It is a fact that misting shocks are often misdiagnosed as failures, however this is a normal and necessary function, and there’s a bit of a debate among mechanics between misting and leaking shock absorbers requiring replacement during shock absorber inspections.
As the shock absorber rod moves out, some of the hot oil coating the rod evaporates before condensing in the cooler outside air onto the shock absorber body. Shock absorber rod seals rely on this thin film of oil to keep the seal lubricated and to prolong seal life.
A leaking shock will show clear signs of fluid leaking in streams from the upper seal, which may drip from the shock. The seals may leak because of extreme wear, contamination or defect. Raising the chassis to fully extend the shock for inspection is a good way to confirm the condition.
In addition, other regular inspection points include:
- The mounting bushes for wear or deterioration
- The shock body for bends or dents
- The mounting bolt tightness and security. A loose mounting bolt will usually leave witness marks around the mounting bolt washer from the relative movement between the parts
- Checking for broken upper or lower mounts. Incorrect ride height, fitting the wrong shock absorber or deteriorated shock bushes can damage the mounts. Damage to the mounting holes by a loose shock bolt must be repaired or it will cause the new shock to come loose as well.
Be sure to wipe any built-up oil and dust from the shock body after every inspection.
As dampening of the suspension movement occurs, shocks produce and dissipate heat. As a result, shocks can operate at temperatures ranging from close to ambient up to 175° C.
Depending on the driving conditions, the shock should be warm to touch in normal operation. Use extreme caution if checking the temperature with the back of your hand. High ambient temperatures and driving over corrugated roads can induce temperatures hot enough to burn you. Do not touch the shock if there appears to be excessive heat with your hand near it.
Checking the shock temperature with an infrared thermometer at the body below the dust cover tube is highly recommended. All shocks should be warmer than the vehicle frame after driving. Any shock that is noticeably cooler than the one on the other side of the axle may have failed. A differing temperature on any axle warrants removal and examination of the shock.
Checking shock function by hand is not recommended due to different designs and the rate of movement. Truck shocks will usually compress easily, while the rebound is firm. Trailer shocks will usually be firm on both compressing and rebound strokes but may vary dependant on the manufacturer’s suspension application.
To inspect for an internal failure, remove and shake the suspected shock while listening for the sound of rattling metal. Replacement is required if a rattling is heard as the sound is an indicator that the shock has an internal failure.
Overall, shock absorbers perform a critical role in dampening oscillation. Checking them regularly and replacing them when required will help prevent suspension system component and abnormal tyre wear.