Standard Oil, Gas, Foam Cell, Mono-tube, Twin tube shocks
Previously we briefly explained how shocks work and what they do. It’s important to know the differences between the types of shocks to best suit your needs and your pocket. The different shock types include Standard Oil, Gas Charged, Foam Cell, Mono-Tube and Twin-Tube shocks.
The majority of shocks available for our 4wds are Twin-Tube shocks. They have an outer tube or canister and an inner working or pressure tube. The piston moves up and down within this inner tube. It is a snug fit and the hydraulic fluid or shock oil is forced to pass through the piston. Excessive oil is stored in the outer chamber between the inner and outer tubes. There is an air space above this oil level in the outer chamber to allow for fluid to move between the inner and the outer tubes. This is necessary due to the fact that the oil cannot be compressed.
A Mono-Tube shock only has an outer tube. The piston assembly moves up and down against the inner surface of this tube. Again, an air or gas space is required somewhere within the body of the shock. This is done by putting a dividing, free floating piston below the shock oil separating the fluid from the gas.
We mentioned before that all shocks use hydraulic fluid that passes through restrictive orifices to dampen suspension movement. This action generates heat and when driving down your favorite Kalahari corrugated dirt road with your laden 4wd, things can get seriously hot. This heat together with the switching low and high pressure areas in the hydraulic fluid around the shock absorber working piston will cause the fluid to aerate or “foam”. This foam is much less dense than the oil. When the foam passes though the working piston, there is much less resistance and the shocks fade. Overheating, fading shocks = dangerous and uncomfortable ride.
In recent years, suspension engineers have come up with a solution to this fading by adding nitrogen gas under pressure instead of normal air. The purpose of this gas is to place the hydraulic fluid under pressure. This pressure decreases the foaming of the oil at high temperatures and these gas shocks therefore do not fade as readily as plain hydraulic shocks.
In both Standard Oil and Nitrogen Gas charged shocks however, the oil and air or gas is in constant contact with each other which accelerates aeration. To counter this, foam cell shocks do not have any air or gas within the body of the shock. Instead, a foam cell tube is inserted around the outside of the inner tube. This foam cell is made from a material similar to wet suit foam rubber and contains enclosed air cells. When excess oil is forced into the outer camber during shock compression, the foam cell is compressed. Foam Cell shocks usually have a larger bore size than gas shocks and a much higher oil capacity. They may however not be available for all vehicles due to their size.
In short, Nitro Gas charged shocks are better than plain Oil shocks and are a great all round performer. In severe conditions, the larger capacity Heavy Duty Foam Cells are worth a closer look.