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A Guide To Inspecting Truck Shock Absorbers

Technical Bulletin - Shock Absorber Inspection

Misting Shocks vs. Leaking Commercial Vehicle Shock Absorbers

Many technicians find it difficult to differentiate between a misting shock absorber that is working perfectly and a leaking shock absorber that needs to be replaced quickly.

Misting is a perfectly normal and necessary function of a commercial vehicle shock absorber. Shock absorbers are designed to carry a film of oil through the piston rod to the rod seal to lubricate the seal lips. This reduces friction and wear at the seal contact area and prolongs seal life. Misting occurs when the hot piston rod is drawn out of the shock body and the microscopic film of hot oil on the rod turns to vapor. This vapor, also called mist condenses when it reaches the cooler outside air and forms a film on the outside of the shock absorber body. The film will attract road dust and debris and over time will often coat the entire body of the shock.

The role of a shock has changed dramatically. Not so many years ago, most suspensions had heavy multi-leaf springs with limited travel and a great deal of inherent friction. These suspensions quickly self dampened and therefore limited the amount of work the shock needed to perform.

Low friction suspensions such as air and taper leaf have dramatically changed the shock's role. Shocks now play a critical role in dampening suspension oscillation.

Properly functioning shocks can help reduce the wear of more expensive suspension components such as air bags, while assisting in the reduction of tire wear and vibration damage to the cab and chassis. They also aid in reducing driver fatigue. Worn shocks simply cannot assist in providing control over today's sophisticated suspension systems.

NOTE: Some minor streaking of oil may appear on the shock body during initial stroking. This is a result of the seal "setting" and purging any oil trapped in the seal during the assembly process. This is not to be mistaken as a failure. This new shock absorber condition is temporary and totals only a few mililiters of oil.

When Should Shocks Be Replaced?

There are many tell tale signs that indicate it may be time to replace shocks.

    Inspect for:
  • Uneven Tire Wear - Balance should be checked first
  • Ride Deterioration
  • Excess Vibration
  • Sagging Taper Leaf Springs - This is not caused by weak shocks
  • Premature wear on cab, electrical, and cooling system components
  • Broken or Torn Air Springs - Shocks will only affect this if they are broken or are missing

Leaking VS. Misting Shocks

Take The Heat Test

Shock absorbers function at temperatures ranging from ambient to 350 degrees Fahrenheit. A shock's role is to dampen the oscillation of the truck's springs. It does this by transforming the energy produced by the spring to heat and dissipating it. As a result the shock should be slightly warm to hot to the touch after normal use.

If ride deterioration is experienced and there is suspicion that a shock has failed internally, which is visually undetectable, perform the following Shock Absorber Heat Test within a few minutes of operating the equipment.

  1. Drive the vehicle at moderate speeds for at least 15 minutes.
  2. Within a few minutes of driving the vehicle check the temperature of the main shock absorber body below the dust tube. Compare this temperature to the temperature of the surrounding chassis frame. Warning: Do not touch the shock absorber as it may be hot and could cause a burn injury. An infrared thermometer gun or similar measuring device is recommended.
  3. All shock absorbers should be warmer than the chassis. Suspect a failure in any shock that is noticeably cooler than its mate on the other end of the axle. Different temperatures from axle to axle do not necessarily indicate failures, but cooler temperatures on any one axle does warrant removal and examination of the cooler shock absorber.

To inspect for an internal failure, remove and shake the suspected shock. Listen for the sound of metal rattling inside. Rattling of metal parts can indicate that the shock has an internal failure.

This documentation was provided by Gabriel.

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