What causes shock absorbers or struts to leak or otherwise fail?
Shock absorber/strut life depends on how and where the vehicle is driven. Original equipment shock absorbers/struts are carefully matched to the vehicle springs and bushings to provide the best ride comfort and control. As the ball joints, control arm bushings, and other suspension components age, the energy built up in the springs of the vehicle is controlled less by the friction of these joints and bushings, requiring more control from the shock absorbers/struts.
Normal suspension systems are called passive suspension systems. Passive systems have either a firm or a soft ride. Their height varies according to mechanical forces on the suspension and they do not adjust to these changes.
The major purpose of any shock or strut is to control ride and handling. The springs support the weight of the vehicle; the shock absorbers control the actions and reactions of the springs. Various styles of dampers are in production, hydraulic, gas-charged, air-inflatable, magneto-rheological fluid, and electronic-controlled to name a few. For the purpose of this discussion, gas-charged dampers will be covered.
The majority of today's vehicles use gas-charged shocks/struts. Pressurizing the oil inside the shock/strut helps smooth the ride over rough roads. This pressure helps prevent air pockets from forming in the shock absorber oil as it passes through the small passages in the shock.
Even under normal conditions on a smooth road, shocks can cycle an average of 1,750 times for every mile traveled. This can add up to almost 9 million cycles in 50,000 miles and that is why inspection of suspension components are so important.
When a shock or strut is cycling and exchanging fluid between chambers, it is generating heat. This heat is produced because the unit is changing the movement of the suspension into thermal energy. The solution for managing this excess heat is nitrogen gas. The gas chambers in the shock or strut keep the shock oil under constant pressure, which reduces foaming. If the gas leaks out of the chambers, the unit will “fade” and not perform as intended. Note that the unit will not leak externally. Instead, the nitrogen will form tiny bubbles in the oil. Since nitrogen gas is compressible and is much thinner than the oil, the unit starts to fade and provides much less resistance and ride control.
The valve size is not constant on most shocks and struts. By using metal discs and springs, a valve can change the size of fluid passages depending on the type of rod movement. The discs and springs in the valves can suffer from metal fatigue due to the constant movement of the suspension and fluid, which can lead to wear and handling problems. This type of wear can’t be seen with the naked eye.
The seal between the piston and bore is one area of wear. The seal must prevent fluid from flowing between the two surfaces without creating excessive amounts of friction. If the seal allows too much fluid to pass, it will influence how the valves perform. Poor sealing surfaces will cause the unit to leak. If the pitting or lost chrome plating is large enough, it can damage the seal, which can lead to water and debris getting into the unit and damaging the valves and piston seal.
If the outside of the shock body is damaged in any way due to road debris or collision. If rubber bump stops show signs of contact, a shock could be damaged or the vehicle might have been lowered or raised excessively.
A preliminary shock test with the car on the ground is the bounce test. Push down hard two or three times on the fender at each corner of the vehicle. After the fender is pushed down, it should oscillate only about 1.5 cycles and then settle.
The bounce test only checks the first stage of shock/strut operation. A failure in the second or third stage valves will not be evident from this test. So even if a shock passes the bounce test it still might be defective.
The bounce test or looking for cupped tires only highlights worst case shock/strut damper failures. Evaluating worn shocks/struts is best done by driving the vehicle over variety of roads. Shocks/struts do not often fail at the same rate. Usually one damper has a problem, requiring the replacement of both.
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