’50s and early ’60s, so you have to look at these cars through the window of that time. During the era when most muscle car design were born, only skinny bias-ply hard tires existed. Speeds were slower, traffic was less, and engines were less powerful, putting less demand on the car. Heck, half the roads in the country weren’t even paved then. Cars like the Chevy II Novas came with 120hp six-cylinder engines for grocery getting. So when you add three to four times the amount of power and factor in the stresses imposed by the grip of modern tires, it’s not reasonable to expect the handling and braking performance of a muscle car to match that of a modern car. Fortunately, by applying the concepts of modern suspension design to muscle cars, they can match or exceed the handling capabilities of late-model performance cars.
Factory Suspension Shortcomings
From a suspension design standpoint, the biggest problem of the GM A- and F-body is that their upper control arm is angled the wrong way, going downhill on the way out to the spindle. A taller spindle changes that angle so that the wheels are tipped into the inside of a curve. This aids handling with a better tire contact patch as well as migrating the center of gravity inboard. The different control arm angle also alters the roll center to a height much closer to the car’s center of gravity, resulting in less body roll. Therefore, you can get away with less aggressive sway bars and still have good handling, resulting in a better ride and tire-to-road compliance. In a nutshell, a deficient muscle car stock suspension requires stiffer springs, shocks, sway bars, and bushings to essentially convert it to a go-kart with minimal body roll. It will handle better but beats you to death. Plus, if the tires skip over the pavement because the suspension is too stiff, they don’t provide much cornering or stopping power. For the best real-world handling and ride, you want to fix the geometry problems and then use the softest springs that will support the car. You want only enough sway bar stiffness to control residual body roll, and just enough shock stiffness to dampen the suspension travel. As for ’62-67 Novas, they share all the aforementioned geometry problems in addition to a weak structure.
Simple changes in geometry can have a dramatic effect on handling. Testing performed by Hotchkis from around 1974 showed that a taller spindle from a front-steer ’70-81 Camaro could be installed on a front-steer ’64-72 Chevelle with a hybrid lower ball joint, and a special-length upper control arm to correct static camber settings. This swap would not work on the rear-steer Camaro, as the Ackerman would be reversed. Nevertheless, the result from these simple changes in geometry was a 20 percent improvement in lateral grip during skidpad testing.
Camber gain is the change in the inward tipping of the tire as the suspension moves vertically in response to the road. Positive camber change tips the top of the tire outward and negative tips it inward. The advantages of leaning the tire inward in a turn includes transferring the car’s center of gravity toward the inside of the turn to induce less body roll, and minimizing the increase in the height of the roll center due to that body roll. With radial tires especially, leaning the tire inward tends to keep more footprint on the pavement. By eliminating understeer through camber gain, you can use less sway bar for a better ride quality while providing improved handling. The confusion comes from when a design is referred to as either a positive or negative camber gain. In truth it is usually both, depending to which tire you are referring. Assuming a level lower control arm, when the car’s upper control arms slopes downward toward the center of the car, the wheels will lean into a curve. Since the outer wheel is going negative and the inner wheel positive, they can be referred to as either positive or negative according to which wheel you refer to. In either case, suffice it to say this is far superior to a situation where both tires lean out of a turn, the tire footprint decreases, the CG migrates outward, and major understeer results.
The leaf rear suspension systems on GM muscle cars work pretty well, despite the fact that they must support the weight of the vehicle in addition to locating the rearend. As for the triangulated four-link rear suspension design of the Chevelle, they are pretty short and flexible for best performance. Since the floor limits the upper bar length, you generally have to live with the basic OEM design. The raised upper mount kits that have been around forever do improve launch geometry, while boxed or tubular control arms combined with tighter bushings improve control. As always, better shocks and properly sized sway bars are mandatory to finish the upgrade, and it’s important to remember that bigger sway bars are not always better. Sway bars are used to achieve balance, and a bigger bar can create as much imbalance as one that’s too small.
The type of spring that a car has really doesn’t affect performance very much. It’s simply a matter of how much weight you want to support while taking into account factors such as suspension leverage ratios, and optimal spring load and rate. With enough effort you can find this ideal balance with a leaf spring, coil spring, air spring, or even torsion bars. Air springs offer the ability to tailor the spring characteristics to the car with the push of a button, although there’s sometimes a premium price to pay. Now that air springs have matured with good ride height control systems, the adjustability they offer in terms of ride height and spring rate are advantages to be sure. If you want to park your car lower than it can be practically driven, an air spring is in fact the only real viable choice. The very adjustability can be a trap for those who don’t learn to make proper adjustments, but any system improperly used cannot provide optimal results.
Leaf Springs vs. Four-Links
Leaf springs will always be the simplest way to mount a rear axle. The leaf springs handle both the positioning of the axle as well as the acceleration and deceleration forces. That is where problems can arise in high-performance applications. When the spring has to store all the energy of the vehicle weight in addition to the acceleration and braking forces, it can become overwhelmed and create wheelhop. The most effective way I have seen to control this is the CalTracs bar. Unlike the old slapper bars, these are mounted securely to the front of the spring rather than floating below it, and also tend to hang down less. Their unique feature is a sliding joint that keeps the bar in a straight line while allowing necessary but small changes in length as the leaf spring flexes. In essence, you are using the front half of the leaf spring as the upper link of a four-bar system.
That said, four-link suspension systems certainly offer advantages in style, unsprung weight, and ultimate power handing. However, unless you are willing to make major mods to the floor, the length of the upper bars must be limited in order to clear the floor. In theory this should not work as well as real world empirical testing has proven. At Fatman Fabrications, we are also big fans of the long trailing arm system originated on GM pickups in the ’60s that are still used to this day in NASCAR. The problem is that it does not fit well under most cars. In recent years, the market has seen the emergence of torque arm rear suspension systems in response to the desire for a longer upper control arm. It does work well, but seems to get overly complex and add considerably to unsprung weight. The benefits are excellent traction and handling.
An independent rear suspension is absolutely a very trick addition to a car that’s worthy of bragging rights. The problem is that many frames lack the arch in the rear framerails to allow for enough hub carrier clearance with a low ride height. That’s why Corvettes don’t have a real trunk. Speaking of Corvettes, I don’t think anyone is going to convince you that early Corvettes ever rode well, although the C6 seems to be a major improvement. When I visit an SCCA race, all the fast C5-and-older Vettes have a 9-inch rearend with a four-link. That suggests to me that the IRS handling advantages are questionable. In the real world, I think your money is far better spent elsewhere if you’re looking to maximize the handling of a muscle car. That said, if you are building a muscle car to win one of the big national awards, an IRS will likely soon become the way to go.
Modern tires offer far more grip than tires from decades past, which has forced aftermarket suspension products to evolve to make the best use of the additional grip. With skinny, hard bias-ply tires, so little grip was available that proper geometry just didn’t matter much. These designs were often as much about being cheap to produce as they were about handling. If you read any of Tom McAhill or Smokey Yunick’s columns in Popular Science and Mechanix Illustrated in that era, they recognized the inadequacies of American cars while they were new. Modern wider tires with better grip impose far greater loads than these cars were designed for. For instance, bumpsteer isn’t an issue when there’s only 4.5 inches of bias-ply tread on the ground. There simply isn’t enough traction to allow a car to stop or turn well in the first place.
To increase the power of these cars without upgrading the brakes or suspension makes them less safe than they were when they couldn’t go so fast. Tires that are already in a partial skid during normal maneuvers cannot deliver on their promise when extra power is added to the equation. Modern aftermarket suspension upgrades are all about maximizing the footprint of the tire, and eliminating tire scrub and bumpsteer. Better camber control, better alignment specs, and improved steering and brakes will all work together to allow the tire to perform to its true potential. As the knowledge of proper geometry and methods of problem solving spreads, and just as importantly are installed by rodders, our favorite cars get better and better.
Autocross vs. Road Course Setup
Driving a car around an autocross compared to driving around a road course requires vastly different setups in order to maximize performance. It’s similar to comparing the differences between a fighter jet and a Cessna. The Cessna is designed to be stable to allow pilots and students with a wide variety of experience and ability to fly it safely. It is especially important that it has no bad habits and that it doesn’t maneuver so radically that it easily gets the pilot in trouble. On the other hand, a dog fighter must be inherently unstable. It can turn best by being on the ragged edge of stability at all times, ready to react instantly. Full-on autocross cars are like a dog fighter. Negative camber, minimal caster, and toe-out are used to make the car want to avoid staying straight. This comes at the expense of tire wear, but on runs that generally last for less than a minute, that just isn’t an issue. Road course cars need to have more stability, so more positive caster and little to no toe-in are common settings. With a longer course, higher speeds, and more laps to be run, tire wear and overheating are very real concerns.
Street cars general do best with minimal positive camber, 3 to 4 degrees of positive caster, and toe-in to produce straight-line stability and minimal tire wear. Turning and stopping ability are well within the needs demanded by the naturally less demanding driving needs on the street. Naturally, compromises can be made with an informed view of the tradeoffs for the performance level desired. When the autocross craze began, it was pretty much a run what you brung series. I think we all knew that the natural progression of competition would edge the faster cars toward purpose-built race cars that can be street driven, rather than street cars that can be raced. Witness the teams, transporters, tire blankets, and trained drivers in specially prepped cars seen winning today. That’s all good, if you can live with a fighter rather than a Cessna every day. As long as your upgraded muscle car can out handle a modern performance production car, as has been repeatedly proven, your car has all the handling it needs for normal use.
With all the aftermarket suspension components that are available today, a very common question is which parts offer the best dollar-per-dollar improvement in handling. First-gen Camaros, second-gen Novas, and Chevelles first need a dropped yet taller spindle, disc brakes, and better shocks and sway bars. A better, quicker steering box is next on the list. Tubular control arms make modern alignment settings easier to achieve, but provide little functional benefit other than style. Likewise, tubular arms offer little change in strength or weight savings, while better bushings can be used in a stock control arm to reduce deflection. Coilovers make adjusting ride height easier without the higher spring rates common to shorter springs, but in fact have little real advantage over upgraded standard shocks and springs other than bragging rights. It’s simply a luxury to have an easier way of tuning spring rate and ride height. If the budget allows, that can be worth a lot in reduced effort. Second-gen Camaros, Chevelles, and third-gen Novas come factory equipped with excellent tall spindle geometry and good disc brakes. In fact, most of the mods that are made to first-gen Camaros merely seek to emulate the superior second-gen design. Dropped spindles, and better shocks and sway bars should be first on the list, with a blueprinted steering box and tubular control arms next.
Another option is Fatman Fabrications’ Camaro Strut independent front suspension conversion. It provides a simple bolt-in upgrade with a front steer rack to allow use of normal rear-sump oil pans, disc brakes as large as 13 inches, and excellent geometry. It can be installed for well under $3,000 by any decent mechanic with hand tools. Although third-gen Camaro build quality has always been in question, they handled well and the MacPherson strut front suspension design has certainly been proven to perform as well as a twin A-arm system.
With full-frame cars like A-bodies and Tri-Fives, owners have the option of bolting aftermarket suspension pieces onto the stock frame, or replacing the entire frame with an aftermarket unit. Front subframe replacement kits are available for cars like the Chevy II as well. While it is certainly possible to upgrade an OEM suspension with better brakes, lowering springs, and improved geometry and steering, there are often drawbacks to rack-and-pinion steering conversions. The biggest issue is that they limit how much you can change over to modern alignment specs. By the time you have done a standard rebuild and made your upgrades, you may have spent more time and money than replacing that OEM suspension with a more modern suspension. An entirely new frame, properly designed, is simply a substantially improved design. Both the front and rear suspension come ready-made with new components that will deliver excellent performance. There is no need for scraping undercoating and sandblasting. Far greater torsional rigidity is often achieved by using superior crossmember designs and materials. A more trick appearance is a natural result of all this fine work, and a new frame can save on installation time. An original chassis upgrade can make all the sense in the world for a nice driver built on a budget, while a no-holds-barred show car just about requires a new chassis to be competitive.
A very effective alternative to a full aftermarket frame is installing a front frame stub. Front stubs can also be installed in some unibody cars as well. Not only are they relatively easy to install but they offer many of the same performance advantages of a full-frame upgrade. For example, when a big-block installation is planned, the rack-and-pinion used will generally solve the header to steering box clearance issues. A small-block or LS1 swap gets even easier as well. Most chassis need the most help up front, and since the body mount and contours from the firewall back are the most difficult part of a new frame to design, a combination of a frame stub joined to an original rear chassis offers the most improvement for the least cost. It is important to note that weld-in stubs need to have the attachment joint properly constructed. Fatman selects framerail tube sizes that closely match the original rails when possible. Internal gussets are then used to construct a 0.25-inch wide by 0.125-inch deep backed-up butt weld. That channel is then filled with multiple weld passes and ground to provide a joint that is stronger than the original rails yet makes a smooth transition. Our frame stub designs are all specific to the car being built, with radiator core support and bumper holes properly located for remounting of the car’s front clip.
Early Chevy II Novas had a very primitive front suspension. Fatman Fabrications has developed a MacPherson strut IFS for these cars which has proven very effective. This arrangement preserves the stress path for the car’s weight as designed, carrying the suspension load through the shock tower and back into the firewall via the strut tower bracing. Since the upper control arm is eliminated, the shock towers can be cut back as much as 4 inches per side, but is not required for proper installation. Our current Chevy II IFS is a second-generation design that uses a front-steer configuration steering rack. This allows a normal GM rear sump oil pan to fit, making LS engine swaps more practical. Likewise, the Chevy II IFS also uses a full 5/16-inch plate crossmember with tubular lower control arms that eliminate the strut rods. Customers like the idea of losing the strut rods, and we supply a replacement front corner gusset plate to serve as the structural function of the stock strut rod bracket. By bolting the main plate into the original lower control arm holes, the frame is greatly strengthened. In stock form, it has virtually no crossmembers taking structural loads. That has always been a problem with the stock suspension holding alignment. The plate also adds rigidity to the total feel of the car, and also serves as a skidplate under the car and allows for two more inches of ground clearance. Since this kit uses ’82-87 Camaro spindles, it’s very easy to upgrade to 13-inch disc brakes. We supply a lefthand engine mount to make the steering connection quite easy, along with the U-joints and shafts needed to finish the job. Excellent header clearance is the result with big- and small-blocks. The strut conversion also offers adjustable height when coilovers are mounted in the strut cartridge.
Aftermarket tubular control arms are very popular upgrades, but the truth is that changing the shape of the arms doesn’t change the geometry unless you alter the position of the bushings and the ball joints. Fatman Fabrications was actually the very first company to build tubular control arms for GM cars. We backed into that because our original purpose was to make narrowed arms to solve the track width problems for street rodders who had unwisely chosen to install a GM subframe in a car too narrow for proper tire clearance. After our first sets of control arms hit the market, the Camaro and Chevelle guys wanted them as well. By making them so long, we found constant improvement by using an offset upper control arm shaft, urethane bushings in some cases, and relocating the upper ball joint. We also came up with a modular spring mount on the lower control arms that accommodate coilovers and air springs. The 3/16-inch offset shaft was first developed by MOOG, as even old lady cars were prone to sagging over time, making it difficult to maintain proper alignment specs. Add to that lower raked cars with altered settings, and the ability to change the length of the upper arm 3/8 inch by flipping the shaft in its bushings can eliminate the crazy stacks of shims that might have been necessary.
Additionally, we make the upper control arms just slightly longer to allow reaching optimal camber, settings on a car with a sagging frame. The offset upper shaft then helps if the arm ends up being too long. In essence, we have found that old cars can be a moving target, so we allow more latitude in fitting them to the car. We also move the ball joint back 3/4 inch in all applications. This allows four more degrees of positive caster to counter the loss of caster that results from having a low front ride height, and to allow more stability at high speed with power steering.
Installing lowering springs and drop spindles, or cutting the stock springs, are all effective methods of lowering ride height. Cutting coil springs works well as long as you don’t go too far. It is true that removing a coil raises the spring rate, but the effect is negligible up to one full coil. A good rule of thumb is that every full coil is worth about a 2 inches of ride height. Cutting more raises the spring rate excessively, but more importantly, can cause loss of travel by bottoming out the shock or bumpstops. It is also true that a lowered car requires a higher spring rate. If you decelerate the same mass within half the travel, you will produce twice the g-forces. That translates to twice the force in the seat. That is another reason why excessively short springs are a bad idea.
Dropped spindles are generally my first choice for lowering a car since all they change is the position of the wheel on the suspension. They preserve full suspension travel and shock stroke, and also make it easier to add disc brakes. The dropped spindles with a raised ball joint height also provide handling advantages. Generally, a 2-inch drop is the limit before outer tie-rod clearance issues come up. You can also combine shorter springs and dropped spindle to get 4 inches of total drop with minimal affect on ride quality. I would always recommend the drop spindles first, except on cars that have too short of a spindle to begin with, which results in the upper control arms sloping the wrong way. In ’88-93 GM pickups, S-10s, and ’78-87 G-body cars that lack a tall spindle option, a short spring may actually result in a geometry improvement. The shorter spring will move the upper control arm closer to level, and reduce the improper camber change normally seen.
Rubber, urethane, and Delrin are materials that are commonly used in suspension bushings. Rubber has the most compliance, urethane has less, and Delrin has the least compliance. Drag racers like Delrin for minimum compliance and friction for maximum suspension rise, while autocross guys like the precision it offers. When it comes to bushings, the real question is what are they trying to do with the car? Compliance is not always a bad thing. The earliest independent front suspension designs in the ’39-53 era often used steel or bronze bushings for virtually zero compliance. This was precise, but wore quickly while providing very little isolation from road shock, jarring people in the car. Since the bushings wore so quickly, they were often allowed to wear loose, thus negating the precision they might have provided when new. Urethane is a good compromise. On all upper control arms, you can use urethane to maintain precise location of the ball joint with little effect on ride quality. On the lower control arms, I personally only like to use urethane when the material wall thickness exceeds 3/16 inch. Otherwise, rubber bushings in good condition will not flex excessively while still maintaining some separation from road shock. Urethane works well on the lower control arms as well if precision is really favored over comfort.
The aftermarket front subframe assemblies we produce for first- and second-gen Camaros offer more ground clearance, and a narrower track for tire clearance. Our first-gen Camaro subframes also yield superior handling due to improved geometry compared to the OEM design. The front-mounted rack-and-pinion setup offers much better clearance for the lefthand exhaust, making a big-block swap much easier. It’s also nice to build a car with a cleaner framerail design that has much better looking welds than the OEM subframe. Likewise, our first- and second-gen Camaro subframes offer substantial handling advantages over stock. It is based on the ’74-78 Mustang II IFS, but only the spindle remains stock. We upgrade the brakes, sway bar, control arms, and crossmember in our subframe design. Some seem to think that this suspension is too weak, but I would counter that the Mustang II was a 3,500-pound car. It has an excellent symmetrical geometry, making fabrication easy and parts less costly. A very strong forged steel spindle is used with wheel bearings larger than on an OEM Camaro, and both the OEM Mustang and common screw-in Chrysler ball joints are larger than those on a stock Camaro. Strength is not an issue, and our Camaros and ’57 Chevys have put down numbers on autocross and slalom courses right in line with the fastest Camaros featuring Corvette-derived IFS designs. We have amply shown to any open-minded person that the Mustang II design can deliver world-class performance.