Spring selection is an important aspect to the setup of any performance car. In a road racing scenario, the choice of springs is an integral decision to the dynamic science of car setup.
Coil springs have their advantages over other types and designs for several reasons; weight, selection, longevity, and adjustability are all top selling points for coil springs. While leaf springs, torsion bars, air bags, and other such suspension components have their places in racing, coil springs are the knee-jerk reflex for the road racer. In order to get the nest holistic perspective on spring theory and selection we consulted with Eibach, Swift Springs, and Vogtland/DG Spec Racing. With their expertise we will guide you through the fundamentals of coil springs.
Selecting the right coil springs for your application is no small decision.
It is important to understand the problem from all angles before starting the process. An understanding of spring construction, design, and theory, is as important as having an understanding of driving styles and experience.
While the vast majority of handling enthusiasts make a buying decision based on Internet searches, or bench racing hearsay, the best compromise of handling and compliance will be through an informed decision, and a custom-tailored package.
Fundamentally, the suspension supports the mass of your car; dynamically as G-loads are applied from all possible axis under movement, and statically at a stop. Suspension systems are diverse in their design and geometry, making the selection of so-called ideal “bolt-on parts” a misconception. Even more wide-reaching are the skill sets of drivers seeking to improve the ride, and handling habits of their vehicle.
The goal of this article is not to give you the formula for the perfect coil spring package to turn your ’60s musclecar into a Formula 1 car in the corners, but to show how far reaching a discussion suspension setup can be. According to Dan Gardner of DG Spec Racing on the topic of spring setup, “There’s only about five right ways and about a million and one wrong ways” (to set up a car).
There’s only about five right ways and about a million and one wrong ways. – Dan Gardner, DG Spec Racing
Spring Construction, Design, And Technology
For the purpose of this article the springs we refer to will be coiled, ferrous construction, and designed specifically for the performance aftermarket. The material springs have been manufactured from has changed steadily over the decades as material technology has improved. As iron gave way to carbon-alloyed steel in the late 1800s, springs became stronger, more resilient, and more consistent. Fast-forward to today and we see metallurgy science has advanced light years in its ability to create boutique alloys with specific properties and capabilities.
Swift Springs controls their own metallurgy, and can therefore control the properties of materials used to make their springs.
Modern ferrous spring construction ranges from plain carbon steels, such as 1070, to chromoly alloys like 4130, 4140, and 4340, to branded alloys like 300M, and proprietary blends from mills controlled by spring manufacturers. Choices of alloying elements such as carbon, molybdenum, chromium, and silicon, add to the versatility of springs and lend advantages from corrosion resistance to vibration dampening. When asked about specific alloy choices, David Cardey of Eibach answered “chrome-silicon,” as an alloy that is available in different tensile strengths. Utilizing the maximum tensile strength Eibach is able to use thinner material, meaning lighter springs and more travel.
Vogtland employs a proprietary alloy, a vanadium and chrome-silicon steel they call VVS. While they would not reveal their formula for this particular alloy they described some of the properties that make this material a superior metal for winding springs.
According to Leila Vizzari of Vogtland, this material was first introduced to the market by the engineering department of the German spring company. She explained that straight across it is 35 percent lighter than a competitor spring because the properties of the material allow the wind to include a lower coil count. With the 1-2 less coils than an equivalent competitor spring, travel is increased and weight it reduced.
“This same material used in our race springs is used in our aftermarket lowering springs — so you’re going to notice a difference there too” said Vizzari. She continued to explain that Vogtland test every spring to make sure they are with two percent of tolerance for rate. Manufactured in Germany on precision equipment, these cold-wound springs are a highly quality-control oriented option for both race, and track cars.
By engineering springs that reportedly don’t lose rate or height, include fewer coil counts, weigh less, and feature longer travel before bind, Vogtland provides an opportunity for racers to reduce unsprung weight, and ultimately improve the responsiveness of their suspension system.
The designers of coils springs hold control over more variables than the casual observer might consider. While the obvious, like coil thickness and material, relate directly to the ultimate spring rating, many other factors are at play. The diameter of the coil, coil count (loops per foot), angle of wire as it spirals, the tightness or looseness of the coil at different points along the spring, heat-treating processes, and other manufacturing considerations all affect spring behavior on your vehicle.
Your selection of complementary components will affect your spring choices as will tires. For instance, Leonard Wang of Swift Springs expanded on this topic by asking “What type of tires [do you have], what are you trying to accomplish? Stickier tires need higher rate springs to counteract roll.” Leonard summed up the experience of suspension modification saying “When you purchase a set of coilovers, that’s when the tuning begins.” This is a truism that should be understood by all racers.
When you purchase a set of coilovers, that’s when the tuning begins. -Leonard Wang, Swift Springs
Vehicle and Suspension Dynamics
Understanding the technical terminology associated with performance suspension is an important book-learning step to take before heading out into the world with your wallet open. By understanding the correct terms for the conditions experienced by your car you will be able to articulate your needs to shops, manufacturers, and anyone else in a position to help tune your suspension. These terms are applicable to all vehicles regardless of suspension design or motorsport.
Heavy, full-fendered cars will have higher sprung weight.
Sprung Weight is the load on your suspension with the wheels on the ground. This is the load supported by your suspension while on the road. It is an important consideration when looking at spring rates.
Unsprung Weight is the load supported by your suspension at full droop. The mass of control arms, spindles, hubs, wheels, tires, and springs are included in the unsprung-weight of a car. Unsprung-weight is very important when considering shock-tuning and valving.
Open-wheel race cars often have inboard brakes, and light suspension components leading to very low unsprung weight.
Because more mass equals more inertia, and Newton’s first law applies (objects at rest tend to stay at rest, and objects in motion tend to stay in motion), a car with excessive unsprung weight will suffer handling consequences that a car with light components will not. Because the suspension will have to overcome the reluctance to move, heavier components contribute to a lack of responsiveness.
Corner Weight is the tire-to-tire load distribution of mass. A front-engined car may tend to have a center of gravity biased towards the front, a rear engine car towards the back, and a mid-engined car my be close to a perfect 50/50 distribution. Corner weight is a function of distribution; passengers, or lack thereof, will change the side-to-side corner weight.
A rear-engined car may have a drastically different corner-weight distribution than a front or mid-engined car.
Corner weight also is affected in dynamic situations; a vehicle under acceleration (a change in speed and/ordirection) will experience synthetically-altered corner weights as a result of the temporary G-loading. With this consideration vehicles that only travel in a straight line, or only turn one direction, will have different springs compared to vehicles that require the versatility of cornering.
Motion Ratio (Mechanical Advantage) refers to location of the spring in regards to the inner pivot of the the control arm, trailing arm, or axle to which it is mounted. The further away from the pivot a spring is located, the less mechanical advantage the car has to overcome the spring rate, and move the wheel up.
Photos Courtesy of Eibach Springs.
Conversely a spring that is mounted extremely close to the inner pivot and far from the load input, out at the wheel, will be more easily compressed and have an artificially-reduced spring rate. Knowing what kind of ration or mechanical advantage is built in to your suspension geometry is important when deciding on an appropriate spring.
Coil Spring Angle is a similar consideration to motion ratio in that it deals with the efficiency of the spring under compression and rebound. A spring compressed along the axis of its length will be the most predictable in terms of performance. Springs mounted at angles lose efficiency, and behaviors are harder to predict. It is important to note spring angle is not a function of the spring angle in relation to the ground, but is a function of the direction of travel of the suspension.
McPherson struts can rarely avoid some shock angle.
In the case of most McPherson strut designs the spring angle is canted slightly inboard for packaging but the wheel travel is vertical. Some suspension systems travel in arcs and have forgiving characteristics. Trailing arm and leaf spring designs will often angle springs forward at the top because the direction of travel is not always straight up and down.
Travel/Stroke is simply the difference between loaded and unloaded lengths of the spring. The amount of distance the spring will be compressed throughout the range of travel is a function of the motion ratio and total wheel travel. It is important to consider wheel travel needs to avoid coil bind.
A “Spring-Dyno” can be used to measure various mechanical properties of a spring.
Spring Rate is the rate at which a spring compresses under load. The compressed length of a spring may lower one inch for every 200 pounds added. This specification is the most common rating for springs and is used to determine the overall behavior of a spring in use.
Spring Load is easily confused with spring rate. Spring load is the force a spring must experience to deflect.
Preload is the compression added to a spring above and beyond the naturally occurring compression resulting from the mass of the vehicle. Preloading a spring can be used to alter ride height and set bump/droop travel ratios.
Linear Spring Rates are spring rates that do not change throughout the travel of the spring. As a linear spring compresses the force required to continue compressing the spring remains constant. This predictability makes linear springs the preferred design for road racing applications.
The rate of progressive springs changes throughout the travel of the spring.
Progressive Spring Rates change as the stroke or travel is compressed. Generally speaking a progressive rate spring will increase in spring rate as more force is applied. This stiffening of the suspension, higher up in the travel stroke, is generally used to prevent bottoming out but is more difficult to tune with a complimentary damper.
Cardey, of Eibach, explained that the way they can wind progressive or digressive rates into a coil involves spacing, alternating steep, and flat pitch of the winds.
Individual Driver Skill And Goals
It’s time to be brutally honest with yourself. How much track experience do you really have? What is your actual realistic ability? Determining your skill as a driver right now will help better select appropriate suspension modifications to complement your ability and ultimately grow you as a driver. This is not to say the beginner should not change coil springs for a performance advantage, but the changes made by a professional will greatly differ from those experienced by the once-a-month weekend warrior.
The suspension upgrades of a beginning to intermediate driver are often a compulsive response to impatience, an and urge to go faster without first understanding the variables of spring choice. Over-lowering and over-springing a car for that hardcore track aesthetic can be a destructive and misguided path.
Gardner explained how spring rates for front wheel drive cars will favor a stiffer rear to encourage some over-steer.
According to Gardner, in the case of most enthusiasts starting with a production vehicle they can modify, motion ratios close to 1:1 are common, and spring rates start between 100 to 200 pounds. It is safe to double or triple the spring rate for track use but more extreme stiffening should be postponed until a better understanding is achieved.
Over-lowering can cause a compromised suspension geometry where the benefits of a lowered CG are outweighed by the consequence of a lower roll center. According to Gardner, “Roll center drops versus how far CG comes down creates a roll couple you don’t want.”
Testing And Tuning
I’d set up the suspension to be softer, more roll and pitching. I’d want that wheel and tire to dig in a little more, but that becomes unstable at high speeds. -Leonard Wang, Swift Springs
Testing and tuning is vital to properly selecting a spring package to suit the needs and goals of your application. There is no substitute for time on the track. Building consistent lap times is a must before diving in and changing suspension. Experience and familiarity with your platform is a necessity to identify true strengths and weaknesses; articulated to us by Gardner “You need to be aware of when you’re driving around a problem and just managing it.”
Setting up a vehicle is a purpose-driven task. Is this your daily driver, or a dedicated track car? Do you plan to autocross, drive road courses, or ovals? Be realistic in your needs and you will be happier with the end result of your purchase. Let’s say you plan to autocross your car; Wang of Swift Springs has set up suspension for numerous applications and said, “I’d set up the suspension to be softer, more roll and pitching. I’d want that wheel and tire to dig in a little more, but that becomes unstable at high speeds.”
You need to be aware of when you’re driving around a problem and just managing it. -Dan Gardner, DG Spec Racing
Gardner made it clear that the ability to identify the difference between a perceived problem with the car, and a problem with a driver technique, is a fundamental in test-tune standard operating procedures.
Testing and tuning can take many forms, and need not simply rely upon the seat of the pants sensation of the driver. The poor man’s telemetry can be incredibly effective, a camera allowing crews, mechanics, and drivers to review footage, identify problems and take corrective actions. A good tire tire pressure gauge and pyrometer will help collect tire data.
Common problems Gardner identified for us are concepts he calls porpoising, carving, and washing out. The first two phenomena are harmonic rocking tip-to-tail and side-to-side, respectively. If you have ever seen a big old car coming through a corner like a yacht in the chop of a windy day, you have seen this effect.
Washing out is a collapse or bounce in the suspension that robs the driver of directional control. The top tips from Gardner were to “find the outside edges by going too soft or too stiff, [and build to] give yourself confidence.” Building suspension that is “compliant” is a safeguard to building the driving confidence of a beginner driver, and allowing them to grow without the threat of a crash every mistake they make. This ideology is a foundation for DG Spec’s testing and tuning tips.
Throw the math out the window, and go test and tune. -Leonard Wang, Swift Springs
“Throw the math out the window, and go test and tune,” said Leonard, further emphasizing the importance of real world experience over calculations that fail to consider a variety of pertinent factors that contribute to handling.