A coilover shock has the outside of the body threaded like a giant bolt with large adjuster nuts that hold the coil in place. These adjusters also allow you to preload the coils if need be, and adjust the ride height of the vehicle. The coil companies and dealers will help you determine the spring rate you need, and remember that the coil diameter is a half inch larger than the shock it is going on, such that a 3-inch coil is for a 21/2-inch shock. Another general rule to start with is that the overall length of coils needs to be twice the travel of the shock, such that a 14-inch-travel shock needs 28 inches of extended coil spring. The number of coil manufacturers is very small with most folks pointing to either Hyper-Coil or Eibach as the guys to go to.
The list of coilover manufacturers is also pretty short. In the world of off-road racing there are just four: King, Fox, Sway-A-Way, and Bilstein. If there is a coilover on a racetruck out there other than these four, it is either custom-built or from a company no longer around. Recently a few other shops have showed up to address the demand for performance coilovers. Walker Evans Racing has taken its experience in both desert and snowmobile shock tuning and has come to market with its own version. Rancho is working at an entry-level coilover that will not be rebuildable, but will have adjustments similar to its RS 9000 shocks. Since we haven't tested every product, we posed the question of which is best to some of the different major retailers and the answer seems to most likely be determined by; (A) Which can you afford? (B) Who has the size you want? (C) What color shock do you want on your rig? And (D) Can you buy your shocks from a shop or dealer you like working with, since you will most likely be returning to them with questions till you have them dialed in? The fact is that the basic technology is near identical between these front-runners, and it comes down to the setup, tuning, and valving of each to really get the most from your shocks.
A very important aspect of a coilover suspension is controlling the movement of your axle. The best way to control the movement of an axle is with a linked suspension. This is a complete science in itself, and though not as simple as a leaf-sprung suspension, it can be designed to better transmit the power from the wheels to the vehicle. Plus then the job of supporting the weight and controlling the movement of the axle is separated from the job of locating the axle. Many folks will argue that a coilover suspension is lighter than a leaf-sprung suspension, but that is not always true. If the coils are mounted directly to the axle then the links can be made very light from some tubing, but if the coils are attached to the lower link arms like in many race or desert buggies and trucks, then they must be very beefy and the weight can increase.
Another issue with coilovers is cost. On average, a coilover with a set of two coil springs is between $400 and $550 per corner, which equates to around $2,000 just for the shocks and coils. That doesn't even address the link materials needed to locate the axles, the brackets for mounting the shocks, the fabrication required to make everything work, and getting someone qualified to help choose the perfect valving and coils for your vehicle. However, when you start to look at the performance of the suspension, you can realize why it is so expensive. Where a leaf-sprung suspension has friction, noise, and inconsistent performance to deal with, coils give you adjustability, little to no axlewrap, and a smooth ride over harsh terrain.