Shock absorbers, one axle

It stops the spring from bouncing. A coil spring on its own would keep oscillating after every bump like a pogo stick, and the tyre would skip off the road on each rebound. The shock absorber, which the trade calls a damper, is a sealed tube of oil with a piston inside. As the suspension moves, the piston forces oil through small valves, which turns the energy of the bump into a little heat and lets the spring settle in one movement instead of five.

That job is invisible until it is not being done. A car on worn dampers still sits at the right height, still steers and still stops, which is why people drive on them for years. What changes is how well the tyres stay pressed onto the road, and that matters most at exactly the moments you need grip: braking hard, swerving, or hitting a mid-corner bump.

Dampers fade gradually, so the owner is usually the last to notice. Somebody else driving your car often spots it straight away. The signs to look for fall into what you feel, what you hear and what you can see.

It depends how much it leaks. Shock absorbers are section 5.3.2 of the DVSA inspection manual, and the manual draws the line clearly: slight seepage that leaves a film of fluid on the damper is not a reason to fail. A damper damaged so that it no longer works, or showing signs of severe leakage, is a major defect.

The tester also fails a damper with negligible damping effect, if that becomes evident at any point in the inspection, and a damper bush that is excessively worn. A damper insecurely attached to the chassis or axle is major; one that is missing, or likely to come adrift, is dangerous and the car cannot be driven away. The manual adds that a missing damper only fails if the car was fitted with one as standard.

The manual sets no measured damping figure for a car. The tester judges by eye and by what the suspension does as the car is jacked, rocked and lowered, so a car with tired but dry dampers often passes. Passing does not mean they are doing their job well.

ClickMechanic publish separate figures for each axle. A front pair averages £300 within £200 to £500 and takes about 2 hours. A rear pair averages £225 within £150 to £400 and takes about 1 hour. Together they make up the £150 to £500 range this page quotes for one axle.

On most front wheel drive cars the front damper is the core of the strut, with the spring and top mount stacked on it, so the whole assembly comes out and the spring has to be compressed before the damper can be swapped. A rear damper on many of the same cars is a separate unit held by one bolt at the top and one at the bottom. Fewer parts and less dismantling is the whole of the price gap.

Where the price leaves this range altogether is electronics. Adaptive or electronically controlled dampers, which change their stiffness at the press of a button or on their own, cost several times what a passive unit does. Air suspension is a different system again and is not what this range covers.

When a front damper is replaced on its own, the garage has to compress the spring, move it and the top mount across to the new damper, and reassemble the lot. The alternative is a complete strut: a new damper already fitted with a new spring, top mount and bearing, ready to bolt in. The parts cost more, but the labour drops because nobody has to compress anything, and every wearing part in the corner is new at once.

A complete strut makes most sense on a high mileage car whose springs and top mounts are the same age as the dampers. On a car with springs that were replaced recently, reusing them with new dampers is the better value. Ask for both prices; a garage that only offers one is not wrong, but you should know the choice exists.

A damper that is simply tired will not leave you stranded, and nothing breaks dramatically. The cost of waiting is paid elsewhere. Worn dampers let the tyres bounce and lose contact over rough surfaces, which lengthens braking on a bumpy road and can make the car feel nervous in an emergency swerve. They also wear tyres unevenly, and cupped tyres are noisy and cannot be put right by rotating them.

A badly leaking damper is more urgent, because the oil can run onto the spring and the tyre. Once the fluid has gone the damper has no damping left at all, and that is a major MOT defect whenever it is found.

Rear dampers on a car where they are separate from the spring are among the easier suspension jobs at home. With the car safely on axle stands and the axle supported, the damper usually comes off with one bolt at each end. Front struts are harder and more dangerous because of the spring compressor, unless you buy complete struts that arrive assembled.

The detail people miss is the final tightening. Rubber bushed mounting bolts on many cars are meant to be tightened with the car sitting on its wheels at normal ride height, not hanging on the jack, or the bushes are twisted and wear out quickly. The workshop data for the car says which bolts need this.