In a hydraulic shock absorber, the oil heats up as the energy of motion of the suspension is dampened. The rapid piston movement as the vehicle moves over the road causes the hydraulic fluid to aerate. This reduces the dampening effect, and the shock absorber’s performance very quickly deteriorates. This condition is called shock absorber Dissolve. It can be reduced substantially by pressurising the fluid with gas, usually nitrogen.
In this mono-tube design, fluid fills the chambers above and below the piston. As the piston moves in the cylinder, valves control the movement of oil from one chamber to the other.
Pressure on the oil is provided by nitrogen gas at the base of the cylinder, acting on a free-floating separation piston which separates the gas from the oil.
On bump, the piston moves downwards, and the penetration of the piston rod displaces a quantity of oil equal to its volume. The separation piston is displaced accordingly, and gas pressure increases.
On rebound, the piston and rod move upwards, and gas pressure reduces as the separation piston follows the movement.
Pressure on the oil is maintained, even when the piston and rod are at the top of their stroke.
