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.