Shocks/struts can either come charged with nitrogen (gas shock) or conventional (without a charge). Both can wear out over time. Shocks should be replaced around 50k miles under normal conditions. That really depends on how you drive your vehicle, though. If all of your driving is done on the freeways of Texas where the straights are longer than long, then you'll find your shocks will last you longer ... maybe even considerably longer. If, on the other hand, you drive in a downtown area where you are stop/go all the time, plus have plenty of potholes to worry about, those shocks are going to have a much shorter lifespan.
What exactly does a shock do? It converts the bouncing energy which happens when you are going over the bumps into heat energy which dissipates through the hydraulic fluid in the shocks. Due to this, there is only so much abuse the shock can take before it wears out. Not only does how the shock is used have an effect on the life expectancy, environmental factors can take a toll as well. If sand or salt is used in wintry conditions, these elements can get up on the shock and wear them out through abrasion or rust.
The best way to know what kind of condition your shocks are in is to pay attention to them. How does the ride feel? Does your vehicle bounce up and down when you come to an abrupt stop? If so, your shocks may be worn out. There are two basic things you can do to see if there may be an issue:
- Take your car into a parking, drive it at about 10mph, then slam on the brakes. If your front end continues to bob up and down after you come to a stop, your shocks (at least the front ones) are probably toast.
- Get under your vehicle and look at the shocks. If you see any oil seeping from where the rod inserts into the shock, it's done. A fine sheen of oil on the rod (no drips) is okay, as this is normal (if it isn't lubricated, it will tear up the seals). You also want to look for dents in the shock casing, as this is a sign of abuse (could be caused by road debris). While you are under there, make sure you check all of the mounting hardware and rubber bushings associated with your shocks. Shocks require solid mounts in order to work correctly. If the bushings are worn out or the bolts are loose, this will degrade their performance.
There are other things which can happen if you ride on worn out shocks. Since the shock is no longer able to buffer the rebound/extension of the wheel travel, the car will bounce (given). Due to this, the tire will have a tendency to bounce up and down as you are going down the road. This wears out the tires by what is called "cupping" or "scalloping". You'll see small, smooth, flat spots on your tires at regular intervals. When this happens, the tire will need to be replaced as well. Here is a list of some of the dangers due to riding on worn out shocks:
- Reduced braking efficiency resulting in longer stopping distances
- Reduced efficiency of Anti-Lock Braking Systems (ABS) and Electronic Stability Control (ESP)
- Increased risk of skidding in the wet
- Aquaplaning occurs at lower speeds
- Less control when cornering or caught in a cross wind
- Increased driver tiredness and reduced speed or response
- Increased wear of tyres and other suspension components
- Uneven/oscillating headlight level causing dazzle to on-coming drivers
- Increased passenger discomfort
- Increased risk of 'snaking' when towing
This site gives an example of real world testing of the differences between good and worn out shocks:
To highlight the danger of driving with worn shock absorbers, Tenneco Automotive - Monroe's parent company Ð has recently conducted a number of comparative tests between vehicles fitted with 50% worn shocks and 100% effective, Monroe Sensa-Trac with Safe-Tech shock absorbers. Having teamed up with TÜV, one of Europe's leading road safety institutes, the first tests showed that a multi-purpose vehicle (MPV) such as a Renault Espace with worn shocks can have a braking distance that's up to 4 metres longer than the same vehicle fitted with new shocks.
The second test featuring a new Volkswagen Beetle found that despite being fitted with the latest car safety systems such as Anti-Lock Braking (ABS) and Electronic Stability Control (EBS), with worn shocks the vehicle can have a braking distance of up to 6 metres longer than when fitted with new shocks. The final set of tests set out to demonstrate what effect ice and snow would have on a car's anti-lock braking and anti-skid systems, when combined with worn shocks. In the braking test, the results showed that a vehicle such as a Mercedes-Benz C250 Estate fitted with ABS but also with 50% worn shocks, had an emergency braking distance of 1.8 metres longer than the same vehicle fitted with 100% efficient shocks.
The same test conducted on a Peugeot 206 with ABS and 50% worn shocks, took an extra 1.2 metres to stop. The braking test results thereby prove that a car's ABS system does not function properly with worn out shocks. Finally, when comparing a car's acceleration on ice and snow, the tests found that a car such as the Mercedes-Benz C250 fitted with an ASR anti-skid system and 100% efficient shocks accelerated to a speed of 35.6 km/h (22.1 mph) in seven seconds, whereas the same car fitted with 50% worn shocks could only achieve 34.2 km/h (21.2 mph), a 16% improvement for the first vehicle. Furthermore, the time period of lost traction was 37% shorter with 100% efficient shocks compared to 50% worn components.
This test confirms the importance to check that a car's shocks are in good condition, even when fitted with ASR.
You'll find that the basic technology which you are used to on your motorcycles is what's in use on cars. Also, because of heat buildup in the hydraulic oil, breakdown occurs due to this. Heat is the major component of shock failure (or when a shock gets worn out). Just like so many other things in the automotive world, shocks have a lifespan. The harder you beat on them, the sooner they die.
Some of this was mentioned already, but I thought I'd consolidate. There are six noticeable signs your shock absorbers need to be replaced:
- Longer stopping distance
- Swerving & nose-dives
- Car sliding & veering
- Rocking & rattling
- Uneven tire wear
Hope this helps.