It is the force needed to compress the spring by a given distance, and it determines load capacity, ride height under load and how the vehicle rides.

A higher rate supports more weight for a given amount of compression, which is what a load-carrying vehicle needs.

It also makes the ride firmer, because the same bump compresses the spring less and transmits more into the body.

Rate must be matched to the damper. A spring stiffer than the damper was valved for leaves the vehicle under-damped and floating.

Springs are colour-coded or part-numbered by rate, and the codes distinguish variants fitted to the same model with different equipment or engines.

Fitting the wrong rate is easy and its symptoms - poor ride, wrong height, uneven handling - get blamed on everything except the spring.

Linear for a consistent, predictable response at a known load. Progressive where the vehicle works both empty and fully laden.

A linear spring has evenly spaced coils and the same rate throughout its travel.

A progressive spring uses variable spacing, a varying wire diameter or a conical form so that the closely spaced coils bind first and the effective rate rises.

That gives a soft initial rate for comfort and a rising rate for support, which is the behaviour a van or pick-up actually needs.

The trade is predictability: handling changes with load in a way a linear spring's does not.

Progressive springs are also usually directional, with the closely wound end fitted a specific way up - fitting one inverted changes the behaviour completely.

Measure ride height against specification. Sag is gradual enough that nobody sees it by eye, including the driver.

Manufacturers publish a ride height, usually measured from the wheel centre to a defined point on the arch or the chassis.

Measure on level ground with correct tyre pressures and the specified load state, and compare side to side as well as against the figure.

A vehicle that has sagged sits closer to its bump stops, so it crashes over bumps that it used to absorb.

Lost height also changes camber and the suspension's working geometry, which shows in tyre wear.

A side-to-side difference is the clearest single indicator, and it is worth recording at service so the trend is visible over time.

Corrosion pitting starts a crack, and the crack grows under cyclic load - which is why breakages cluster on vehicles used where roads are salted.

The spring is under continuous varying stress, so any surface defect concentrates that stress.

Stone chips and corrosion both create such defects, and salt accelerates the corrosion.

Breaks usually occur within a coil or two of an end, where stress is highest and where debris and moisture collect.

A broken coil can drop onto the tyre or foul the steering, so it is an immediate defect in most roadworthiness regimes.

Coated and painted springs resist it better, and clearing debris from spring seats at service genuinely extends life.

Yes. A new spring on one side and a sagged one on the other tilts the vehicle and puts the alignment out on both sides.

A new spring has its full free length and rate; its opposite number has neither.

The vehicle then sits at different heights left and right, which changes camber on both corners.

Handling becomes asymmetric, and the vehicle pulls or feels different turning one way against the other.

The old spring is also the same age and has the same corrosion exposure, so it is the next one to break.

Front and rear axles are independent, so an axle pair is the unit of work.

Yes. Shortening the spring without matching damper travel, bump stops and geometry leaves the suspension running out of travel.

A lowered vehicle sits closer to its bump stops, so ordinary bumps are taken by the stops rather than the spring.

Standard-length dampers may be over-extended at full droop or fully compressed too early, and either destroys them.

Altered geometry changes camber, castor and roll centre, and the resulting handling is not what the vehicle was engineered for.

Load capacity falls, which matters on any vehicle that carries or tows.

There are also approval and insurance implications in many markets for altering suspension height - worth confirming locally before it is done.

Spring seats and insulators, bump stops, and the damper if it has done the same mileage.

Rubber or plastic insulators sit between the spring and its seat, and a perished one lets the spring contact metal directly, which is noisy and starts corrosion.

Spring seats corrode, particularly where debris collects, and a corroded seat is a structural repair rather than a parts swap.

Bump stops perish and disappear, and a vehicle running without them hammers the suspension at full compression.

A damper of the same age is usually near the end of its life, and access is already open.

Clean debris from the seats before assembly. It is the single cheapest thing that extends the new spring's life.