If drum brakes and flat-tappet camshafts are old-school technology, then springs are quite literally ancient. In some respects the wooden bows and catapults used by medieval knights to bludgeon their adversaries into submission were very much the precursors to the modern leaf spring. In the days of the horse and carriage, leather straps were commonly used as a springing medium until they were replaced by steel leaves during the Industrial Revolution. With centuries of development under their belt, the springs bolted to modern vehicle suspension systems are simple-looking yet highly refined components.
To get a grasp on the science involved in designing springs, how they work, what they do, and their role in overall suspension performance, we consulted with Kyle Tucker of Detroit Speed and Engineering, Tom Brown of Heidt's, Chris Alston of Chassisworks, and Lang Paciulli of LP Racecars. Since leaves and coils were the springs most commonly used in Chevy muscle cars, we'll leave it to the Mopar guys to yack about torsion bars. Ultimately, selecting the correct springs for a given application is very complicated, but the knowledge gained in this story will steer you down the right path.
A Spring's Job
When attempting to understand the role of springs in regard to cornering performance, the first step is disregarding preconceived biases. That's because much of how a spring functions in the scope of overall suspension dynamics is counterintuitive. Although proper spring selection is critical to maximizing handling potential, a spring's fundamental purpose isn't related to cornering at all. "The job of the springs is to merely support the weight of the car, and allow the wheels to follow road irregularities. Once the correct spring rate has been established, the bulk of suspension tuning is done with the shocks and sway bars, not the springs," explains Chassisworks' Chris Alston.
Since most aftermarket performance springs utilize a stiffer rate than stock, the prevailing sentiment amongst many enthusiasts is that stiffer is better. Surprisingly, however, our panel of experts unanimously agrees that the opposite is true: A car's tires are its only source of generating cornering force, and a tire that momentarily lifts off of the pavement due to springs that are too stiff will generate no cornering force at all. Likewise, a stiffer spring rate can also limit suspension travel in both compression and rebound, which further limits the tires' ability to follow the contours of the road. On the other hand, softer springs allow the tires to follow the dips and bumps of the road more of the time and, in turn, have the potential to produce more grip.
While it's true that a stiffer spring increases a suspension's roll stiffness, hence its ability to resist body roll, this approach degrades ride quality. As we touched upon in last month's story "Turning the Corner," cutting a car's body roll in half requires doubling its spring rate, which almost guarantees an unbearably harsh ride. The preferred setup amongst the suspension experts we consulted is to achieve the desired roll stiffness with bigger sway bars, and match them with softer springs to preserve ride quality and keep the tires in contact with the road.
Still, some tuners prefer the stiff springs/small bars method, especially in autocross applications, and sometimes the ideal setup is a combination of both. "For the front suspension, we like to run soft springs and big sway bars," says Kyle Tucker of DSE. "This takes the burden of roll resistance away from the springs and minimizes body roll, suspension movement, and camber change. For the rear suspension we often use stiffer springs and a smaller sway bar, or no sway bar at all. This helps to decouple the rearend from side to side, but the stiffer rate requires a very good shock package."