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."
Load Vs. Rate
Two of the most basic characteristics of a spring are load and rate. Spring rate is the amount of force required to compress a spring 1 inch, while spring load is the force required to compress a spring to a certain height. For example, a 350-lb/in spring requires 350 pounds of force to compress 1 inch. A spring that compresses 2 inches under 200 pounds of load will compress 4 inches under 400 pounds of load. As this scenario illustrates, load changes as a spring is compressed, but rate does not. This explains why aftermarket lowering springs, which reduce suspension travel, must be stiffer than their stock counterparts to prevent the suspension from bottoming out over bumps. Ultimately, spring rate affects a car's weight transfer characteristics while spring load determines how much weight a spring can support at a given height.
Just as the gross lobe lift of a camshaft is vastly different from net valve lift at the seat, spring rate and wheel rate are different animals entirely. To put it succinctly, wheel rate is the effective spring rate once you take into account the linkage ratios of the control arms. The only way to achieve a wheel rate that's in direct proportion to spring rate is by mounting the springs directly to a solid axle, which is possible in the rear suspension but not practical in the front. The formula for determining wheel rate is outlined below, where "a" is the distance from lower control arm's inner pivot point to the spring center, "b" is the distance from the lower control arm's inner pivot to the ball joint, "c" is the distance from the lower ball joint to the front instant center, and "d" is the distance from the tire's contact patch to the front instant center.
Wheel rate = Spring rate x (a b) x (c d)
Regardless of the numbers plugged into this equation, the bottom line is that the effective wheel rate will always be less than the spring rate. This relationship is important to keep in mind when upgrading the springs in your car, since adding 300 lb/in of spring rate may only increase the effective wheel rate 150 lb/in. Furthermore, comparing spring rates from different cars is almost useless. That 600-lb/in spring rate that works so well in your buddy's autocross Camaro might not be the best setup for your Chevelle.
With an independent suspension, the best way to minimize the disparity between wheel rate and spring rate is by mounting the spring as close to the lower ball joint as possible. Increasing the distance from the spring to the ball joint reduces wheel rate, which necessitates a stiffer spring rate to compensate. While aftermarket control arms can tweak the positioning of the springs, how much they can be moved is very limited.
"The packaging constraints of a front suspension are a disaster, and to a certain extent you're stuck with what the factory gave you. The spring, shock, sway bar, endlink, bumpstop, spindle, tie rod, and control arms are all crammed into a the same small space," Chris Alston explains. "A larger-diameter spring eats up precious real estate as well. By starting with a clean-sheet design, one of the big benefits of an aftermarket front clip assembly is that you have more flexibility of where to position each suspension component. However, when designing a suspension around the factory design and pick-up locations, you can't move things around much at all."
Fortunately, the outlook isn't nearly as bleak at a car's posterior. Since most muscle cars employ solid rear axles, the springs can be mounted directly to the rearend housing very easily. This applies to both coil and leaf-spring suspensions, and yields spring rates that are equal to the effective wheel rate. However, a solid rear axle doesn't always ensure a wheel-to-spring rate ratio of 1:1, as certain suspension designs position the springs on the control arms. The good news is that GM got things right by mounting the springs directly to the rearend housing in both its four- and three-link coil spring suspension designs, such as in the Chevelle and third/fourth-gen Camaros, respectively.
A sometimes overlooked factor in spring performance is its orientation in relation to the chassis. Springs absorb suspension loads most efficiently when mounted vertically. While mounting springs at a slight angle will have little consequence, anything greater than 25 degrees substantially reduces their effectiveness.
Calculating Spring Rate
The easiest way to measure spring stiffness is with a spring rate checker tool, but if you don't have one, the rate can be determined using simple math and a ruler. Calculating spring rate-whether for coils or leaves-is far from abstract, and directly related to a spring's physical proportions. In the equation below for coil springs, "G" represents the torsional modules for steel, which is 11,250,000. This figure is always the same for steel coil springs. Next, "d" is a spring's wire diameter in inches, "N" is the number of active coils, "D" is the mean coil diameter, and the number 8 is a constant for all coil springs.
Coil spring rate = Gd4/8ND
The formula may be simple, but what it illustrates about spring design is profound. One of the most obvious lessons is that cutting coils from a spring increases its rate due to the reduction in active coils. Furthermore, increasing the wire diameter by a few hundredths of an inch substantially increases the spring rate. Ultimately, spring rate is determined by the material it's made of and its dimensions.
A bit less apparent in examining the formula is the fact that a spring doesn't lose its rate over time. Although a spring may wear out and sag as it ages, losing its load capacity, its rate never changes. Consequently, if the 40-year-old springs in your muscle car are sagging, they don't need more spring rate. They need more spring load.
As with coil springs, calculating the rate of leaf springs is based on simple math. In the formula below, "W" represents the width of the leaves in inches, "N" is the number of leaves, "t" is the thickness of each leaf, "L" is the length of the spring, and the number 12 is a constant for all leaf springs.
Leaf spring rate = (WN 12) x (1000t L)
One of the most difficult aspects of setting up a suspension is selecting the ideal spring rate. Not only is rate selection contingent upon a car's intended application, any component that adds or removes weight changes the equation entirely. This is why GM used more than 100 different rear springs for first-gen Camaros and X-bodies.
"Whether you run a small- or big-block, a stick or automatic, cast wheels or billet wheels, A/C or no A/C, move the battery to the trunk or not-literally everything on your car affects what spring rate you should run," says Chris Alston. "Shocks determine how fast weight is transferred from side to side and front to back, but the springs determine how much weight is transferred. Therefore, if you have the wrong springs on your car you're never going to get the shock valving and wheel damping correct. In fact, 80 percent of the shock problems we see is because people have the wrong springs on their car."
With so many factors involved, there is no rule of thumb when it comes to rate selection. Our expert panel suggests calling their tech line for advice. "Over the years we've compiled an extensive database of customer cars and setups, so we can get your spring rate pretty close to ideal right off the bat," says Tom Brown of Heidt's. "Let's say you're building a big-block Camaro with aluminum heads, a six-speed trans, and 18-inch billet wheels. We can look up which of our past customers had a similar setup and recommend a spring accordingly. Part of the process is trial and error, so if the springs we send you are too soft or too stiff, as long as they're in good shape we'll be more than happy to exchange them for another set with a slightly different rate."
Despite all the factors involved with selecting the perfect spring rate, some generalizations can still be made when it comes to the intended use of a car. Naturally, a strictly street-driven car will benefit from a softer setup, but the needs of an autocross and road race application aren't necessarily one in the same. "An autocross course is very smooth and flat, so you can get away with running very stiff springs. On a road course that has bumps, uneven pavement, and elevation changes, you need a softer spring to help keep the tires in contact with the road and a larger sway bar to provide roll stiffness," explains Kyle Tucker.
Coils Vs. Leaves
Whether they take the form of a coil or a leaf, springs serve the same function of supporting a vehicle's weight. Coil springs are lighter and more compact than leaf springs, making them easier to package. On the other hand, leaf springs spread their load over a larger area of a vehicle's frame, which is beneficial in heavy-load applications, such as trucks. Nonetheless, neither design has an appreciable advantage over the other in a production car. In the case of muscle cars, however, leaf springs are the principal component in the ubiquitous Hotchkiss Drive rear suspension design, wherein leaf springs positioned on each side of the rearend housing serve as the control arms as well. Since the leaf spring is used almost exclusively in Hotchkiss Drive suspension systems, determining the virtues and limitations of the spring itself requires studying the actual suspension design.
If a leaf spring suspension seems rudimentary, it's because the basic design predates the automobile itself. A remnant of the days of the horse and carriage, the simplicity and low cost of the design are why it was so widely used in muscle cars. That said, there are several downsides of the venerable Hotchkiss Drive setup. It has no control arms to help locate the rearend under acceleration, cornering, and braking. "Adding extra leaves helps resist these forces, but the tradeoff is a stiffer spring rate and a harsher ride. You can add leaves to front half of the spring only to minimize the increase in spring rate while still reducing rearend windup, but it's far from an ideal solution," explains Kyle Tucker. "To accurately control the motions of the wheels and keep brake hop in check, you need coilovers, which usually requires upgrading to a street four-link suspension with a leaf-spring car. Achieving proper ride height often requires lowering blocks with leaf springs, but coilovers give you much greater adjustability."
It's important to note, however, that these limitations aren't due to the design of a leaf spring, but rather the means in which a leaf spring is used in a Hotchkiss Drive suspension. When used in a sophisticated twin A-arm suspension in fourth-, fifth-, and sixth-generation Corvettes, leaf springs are more than up to the task of providing world-class handling.
Granted, a leaf spring suspension certainly has its drawbacks in the handling department, but that certainly isn't the case for straight-line applications. The fastest small-tire leaf-spring car in the country-PSCA Wild Street champ Al Jimenez's '73 Camaro-hooks 1,500 hp on 275/60R15 M/T drag radials to the tune of 7.56 at 188 mph using nothing more than Calvert Racing monoleaf springs and traction bars, with an otherwise stock rear suspension. According to chassis builder Lang Paciulli of LP Racecars, these stunning results are a combination of suspension tuning and power management. "With modern electronics like the MSD Digital 7 ignition box, small-tire racers can now pull out timing coming out of the hole and slowly add it back in as a car progresses down the track. Throw a good set of springs, traction bars, and shocks, and you can plant insane levels of power," he explains.
By reinforcing the front half of a leaf spring, connecting the rearend housing to the front spring eye location, traction bars prevent axle windup and help plant the rearend into the ground. "In certain situations, there's very little, if any, advantage of an aftermarket four-link to a leaf-spring suspension. Four-links tend to be very finicky, and it's easy to get lost with them during the tuning process."
Leaf Spring Design
Although they look like nothing more than arched strips of steel, leaf springs incorporate several important design factors than ultimately affect their performance. The multileaf design is most common in production muscle cars and features several leaves of diminishing lengths stacked together and held by shackles. Due to the use of multiple leaf elements, load, rate, and ride height can be more finely tuned.
A monoleaf, on the other hand, consists of a single leaf whose thickness is uniformly tapered from the center outward. This arrangement reduces mass while matching or exceeding a multileaf in strength. However, the method of manufacturing a monoleaf is extremely involved, which severely limits the spring rates and ride heights in which they're offered.
Lastly, a parabolic spring is a spin-off of a monoleaf that utilizes two or more tapered full-length leaves, but is uncommon in performance applications due to cost and manufacturing complexities.
How the ends of the leaves are shaped affects performance as well. A square end is the simplest design, but suffers from excessive friction between leaves as they flex. Some interleaf friction is desirable for high-load applications such as trucks, but result in a poor ride in lighter weight cars. Cutting off the corners of the ends in the shape of diamond points reduces friction by reducing surface area, but this design still produces a rough ride and is best reserved for trucks. For performance cars, rolled ends are ideal. With this setup, the leaves gradually decrease in thickness near their ends, which reduces interleaf friction and increases flexibility for a smooth ride. Placing Delrin inserts between the leaves is common in performance leaf springs to further reduce friction.