What Really Wears Out Inside Shocks & Struts - Tire Review Magazine

What Really Wears Out Inside Shocks & Struts

When shocks and struts wear out there are almost no visual clues. When a unit is leaking, it is a sign of failure and not wear. 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.

Next to performing an autopsy or throwing the unit on a dynamometer, there is no conclusive way to determine if the internal components are worn to a point where it could compromise vehicle stability. Even the “knee on the bumper” test can give false results due to how some units handle low frequency movements.

Shocks and struts wear and thus their performance degrades, gradually and even imperceptibly. After more than three years of durability testing, the Automotive Maintenance and Repair Association (AMRA) offers its take on shock/strut wear. 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. Some of the first items to wear on a shock or strut are the boots and bellows that protect the shaft from road debris.

MAP has expanded its Uniform Inspection & Communication Standards 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.

This standard is for OE hydraulic fluid and/or gas charged shocks and struts only, not for electronically controlled units. This replacement recommendation, AMRA said, 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. When a stone hits the chrome shaft of a shock or strut, it will first cause a pit or scratch. This might later 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.

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

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 render 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.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 they had brake problems and did not notice the changes in vehicle behavior over the period of time it took the strut to degrade.

Worn bushings and joints can change the travel of the shock or strut. Damaged control arm bushings could cause the unit to bottom out and damage the valves and gas chambers.

Inspecting the tires may reveal suspension issues. Uneven wear or toe wear would tell you the tire/wheel assemblies are out of alignment. If the strut rod is bent, it could cause excessive camber or caster. One key 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 too much fluid to pass, it will influence how the valves in the piston and base perform.

Uneven surface wear across the face of a tire can indicate weak ride control components. One sign is tire cupping, which could be the result of improper tire balancing or improper damping force in the shock absorber. Also, tires may have inside or outside excessive edge wear from improper wheel alignment. 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.The seal at the top of the body is the barrier between the harsh environment outside and the fluid and gas within the unit.

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