To start, the idea of a coilover shock is pretty simple. A coil spring (or two or more as in dual-rate or triple-rate coilovers) is mounted through various methods to a shock or strut. The coil spring(s) support the weight of the vehicle and compress or extend as the vehicle goes over bumps. The shock dampens the action of the shock by controlling fluid movement, and this is usually where things get a bit more complicated. Within the shock's body, a piston rides on the shock shaft, moving up and down as the shock compresses and extends. On either side of this piston is a fluid, or shock oil. The compression and extension (rebound) of the shock is controlled by how much fluid can move through or sometimes around this piston. The rates of this fluid movement through the piston is controlled mainly by shims that control the valving. All these parts and exactly how they do their job can get complicated. So for now we will glaze over the finer details of shock valving to focus on the larger picture of how a coilover shock does what it does in general. We'll also talk about how it is set up, how it works, and why folks want them on their 4x4s.

Coilovers are Tunable

The main benefit of an aftermarket coilover shock is they are rebuildable and tunable. By that we mean the end user can choose the shock travel length, choose from a wide variety of spring rates, and adjust the valving or how the shock dampens compression and extension (rebound). All these parts can be fine-tuned to do their job exactly as you want them to. That beats the pants off standard shocks and springs, which are generally aimed at some hypothetical middle-of-the-road version of your 4x4. That middle-of-the-road tune is not one for relatively light-weight or heavily loaded vehicles, but one somewhere in the middle. Not one for aggressive drivers or overly calm ones, but a hypothetical average driver. That works well for general suspension performance, but if you want to get the most out of how your suspension works for comfort on-road and off- you need to fine-tune it. That tuning isn't necessarily easy, but it's only possible if your shocks are tunable or rebuildable. To learn more, a lot more we recommend checking out the tech info on AccuTune Offroad.

Coil Spring Rates

Coil rates, or how much weight each coil spring can hold, is one of the easier things to tune on a coilover shock. Coil rate is usually measured in pounds per square inch and is determined by the wire diameter, how it's coiled (coil size), and a few other parameters. To get you in the ballpark, you need to know the amount of travel your shocks have and the corner weights of your 4x4. Corner weights can be estimated with existing coils, or you can get a front and rear vehicle weight at a truck stop with a scale and then divide that weight by two. You will also need to subtract the unsprung weight, namely the weight of the axle, wheels, tires, shocks (to get the sprung weight). Alternatively, a high-end fabrication shop or race shop should have scales that can give you very accurate corner weights (which can vary side to side). Once you have weight info and some parameters on how your suspension is set up (links, A-arms, trailing arms, etc. ), you can use an online calculator to get an estimate of coil rates, and a shock tuner or shock manufacturer should be able to get an idea of a good place to start with valving. Changing coil rates is as easy as unloading the shock, removing at least the lower-end spring retailer, and changing springs. The springs of course cost money but aren't too expensive relative to the full cost of coilover shocks. Some shock retailers and installers like AccuTune Offroad will calculate spring rates and valving for you when you buy coilovers from them or have them rebuild your shocks.

Dual- or triple-rate coilovers

Many coilover shocks have two (or more) coils on the body of the shock. These two coils ideally have different coil rates. The two coils push on each other via a coil slider, with the lighter coil compressing (on top) faster than the harder coil (on the bottom). That allows the suspension to be relatively soft until the softer coil reaches some level of compression. At this point the slider between the two coils stops at the dual-rate nut, and the shock jumps to the higher coil rate of the heavier coil spring. This point is adjusted by adjusting the dual-rate nuts that are threaded onto the body of the shock. Dual-rate springs help prevent hard bottoming and prevent a harsh ride, as the lower rate rides soft and the higher rate slows the suspension as the compression ramps up. Triple-rate coilovers are reserved for lighter-weight rigs where there is negative spring pre-load and the springs can be fully extended before the shock is fully extended. In this case a very light weight "tender" coil spring keeps the other springs from moving around when unloaded.

Spring Preload

Spring preload is set by lowering the adjusters on the top of the coilover shock to put the springs under a load without weight on the shock. Preload is set with the shock unloaded. Then the preload nut is tightened down until it touches the coil, holding it in place. That is the zero-preload setting. Preload is then added by tightening the preload nut down, compressing the spring. If you tighten the preload nut 1 inch and the spring is a 200-pound/inch spring, you've preloaded it 200 pounds. If you loosen the preload nut, then you have negative preload, which usually isn't a good idea except in some circumstances. Preload can affect the ride height of the vehicle to a small extent but isn't the main way that ride height is adjusted. If you need to add lots of preload to get the ride height you want, then you need higher-rate coils. If preload lifts the vehicle more than you want, you have springs with too high of a coil rate. If your shock tops out harshly, that could be because you have too much spring preload, and if the spring floats on the shock when unloaded, you don't have enough spring preload or require a spring tender coil.

Damping

As said, the damping properties of your shocks are set by valving of the piston. Generally, coilover shocks should have some dampening tuning to fine-tune the ride, but tuning the dampening is a bit more complicated than the other tuning we are talking about here. Like with spring rates, shock valving can be estimated based on your suspension setup and corner weights, but to quote a friend of ours, if your shock valving on a custom rig hasn't been tuned at least once, it isn't right. Chances are if your shock valving is off, you will need to learn a lot more about how shock valving works, or you'll have to bring in an expert like AccuTune Offroad to rebuild your shocks. For revalving, the shocks will need to be fully disassembled. Changes might be made to valving or coil rate based on how the vehicle works and what has worked for similar vehicles in the past. Keep in mind that each vehicle is different; very rarely will two custom-built rigs share the same exact spring rate and shock valving. Tuning is based on personal preference, vehicle weight, and driving style. This does become a bit easier on commonly built vehicles that use the basic OEM suspension design, but even in that situation, vehicle weight can fluctuate greatly.

Gas Charging

Coilover shocks come in at least three flavors of gas charging. First, emulsion shocks, where the oil is directly charged with nitrogen, the two are in the same chamber and can mix. Second, an IFP, or internal floating piston, where a floating piston (not the piston on the shaft) separates the oil from the nitrogen charge. And last, a remote-reservoir shock, where the remote reservoir has a floating piston that separates the shock oil from the nitrogen charge. Emulsion shocks are the least expensive and have the least amount of nitrogen, which requires higher pressures. Higher pressures mean more heat, and more heat is bad for a shock and performance. Also, the nitrogen and oil will mix, and both will pass through the piston, making damping inconsistent. IFP shocks are slightly more expensive but usually require a longer body for packaging the floating piston and nitrogen, again at a higher pressure. Emulsion and IFP shocks are usually not rebuildable. Remote reservoirs (and piggyback reservoir shocks) are more expensive, usually rebuildable, and generally work better off-road. They run cooler because there is more oil to absorb heat, and pressures can be lower because there is more space for the nitrogen in the reservoir. The main body of the shock in a remote-reservoir shock has only oil in it, with the reservoir containing both oil and the nitrogen (again separated by a floating piston). The amount of pressure added by the nitrogen also impacts how the shock works. Too much may cause a harsh ride (effectively adding spring rate), and too little may cause the seals to not seat properly (allowing leaks). Generally, the nitrogen pressure is set by the manufacturer.

Anatomy of a Coilover Shock

Body: This seems obvious, but the body of most coilover shocks is the main part and is often threaded for adjusters.

Shock shaft: This is the hard metal rod that moves in and out of the shock body.

Shock shaft bumper: This is a rubber bumper that's designed to prevent damage to the shock if the shock compresses fully. You want the bump stops to prevent this from happening to completely avoid damage to the shock.

Piston and valving: This is the part of the shock attached to the shock shaft that fits inside the body of the shock and controls fluid movement via valving. How these parts do exactly what they do is outside the scope of this article—it's complicated. But it is tunable for many of the larger remote-reservoir coilover shocks.

Coils: These are the springs that support the weight, available in different spring rates usually in 50-pound/inch increments.

Preload nut: This nut, or pair of nuts, sets the spring preload and fine-tunes ride height when the correct spring-rate springs are installed. Note that the preload is not the distance between the preload nut and the top of the shock, but rather the distance from zero-preload (nut snugged on the coils) to your preload setting, generally an inch or two below the zero-preload point.

Dual rate nut: This nut, or pair of nuts, sets the stop point for the lower spring-rate spring on a dual-rate coilover. At this point the shock switches to a firmer spring rate, slowing compression.

Sliders: The main slider is a plastic tube with a shoulder that holds the two coil springs together and slides over the body of the shock to play a role in the dual-rate function.

Remote Reservoirs: A remotely placed reservoir for the oil, a floating piston, and the shocks' nitrogen charge.

Compression adjuster: Some remote-reservoir coilover shocks have a compression adjuster available for fine-tuning the ride of the shocks. These generally don't work well for coarse adjustment on shocks that are not properly valved from the start.