When it comes to components involved in a vehicle's suspension and steering unit, there's a lot of talk about Shocks and struts. In fact, there's a great deal of confusion about these items. The discussion is usually centered around their function, their potential interchangeability and when it may be time to replace these particular items.
The first thing to understand is shocks and Struts are both quite different. A shock helps to absorb bumps in the road and provides passengers with a smooth ride. A strut is a shock absorber that is a permanent part of the vehicles suspension unit. It works similarly to a shock absorber, but it also controls things such as the caster and camber of the front wheels, as well as controlling the ride height of the vehicle. With a strut, there is also a spring coil, which helps to limit the movement of the vehicle when driving on a bumpy road.
It's also worth noting that these systems are not interchangeable. In fact they are quite different in terms of ease of replacement. To change a shock absorber, it's a very simple and straightforward removal of the old absorber and the installation of a new one. Replacing a strut is a bit different and more involved because it will also involve having to properly align the tires once a new strut has been put in place.
Both a shock absorber and a strut have similar problems with the wear and tear. While there is no magic number to when these parts need to be replaced, anywhere from 70,000 to 80,000 miles is usually when failure begins to occur. For shock absorbers, seals within the absorber may begin to break down and allow hydraulic fluid to leak out. This is a good indication that the absorber needs to be replaced. This is also true for a strut. However, the mount for the strut as well as the coil spring can slowly begin to fail with excessive wear and tear.
The mount as well as the spring don't typically fail all at once and it is more gradual failure. As the car begins to travel more over bumpy roads or the bumps are felt more significantly than in the past, this is a good indication that the system is beginning to wear down and may require replacement. A mechanic will be able to best determine when it's time for a replacement.