Your car or truck's strut cartridges are the parts that are doing all the "dirty work" for your vehicle's suspension. When there is a problem with your ride, you will know it right away. The body or chassis may bounce a few more times when going over a bump, you may notice a rough response to those same bumps - almost like your vehicle is bottoming out, or you'll hear squeaking and whining while you drive. These all point to a problem with your struts, and can be fixed if you have the appropriate strut inserts installed. With age, all of your dampening devices will begin to leak. That is the nature of beast when it comes to hydraulics, but it is very easy to see. If you have a bad strut or shock, a simple visual inspection of the unit will tell you if oil is leaking out. This means danger is right around the corner, and if you do lose control of your car or truck while making a fast turn or dodging an obstacle then people could get hurt.
Hydraulic dampening has been used for over a century. The most common form of suspension on cars today is a MacPherson strut assembly suspension, because it provides a great amount of flexibility, 360-degrees of motion at the upper mount, and it eliminates the need for an upper control arm. The problem is that these strut assemblies are expensive to replace, and it would be much less costly to simply replace a leaking strut cartridge. Today, strut cartridges include a stem that travels down into a reservoir of hydraulic fluid. On monotube designs, that oil presses down on a floating piston that compresses a pocket of gas. That gas will "fight back" and rebound against the piston and push the stem back up. Twin tube designs have pockets of gas running along the sides of the strut body that the oil compresses without the use of a piston.
Inside your strut assembly lies the strut cartridge insert. Here is how it's replaced: