Shock Compression and Rebound In the rest of the world, shocks are called spring dampers. That’s because the springs control the motion of a car’s body, while the shocks control the gyrating motion of the springs. This is accomplished by pushing the piston into the shock body during compression, and after dissipating the shock load, it extends back out of the body during rebound. “When designing a shock, we put it on a shock dyno to test how it reacts to carefully measured units of force in both compression and rebound. The pounds of force exerted on the shock rod is charted on the Y-axis of a graph, and the acceleration of the rod in inches per second is mapped on the X-axis,” Stacy Tucker of Detroit Speed and Engineering (DSE) explains. “A shock must be matched to the spring rate, and the rest of the suspension. After we build a prototype shock and install it on a car, we take it on a very specific street route, test it at the track, and drive some more on the street. During this process, we’ll fine-tune the compression, rebound, and jounce bump characteristics. This can take several months to complete. Overall vehicle weight, front and rear axle weight, wheelbase length, track width, suspension geometry, and spring location relative to the tire all affect how a shock must be valved.”
The amount of force required to compress a spring 1 inch is its spring rate. For example, a 400 lb-in spring requires 400 pounds of force to compress 1 inch. Wheel rate, on the other hand, is the actual spring rate as measured at the wheels. The length of the control arms creates a linkage ratio in the suspension, which means that adding 100 lb-in of spring rate increases wheel rate by a lesser amount. Furthermore, for any given wheel rate, positioning a spring closer to the control arm mounting points requires using a stiffer spring rate. As such, it’s impossible to compare the spring rate from one model of car to another. Moreover, when shopping for springs, hot rodders have the option of linear and variable rate units. “With a linear rate spring, the rate is constant throughout the suspension travel, whereas a variable rate spring starts out soft and gets stiffer as it’s compressed,” explains DSE’s Stacy Tucker. “Variable-rate springs are used more in OEM applications because they require very specific suspension tuning requirements. Since ride heights, wheel sizes, and vehicle weight are all over the map with hot rods, linear rate springs often work more effectively.”
Let’s say you’re in a helicopter looking straight down at the roof of a car. The suspension’s toe angle is how much the front of the tires point inward or outward. Toe-in is where the front of the tires points inward, and toe-out is where the front of the tires points outward. Changing this angle is easily accomplished by adjusting the tie rods. “Toe-in increases stability, but the trade-off is that it makes a car less responsive. If you have very wide front tires, you can dial in some toe-in to intentionally scrub the tires to build heat,” CPP’s Danny Nix explains. “Conversely, toe-out reduces stability and makes a car feel darty. For a car that’s driven on an autocross or a slow road course, that’s not necessarily a bad thing. Dialing in .25 to .50 inch of toe-out is good for low-speed track use because it allows a car to change directions quickly.”
As a car turns left or right, it rotates around a specific point located in the center of the chassis. Drawing an imaginary line through this point, from the roof to the pavement, establishes the vertical axis around which a car understeers or oversteers. The movement around this axis is called yaw, and how the mass of a car is distributed within the chassis plays a major role in yaw rate. “If you take a dumbbell and try to rotate it end over end, it’s very difficult because the weight is distributed on the far ends of the dumbbell. However, if you could move all that weight to the center of the dumbbell, it would be much easier to rotate even though the overall weight of the dumbbell has not changed,” says Danny Nix of CPP. “That’s why it’s important to reduce the polar moment of inertia in a car, which is a measure of a car’s resistance to rotating around its vertical axis. A mid-engine car is very responsive and rotates extremely well because the weight of the engine is concentrated near the center of the chassis, which reduces the polar moment of inertia. Moving weight that’s located on the far left or right sides of a car closer to the center of the chassis will net a similar effect.”