There is a great deal of math and physics that goes into designing performance suspensions systems and upgraded parts. Fortunately, you don't have to know or understand any of that to understand what they do and how aftermarket performance parts can improve the handling, ride, and enjoyment of your project.
We never cease talking about all the great new products available in the aftermarket for vintage cars, but we don't often have the space to go into exactly why all of those products actually exist. We promise it's not just prettier versions of the stock stuff. The evolution of the automobile has brought a deeper engineering understanding of what works and what doesn't, and advanced technology has made accessible many products that were previously not possible. To give you a better grip on suspensions, we're going to drive through the most common upgrades in the performance handling world, how they function, and what benefits they offer.
One overriding thing to take away from this is that it all really boils down to geometry and making sure various parts function together harmoniously. That's the biggest reason we don't advocate randomly mixing and matching parts from different manufacturers unless you know firsthand that the specs work together. It's entirely possible to piece together a group of quality parts that result in poor handling if they were not designed to work with one another. You could be basically back to square one, but with a lighter wallet! There's just so much to talk about, so let's get right to it.
The upper and lower control arms are arguably the most defining parts in the front suspension since they establish all of the geometry. Their orientation to the chassis and one another, plus length, can completely change the feel and capabilities of a car.
It's never about weight in the street or performance-handling world. In fact, it's not uncommon for well-built tubular arms to be a bit heavier than the stamped steel stockers. Gaining unsprung weight is not ideal, but there is a very good reason behind it: strength. Stock stamped arms were designed to deal with 14- and 15-inch wheels, and very tall, skinny bias-ply tires. Modern tires and large-diameter wheels generate far more forces than would have been conceivable outside the world of Trans-Am racing back then. The tubular design gives arms greater strength and resistance to deflection, cracking, and failure under hard use. That's also why you don't use drag-race specific parts on the street despite the fact they often save weight; they're not designed to withstand the rigors of cornering.
Beyond strength, the key point in a well-engineered performance arm is altering the geometry of the suspension and steering. There is a reason new cars drive better out of the box, and it's mostly about a better understanding of what creates confidence inspiring control. In vintage cars, it's very common to have far too little caster, positive camber curves, an ultra-low unstable roll center, and massive bumpsteer, all of which will get worse when paired with modern performance parts. A well-engineered set of control arms will address all of these concerns and transform the way the car drives immediately.
All but the most aggressive factory cars ride on springs designed with soft rates to provide smooth, cushy rides for the masses while sacrificing performance. They are also typically pretty tall to provide ground clearance for all road conditions at the cost of raising the vehicle's center of gravity.
Performance springs work by increasing the rate of the spring (i.e. from 350 lb/in to 600 lb/in) to resist the transfer of weight from one side of the vehicle to the other during cornering. For maximum grip, all four tires need to maintain as much contact with the road as possible, which they cannot do if too much weight is applied or lifted (due to body roll) during a corner. Increasing the spring rate will diminish the weight transfer, and using lowering springs will bring the vehicle's center of gravity lower, which will also help diminish roll.
Also known by the technically more accurate names antiroll bar, antisway bar, or stabilizer bar, these tubes of steel are essentially torsion springs that couple the two independent sides of the suspension together. Their primary function is to reduce body roll by supplementing the roll resistance of the springs. The mounting points will always be on the chassis, while the ends are typically connected to the lower control arm on independent suspension systems, and directly to the axlehousing on solid axle systems. In a turn, more force is applied to the ground by the outboard wheel than the inboard. The bar reacts by adding force to the inboard wheel and subtracting force from the outboard. This helps equalize the force and reduce body roll. Technically, you can also reduce body roll through very stiff springs, but sway bars allow you to accomplish the same effect without creating a bone-jarring ride that doesn't handle bumps or irregularities well.
That coupling by definition reduces the independence of the two sides of the suspension, though it's less noticeable on a solid axle. That means that bumps on one side also affect the other. Outside of factory-prepped track cars, the sway bar is typically thin and chosen to create a compromise between comfort and handling, however, sway bars are simply a tuning tool that must be tailored to work with the rest of a suspension system, so the answer is usually not to bolt up the biggest bar possible. The diameter needs to be chosen in respect to the car's weight, use, and driver preference.
Sway bars come in two styles: standard one-piece bars, and three-piece splined bars, often referred to colloquially as NASCAR bars. Three-piece bars offer the benefit of interchangeable torsion spring sections, as well as a splined connection with the arms that eliminates deflection that can occur with one-piece designs. Adjustable holes on the arm allow quick adjustments to the effective length of the arm, which alters the force it exerts on the torsion spring.
A spring alone in a suspension system would be unpredictable and would generate too much compression and rebound over road irregularities and during cornering, resulting in a car that would be impossible to control. A shock works as a damper that slows and reduces the spring's motion to a more controllable and predictable state by changing kinetic energy (the spring motion) into thermal energy inside the shock as the oil is forced through the valves by the piston. A pair of shocks that can effectively dampen the action of the spring on both compression and rebound will allow a good spring to shine.
The type of valving and fluid control in the shock makes a huge difference, so always go for shocks that are designed for performance and handling characteristics rather than a generic passenger car replacement. You very much do get what you pay for in this category and cheap shocks will make your upgraded parts feel less confidence inspiring. Shocks should be paired with springs, such that the shock is neither overwhelmed by the stiffness of the spring, nor is the spring's motion overly restrained by too much shock damping. The level of tuning can make a large difference in performance. For a mildly upgraded street car that sees rare autocross duty, a quality non-adjustable or single-adjustable shock is simple and will do the trick. As parts get more aggressive and speeds get higher, those seeking maximum tunability should definitely seek out single-, double-, or even four-way (high- and low-speed rebound and compression) adjustable shocks. Control over rebound and compression allows the driver to change how the suspension reacts to changes in spring height from cornering and road irregularities.
Standard spring and shock combos typically place the spring inboard of the spindle by quite a few inches, while the shock and spring are mounted independently. A coilover places the spring directly on the body of the shock, creating a single unit with one mounting point.
There are several benefits to a coilover swap, but they vary depending upon the application and mounting style. Since the spring is mounted directly to a threaded sleeve on the shock, all coilovers offer the benefit of relatively easy ride height adjustment, as well as swapping the entire unit out if need be. Additionally, coilover springs are much smaller and the entire assembly will weigh less than a typical separate coil spring and shock combo. The threaded sleeve also offers the ability to set preload on the coil spring, meaning that the spring is compressed between its upper and lower perches on the strut body before the assembly is placed in the car. Since the springs are tightly wound to the shock body and their compression will be totally linear with the shock, spring deformation or deflection during travel is essentially eliminated.
Coilovers can also offer the benefit of better transfer of the spring rate to the wheel rate if the mounting position is moved closer to the ball joint. The wheel rate is the force the tire actually sees and is rarely equal to the spring rate. To get the wheel rate of a spring, the spring rate is multiplied by the square of the motion ratio. The motion ratio is the mechanical advantage that the wheel has over the spring in compressing it and is found by dividing the distance from the control arm mount by the distance from the control arm mount to the ball joint. The angle of the centerline of the coilover from the horizontal of the control arm or axle also affects the motion ratio, though typically not as drastically.
The exact number varies by chassis and suspension configuration, but on a car with a conventional inboard-mounted coil and shock arrangement, the wheel rate could be as low as 25 percent of the spring rate. That would mean a 600-lb/in stock-location spring would have a wheel rate of 150-lb/in. Due to the smaller diameter of the assembly, if the mounting point of a coilover can be moved toward the ball joint significantly, the value could be more like 90 percent of spring rate, or even over 100 percent on some solid axle locations. That means a much lower spring rate could be used while still yielding a higher wheel rate. Due to reductions in unsprung weight and friction in the bushings and ball joints, a higher wheel rate coilover will often not only handle better, but also ride better than a conventional arrangement.
Leaf springs—a carryover from the horse and buggy era—are the oldest and most basic version of automotive suspension. Using arched strips of spring steel, they can be arranged as single leaves, or multileaves depending upon application and desired load-carrying capacity.
To be perfectly honest, a parallel leaf spring only does one thing really well: carry heavy weight. That's why you still see them commonly used in modern pickup trucks and SUVs. On the performance front, they are fairly compromised since the chassis requires them to be both springs and control arms at the same time. This can result in spring wrap, which can cause axle hop on acceleration or braking, and roll bind when cornering. That certainly doesn't mean they can't be dramatically improved. If you're sticking with leaf springs, look for modern-design replacements offered by companies that have put in the homework to determine what will work for your chassis. At a minimum, we always recommend upgrading the shackles and bushings since this is a major deflection point. Also, adjustable shocks can be particularly helpful with controlling leaf springs.
Adding extra links to the suspension can dramatically improve leaf spring characteristics as well. Slapper-style traction bars can improve straight-line acceleration by limiting axle rotation and spring wrap in front of the axle. A fixed length or sliding bar mounted below the spring and connected to the forward spring eyelet will act as a lower control arm and prevent axle rotation. Race-prepped vintage Shelby Mustangs also used a version of this mounted above the spring that was reportedly better at controlling brake-induced wrap. A center-mounted torque link attached to the rearend centersection can also be a very effective device that will experience zero bind during suspension movement.
Adjustable Four-Link Arms
A trigulated four-link rear suspension is very effective at controlling the pinion angle of the rearend during articulation since both the upper and lower sections are captured. For street cars at stock ride height the factory setting are generally fine.
The design of the four-link controls the pinion angle through suspension travel, which means that it is dependent on the vehicle's ride height. At stock or slightly lower ride height, non-adjustable arms will suffice, however, the lower the ride height the more the pinion angle is altered, which will alter the traction under acceleration. Adjustable control arms can compensate for this by allowing the length to be altered to bring pinion angle back into check. Racers also use adjustable arms to dial-in exactly the angle they want in the driveline when the car squats to accelerate off the starting line.
For handling, extremely lowered cars that also have a lateral location device installed may benefit from replacing the bushings at one end of the arms with a Heim joint since they will allow full articulation of the suspension without bushing bind.
Panhard Bar & Watt's Link
Both Panhard bars and Watt's links are lateral location devices used on solid-axle cars. While leaf springs and four-link rear suspensions work well for straight-line acceleration and mild street use, they lack positive location for the axle under high lateral loads experienced when cornering.
Panhard bars are simple devices that are literally nothing more than a straight rod with a bushing at either end that connects one side of the axlehousing to the opposite side of the chassis. The bar prevents sideways movement of the axle while still allowing articulation. Though they work well, Panhard bars inherently induce some suspension bind and slightly differing characteristics in left versus right curves since they cause the suspension to travel in an arc when moving up or down. To minimize the arc, Panhard bars are always designed to be as long as possible. This can result in an arc that can be as little as .25 inch over 3 or 4 inches of travel.
Watt's links have the same principle goal as a Panhard bar, but the locating bar is split by a central bellcrank between the chassis and axle mounts. The crank is mounted to either the rearend housing or an independent structure, and rotates during suspension articulation and allows the rear suspension to move without bind. This is the ideal lateral location device for solid-axle performance irrespective of spring choice, but packaging can be a challenge and impractical in some cases.
A car's handling, steering, braking, and ride quality are all dependent upon the bushings being tight and within spec. All suspension and steering systems are interconnected through bushings in some way, and slop at any point in the chain will result in decreased vehicle dynamics. As a matter of fact, just replacing control arm, sway bar, and body bushings (if your car has them) can make a dramatic improvement in how a car drives with no other new parts. If you're on a tight budget, start here.
As far as material, we're advocates of using increased durometer polyurethane bushings at most points since they offer less deflection while keeping NVH levels acceptably low. The less deflection in the bushings, the more precisely the components can do their jobs; that's why you'll see Heim joints and solid bushings in race cars. We wouldn't recommend the full solid bushing route on a street car since you'll feel every pebble, but you can safely opt for solid body bushings and near-solid Delrin bushings without creating excessive NVH.
Torque arms are another traction device used to control the motion and wrap of the rearend in a solid-axle car. Every vehicle has an imaginary point where the suspension links would converge if they were extended forward, known as the instant center. Dialing in the location is key to both straight-line acceleration and braking. Whereas the instant center can move around on three- or four-link rear suspension, torque arms have the advantage of a fixed instant center location and a very long swing arm length.
The length of the torque arm matters, because it directly affects the antisquat of a vehicle. A shorter torque arm increases the antisquat, which provides more bite during acceleration, but for a road race suspension it's possible to have too much antisquat, which can lead to rear brake hop under threshold braking. In general, a torque arm suspension allows a significant amount of antisquat to be designed into the suspension while still maintaining good roll steer characteristics.
Though generally paired with a coilover conversion, a torque arm actually works very well for leaf-spring suspensions as well. Under-links on leaf springs only help significantly with acceleration, while overriding links help with braking. A torque arm does both by not allowing anywhere near the amount of axlewrap possible from short three- or four-link designs. Additionally, since the arm is centrally mounted to the gear case of the rearend, there is very little pinion-angle change or driveshaft slip during suspension articulation.
Subframe Connectors & Chassis Braces
A unibody chassis is a great way to engineer a vehicle, and it also allows easier integration of impact-absorbing crumple zones for safety, however, in the early days of such technology, the elimination of a full frame without properly reinforced substructures created very flexible chassis. That loss of rigidity results in twist in the midzone, compromising performance and control on all fronts.
Luckily there is a brutally simple solution that works in all cases: subframe connectors. Subframe connectors tie the front and rear of the unibody together with a weld-in or bolt-in set of steel rails that effectively become frame members that reduce chassis twist (see “Weld-In Subframe Connectors," p.70). Subframe connectors can be purchased for many applications, or easily created with standard 2x3 rectangular stock and a little cutting. When practical it's recommended to keep the connectors as straight as possible and cut the floorpans to accommodate them. This makes them a true structural element of the chassis.
Engine compartment braces function on the same principle of eliminating deflection in an area with little reinforcement. Different platforms have differing needs for braces. For example, shock tower–equipped cars like '60s Fords benefit from tower-to-cowl braces that prevent both tower sag and movement from hard driving. Subframe-equipped GM products benefit from down bars that connect to the upper part of the firewall and extend forward to the end of the factory subframe.