A vehicle suspension maintains control and comfort of ride. As a wheel goes over a bump, heavy springs in the suspension between the wheel and frame compress providing the needed give to keep the vehicle level. The nature of springs however is once compressed they will re-expand and re-compress over and over until the energy put in by the bump is worked out. If allowed to cycle like this, it’s possible for the tire to loose contact with the ground and compromise control.
Shock absorbers like suspension springs are located between the wheels and frame and removes the energy in the cycling spring by moving a piston in the shock tube that forces oil through small holes. This action converts the energy of movement (also known as kinetic energy) in to heat. The effectiveness of a shock absorber is in how quickly it can remove the cycling energy & return the suspension to its stable (non-cycling) state.
The engineering of shocks can get esoteric. This short article will stick to the more common technologies that are road, trail and race proven.
These are the most common shocks. The “two” tubes might be thought of as a tube inside of tube design. The piston chamber is located in the inner tube and reservoir for the hydraulic fluid the outer tube. Depending on design, valving between the outer and inner can provided damping force in addition to the piston.
Sometimes marketed as Nitrogen charged or “Nitro” are shocks whose piston chambers are charged with a high pressure gas. In addition to oil, all twin tube shock piston chambers have air spaces. As the piston churns through the oil, bubbles can form reducing the shock’s effectiveness, a condition known as shock fade. By charging the cylinder with a gas under high pressure, oil foaming is inhibited.
Unlike a twin tube design, the external wall of the tube forming the piston chamber is exposed to the elements and so better able to cool. Also in most designs, the gas section is separate from the fluid avoiding the foaming issues giving these shocks higher performance characteristics.
Typically added to mono tubes, these are shocks with increased oil capacity which often equate to greater heat dissipation and thus better high cycle rate performance.
These add external valving and potentially multiple valving tubes (perhaps separate for compression and expansion). As shock solutions go these are highly tuneable and are often the choice of high budget racers.