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.
Rear Suspension Design
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.