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.