A spring alone in a suspension system would be unpredictable and would generate too much compression and rebound over road irregularities and during cornering, resulting in a car that would be impossible to control. A shock works as a damper that slows and reduces the spring's motion to a more controllable and predictable state by changing kinetic energy (the spring motion) into thermal energy inside the shock as the oil is forced through the valves by the piston. A pair of shocks that can effectively dampen the action of the spring on both compression and rebound will allow a good spring to shine.
The type of valving and fluid control in the shock makes a huge difference, so always go for shocks that are designed for performance and handling characteristics rather than a generic passenger car replacement. You very much do get what you pay for in this category and cheap shocks will make your upgraded parts feel less confidence inspiring. Shocks should be paired with springs, such that the shock is neither overwhelmed by the stiffness of the spring, nor is the spring's motion overly restrained by too much shock damping. The level of tuning can make a large difference in performance. For a mildly upgraded street car that sees rare autocross duty, a quality non-adjustable or single-adjustable shock is simple and will do the trick. As parts get more aggressive and speeds get higher, those seeking maximum tunability should definitely seek out single-, double-, or even four-way (high- and low-speed rebound and compression) adjustable shocks. Control over rebound and compression allows the driver to change how the suspension reacts to changes in spring height from cornering and road irregularities.
Standard spring and shock combos typically place the spring inboard of the spindle by quite a few inches, while the shock and spring are mounted independently. A coilover places the spring directly on the body of the shock, creating a single unit with one mounting point.
There are several benefits to a coilover swap, but they vary depending upon the application and mounting style. Since the spring is mounted directly to a threaded sleeve on the shock, all coilovers offer the benefit of relatively easy ride height adjustment, as well as swapping the entire unit out if need be. Additionally, coilover springs are much smaller and the entire assembly will weigh less than a typical separate coil spring and shock combo. The threaded sleeve also offers the ability to set preload on the coil spring, meaning that the spring is compressed between its upper and lower perches on the strut body before the assembly is placed in the car. Since the springs are tightly wound to the shock body and their compression will be totally linear with the shock, spring deformation or deflection during travel is essentially eliminated.
Coilovers can also offer the benefit of better transfer of the spring rate to the wheel rate if the mounting position is moved closer to the ball joint. The wheel rate is the force the tire actually sees and is rarely equal to the spring rate. To get the wheel rate of a spring, the spring rate is multiplied by the square of the motion ratio. The motion ratio is the mechanical advantage that the wheel has over the spring in compressing it and is found by dividing the distance from the control arm mount by the distance from the control arm mount to the ball joint. The angle of the centerline of the coilover from the horizontal of the control arm or axle also affects the motion ratio, though typically not as drastically.
The exact number varies by chassis and suspension configuration, but on a car with a conventional inboard-mounted coil and shock arrangement, the wheel rate could be as low as 25 percent of the spring rate. That would mean a 600-lb/in stock-location spring would have a wheel rate of 150-lb/in. Due to the smaller diameter of the assembly, if the mounting point of a coilover can be moved toward the ball joint significantly, the value could be more like 90 percent of spring rate, or even over 100 percent on some solid axle locations. That means a much lower spring rate could be used while still yielding a higher wheel rate. Due to reductions in unsprung weight and friction in the bushings and ball joints, a higher wheel rate coilover will often not only handle better, but also ride better than a conventional arrangement.
Leaf springs—a carryover from the horse and buggy era—are the oldest and most basic version of automotive suspension. Using arched strips of spring steel, they can be arranged as single leaves, or multileaves depending upon application and desired load-carrying capacity.
To be perfectly honest, a parallel leaf spring only does one thing really well: carry heavy weight. That's why you still see them commonly used in modern pickup trucks and SUVs. On the performance front, they are fairly compromised since the chassis requires them to be both springs and control arms at the same time. This can result in spring wrap, which can cause axle hop on acceleration or braking, and roll bind when cornering. That certainly doesn't mean they can't be dramatically improved. If you're sticking with leaf springs, look for modern-design replacements offered by companies that have put in the homework to determine what will work for your chassis. At a minimum, we always recommend upgrading the shackles and bushings since this is a major deflection point. Also, adjustable shocks can be particularly helpful with controlling leaf springs.
Adding extra links to the suspension can dramatically improve leaf spring characteristics as well. Slapper-style traction bars can improve straight-line acceleration by limiting axle rotation and spring wrap in front of the axle. A fixed length or sliding bar mounted below the spring and connected to the forward spring eyelet will act as a lower control arm and prevent axle rotation. Race-prepped vintage Shelby Mustangs also used a version of this mounted above the spring that was reportedly better at controlling brake-induced wrap. A center-mounted torque link attached to the rearend centersection can also be a very effective device that will experience zero bind during suspension movement.