How To Choose Springs For Your Next Hot Rod Project
When it comes down to it, your vehicle’s suspension has a pretty tough job. It has to cushion the occupants from road shock, as well as maintain the optimum balance of handling and directional control. And among the components that make up the suspension system, the springs have a pretty tough job, having to maintain ride height while absorbing energy transmitted from the wheels over varying types of surfaces.
During the course of automobile history, there are three main types of springs that have been employed for suspension use (we won’t include rubber or hydro pneumatic systems in this article as their relative complexity deserves a separate topic), each with its own inherent strengths and weaknesses. And when it comes to building a hot rod, choosing a specific spring will have a major effect on how the vehicle behaves out on the open road.
Leaf springs were fitted to horse-drawn carriages, and when self-propelled vehicles came along, they found their way underneath most cars. They were in widespread use through the 1940s but were slowly phased out in favor of coil springs on most automobiles. Today, their use is generally restricted to pickup trucks and larger commercial vehicles. Leaf springs consist of a long piece of flexible material (the main leaf - usually steel) that is rolled at each end to form an eye where it connects to the vehicle structure via hangers or shackles. This main spring usually has a number of additional leaves mounted to it above and below (usually by clamps). In order to maintain constant resistance (energy) across the length of the spring, these additional leaves are of varying lengths (the longest closer to the main leaf, the shortest farthest away). A pair of rebound clips mounted over the leaves toward each end is designed to transmit stress from the upper to the lower leaves after the suspension is compressed and begins to return to its normal position. A rubber stop is often mounted above the axle to prevent frame damage when the spring is fully compressed.
Leaf springs became popular because they are cheap to manufacture, strong and easily adjustable to suit specific loads and requirements. By altering the length, width and thickness of the leaf, along with the number of leaves, it is easily possible to change the amount of resistance, which alters the ride and handling qualities of the vehicle to which it is attached. Leaf springs are also used because the spring acts as an additional link for the suspension, much like a control arm.
For the purpose of hot rods, many cars built in the 1920s through the early 1940s (particularly Fords) used a single transverse leaf spring on each axle, shaped in the form of a yoke, which formed the upper link on the solid beam dropped front suspension, and live rear axle that linked the left and right wheels. In these cars, the transverse springs were also designed to act as a damper, dissipating energy to prevent excessive bouncing. The design was simple and strong, allowing for sizeable wheel travel. This made it ideal for coping with rough, uneven roads, common in the first half of the 20th century.
US-market Ford vehicles used transverse springs all the way until 1948, while several other manufacturers continued to use them afterwards, notably Chevrolet and European automakers Fiat and Triumph. However, their ideas were slightly different than Ford. In this case, a single, transverse-mounted spring represented a cost- and weight-effective method of utilizing a fully independent rear suspension. This is because the spring, with a solid center mount, actually operates like two independent “quarter elliptic” springs, aiding wheel control and supporting the lower links. Many early leaf springs required periodic maintenance by spraying penetrating oil between the leaves. In more recent years, the advent of springs with fewer leaves, low-friction inter-leaf plates and synthetic rubber inserts have rendered this procedure obsolete. Although originally manufactured from high carbon steel, today, most leaf springs are made from alloy steels like silico manganese and, increasingly, composite or plastic materials. Since 1980, Corvettes and several other cars have used composite transverse leaf springs, which have demonstrated remarkable longevity with few reports of fatigue.
Longitudinally-mounted leaf springs are far more common, especially on the rear suspension of many cars built from the 1940s through the 1980s. These springs, mounted parallel to the vehicle’s frame or wheel on each side, have the advantage of spreading the weight of a carried load over a greater area of the vehicle, one reason why they are still widely used in pickups and heavy duty commercial vehicles.
Despite their inherent simplicity and cost savings, leaf springs also have their drawbacks. Longitudinal springs take up a lot of space, and because they’re mounted directly to the frame, behind the tires, they don’t offer a lot of clearance when it comes to installing bigger rims and rubber. If you’re planning on building a car with a narrowed rear end and wheel tubs to accommodate wide tires, you’ll likely have to look for alternatives. In addition, although the spring rates are easily adjustable, leafs also deliver a fairly harsh ride by modern standards, especially if they aren’t supporting a load (think about the behavior of a pick up truck over rough roads with an empty bed).
Helical or coil springs are torsion springs wrapped around a central axis. In automotive suspension applications, they are designed to resist compression in order to provide a barrier between the road surface and the vehicle’s structure. Coil springs began to replace leaf springs, particularly at the front of most passenger cars, in the 1950s and have largely supplanted them at the rear as well. Coil springs are more efficient at storing energy than leaf springs. They are also much more compact in design, making them a popular choice in modern automotive applications, especially where space is limited, such as in small, front-wheel drive cars, where the spring actually forms the upper arm and is wrapped around the top of the damper or shock absorber (MacPherson strut).
Another advantage is that coil springs, because of their ability to store more energy in a smaller space, are able to deliver improved ride qualities over leaf springs in conjunction with shock absorbers. They’re also cheap to manufacture, take less time to install and, combined with the right combination of suspension components, are generally much more suitable for high performance use. Most coil springs used in cars come in two forms – linear or progressive rate.
Linear versus Progressive
Linear springs have the coils evenly spaced along the length of the spring. This allows a great deal of energy to be stored in a small space, one reason why linear rate springs tend to be fairly short. Linear rate springs are most commonly used on high performance cars and specific racing applications, where maximum resistance is crucial to delivering minimal body roll, enhanced grip and cornering ability, as well as less chance of “bottoming out,” where the suspension reaches the limit of its compression (common on soft or worn out springs).
Progressive or variable rate springs tend to be longer and feature some coils that are closer together than others. Essentially, the wider the coils are spaced, the higher the energy and the greater the resistance. By having some closer together, it achieves a good compromise between greater resistance (for handling) and less resistance (for comfort). The closer coils are usually positioned at the top and bottom of the spring, since this is the part that is most subjected to compression under normal driving conditions, allowing it to absorb energy before the wider spaced coils in the middle. Progressive rate springs are most commonly found on regular street-driven cars, where the objective is to strike a balance between adequate handling and ride comfort.
As with leaf springs, coils also have their drawbacks. They are more susceptible to fatigue since they tend to absorb more energy. And because of that energy, great care needs to be taken when removing and replacing them, using spring compressors to control the resistance. Otherwise, a tensioned spring could bounce off the vehicle and cause tremendous damage to the surrounding area, possibly even killing you. Because they focus energy at a single point, coils aren’t as suitable for use in heavy duty applications, since they aren’t able to distribute the weight of a large load across a big area like leaf springs. On independent suspension, high performance or heavier duty applications, coil springs require additional suspension links to provide improved tire contact, better handling and increased durability, which usually adds cost, complexity and weight to the vehicle.
The third type of spring is the Torsion bar, which works in much the same way as a coil spring. This consists of a long tubular or rectangular bar that is attached to a control arm or other form of lever linked to the wheel at one end and attached to the chassis or unibody at the other. At each end, the bar is larger in diameter than the center section, with special serrations or ruts used to link it with the moveable wishbone or lever. At the chassis end, the torsion bar can be adjusted to raise, lower or level the vehicle’s suspension. Like a coil spring, length and diameter are critical to resistance. The thicker and longer the bar, the greater its resistance. In North America, the most common use of torsion bars can be found on Chrysler cars from the late 1950s through the 1980s and on light trucks like GM’s S-10/Sonoma and late-model full-size pickups.
Torsion bars are a good option because they’re extremely simple and strong, and suspension adjustment is easy, requiring little more than a simple wrench to either remove and replace them or make alterations to ride height and handling. They’re also extremely compact, taking up very little space under the vehicles.
However, the very simplicity of torsion bars means that they can’t be as finely tuned as a suspension system using coil springs. Because it isn’t really possible to use progressive rate torsion bars (strength would be compromised), the result is a generally harsher ride and, in some cases, more limited handling potential than coil spring suspension designs.
What to Choose
It isn’t always easy to figure out what type of suspension system to use on your next hot rod project, and that includes the type of springs. It largely boils down to your plans for the vehicle. Will it be driven regularly? Will it be built in a traditional or high tech style? Will it be raced? Will it be built primarily for show duty?
As for cost, the overall suspension design you go with will often tend to influence the choice of spring. Today, there are numerous companies that can come to your aide when it comes to choosing the right springs for your car.
Classic Hot Rods
In this case, we’re referring to cars built before 1948, or those with separate fenders. For vehicles like these, there are a number of different options. For a traditional look, especially since the front suspension will be exposed, many enthusiasts adopt transverse leaf springs in conjunction with dropped front axles. Companies like Eaton Detroit Spring can supply you with affordable leaf springs, tailored to your specific requirements. Many owners fit a pair of small shocks to help control spring bounce and parallel links to help deliver as smooth a ride as possible, while maintaining the original configuration so beloved of ol’ Henry Ford. At the other extreme, some early-style rods built primarily as high performance street machines have used late-model Corvette suspension (C4-C5), which is fully independent and uses transverse composite leaf springs. However, engineering such a setup in something like a 1932-34 Ford requires a considerable amount of time and expertise, though the results are often sublime levels of handling and grip.
Torsion bar suspension systems are less common, but if you’re looking to stand apart from the herd, they can provide a good alternative to buggy springs with their durability and ease of adjustment. Coil springs at the front seem to be a love-it or hate-it deal among many rodders. On early cars, they tend to look out of place unless the vehicle in question is designed for ultra high performance or is built using a modern theme, where the handling advantages offered by coils can be fully exploited. However, using coils and independent front suspension adds cost, complexity and weight to the car, the antithesis of rodding in many respects.
At the back, some rodders will use transverse leaf springs on their 1930s and 1940s cars if seeking authenticity. However, ride quality is harsh, and tire and wheel choice is severely limited because the spring spans the entire width of the axle, restricting rim offset. As a result, many rodders use multi-link suspension with coil springs, or even coilover shocks, to improve both ride quality and options when it comes to wheel and tire fitment. Coilovers, although often expensive, are a great choice for cars with narrowed rear ends, since their compact nature allows them to be mounted in-board, linking the third member and body/chassis, without adversely affecting ride and handling. Coils promote not only an improved ride quality but also better suspension compression, improving tire grip under acceleration and cornering, one reason why this type of rear end setup became so popular on hot rods during the 1950s and remains so today. Torsion bars are seldom used at the rear on hot rods, though sometimes they can be added to improve rear axle location, especially to aid handling or reduce axle tramp in conjunction with coil or leaf springs.
When it comes to full-bodied cars, essential those built from 1949 onwards, aesthetics in suspension design are generally considered less important, so in some respects there are more options. Most Detroit cars from the 1950s through the 1970s feature coil sprung independent front suspension and live rear axles, supported by either longitudinal leaf springs or coils (as on many GM cars). As mentioned, many Chrysler products from the 1950s onward used front torsion bar springs along with leaf springs and a solid axle at the rear, though some cars, like 1955-56 Packards, used torsion springs both front and back. The choice of springs will largely depend on the kind of modifications performed on the car and whether it will be regularly driven or not.
Given the profile of many late 1940s, 1950s and 1960s cars, a lot of them can benefit from a lowered stance. If the vehicle is equipped with coil springs, this can be achieved by simply installing shorter coils, commonly known as lowering springs, though you might have to search for your particular application. Prolific suppliers Eibach and H&R Springs are a good place to start. For cars equipped with torsion bars and leaf springs, lowering can be effectively achieved by turning the bars to a lower setting, which can drop the ride height. If your car has arched leaf springs, removing the shackles and replacing the rear leaf units with reduced arc springs is a cost-effective way of lowering it. Another option is to use an entire front clip from a later model car. Mustang II front suspension comes to mind, though late-model Corvette and even custom aftermarket IFS systems are quite popular, particularly on cars like early 1950s Chevys.
Although it is really beyond the scope of this article, another option is using air suspension. With air suspension, the coils are replaced by rubber bags, in which air pressure, controlled by an onboard compressor, maintains the ride height or allows it to be adjusted for show or go. Even on cars with leaf sprung rears, air bags can often be used with custom fabricated links to maintain ride height and the adoption of a Watt’s Linkage. Many dramatic customs use airbags to give that slammed in the weeds stance when parked, without causing damage to the underside of the vehicle when it is driven. Although complete air suspension kits often start at more than $1100, it can be a good option to consider if you want both style and versatility. Popular vendors include the Airlift Company, Arnott Air Suspension, KMW Performance and RideTech.
Another (albeit expensive) option is the recently developed Bose suspension system, which uses a linear electromagnetic motor at the wheel in place of a conventional spring/shock. Amplifiers supply energy to the motors, so energy is regenerated each time the suspension compresses. The advantages of this system are said to be much better wheel control and smoother ride without sacrificing handling, since the motors allow the suspension to compress at much greater speed.
Sprung and Unsprung weight
When it comes to suspension systems, there is often a lot of talk concerning sprung and unsprung weight. Sprung weight or mass refers to the actual weight of the vehicle supported on the springs, while unsprung weight refers to the weight load between the road surface, tires, suspension and springs.
When engineers design a vehicle, the amount of sprung weight relative to unsprung weight has an effect on how the vehicle rides and handles. A greater ratio of sprung weight to unsprung weight is designed when a softer ride quality is the number one goal, and so springs with lower resistance are used. However, that softer ride often results in a car that wallows, pitches and often dives hard under braking. Examples of vehicles with a relatively high sprung weight in relation to unsprung weight include most large Detroit cars of the 1950s through the mid 1980s.
If the relation of sprung weight to unsprung weight is smaller, the result is a vehicle that delivers a firmer ride but with improved cornering and reduced body roll, since springs with a higher resistance can be used. Many performance cars feature a higher unsprung weight relative to sprung weight. In order to minimize harsh vibrations and reduce the risk of wheel hop, engineers often use soft compound tires and lightweight wheels, along with improved suspension location.
Eaton Detroit Spring
Legendary Motorcar Company Ltd
Leaf springs have been around for centuries and were standard fare on most cars built through the mid 20th century. Fords used transverse leaf springs for many years, because they were strong and simple. Pictured is the front suspension and spring setup on a 1940 Ford Deluxe convertible.
At the back, it was a similar situation. Ford continued using transverse leaf springs all the way through 1948. Most early hot rods utilized the same setup, as shown on this traditional style Model A.
The next evolution was longitudinally mounted leaf springs, as shown on a 1969 Chevy Camaro. In this case, the springs provide additional location and act as lower control arms.
In the post war environment, transverse leaf springs increasingly became popular with a number of manufacturers as part of an independent suspension setup, as they were able to provide superior wheel control. Most notable was the Chevy Corvette, which received IRS for the 1963 model year.
In fact, Corvettes still use leaf spring technology today, employing composite monoleaf units, both front and back, as shown on this C6. These springs are incredibly durable and are becoming increasingly popular on high tech custom and rod projects that utilize late-model Corvette suspension.
Leaf springs feature “eyes” at each end where they attach to a shackle or hanger. Shown is a single eye.
By contrast, here is a double eye. Note the bushing housed within the eye and also note the rebound clip that encases the spring. This is designed to return the spring to its normal position once the suspension is compressed.
Here is another view of the spring and suspension setup of the Model A hot rod shown previously. One of the drawbacks of transverse leaf springs is that they severely restrict tire size and wheel choice, a problem rodders discovered when greater traction and a larger contact patch was needed as the result of fitting more powerful V-8s into their cars.
Another issue concerning leaf springs is wheel hop, which became prevalent on many 1960s hot rods and muscle cars. To combat the problem, it was common to fit traction bars, which provided additional location for the rear suspension as well as improving tire contact with the pavement.
Today, leaf springs are still popular for traditional style custom cars and for restoration projects. Enthusiasts can have brand new springs made to specific requirements. One of the most notable vendors is Eaton Detroit Spring, based in the heart of the Motor City.
As automobile suspension design became more sophisticated, other types of springs were employed. Torsion bars, essentially a circular or rectangular bar attached to a control arm at one end and the vehicle frame or body structure at the other, were adopted. Simple and rugged, they were a mainstay of Chrysler passenger cars from the late 1950s through the early 1980s. Torsion bars deliver a smooth ride, but the design places restrictions on handling capability.
This 1970 Plymouth ’Cuda originally came from the factory with torsion springs up front and longitudinal leaves in the rear.
In the quest for maximum handling (considering it makes over 1000 horsepower and can reach 200 mph), the ’Cuda now boasts a state of the art independent suspension system with coilover shock springs at the front.
In back, the once-leaf-sprung live axle has given way to a fully independent setup with coilover shocks. These springs allow minute adjustments in both spring rate and damping, in order to deliver maximum handling performance depending on the road/track conditions.
Most modern, mainstream passenger vehicles utilize coil springs. Coils are the preferred choice because they are compact, can store a great deal of energy and can easily be tailored to specific requirements. Shown is a Ford Racing progressive rate spring, so named because some of the coils are closer together than others, altering the resistance as the suspension is compressed.
One of the most popular manufacturers of both OE and aftermarket coil springs is Germany-based Eibach. This company’s springs have been used in just about every kind of motorsport and performance application imaginable.
Eibach’s line of Pro Kit performance springs are hugely popular with auto enthusiasts today. These springs drop ride height around one inch from stock on most vehicles and provide a good blend of performance handling and ride comfort.
By the 1960s, most American passenger cars featured independent front suspension with unequal upper and lower control arms and coil springs. This configuration offered good tire contact with the road, and variations are still in widespread use on many higher-end performance and luxury vehicles.
For many hot rodders, coil sprung front ends are a love or hate deal. Although traditionalists tend to shun the concept, a well-designed independent front suspension employing coils or coilover shocks can be very effective, especially on a high-powered street rod like this 1934 Ford roadster.
A close-up of the rear suspension on the Ford roadster reveals coilover shocks, too. Like everything else, the springs are designed to form part of an overall system, so when you’re designing and building your latest hot rod project, you need to take into consideration the overall objectives for the car. In this case, the goal was to build an ultra-high performance rod that was also safe and easy to drive on the street. Thus, modern coilovers and multilink axle location were prerequisites.
Published Dec 7th, 2015