"Everything" may be a rather vague term, but when trying to figure out what factors determine the ideal spring rate for a car, it's pretty darn accurate nonetheless. To put it succinctly, anything that adds or removes weight from a car affects the optimal stiffness of a spring. Aluminum flywheels, A/C systems, fiberglass hoods, power steering boxes, aluminum heads, and a driver's midsection girth all swing the weight total in different directions, but each and every component that has mass plays a role in spring rate selection. With the countless variables that can alter a vehicle's weight, it's not surprising that GM had to use more than 100 different rear spring rates on the first-gen Camaro and its Nova platform-mate at the factory.
The OE suspension designers back in Detroit-equipped with mechanical engineering degrees, chassis schematics, CAD/CAM software, and really big brains-can precisely calculate the perfect spring rate by feeding data such as control arm lengths and suspension pickup points into exotic math formulas. Those of us that can't are stuck with test-fitting boatloads of springs onto our project cars and hoping for the best, or settling for a setup that compromises ride quality, handling, or both. Obviously, neither option is particularly appealing.
So when Chris Alston of Chassisworks informed us that he's devised a method of selecting the ideal spring that's both 100 percent accurate and easy to execute, we were all ears. "People call us up all the time and want us to recommend the perfect springs for their car. If I were that brilliant, I'd be in the business of buying lottery tickets, not building race cars," Chris quips. "We can get you in the ballpark by comparing your car's setup to other cars we've supplied springs for in the past. However, no two cars are the exact same, so you can't get it perfect that way. For people who have already made the plunge for coilovers, our method involves starting out with a baseline spring to get you in the ballpark, making a few simple measurements once they're installed, then plugging those numbers into a simple formula to determine the ideal spring rate for your car. That still requires buying a second set of springs, but if you think it's worth the money we'll sell them at a discount. That's still better than trying out dozens of different springs to no avail."
It's All About Support
Unless the role of a spring in relation to overall suspension dynamics is made clear, the procedure required to select the right rate-and why it's necessary-won't make any sense at all. Consequently, at the risk of being redundant, we'll briefly recap some info covered in previous installments of CHP's ongoing suspension tech series. According to the vast majority of chassis tuners we've consulted, a spring's job is to simply support the weight of the car. Although a higher spring rate reduces body roll, it comes at the expense of a harsher ride. "Not only will a softer spring improve ride quality, it will also enable the tires to follow uneven road surfaces more precisely for improved grip. A tire that momentarily loses contact with the road due to having too stiff of a spring won't produce any grip at all," Chris explains. "Upgrading to bigger sway bars is a far more effective method of limiting body roll and managing weight transfer. Once the spring rate is dialed in, the bulk of track tuning is accomplished by adjusting the shocks and trying out different sway bars. If you prescribe to this school of thought, where the purpose of the springs is to merely support the weight of the car, then picking out the right rate isn't hard."
Hitting The Sweet Spot
If the role of the springs is so simple, then why the big stink about picking the perfect set for your car? It all comes down to making sure that the shocks can operate in their ideal range of motion, and the importance of the shocks in relation to overall handling performance. "In my opinion, the only thing more important than the shocks in regards to handling is the tires. By dampening the motion of the springs, shocks control the rate of weight transfer in a corner," Chris explains. "Optimizing the rate of weight transfer is the essence of good handling, and the shocks control how the entire suspension operates. That said, shocks have a very limited range of motion, and their performance degrades dramatically as they reach the extremes of their travel. It's up to the springs to make sure the shocks operate in their sweet spot, which is why proper spring selection is so important. If your car sits too high or too low due to having the wrong springs, it will take the shocks out of their sweet spot and they'll never perform well. People often think there's something wrong with their shocks, but 80 percent of the time perceived shock problems are a result of not having the right springs on their car."
A typical A-arm-style front suspension has between 5 to 7 inches of wheel travel. However, the wheels are mounted farther outward than the springs and shocks, and travel in a longer arc. That means that the 5 to 7 inches of wheel travel necessary for a smooth ride and secure handling equates to just 3 to 4 inches of total shock travel. When attempting to determine the correct spring rate for a car, the trick is to set the ride height so the chassis rests in the shocks' sweet spot within this small 3- to 4-inch window. A solid-axle rear suspension, on the other hand, is a bit more forgiving. Since the shocks are usually bolted directly to the rearend housing, the motion ratio of the wheels is more proportional to the total travel of the shocks. As a result, the shocks mounted to a solid-axle rear suspension typically have 5 to 6 inches of travel.
How much of a shock's range of travel is allocated to compression and rebound depends on a car's intended use. Street cars require more compression than rebound, while drag cars are the exact opposite. Cars built for the road course or autocross, on the other hand, can benefit from an even split of compression and rebound. "A street car needs about 60 percent of its travel in reserve for compression, and the other 40 percent for rebound (60/40). The bias toward compression improves ride quality and has a built-in safety guard for unexpected road hazards," Chris explains. "Street/strip cars need roughly 40 percent of their shock travel for compression and 60 percent for rebound, as the extra extension assists in front-to-rear weight transfer. Since road course and autocross cars usually run on smooth surfaces which require less compression, they can benefit from a 50/50 split. However, variations in suspension geometry or track conditions may necessitate altering the travel percentages to prevent the shocks from bottoming- or topping-out."
Whether you bias the shock travel toward compression, rebound, or keep it neutral, the first step in accomplishing this is determining the total travel of your shocks. Chassisworks publishes the shock travel specs of all its shocks and struts, as do many other manufacturers. It just takes some research. Next, figuring out how much the springs and shocks should be compressed at ride height to get the shocks in their sweet spot is merely an exercise in simple math. A street-oriented setup (60/40) requires that the shocks and springs collapse 40 percent from their free length at ride height. Consequently, a shock with 4 inches of travel should compress 40 percent, or 1.6 inches, at ride height. That results in 40 percent of travel reserved for rebound and 60 percent, or 2.4 inches, reserved for compression.
With all the foreplay out of the way, it's time to finally get into some number crunching. Calculating the optimal spring rate for any car can be expressed in the following equation, where "F" is the free length of the spring, "L" is the loaded length of the spring, "R" is the baseline spring rate, and "T" is how much the shock collapses at ride height.
For the sake of illustration, let's presume that a 500 lb/in spring (R) measuring 8.94 inches (F) compresses down to 6.50 inches (L) at ride height. The difference between the two spring height figures is 2.44 inches, which yields a product of 1,220 when multiplied by the spring rate of 500 lb/in. When matched with a shock featuring 4.25 inches of travel, setting it up for a 50/50 cornering application requires the shock to collapse 2.13 inches (T) at ride height. Finally, dividing 1,220 by 2.13 results in a quotient of 572.77, which is the ideal spring rate for this application.
In this example, the baseline spring rate of 500 lb/in was too soft, which resulted in a shock that collapsed more than the desired amount at ride height (2.44 inches). Not only would this spring move the shock outside its sweet spot, it would also bias its travel too far toward rebound, not leaving enough travel for optimum compression valving and potentially degrading ride quality. Although it may seem trivial, the extra 73 lb/in as calculated in Chassisworks' formula would effectively decrease the amount the shock collapses at ride height to hit the target sweet spot of 2.13 inches. That yields an equal amount of shock travel in each direction for compression and rebound (50/50), which is perfect for many road course or autocross setups.