It converts the spring's stored energy into heat, so the suspension settles after one movement instead of bouncing several times.

A spring alone stores and returns energy. Hit a bump on a springs-only vehicle and it keeps oscillating until friction eventually stops it.

The damper forces oil through valves as it moves, and that resistance turns the oscillation into heat that dissipates through the damper body.

Damping is deliberately different in each direction: compression damping is usually softer, rebound damping firmer, because controlling the spring's return matters more.

The purpose is keeping the tyre on the road. A wheel that is still moving vertically is not applying its full share of grip.

So the visible symptoms - float, wallow, nose dive - are secondary. The real cost is traction, and it is invisible until it is needed.

Because the oil and the valving degrade with heat and use long before a seal fails - a dry damper can be completely ineffective.

Damper oil shears and thins over millions of cycles, and heat accelerates it.

The valve shims that set the damping characteristic wear and lose their calibration, so the damper still moves but no longer resists correctly.

Gas pressure bleeds away past seals slowly, which lets the oil foam and produces a damper that fades badly when worked hard.

None of this shows externally, which is why 'it is not leaking' is not an inspection.

The bounce test finds only gross failure. Meaningful assessment comes from tyre wear patterns, body control on a road test, or a suspension tester where one is available.

Twin-tube for ride comfort and cost, which is what most vehicles use. Monotube for heat capacity, consistency and mounting freedom.

A twin-tube damper has an inner working cylinder inside an outer reserve tube holding spare oil and gas. It is compact, compliant and inexpensive.

Its weakness is heat: the outer tube insulates the inner one, so sustained work makes the oil hot and the damping fade.

A monotube uses one larger-bore tube with a floating piston dividing oil from high-pressure gas. The larger piston gives finer damping control and the single wall sheds heat directly.

A monotube can also be mounted inverted or at any angle, because the floating piston keeps oil and gas separated regardless of orientation.

The trade-offs are cost, a firmer ride, and vulnerability - a dented monotube body jams the piston, where a dented twin-tube outer often still works.

Because damping determines how each side of the vehicle responds, and mismatched sides make the vehicle behave asymmetrically.

A new damper controls the wheel far better than a worn one, so the two sides of an axle settle at different rates over the same bump.

That shows up as pulling under braking, unpredictable behaviour mid-corner, and uneven tyre wear.

The old damper is also close to failure by definition, so the second job follows within months and pays the labour twice.

Front and rear axles are independent, so replacing one axle's pair is entirely legitimate.

Where the vehicle has covered high mileage, doing all four at once restores a balance that has drifted so gradually the driver will notice the difference immediately.

Only if the vehicle is genuinely under-damped for its load. Fitting firmer dampers to a lightly laden vehicle degrades both ride and grip.

Heavy-duty dampers are valved for higher loads, and under those loads they control the suspension properly.

On an unladen vehicle the same damper over-controls: the wheel cannot follow the road surface, so it skips rather than tracking, and grip falls.

For vehicles that run laden most of the time, matching the damper to the working load is a real improvement rather than a preference.

For vehicles that alternate, self-levelling or load-adjustable dampers are the honest solution rather than a compromise valving.

Firmer is not better. The correct damper is the one matched to the spring rate and the load the vehicle actually carries.

It produces cupping or scalloping - regular patches of wear around the tread - because the tyre is intermittently unloaded as the wheel bounces.

An uncontrolled wheel oscillates vertically after each input, alternately pressing hard and lifting slightly.

The tyre scrubs where it is loaded and skips where it is not, cutting a repeating pattern around the circumference.

Cupping is nearly diagnostic of worn damping, and it is often felt as a rumble mistaken for a wheel bearing.

Once cupped, the tyre is noisy and will stay noisy - replacing the dampers stops it worsening but does not restore the tread.

For fleets, recording tyre wear patterns at every service catches damper wear at no extra cost and before the tyres are ruined.

Top mounts, bump stops, dust boots and the fasteners - all cheap, all in the way, and all capable of causing a comeback.

The top mount carries the damper load into the body and usually contains a bearing or bonded rubber. A worn one knocks and can make a new damper noisy immediately.

Bump stops limit travel at full compression. A perished or missing one lets the suspension bottom out metal to metal.

Dust boots keep grit off the piston rod, and grit on the rod destroys the seal - which is how a new damper starts leaking within a year.

Many suspension fasteners are single-use or torque-to-yield, and bush-mounted ends must be torqued at ride height, not with the suspension hanging.

Pre-assembled strut units bundle several of these together and remove the need to compress a spring, which is frequently the better purchase.