There are three types of shock absorbers -- hydraulic twin-tube, twin-tube low-pressure gas and mono-tube high-pressure gas. The twin-tube hydraulic shock is the oldest and least complex design and the cheapest to produce. That's why nearly all American cars receive them as original equipment. The twin-tube has an inner pressure chamber that houses a piston and oil; a concentric outer tube serves as oil reservoir. Depending upon piston speed, oil is forced through orifices in the piston or through a valve in the base of the pressure tube. The twin-tube shock can easily be valved to deliver good ride quality and it works well with the long suspension travel and moderate spring rates found in most stock vehicles. Properly designed and built, such a shock can give admirable performance with good longevity.
   One design limitation of the twin-tube shock is that the outer chamber acts as an insulator, slowing the rate of heat transfer from the oil inside as it's churned by the fast-moving piston. Heat buildup is the bane of shock absorbers, eventually leading to aeration of the oil. Oil can't be compressed but air certainly can; mixing the two causes a sharp drop in damping performance. Aeration can be kept in check by using a larger-diameter piston, high-quality oil and premium materials, particularly for the piston seal. But a bargain-basement shock--constructed with low-rent materials and indifferently assembled--will quickly overheat and wilt at the first hint of spirited driving.
   Efforts to limit heat build-up led to the creation of the mono-tube