You want them, but why?
You want coil-over shocks. You don't know why, you don't know how they work, and you don't know why they might or might not be better, but you've seen them in the magazines, on race trucks, under rockbuggies, and on the Web, so you think you want them. Well, let's figure out if they are more than just cool-looking.
A coilover shock is basically a shock absorber with a coil spring or multiple coil springs wrapped around the shock so that it not only supports the weight of the vehicle, but also dampens the suspension to help control the spring. Seems pretty simple, eh?
Well it can be, but the major benefit of a coilover shock is that it can be adjusted and dialed in to achieve a far superior working suspension over any given terrain, but this also requires that you take the time and have the knowledge to dial in the coilover to get the best performance out of it. Dialing-in most coilovers involves adjusting both the coil-spring rates and the internal valving of the shocks. Oftentimes the company you purchase your coilovers from can help with this setup, and in the future we will give you a more technical breakdown of how to do this. But for now let's hit some coilover basics.
A coilover shock is fundamentally a cylinder (the shock body) filled with oil and an inner shaft with a piston that moves up and down within the body. The piston has small holes and very thin washers or shims designed to allow the oil to flow through it, but at the same time slow or control the speed and volume at which it moves. These shims can be changed to adjust the rate that the oil flows in each direction, and this gives the shock tunability on both the compression and rebound stroke. In addition to the shock oil, most of these rebuildable shocks are nitrogen charged. The nitrogen applies pressure to the oil to help force it through the piston, and also prevents the vacuum formed by the movement of oil, also known as cavitation. There are also two major styles of coilover: an emulsion shock, where the nitrogen charge is in the same area as the oil; and a dividing piston shock, where the nitrogen is separated from the oil by a floating piston. The floating piston can be found either in the shock body (internal floating piston) or in a remote reservoir. An emulsion shock is adequate for most four-wheelers looking for a slow speed or rockcrawling shock, whereas the reservoir and IFP shocks are better for high speed since the nitrogen and oil are less likely to mix and foam under extreme use.
Most coilovers are available in 2-, 21/2-, or 3-inch-diameter shock bodies and various travel amounts from 6 to 18 inches. The average American would instantly assume that he needs a 3-inch shock with 18 inches of travel, but that is not always the case. In fact, most of us only need a 2-inch shock with 10 to 16 inches of travel. The larger shock does allow for greater heat displacement due to the increased oil capacity, but most 4x4s are not put through the abuse where they would need that. If you are in a lightweight rockcrawler, basic trail rig, or street truck, then a 2-inch is fine or at the most a 2 1/2. A fullsize desert prerunner or race buggy heading for 800 miles of hard desert roads could step up to a 21/2-inch. A 3-inch is mostly needed for trophy trucks. Also look at the coils available to determine if the spring rates you need are available in the shock size you want. The smaller shocks and coils are also cheaper and have more available spring rates.