Car Shock Absorber Gas Vs Oil. Pressurizing the oil inside the shockstrut helps smooth the ride over rough roads. Out through the oil into the air.
Many shock absorbers contain pressurized nitrogen gas in them as well as the hydraulic oil. If it were to mix with the oil and then pass through the valves it would create a loss of resistance. This type of shock absorber is suitable for sports car models or if you like to drive at high speeds.
Most modern shocks also contain nitrogen gas to reduce shock absorber fade that happens if oil becomes aerated and bubbly due to heat buildup.
But they do offer much better control at higher speeds. The majority of todays vehicles use gas-charged shocksstruts. The shock transfers the energy of the cars motion into heat and then dissipates the heat. If a shock has to react very rapidly to an up and down motion the hydraulic oil can start to foam.