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Why Shocks And Struts Wear Out

Ask any one working the front counter what is the most difficult items to sell and two items will always come up, diagnostic fees and struts. When shocks and struts wear out, there are almost no visual clues. The car will not turn on a light or leave the customer stranded. The clues are all in the “seat of the pants” of the driver. When a unit is leaking, it is a sign of failure and not wear. Even the “knee on the bumper” test can give false results due to how some units handle low-frequency movements.

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Ask any one working the front counter what is the most difficult items to sell and two items will always come up, diagnostic fees and struts. When shocks and struts wear out, there are almost no visual clues. The car will not turn on a light or leave the customer stranded.
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The clues are all in the “seat of the pants” of the driver. When a unit is leaking, it is a sign of failure and not wear. Even the “knee on the bumper” test can give false results due to how some units handle low-frequency movements. 

What has happened inside the shock or strut is what makes shocks and struts lose their ability to control the motion of the suspension. It could be wear on the bore and piston. But, more than likely, what has happened is that the valves have worn out or the gas inside the unit has leaked out.
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Shocks and struts wear, and their performance degrades, gradually and even imperceptibly. After more than three years of durability testing, the Automotive Maintenance and Repair Association (AMRA) says the answer is 50,000 miles. AMRA represents the automotive repair industry and promotes a consumer outreach effort, the Motorist Assurance Program (MAP), that’s meant to strengthen communication and trust between the industry and its customers. MAP has expanded its Uniform Inspection & Communication Standards (UICS) to include a standard for shocks and struts. The standard says most ride control units degrade measurably by 50,000 miles and replacement for improved vehicle performance may be suggested to the customer after that point. 
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This standard is for OEM hydraulic fluid and/or gas charged shocks and struts only, not for electronically controlled units. This replacement recommendation was determined through more than three years of testing performed by four major manufacturers of shocks and struts. In laboratory testing, shocks and struts were tested more than 70 million cycles of compression and rebound. 

The wear incurred during the 70 million cycles was not on the outside, but on the inside.  

on most gas-charged mono-tube shocks and struts, nitrogen gas is ­contained within a chamber in the body. the gas ­prevents aeration or ­foaming of the fluid. over time, the seal can become ­damaged and leak.

 
 

when a stone hits the chrome shaft of a shock or strut, it will first cause a pit or scratch. this might cause damage to the seal, which might cause a loss of fluid. if the damaged area starts to corrode, it could cause the shaft to lose even more chrome. some of the first items to wear on a shock or strut are the boots and bellows that protect the shaft from road debris.

 
 

this rear strut shows the ultimate result of neglect and/or lack of inspection. the seal and upper part of the tube are completely gone. the housing has been distorted by braking forces.  the driver thought she had brake problems and did not notice the changes in vehicle behavior over the period of time it took the strut to degrade.

 

if the fluid becomes contaminated, it can cause wear to the bore and piston. according to some shock and strut suppliers, wear on these surfaces does not happen until the unit has become significantly degraded.

 
 
 
 

 

here is where most of the wear occurs in a shock or strut. these small discs of metal are mounted on the shaft and are held in place with either a nut or a nut and spring. as the piston moves, the discs deflect and fluid moves between the two chambers. even under normal conditions on a smooth road, shocks stroke on average 1,750 times for every mile traveled. the action causes a “shearing” action on the fluid that is not unlike what motor oil is subjected to when it is between engine bearings. this action can break down the base oil and additive package. the wear on the fluid can change the viscosity of the fluid and make the unit  unable to dampen suspension movement. the discs and springs in the valves can also suffer from metal fatigue due to the constant movement of the suspension and the passing of the fluid. the seal at the top of the body is the barrier between the harsh environment outside and the fluid and gas within the unit. the seal can not be effective if the surface of the shaft is pitted or damaged. poor sealing surfaces can cause the unit to leak. if the pitting or lost chrome plating is large enough, it can damage the seal, this can lead to water and debris getting into the unit and damaging the valves and piston seal. to protect the seal, it is essential to replace the boots, dust covers and jounce bumpers. if any of these items are missing, it could cause the premature ­failure of the new unit. One area of wear is the seal between the piston and bore. The seal must prevent fluid from flowing between the two surfaces without ­creating excessive amounts of friction. If the seal allows to much fluid ­­ to pass, it will influence how the valves in the piston and base perform.

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