Springs and dampers: what each one does, and how a tired damper hides

A spring and a damper are fitted together, replaced together and usually talked about as one thing. They do opposite jobs, and the difference between them is the single most useful idea in the subject:

A spring resists how far you move it. A damper resists how fast.

Push slowly on a damper and it gives way almost without protest. Push fast and it fights hard. That one property explains why a damper can be completely dead without the car sitting any lower, why the bounce test everyone knows is close to useless, and why the most dangerous suspension fault on the road is one that nobody notices.

This is the biggest merge in the archive — five pieces, the largest with 520,000 views — and between them they explain the components carefully and never once say how you would know that one had failed. That is the gap this fills.

What each one is for

The spring holds the car up and swallows the bump. A wheel hits a ridge, the spring compresses, and the body barely moves — that energy has gone into the spring instead of into the passengers.

And now the car has a problem, because the energy is still there. A spring gives back everything you put into it. Left alone it would return that push, overshoot, come back, and keep going: a car on springs alone does not settle after a bump, it wallows for several seconds and arrives at the next bump already moving the wrong way.

The damper's whole job is to destroy that energy, and it does it by turning motion into heat. Inside is oil forced through small orifices by a piston; move it fast and the oil cannot get through fast enough, so it resists. The heat goes out through the body. A damper working hard on a rough road gets genuinely hot to the touch, and that heat is the bump, gone.

Which is why the sizing is a compromise the original states well: too soft a spring and the car moves too much, too stiff and it rides badly, and the practical answer is a moderate spring with a damper to control it.

Sprung and unsprung mass, finished

The original introduces the distinction and stops there, so here is the half that matters.

Sprung mass is everything the springs hold up — body, engine, passengers. Unsprung mass is everything below them: wheels, tyres, brakes, hubs, and roughly half of each suspension arm.

The reason to care is that the two masses are trying to do different things. The body wants to travel in a straight line and be left alone. The wheel needs to follow every contour of the road, upward and downward, because a wheel that is not touching the road is doing nothing at all — no grip, no braking, no steering.

And a heavy wheel is bad at following. When the road drops away, the spring has to accelerate the whole unsprung mass downward fast enough to keep the tyre in contact, and inertia resists. So every kilogram added below the spring costs both ride comfort and grip, which is the honest answer to why enormous wheels with thin tyres usually make a car worse at everything except appearance.

How a tired damper hides

This is the point of the article, and it is worth being precise about why this particular failure is so easy to miss.

It changes nothing you can see. The spring still holds the car at the right height, so the car sits normally, looks normal, and passes a glance.

It fails gradually. Damping does not stop; it fades, over years, as the valving wears and the oil degrades. There is no day on which the car became worse.

And it fails symmetrically. Both front dampers have done the same mileage over the same roads, so the car does not pull or lean to one side — it simply becomes worse everywhere at once, evenly, which is exactly the change a driver recalibrates around without noticing.

What it costs is not comfort. Worn dampers have been measured to add as much as twenty per cent to a car's stopping distance, because braking hard makes the nose dive and pitch, and a wheel that is bouncing spends part of every second pressing on the road with less than its share of the weight — or none of it. The brakes and tyres are fine. The wheel is not always there.

The signs that are actually reliable

• Cupping on the tyres. Scalloped, wavy patches of wear spaced around the circumference, which you feel with a palm more easily than you see. It is the tyre being bounced, and it is the most reliable single indicator there is.
• Floating over a crest, and a second small movement after the first — the body carrying on after the road has finished.
• Nose-dive under braking that is worse than it used to be, and a tail that squats more on acceleration.
• A busy, jittery ride over small ripples while big bumps feel soft. That combination is characteristic.
• Oil on the damper body. A wet, dirt-caked film running down from the seal means it has lost fluid, and that one is definitive.

And the test everybody knows, which is weak

Push down hard on a corner and let go, and the car should return and settle without a second bounce. It is a real test and it does find a completely dead damper.

But a modern gas-pressurised damper resists a slow hand-push mostly with its gas pressure, and gas pressure is not damping. A unit whose valving has degraded badly can still pass a bounce test convincingly. Use it as a quick check that can find a fault, never as a check that clears one — and if it matters, the shaker plates at a test station measure the thing itself.

What springs do instead

Springs almost never sag quietly; they break. Usually near the bottom coil, usually from corrosion where road salt and grit collect, and often with no warning beyond a single clunk that gets dismissed.

Two consequences worth knowing. A broken end coil can turn and cut into the tyre, which is a blowout on a moving car and is the reason a suspected broken spring is not something to leave until the weekend. And a settled or broken spring changes the wheel's geometry, so the car pulls, wears a tyre edge and does not respond to an alignment until the spring is dealt with.

The original's account of spring types is sound: steel in leaf, coil and torsion-bar form, and non-steel in rubber, air and hydro-pneumatic. One number in it is wrong in the same way the CVT article's was — it says leaf springs came into wide use in American cars in 1985, which is around the time they were finishing, not starting. It is a lost digit; the carriage-derived leaf spring was universal from the beginning of the motor car.

Gas pressure in a damper, while we are correcting things, is not there to make it firmer. It is there to keep the oil from foaming: a piston thrashing through oil at speed makes bubbles, bubbles compress, and a damper full of froth has no control at all. Pressurising it suppresses the bubbles. The slightly firmer feel is a side effect.

Three things that get blamed on the damper

A knock from a corner is the commonest suspension complaint, and it is usually not the damper.

The method for chasing these is the one in diagnosing engine noises, applied downwards: establish first whether the noise tracks road speed or suspension movement, because that distinction eliminates most of the car before you have jacked it up.

Replacing them

In pairs, across an axle, always. They wear at the same rate, so a new one beside a tired one leaves the car damped unevenly side to side — which is worse than two equally tired ones, because now the two ends of an axle respond differently to the same bump.

And while the strut is off, the top mount, its bearing and the bump stop are inexpensive, are usually as old as the damper, and are the parts that produce the knock people replace a second damper trying to cure.

A note on layouts, which is a different article

One of the merged pieces is a survey of suspension geometries — MacPherson strut, double wishbone, multi-link, torsion beam — and it is good material. It is also a subject in its own right, and folding six thousand words of it in here would bury the springs and dampers this article is named for.

The short version an owner needs: a MacPherson strut makes the damper a structural part of the suspension, which is why replacing one is a bigger job than unbolting a shock absorber and why its top mount matters so much. A torsion beam at the rear is simple, cheap and space-efficient, and its bushes are usually the only thing that wears. Double wishbone and multi-link control the wheel's angles more precisely through their travel, at the cost of parts count — which means more bushes and ball joints to wear out, and more of the knocks in the table above.

Video guides

Video, for the parts of this that are easier watched than read.