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.
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.
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.
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.
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.
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.