The front strut of a motor vehicle, a part of the suspension system used to absorb shock, should be replaced if it looks wet and oily, which can indicate a leak of hydraulic fluid from inside the strut. If a vehicle seems to bounce or knock going over bumps, seems to swing up and down, dips forward at braking or dips significantly toward one side on turns, it's probably time to replace a strut or shock absorber.
Another indicator that replacement may be necessary is an inconsistent tread pattern, smooth patches alternating with tread, on the tires, which is usually caused by bouncing due to failing struts. Small pieces of the tire are scraped off with each bounce, and the holes left by the missing rubber become smooth with driving. Mechanics use a bounce test at each corner of the vehicle to identify a problem strut or shock absorber, more than two bounces indicating a problem. After hitting a pothole or curb it is important to get a wheel alignment, which also determines if a strut is bent. A bent strut, putting full weight on the other struts or shock absorbers, is a safety hazard that may not have any clear symptoms.
Strut failure is gradual and sometimes hard to detect. While struts do not need regular maintenance or replacement, it is good to request a regular inspection and a test drive by a mechanic to ensure they are performing properly, suggests Brake and Beyond.