Twin Tube Gas
This type of shock absorber has two tubes. An outer reservoir tube and inner pressure tube. The piston moves up and down inside the pressure tube and oil is forced through the compression valve or the rebound valve.
As the oil displaces, an inert gas such as nitrogen at relatively low pressure (around 5 atmospheres) is maintained on the oil in the reservoir tube.
The reason why an inert gas at pressure is used, is to reduce the chances of aeration. This type of shock absorber eventually will fade if used hard and simply requires time to cool for full damping to return.
As the name implies, this type of shock absorber uses only one tube and the piston valving controls both rebound and compression damping. A floating piston separates the nitrogen gas from the oil and the gas is at a high pressure generally above 20 atmospheres.
This type of shock absorber is more prone to stone damage than the twin tube types. Once the tube is hit, the rod piston and/or floating piston will no longer seal properly against the cylinder wall.
With the gas at the bottom of the shock absorber, it's difficult to design one with as much travel as the twin tube designs above. As such it's a popular shock absorber on road cars and as it heats up, the gas pressure increases and exerts more force on the oil. The end result is that the damping rates increase with heat.
More info here...http://www.safari4x4.com.au/80scool/...absorbers.html