Shock Absorber FAQ

Hydraulic shock absorbers have been in common use for over a century but still produce a lot of enquiries, so here is the Tech Know shock absorber FAQ, from Hendrickson. To help clear up uncertainties, Hendrickson have put together a list of Frequently Asked Questions to help clear up common shock absorber misconceptions.

What Does A Shock Absorber Do?

Shock absorbers dampen suspension movement. Without them, suspensions will overreact to movement and vehicles could literally bounce off the road, particularly around undulating corners. They control movement and make the ride more comfortable for the driver and safer for cargo. Excessive suspension movement also has a detrimental effect on our road surfaces, which is why firmer shocks are often required to meet ‘Road Friendly’ certification.

How Does A Shock Absorber Work?

Shock absorbers work by converting kinetic energy from body movement and wheel vibrations into heat. Movement of oil within a shock absorber is restricted by calibrated shims and valves, which are built into the foot valve and piston. (Refer to illustration.) The resultant heat is absorbed by the oil and released through the shock absorber body, which is cooled by the passing air.

Are Shock Absorbers Double Acting?

All current shock absorbers are double acting. Most of the first hydraulic shock absorbers were single acting, meaning there was no restriction on compression and only offered resistance on rebound. However, they are now all double acting. If you look at the chart of the dynamic shock force versus velocity, you’ll see that while the extension and compression curves are different, the shock still dampens in both directions.

What Could Cause Shock Mountings To Break?

There can be a few causes of shock mounts breaking. One of the most common causes is incorrectly set vehicle ride height. Too high a ride height will cause the suspensions to constantly ‘Top out’. Eventually something will give way, either the mounting, the shock eye mount, or the shock itself will come apart. (Refer photo.) Raise/lower valves can cause the same damage if the trailer isn’t dropped back to ride height before the trailer is put back into use. Shock mounts that are not regularly torque-checked can also break because movement between components can cause thumping which will eventually cause something to break. Shocks in severe service applications, such as tippers, may need extra support from rebound limiters to prevent damage to shocks.

Should I Replace Shocks Coated With A Thin Film Of Oil?

A fine coating of oil is quite normal for a working shock absorber. Shock absorber rods have seals to keep the dust and dirt out and the oil in. However, these rod seals rely on a thin film of oil to keep them lubricated and in good condition. As the shock absorber extends, some of the hot oil coating the piston rod evaporates before condensing in the cooler outside air onto the shock absorber body.

This forms an oily film on the outside of the shock absorber body. Over time, this film will collect dust and grime, which will often coat the entire body of the shock absorber. Misting is a perfectly normal and necessary function of the shock absorber. If you are unsure about the condition, then raise the chassis to fully extend the shock absorber, which may then be closely inspected without needing removal.

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