Automotive engineers have been using hydraulic dampers on vehicles for close to a century. The basic idea is to move a constriction through an oil volume and use the viscosity of the oil to convert mechanical energy to heat energy, thereby slowing the movement of the spring in a vehicle’s suspension. While we’ve been using this simple concept for a long time, the rise in technology and capability of modern shocks has greatly changed the ability for vehicles to rapidly traverse terrain while maintaining good stability and comfort.
Initially, hydraulic shocks were simply a cylinder filled with oil through which a piston rod moved an orifice plate, causing restriction or damping the rod movement. Later, with the invention of the gas-charged damper, shock performance was greatly improved. Future advances brought about various means by which to regulate the flow of oil within the shock to accomplish the damping action needed for specific applications. High-performance race applications have further fueled innovation and today there is a variety of fully rebuildable and tunable shocks available to us.
How a shock behaves on a particular vehicle is dependent on a number of factors. These include oil viscosity and volume, valving setup (compression and rebound). Nitrogen-charge pressure, and spring rate.
Today, there is a wide range of available shock types and we’ll discuss a bit about each to explain their construction, relative cost, and off-road application.
Basic Hydraulic Shocks
A common hydraulic shock found on some factory stock vehicles is that of a twin-tube shock. In this design, there is an inner tube where the shock piston moves and the outer tube which serves as a fluid reservoir. As the shock is compressed and extends, oil moves back and forth through valving in the inner tube. These shocks may use low nitrogen pressure inside, but when the piston moves, it creates a vacuum behind it and may cause air bubbles in the oil. This aeration of the oil causes inconsistent movement of the fluid through the piston valving and can cause fade (where the fluid damping of the aerated oil becomes much less effective). These are relatively inexpensive shocks, but will offer the lowest level of performance and resistance to heat dissipation.
Monotube shocks use only a single wall tube which allows for greater oil volume than a twin tube design. As such, they can tolerate more abuse before they build significant heat and start to fade. Monotube shocks are typically gas-charged. All the oil resides in the single tube and there is a floating piston inside that keeps the oil separated from the high pressure nitrogen gas. As the shock is compressed, the oil pushes against the floating piston and further compresses the gas charge behind it. This added pressure from the gas charge keeps the oil under pressure at all times to reduce foaming as the shock cycles rapidly. A monotube shock is also lighter than a similarly-sized twin tube shock, and often has the advantage of being mounted in any orientation. There are numerous quality aftermarket shocks for off-road vehicles that use a monotube shock design.
When we start to look at aftermarket high-performance shocks, there are four basic classes we’ll consider. These are: non-coil, coilover, air shocks, and bypass shocks. Each has its desirable applications and there’s a wide range of variation that can be used in the off-road world.
Non-coil performance shocks are used in applications where a separate spring is used to hold up the corner of the vehicle. This could be a leaf spring, coil spring, air bag, or torsion spring assembly of some type. Non-coil shocks come in two basic varieties: emulsion and reservoir. The reservoirs may be mounted to the shock body in piggy back style or they may be remote mounted and connected with a hose.
A coilover shock consists of an oil shock body that also has a coil spring (or springs) placed over the body. The ends of the spring(s) are captive at the shock ends. The spring function of this acts in the same way as a regular coil spring. However, coilovers can provide more than one spring rate as the shock moves through its range of travel. Dual-rate coilovers use two stacked springs, while triple-rate coilovers use three stacked springs. As with non-coil shocks, a coilover can be either an emulsion type, reservoir type, or sometimes other slightly different design.
Non-coil and coilover shocks may be of the emulsion type. These are both tunable and rebuildable. This type of shock internally uses a combination of oil and high pressure nitrogen to control the damping action. The term emulsion, in the simplest terms, means that the nitrogen and oil are mixed together and the mixture passes through the internal piston.
Fox Racing describes their 2.0 Emulsion shocks as designed for prerunners, "limited classes," and recreational vehicles. In general, an emulsion shock works best with less aggressive shock piston movements and situations where shock fade is not a large concern. Emulsion or "non-reservoir" coilovers are also common on many rock crawlers where the vehicle is used more at low speeds and for relatively slow suspension cycle speeds.
If an emulsion shock becomes very hot from use due to high shock shaft velocities, the shock may begin to cavitate to some extent due to the nitrogen bubbles moving through the shock piston and shims. As such, an emulsion shock is less resistant to heat fade as compared to the reservoir shocks we discuss next. Sudden changes in vehicle orientation, such as a rock buggy might see, can also cause the oil and nitrogen to change mix drastically and affect the consistency of the mixture metered through the piston and shims.
Non-coil and coilover shocks may also be reservoir shocks. These are fully serviceable and tunable as well by varying the oil weight and volume, and by changing the internal valving components and configuration. Reservoir shocks differ from emulsion shocks in that the oil and pressurized nitrogen in the reservoir are kept separate by a dividing piston that can move based on nitrogen pressure and shock action.
The pressurized nitrogen impinging on the floating piston applies pressure to the oil on the other side, effectively raising the boiling point of the oil and keeping it from mixing with air. Nitrogen pressure can be adjusted to increase or decrease this effect and provide minor tuning adjustment to the shock. Without this configuration, outside air could enter the oil chamber and cause the oil to foam. As this air/oil mixture moves through the shock piston and shims, the damping diminishes (effective viscosity drops) and your suspension is soon bouncing out of control.
As the shock compresses, oil displaced by the shaft is forced into the reservoir. This oil pushes the floating piston further into the reservoir and the nitrogen on the opposite side of the piston compresses further. In this way the oil is kept in a confined volume under pressure to prevent aeration or cavitation, which yields unpredictable damping action. It also allows room for fluid expansion as the oil rises in temperature. As the shocks are used heavily, the temperature of the reservoirs will elevate as the fluid inside becomes hotter. As such, it’s a good idea to locate them away from other heat sources and in a good area for moving air flow.
There’s another type of rebuildable and tunable shock used on go-fast vehicles. Bypass shocks are dampers only used in conjunction with a coilover shock or other suspension spring. It is a shock that takes a portion of the oil within the shock body and reroutes it to another area within the shock. In this way, the bypass shock allows you to specifically tune the shock to behave differently at various points in its range of travel. While all shocks are velocity sensitive (providing greater resistance as shock speed increases), bypass shocks are also position sensitive. Adjusters are provided to set compression and rebound rates. Bypass shocks can utilize a piggy back reservoir, a remote reservoir, and might even be of the internal bypass variety.
The idea behind a bypass shock is to have a damping rate that is light in the initial inches of travel, but rises to a higher rate further into the travel range as the shock compresses. Oil is taken temporarily outside the main shock body via a tube housing a one-way check valve and then reintroduced into the other end of the shock. By doing so, the valving rate is reduced by allowing some oil to "bypass" the piston and shims. This only occurs over the shock travel range where the bypass tube is effective. Once a bypass tube is no longer open to oil flow, the full valving rate is in effect for the shock. With external bypass tubes on the outside of the shock, the bypass rates can be easily adjusted when needed. These shocks often start with two compression tubes and one rebound tube, but additional tubes can be added to further refine the ability to tune the shock behavior at more travel positions.
Internal bypass shocks are becoming more common and behave similar to external versions, but offer the advantage of being able to use them as a coilover shock to support the vehicle weight. In this way you can have the benefits of a bypass action shock in a smaller space (not requiring a separate coilover). King Shock recently released a new product that offers velocity- and position-sensitive damping, plus an internal hydraulic bumpstop in a monotube shock design. The internal bypass design cleverly routes the bypass oil through passages in the shock’s hollow shaft. An internal tapered metering rod controls the lighter valve rate until the rod gradually becomes seated in a valve seat as the shock is partially compressed. Once this occurs, the oil is all forced to go through the primary piston and shims. The meter rod length adjusts the position where the bypass is effective and the rod taper profile defines how fast the valving rate rises.
In the scheme of budgets, a suspension system using bypass shocks will hit your wallet the hardest. One reason is that they are often used alongside a coilover shock, or at least a spring of some sort. But, when well tuned, they offer the greatest range of suspension control of all the shock types.
As with a coilover shock, a nitrogen-charged "air shock" can be used to provide both spring and damping functions. These units look much like a fat hydraulic shock but incorporate beefier shafts. Inside the shock, the oil and nitrogen are mixed together and move through internal orifices.
The amount of nitrogen charge determines the effective spring rate, plus the internal valving uses the oil to dampen the movement. The mixture is confined inside the shock. The oil is incompressible but the nitrogen can be compressed as the shock is compressed. This nitrogen compression provides a fairly constant spring rate over perhaps the first two-thirds of travel but then the rate rises almost exponentially as the shock is compressed towards the end of its travel.
Behavior can be tuned by changing the internal oil passage valving, the amount and weight of oil in use, plus the nitrogen charge pressure. More oil in the shock will cause the spring rate to rise faster and help prevent bottoming. Less oil will allow the shock to compress further before the shock starts to go to a hydraulic lock condition. Nitrogen pressure can be adjusted to dial in the base spring force.
Due to their simpler design as a suspension component, air shocks are a considerably cheaper alternative to coilover shocks, when the need is to provide a suspension spring and damping in a single component. Air shocks are also smaller and lighter than comparable coilovers. However, air shocks offer less tunability and are generally best applied to applications on lightweight vehicles.
While we’re talking dampers, it’s fair to mention gas-pressurized nitrogen bumpstops (often called airbumps). These cylindrical units consist of a short stroke shock mechanism that is velocity sensitive. Oil is used inside and moves through orifices much like a standard shock. This allows the bump to effectively dampen, or slow, the suspension movement through its final inches of travel.
Airbumps offer several advantages over a fixed bumpstop. First, the hydraulic action offers a smooth exponential rising rate of resistance as the bump is compressed. This eliminates that hard, sudden end of travel you often experience with a fixed material bumpstop. Second, the air bumps usually don’t push backwards on the suspension as the bump rebounds. Their action can be controlled by varying the oil weight grade, oil volume, and nitrogen charge pressure through a Schrader valve on the body of the bump can. Additionally, the internal components can be changed to affect the oil flow rates and damping characteristics.
Nitrogen bumpstops are most commonly used on go-fast vehicles such as desert racers, but they are becoming more common on rock crawling rigs and daily drivers, as well. They provide some extra suspension control when running fast dirt roads or bumpy terrain at speed. When used on a rockcrawler, or other trail vehicle, they allow for a softer spring to retain suspension flex and articulation while preventing hard bottoming during large hits.
These bumps are typically available in several lengths, often having between two and six inches of travel. Depending on the amount of up-travel you have on a suspension, each has its pluses. The longer version obviously applies the stop function over a longer distance. However, if you have minimal up-travel distance, you may find the suspension always tapping into the stop and using it too early in the suspension range. You may only want them to come into play at the very last of the suspension travel to help absorb really big bottoming hits.
Light Racing JounceShocks are another example of an active stop. They are nitrogen-charged secondary shocks that essentially add additional compression damping and controlled rebound to your vehicle. The bottoming limit behaves exponentially. Kits are available from Light Racing to fit a number of factory applications.
The Light Racing stops also have separately adjustable compression and rebound damping that is accessible on the side of the shock body. The rebound damping is designed to be the greater of the two to allow the springs in the suspension to rebound the suspension without the bump shock pushing on it further. At the bottom end, the treaded piston can accept a number of tip styles with materials such as urethane and rubber. For a non-race vehicle, the rubber tips offer a more quiet action when they contact the landing pad but the rubber wears a bit faster than the urethane style tips.
Getting the Most From Your Shocks
It’s easy to see there is a wide expanse of shock options for off-road vehicles. Choice of shock can depend on application (street, trail, race, etc.), vehicle weight, tire size, suspension type, mounting configuration, and certainly your budget.
Fitting replacement shocks to a near-stock vehicle or one with a reasonable lift for mild use means you can usually turn to one of the many manufacturers for bolt-on upgrade shocks. However, once you get deeper into customizing or setting up competition rigs, the choice and setup possibilities are far more varied.
In any case, when you venture beyond a known application replacement, buying and mounting the shocks is simply a start to the process. Dialing in the suspension by tuning the piston valving and other variables can mean the difference between having just a bunch of cool-looking shocks and being able to soak up and traverse whatever terrain you’re out to tackle.
Coilover Spring Rates
Let’s briefly look at the spring rate for a typical coilover shock. For example, a dual-rate coilover is one that has two springs stacked one atop the other. During the initial portion of the compressive shock travel, both springs compress. The effective spring rate is a combination of the two springs and is less than the rated value of either coil, based on this formula:
Spring Rate = (Upper coil rate x Lower coil rate)/(Upper coil rate + Lower coil rate)
The lower-rated coil will compress more than the higher-rated coil as the suspension compresses and a coilover has an adjustable stop ring that can be set to stop the compression movement of the upper (lower rate) coil. Once the upper coil is stopped (when the dual-rate slider hits the stop ring), the spring rate of the shock is simply the spring rate of the lower (higher-rated) coil.
For example, suppose we’re using a 200 lb-ft/in upper coil and a 300 lb-ft/in lower coil. The initial effective spring rate would be 120 lb-ft/in. Once the upper coil is stopped, the final spring rate would jump to 300 lb-ft/in for the remaining distance of shock travel.
By varying the rate of the two coils and adjusting the stop point of the upper coil, the shock can be dialed in to provide a soft ride over small bumps, but then use the higher final spring rate to soak up the bigger hits and prevent bottoming. Additionally, nitrogen pressure is used in the shock body to provide another compressible medium.