Hitting The Sweet Spot
Unlike the front suspension,...
Unlike the front suspension, the shocks on a solid-axle rearend are usually mounted directly to the housing. This virtually eliminates the effects of the motion ratios imparted on an A-arm front suspension, and yields wheel travel that's very similar to shock travel. Consequently, most rear shocks have between 5 and 6 inches of travel.
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.
In order for Chassisworks'...
In order for Chassisworks' procedure to work properly, a car must be 100 percent complete with all interior components, glass, and fluids in place, in addition to ballast to simulate the weight of the driver. "A lot of times you don't even know what specific parts are going to be on your car until you're done building it, so fine-tuning the spring package should be the last thing you do," Chris explains.
With the coilover assembly...
With the coilover assembly removed from the car, all preload must be removed from the spring before recording any measurements. To accomplish this, the lower spring seat should be tightened just enough to remove freeplay from the spring. Accurately measuring the free length of the spring involves recording the distance from the upper spring seat slot to the bottom edge of the spring within 1/16-inch. This length may differ slightly from the spring's published free length. For instance, the 9-inch spring pictured here actually measures 815/16 inches. This will be used as the "F" value in the spring rate equation. Chassisworks recommends taking measurements for one front coilover and one rear coilover, as most production cars will have very little variation from right to left.
After reinstalling the coilover...
After reinstalling the coilover assemblies, wheels and tires, the next step is measuring the height of the springs once again with the car resting on level ground. This will be used as the "L" value in the spring rate equation. Anything that may interfere with the movement of the control arms and rearend, such as bump stops, should temporarily be removed. On most cars, there will be very little front-to-rear variation in terms of how much the springs collapse at ride height.