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.