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.