Struts may seem like very complex components, but they're actually easy to diagnose if you know what you're looking for. Struts are combination components, equal parts of spring, shock absorber and upper control arm. This multipurpose capability means that struts can exhibit at least three different types of failure. The trick is to break the strut down into its individual components and diagnose it from there.
Just as a paper clip will get hot when you bend it back and forth, the spring gets hotter when it cycles up and down. Because spring material receives its springiness via heat-treating -- repeated heat cycling can cause it to get springier and softer. Sagging is one sign of a soft spring, but pushing down on the bumper will tell you more. To test the springs, slowly push down on one corner of the car with three fingers; if you can move it more than a couple of inches -- then odds are that the springs are shot.
Shock absorbers work like medical syringes, slowing suspension movement by forcing oil through a number of small holes between the shock's two fluid chambers. Over time, the shock oil will break down and get thinner, allowing it to pass through the holes easier and reduce the chock's dampening effect. Without shock absorbers, the wheels and car body would just pogo up and down on the springs until they run out of energy. To test the shock, place both hands on the car's bumper and give it a sharp downward shove. The car should bounce upward until the spring extends and then settle down again. If the car continues to bounce past one cycle, the shocks are probably bad.
Mechanical Failures and Leaks
Struts can also fail in a number of mechanical ways, from loose bolts to bent brackets to worn out clips and rubber stops. Aside from a visual inspection, the mechanical failure's primary symptoms are squeaks, rattles, bumps and groans while cornering or going over bumps. If, when traversing a large pothole, your car suddenly slams down like the springs were made of concrete, then odds are the strut's rubber bump-stop has degraded and hardened. If you diagnose a bad strut, then clean the struts with a pressure washer and check the shock body for leaking oil.
Shocks and springs exhibit two different but related symptoms while driving. Bad springs will not resist vehicle movement as well as they should, which will allow the car's body to lean more than it should when cornering. Bad shock absorbers will allow the car's body to lean quicker than it should, and to rock back and forth after exiting a hard corner. Both bad shocks and springs will allow the car to nose-dive more under braking, to lose traction under acceleration and turning and to make the ride excessively soft and compliant. If you hit a speed bump at 30mph and don't spill your latte, then its definitely time to invest in a new set of struts.
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