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Peter Hart

Absorbing the shock

March 2020

Shock absorbers in good condition are important to road safety and comfort.

This article explains some basics of hydraulic shock absorbers that are used on heavy vehicles.

Shock absorbers provide damping in suspensions and in some steering systems. Old style stacked-leaf spring suspensions are damped by the friction between the spring leaves, and do not have shock absorbers.

Stacked leaf spring suspensions produce a harsher ride than airbag or parabolic spring suspensions, which use shock absorbers.

Shock absorbers used on heavy vehicles are generally sealed and not adjustable.

In contrast, shock absorbers used on lighter-duty trucks may be. Some shock absorber offerings for trucks with GVM up to about 7 tonnes can be adjusted by turning the top eye through 180° or 360° to alter the adjustment of the fluid openings in the internal plunger.

Soft, Medium and Firm adjustments are available.

Shocks are tuned to the suspension geometry.

If they are leaning forward, the characteristics are chosen for that orientation.

Uninformed substitution of the shock absorber is likely to change the suspension response.

For this reason, the road friendly suspension certification rests on replacing a worn shock absorber with the original specification.

Use of adjustable shock absorbers will also affect both the certification and ride quality.

By the way, it does not follow that a suspension design that meets the road-friendly technical standard (Vehicle Standards Bulletin No 6) will necessarily produce a comfortable ride.

The road and the driver’s rear-end are in different locations!

Suspension engineers have their own terminology. Jounce means bump. Jounce condition is the most compressed condition.

It is the limit of compression. Suspension blocks or stops are designed to prevent suspension slam or shock absorber damage. Rebound is the response of the suspension after being jounced.

Oscillation is rhythmic or irregular movements up and down. Suspension damping is provided to minimise jounce and rebound oscillations.

Unsprung Weight is the total weight of the wheels, tyres and axles below the suspension. Unsprung weight falls into holes on the road and it makes the suspension work as it travels on rough roads.

The suspension’s role is to minimise the transmission of these vibrations to the Sprung Weight.

The level of Unsprung weight is a factor in this success.

Heavy vehicle shock absorbers are usually softer in compression than in rebound so that shock forces into the chassis rails are minimised. This is called asymmetric damping.

There is an optimum damping level for ride quality, and this can be at odds with the damping required for road friendliness, so the damping is mainly provided on the rebound stroke.

Asymmetric damping factors of up to 5 may be used. This characteristic is achieved by changing the restriction of the plunger(s) to oil travel depending on direction and speed of travel.

Some shock absorbers also contain a gas space, so the oil pressure compresses gas in a sealed compartment.

Others have an inner and outer tube design that allows oil to be forced through a second fixed valve at the bottom, so the oil pressure works against trapped gas in the outer tube. The diagram shows the cross section of a typical heavy-duty vehicle shock absorber.

Suspensions are typically designed around the shock absorber length.

As the shock absorber in many suspensions limit the rebound travel, using a longer shock absorber may result in damage to suspension components, such as air springs pulling apart.

The shock absorber must not limit the jounce travel of the suspension, because the shock absorber will be crushed, or the mount will fail.

Shock absorber mounts must be carefully designed with a strength factor of safety of at least three.

The bushes add some flexibility, which is essential because the shock absorber mounts at top and bottom are moving out of sync.

There are pros and cons in the choice or bushing material between urethane and rubber.

Rubber is usually softer, and it provides damping and vibration isolation. Urethane has become the preferred bushing material over rubber because of its toughness and longevity.

Urethane also provides a more direct road feel and is easier to install than rubber bushes because of its toughness. Follow the suspension manufacturers recommendations with bush choice.

Loss of damping will result in greater magnitude oscillations. I recently drove beside a tri-axle group on an A-trailer that had the centre wheels oscillating noticeably more than the other two.

Shock absorber failure is indicated. In the extreme case, a wheel might lift off the road. So how can you tell if the shock absorber is worn out?

It is difficult to make this assessment based on the ability or inability to move the shock absorber by hand.

Misting of shock absorbers is often misdiagnosed as failure. Shock absorber rod seals rely on a thin film of oil to keep the seal lubricated.

If the internal oil is hot, a thin coating of hot oil becomes exposed outside when the shock absorber is compressed.

The oil evaporates and some of it condenses onto the cooler outside shocker body.

This is normal misting. Over time the film will collect dust and grime. In contrast, seal failure results in oil dribbles or streams from the upper seal. Oil dripping occurs.

To check the condition without removing the shock absorber, raise the chassis to full suspension travel and inspect the full length for leakage.

A visual inspection of the in-situ shock absorber might reveal oil leakage; loose, bent or broken mounting bolts; deteriorated or missing mount bushes; broken or damaged dust shield; body tents or bends.

Any of these observations require remediation and probably replacement of the shock absorber.

The National Heavy Vehicle Inspection Manual (see NHVR website), which is an authoritative document, lists all the above as reasons for rejection.

Note that superficial surface cracks on rubber or urethane bushes can be tolerated.

Shock absorbers absorb vibration energy. They heat up.

A working shock absorber will be warm or hot to touch after driving on sealed roads.

On an unsealed road, shock absorber outer body temperature might reach an extreme temperature of 175°C.

Road testing of shock absorbers is easily done. Drive the laden or unladen vehicle for about 15 minutes on a road with moderate imperfections.

Within a few minutes of the road test, check the shock absorber temperatures using a non-contact (infra-red) heat gun. Use the frame steel temperature as reference. Cold shock absorbers probably need to be replaced.

Remove the cold shock absorber and shake it – the sound of free metal parts rattling may be heard. By the way, the greater the oil volume and the greater the outer surface area, the lower should be the temperature.

Suspensions are non-linear mechanical systems so their road friendliness and ride-comfort performance changes with mass, speed and road profiles.

Adjustable damping characteristics could allow on-the-road tuning of suspension performance.

Electric shock absorbers are often talked about but are still a long way off being robust enough and stiff enough for heavy-vehicle use.

The principle is that a powerful magnet (or solenoid) moves inside a coil. Thereby current is pushed through the electrical load connected to the coil.

The electrical load can be changed depending upon conditions. The current level needed to produce useful damping on a heavy truck is hundreds of amps. Albeit it is a low voltage.

However, the management of such currents is expensive and the energy that could be scavenged is probably not worth the complication.

A successful electric shock absorber will probably also have a hydraulic sub-system.

I am grateful to Hendrickson Australia for providing some of the information in this article.

Dr. Peter Hart,
ARTSA

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