Shock Absorbers: Hydraulic vs. Gas-Charged

Car shock absorbers are usually divided into two main groups: hydraulic and gas-charged. In reality, this classification is somewhat arbitrary, because even hydraulic shocks contain gas (air), which acts as a compensating element. A shock absorber can't work without gas. Oil, like any liquid, is practically incompressible, so without gas the shock would act far too stiffly and its parts would be subjected to heavy loads. In addition, the oil heats up as the shock works, sometimes to very high temperatures, and expands as a result. Gas is needed in the shock to make room for this expansion. When a hydraulic shock absorber works, the gas inside it is the first thing to compress. Only then does the oil come into play, flowing at a controlled rate through calibrated orifices into the outer cylinder. This design, with an additional outer cylinder, is typical of hydraulic shocks and so-called "low-pressure gas" shocks (those with low-pressure gas charging). Gas-charged high-pressure shocks fit everything into a single cylinder. Given these design differences, shocks are also divided into mono-tube and twin-tube types, which is arguably the more logical classification. When comparing how hydraulic and gas-charged shocks work, it's worth noting that hydraulic ones feel softer. This is because hydraulic shocks use a double valve system, while gas shocks use a single one. On the other hand, hydraulic shocks have more inertia: they react much more slowly to wheel movement, especially when the wheel's movements and oscillations are small in amplitude and frequency. In gas-charged shocks, the gas is under some pressure, so the piston valve is constantly "spring-loaded" by the gas. As a result, this kind of shock reacts much faster to wheel movement caused by road irregularities. Hydraulic shocks have some other characteristics too, and under certain conditions these can become drawbacks. For example, if the piston moves sharply down or up, zones of strong vacuum can form in the liquid, meaning the oil. These zones create so-called cavitation voids, or bubbles. This is a very undesirable effect: it can change the shock's performance characteristics and even cause it to fail. One way of solving this problem was the development of gas-charged shock absorbers. In these shocks the oil is "spring-loaded" by the gas, so it is practically immune to this kind of cavitation foaming. Another advantage of such shocks is that they can be mounted in any position, even upside down, with no effect on their performance. As mentioned above, a shock absorber generates a lot of heat when working, so getting rid of that heat is a real concern. The problem is addressed, among other things, by using oil that can work without changing its properties at temperatures of at least 160°C (320°F). Incidentally, twin-tube hydraulic shocks are noticeably worse at shedding heat, because the extra outer cylinder hinders heat dissipation. Mono-tube high-pressure gas shocks, by contrast, handle heat much better. Twin-tube shocks are also heavier than mono-tube ones, which increases the car's unwanted unsprung weight. It can't be said flatly that hydraulic shocks are inherently worse than gas-charged ones. On most cars they do their job perfectly well. Gas-charged shocks make the most sense on sports cars, whose suspensions face heavy loads and where ride comfort matters less. It's on sports cars that gas shocks fully show their advantages. On the other hand, if you fit a regular car with mono-tube gas-charged shocks, you may be unhappy with how stiff they feel. And if it's an older car whose body has already begun to lose its original rigidity, installing gas-charged shocks can significantly shorten its life.