You want to update the suspension on your hot rod, and you want to do so by adding coilovers. It’s a great idea, and choosing coilovers offers a myriad of options regarding adjustability. But, one of the first questions many people ask, “is what spring should I get”? That is a valid and sometimes tricky question, as there are so many options. So, to help you make an informed decision and get the correct spring with the correct spring rate, we spoke with Steve Smith, lead technical writer at QA1.
When your car was first designed by engineers and the brain-trusts at GM, they were able to easily calculate the perfect spring rate that offered “comfort,” by feeding data into confusing math formulas. Years ago, when we wanted to increase handling performance of our classic rides, we had to rely on test-fitting various springs and hoping for the best. Many times, the result was enthusiasts settled for an ill-selected spring that compromises ride quality, handling, or both. Thankfully, those days are gone.
Clearing The Confusion
“There are a couple considerations when choosing the right spring for a given application,” Steve stated. “Most coilover-shock lengths will determine the length of spring to be used, but within that spring length, there are other considerations like the spring’s design, features, and spring rate. A captured-spring coilover with an upper spring cap will use a spring with the same inside diameter on top and bottom. A GM-style front coilover will use a tapered spring, where the upper part of the spring sits in the factory spring pocket within the frame.”
The term spring-rate, refers to the amount of weight that is needed to compress any spring 1-inch. If the rate of the spring is linear, that rate is not affected by the load placed on the spring. As an example, if you have a 200 lb-inch spring, it will compress 1-inch when a 200-pound load is placed on it. If another 200 pounds is applied, the spring will compress another inch. At this point the load on the spring is 400 pounds, but the rate of the spring, however, remains constant at 200 lbs/inch.
A typical GM A-arm front suspension has between 5 and 7 inches of wheel travel. In this configuration, the wheels are mounted outward of the springs and shocks, which allows them to travel in a longer arc as the suspension “works.” Although you have 5 to 7 inches of wheel travel, that equates to roughly only 3 to 4 inches of shock travel.
When choosing coilovers with the correct spring rate for the front of your car, you will want to first have the vehicle at ride height so the shocks are positioned within the aforementioned small 3- to 4-inch window of travel. A solid-axle rear suspension is a bit more forgiving, because the shocks are bolted directly to the rearend housing. This creates a motion ratio that is more proportional to the total travel of the wheels. The shocks used on a solid-axle rear suspension typically have 5 to 6 inches of travel.
…keep in mind is there isn’t one magic spring-rate that will work for your vehicle. – Steve Smith, QA1
“The first thing we need to know when choosing a spring rate is the weight of the vehicle,” Steve said. “Determining the correct spring-rate for the vehicle will also depend on the suspension type (independent vs. solid axle), and the purpose of the vehicle (street driving, drag racing, road racing, etc.). Even if a set of automotive scales aren’t available, you may still be able to weigh the vehicle locally. Waste yards, gardening centers, and truck stops are great resources for weighing vehicles.
“Outside of actually weighing the car, the next best way would be to utilize the “known spring” method. If a particular vehicle’s spring length and rate are known, measuring the compressed length of the spring will show how much spring force (weight) is on that particular corner. For instance, if you have a 12-inch spring with a 150-pound spring rate that compresses to 8 inches at ride height, that spring is supporting (150 lb-in x 4 inches = 600 pounds) of sprung weight. The known spring method is great, because it takes the guesswork out of how much of your corner weight is supported by the spring vs. the unsprung weight not supported by the spring (wheels, tires, axles, brakes, etc.) The last and least scientific way to figure out your approximate vehicle weight is researching online what the factory weight is, plus/minus any modifications the vehicle has received.”
Once you have determined your overall vehicle weight or corner weight, you can then figure the spring rates needed. “It is typical to work with the front- or rear-weights derived from the overall weight,” Steve affirmed. “If you only have the total vehicle weight, you’ll need to split it front/rear. Generically, you’ll call the front weight as 55-percent of the total weight and 45-percent for the rear. If your vehicles front/rear percentage is known, you should use those figures. Take your overall vehicle weight and multiply that by .55 for the front weight. Multiply the vehicle’s overall weight by .45 to find the rear weight.”
When working with a solid axle you will need to generically subtract 300 pounds from the vehicle’s rear weight, to compensate for unsprung weight (wheels, tires, axle, and brakes). These parts are not supported by the springs. Then, divide that number by two to figure corner weights. A 1,500-pound rearend example would be: 1,500 pounds – 300 pounds/2 = 600 pounds of spring-force per corner.
The motion-ratio of an independent suspension – and the effect it has on spring force – is a little more complicated than simply knowing corner weights. First, you will need to divide the frontend weight (for IFS) or rearend weight (for IRS) by two. This will give us the corner weight, including the unsprung weight. Subtract a generic 100 pounds for unsprung corner weight (wheel, axle, brakes, etc.). A 1,200-pound independent frontend weight would look like 1200 pounds/2 = 600 pounds (corner – 100 pounds (unsprung weight) = 500 pounds actual corner weight). After applying the motion ratio, you will have your actual spring force the spring will see.
When figuring the motion ratio of the control arm with an independent axle, you need a measurement from the control-arm pivot point, out to the shock mount. Then, divide that number by the measurement from the control arm pivot point to the ball joint. That will give you the motion ratio. Next, divide your corner weight by the motion ratio to figure the actual spring force (weight) the spring will see. Example: A 500-pound corner weight/.57 motion ratio = 877 pounds of spring force.
Figuring Spring Compression Targets
With a typical street car, you’re looking for the weight of the vehicle to compress the springs 25- to 30-percent. For drag cars, you will typically want between 30- and 35-percent of the spring to compress. “Using the actual corner weights (spring force) and dividing them by the target spring compression will lead to the spring rate that is right for your vehicle,” affirmed Steve. “As an example, if the rear coilover shock on your street car calls for a 12-inch spring, 3 inches would be 25-percent of a 12-inch spring. If the spring force for the rear corner of the car is 650 lbs-in, we would divide 650 by the 3 inches of compression we’re looking for. This comes out to 216 lb/in as the mathematical spring rate. Rounding down to the closest available rate would show a 200 lb-in spring rate as correct. The amount of spring being compressed at 25-percent will change based on the spring length your shocks call for.”
Steve also iterated that, “Another thing to keep in mind is there isn’t one magic spring-rate that will work for your vehicle. Going from a 150- to a 170-pound spring rate isn’t going to mean much of anything in terms of how the car rides. Using spring compression targets is a good general guide, but a drag spring compressing to 40-percent or a street car’s spring at 37-percent isn’t cause for alarm. Ultimately, we’re looking for the street-car spring to have available spring travel and support the vehicle at ride height, and the drag-car spring to have enough travel and stored energy for the launch. The shocks will be called on to control the ride quality and motion control.”
Linear or Progressive?
When deciding on a spring rate, you’ll also want to decide whether a linear- or progressive-rate spring is best for you. Ask three people which they prefer, and you might even get three different answers (think about that a while). As with any aftermarket product, a delicate balance of characteristics is always trying to be maintained. You have to realize: it is almost impossible to design a product that has no downside.
As previously described, a linear-rate spring has one defined spring rate-per-inch of deflection throughout most of the range of deflection. This consistent rate makes it quite easy to calculate a given spring length at ride height, and this helps when setting up the ride height of a specific vehicle.
Progressive-rate springs can be classified into two subtypes: a constantly increasing rate and a “dual-rate” spring. The constantly increasing rate is most often used as load-compensating spring on the rear of a vehicle. These are most often stock replacement type suspension springs. They are easily identified by their construction of continually varied spacing between the coils.
The dual-rate spring has two linear-rate sections that are connected with a rate transition range. This design has a much more focused use. These springs are used primarily in road racing and high-performance street applications. These springs are easily identified by having a few closely wound coils at one end, and then wider, equally spaced coils at the other end. Spring rates for these springs will typically be identified by both rates (i.e. 200/425 lb-in) This means the spring has an initial rate of 225 lb-in and then transitions to 425 lb-in as compression continues.
How you actually mount your coilovers will also have an effect on the spring rate. “The coilover’s angle has a slight effect on the spring rate, but you usually won’t need to factor in a spring-rate change unless you have more than 20 degrees of shock angle,” Steve stated. “With spring rates typically available in 50-pound increments, your target spring compression math may land on 172 lb-in when 150 lb-in and 200 lb-in springs are available. We would generally lean toward the softer side, but taking the shock angle correction into consideration might lead to using the 200 lb-in spring.
Hopefully, this outline will help you with your upgrade to coilovers, because they truly are a great way to improve your car or truck’s road-handling personality. With the help of QA1 you can help you choose coilovers that create a road-handling hot rod you’ll be even more proud of when you drive.