Twin-Tube Dampers
Twin-tube dampers are the most commonly found dampers on the market. Twin-tube dampers are just that; they have two tubes, one inside of another. The damper piston and shaft ride in the inner tube. The oil flow in compression and rebound is controlled by valves in the piston and in a valve that controls oil flow between the inner and outer tubes known as the foot valve. Twin-tube dampers can be made with looser tolerances and are usually cheap to make. For these reasons most stock dampers are of the twin-tube variety.
The problem with using oil inside a damper that needs to change volumes is that oil, like all liquids is essentially incompressible: it occupies the same amount of space regardless of how much pressure it's under. To create a space that will change with pressure, gas is pumped inside the upper portion of the outer damper tube. This gas volume will grow or shrink as the pressure changes inside the damper and as oil is displaced back and forth. Without this, the damper would not compress because there wouldn't be anywhere for the oil to go, since more volume is occupied by the shaft as the damper compresses. To allow for shaft displacement, most twin-tube dampers have a generous volume of air or nitrogen gas in the outer tubes. Gas charged twin-tube dampers are pressurized to around 75psi and are known as low-pressure gas shocks (whack!).
Obviously, if there are low-pressure shocks, there must also be high-pressure shocks, but these are normally found in mono-tube designs, which are usually compressed to about 150-300psi. The gas pressure in a shock creates a gas reaction that acts like spring preload. It does not affect the suspension spring rate, but it does affect the amount of force it takes to move the suspension initially.
Most twin-tube dampers control damping force through orifice sizing and the spring rates of check valves that control flow through both the piston orifices and foot-valve. The foot-valve meters oil that is displaced by the shaft as it is pushed from the inner tube to the outer. The piston-valve meters the oil as it's forced back and forth through the oil by the piston shaft.
The main drawback of a twin-tube damper is that its damping force is not as sensitive nor as repeatable as a mono-tube. Its lower internal gas pressure means the oil can still foam under extreme usage as the gas and oil mix. By design, the oil is forced between two sets of valves, meaning both see a low amount of flow. This makes the valving less sensitive to shaft movement. Because the inner tube is usually pretty small in diameter, the twin-tube also has a small piston with less bearing or contact area with the tube's inner wall, which leads to greater wear.
With an inner and outer tube (and a bubble of insulating gas surrounding the inner tube), the heat path out of the damper is long, making twin-tubes less efficient at dissipating heat. A hotter damper means hotter oil, which tends to thin out, making damping inconsistent. Higher temperatures also make the oil more prone to cavitation because the oil is closer to its boiling point.
Another drawback of the twin-tube design is its lack of a physical barrier separating the gas in the outer tube and the oil. In other words, the oil and gas can mix, especially under hard use, causing erratic and unpredictable changes in the damping force. The damper must also be mounted vertically with the shaft up and the foot-valve down, so that the valve remains submerged. If the foot-valve is exposed to the gas, it's rendered completely ineffective. Sometimes the gas is contained within a plastic bag wrapped between the inner and outer tube so it cannot mix with the oil. This is only a partially effective solution because movement soon wears a hole in the thin plastic, allowing the gas to escape and mix with the oil.
Sometimes damper makers take liberty with the term "gas shock" (duck) and use a piece of closed-cell foam placed in between the inner and outer tubes. The "gas" is the trapped gas in the foam! As the damper compresses, the gas bubbles in the foam compress to make room for the shaft's displacement. Eventually the oil in the damper attacks the foam, breaking it down and allowing the gas to mix with the oil and it's back to square one. These are pretty cheesy (and unfortunately common) attempts at stretching the marketing term "gas shock."
On the plus side, a twin-tube damper is less prone to damage from road debris. The outer tube protects the inner tube, which contains the piston, from dents. Twin-tube dampers are also easier to make adjustable because it's easy to get an adjusting rod to the foot-valve from the outside of the damper. Most damping-force adjustment is made through a needle valve that affects flow in the passage leading to the foot-valve, or by changing the preload of the spring controlling the foot-valve itself. Thus, most of the low-cost adjustable dampers on the market are twin-tubes.
Another reason twin-tubes cost less is because they don't have high-pressure internal seals, which means they can be built to looser tolerances for less money. That's why the OE's prefer this design for production cars. An advantageous side-effect of the twin-tube's sloppy nature is that it is relatively insensitive to small high-frequency movements like cracks in the pavement, tar strips and botts dots. Although from a high performance perspective, this sucks, the low internal gas pressure or lower level gas reaction preload force of a twin-tube tends to have a slightly smother ride on ripply bumps compared to a mono-tube brethren.
Some of the better known twin-tube dampers on the market are the KYB GR2 and AGX, Tokico Illumina, Koni Sport Yellow and Red, Rancho, most Monroe and Gabriel HD replacement shocks and the cheaper models of the TEIN and KW line up. Although the twin-tube has many disadvantages over a mono-tube, the drawbacks are actually minute and many better twin-tube shocks work quite well for high-performance street and low-budget club racing use. Twin-tubes remain a good value and can offer quite a bit of performance improvement for the serious driver.