Home / Blog / Suspension & Steering

The shock absorber is one of the parts vehicle owners misunderstand the most. The common belief is that it's there for comfort, and that it's to be expected to feel "a bit stiffer" over time. In fact, the shock absorber's real job is to press the tyre onto the road. The moment the tyre loses contact with the road, braking and steering stop working. That's why a worn shock absorber is a safety issue before it's ever a comfort issue.

What does a shock absorber do?

The spring carries the weight of the vehicle, not the shock absorber. The shock absorber's job is to dampen the energy the spring produces as it compresses and extends. Inside it there's a piston, hydraulic oil, and calibrated holes the oil passes through. When the wheel hits a bump, the piston moves, the oil is forced through these narrow passages, and kinetic energy is converted into heat. The shock absorber is essentially an energy converter.

What happens if the shock absorber weren't there, or stopped doing its job? After the vehicle passes over a bump, the spring keeps bouncing. Instead of being pressed onto the road, the wheel bounces above it. A bouncing wheel cannot brake. This is why the most serious consequence of a worn shock absorber is an increased braking distance, especially on rough or wet surfaces. Even ABS can't do anything while the tyre is in the air.

How does a shock absorber wear out?

A shock absorber doesn't fail suddenly; it gradually loses its damping ability. There are several ways this happens:

• Oil leakage: The seal that keeps the piston rod oil-tight wears down and oil leaks out. As the oil level drops, damping decreases. A shock absorber with its body wetted by a mix of oil and dust is the clearest visible sign of failure.
• Oil fatigue: The properties of the hydraulic oil, constantly heating and cooling, change over time; the same pressure no longer produces the same damping.
• Internal wear: As the valves and piston surfaces wear down, oil passes through more easily than it should, and the shock absorber softens.
• Physical damage: An impact to the piston rod, or corrosion, wears out the seal quickly. A shock absorber with a torn dust boot has a noticeably shortened lifespan.

Road conditions affect service life far more than mileage. A vehicle constantly driven on rough roads won't reach the same point at the same time as the identical model driven on smooth tarmac. For this reason, it isn't correct to give a single replacement mileage that applies to everyone; manufacturers recommend periodic inspection for most vehicles, and the figure in your vehicle's owner's manual is what counts. The general approach is to replace the shock absorber based on its condition and test results, not on mileage.

How is a shock absorber tested?

The push-down test and its limits

The best-known method is to push down on a corner of the vehicle and release it, watching how many times it bounces. On a healthy shock absorber, the vehicle rises once when released and then stops; if it bounces several times, damping has decreased. This test provides useful information but isn't conclusive: modern vehicles are heavy, the force you can apply by hand may not compress the spring enough, and a shock absorber that isn't yet significantly worn can still pass this test. In other words, passing the test doesn't mean it's "fine" — but failing it is a clear warning.

Visual inspection

Oil leakage on the body, scoring or corrosion on the rod, a torn dust boot, a dented body, a worn upper mount. These can be spotted within minutes on a service lift and are the most reliable clues.

Driving behaviour

If the nose dips excessively under braking, the rear squats heavily on acceleration, the body is slow to settle through corners, the vehicle feels like it's floating on a wavy road at motorway speed, or you hear dull impact noises from the rear on rough roads, the shock absorber is worth investigating. Irregular, scalloped tyre wear is also a common secondary symptom; a bouncing tyre doesn't press evenly onto the road.

Test equipment

The vibration-platform test rigs found in some workshops measure damping performance for each wheel and give a numerical left-right difference. This removes the subjectivity of the manual test. Interpreting the rig's results depends on the vehicle type, and they should be assessed together with visual inspection, not on their own.

Why are both shock absorbers on the same axle replaced together?

This isn't the workshop trying to generate extra work — it's a matter of physics. The right and left shock absorbers have shared the same road, the same time, and the same load; if one is worn, so is the other, it simply hasn't shown symptoms yet. A new shock absorber's damping is noticeably higher than a worn one's. When one side on the same axle is new and the other worn, the vehicle responds differently on each side under braking and cornering. This asymmetry unsettles the vehicle's balance during sudden manoeuvres, especially on wet surfaces, and creates a pulling sensation to one side under straight-line braking. What's more, the worn side will need replacing again shortly afterwards anyway; labour ends up being paid for twice.

The same logic doesn't apply between front and rear: the front pair and the rear pair can be assessed independently of each other. Front shock absorbers generally see more load and brake dive, so on most vehicles they wear out sooner. When replacing shock absorbers, the condition of supporting parts such as the upper mount, dust boot, and mount should also be checked; if these are worn, the new shock absorber's lifespan will be shortened.

After a shock absorber replacement, checking the wheel alignment is good practice, since the suspension geometry is affected.

If you're noticing any of the symptoms described here in your vehicle, you can book an appointment with our workshop for a shock absorber inspection.