Because the spring locates the axle as well as carrying the load, which removes the need for separate links and makes a very robust, simple, high-capacity arrangement.

A coil-sprung axle needs trailing arms, a lateral location device and their bushes and mountings. A leaf-sprung axle needs the springs.

That simplicity is durable, cheap to repair anywhere in the world, and easy to inspect.

Leaf packs also carry very high loads for their cost and can be uprated by adding leaves.

Interleaf friction gives some inherent damping, which is why some trailers run acceptably with minimal or no dampers.

The costs are weight, a harsher unladen ride, and the friction that makes the spring rate less predictable than a coil's.

A multi-leaf pack is many leaves in contact along their length; a parabolic spring has few tapered leaves touching only at the ends.

Multi-leaf packs give a rising rate as shorter leaves engage, and interleaf friction adds damping - along with squeaking and stiction if the leaves are dry.

Parabolic leaves are individually tapered so each carries load along its whole length, letting far fewer leaves do the same work.

The result is lighter, rides better and has almost no interleaf friction, so it needs proper dampers to control it.

Parabolic springs cost more and are less tolerant of overload, because there is no stack of shorter leaves to take up the extra.

The two are not interchangeable within an axle - mixing them side to side gives different rates and an unlevel vehicle.

Measure axle squareness and check for uneven tyre wear and crabbing - the spring locates the axle, so spring wear is axle misalignment.

A worn spring eye bush, an elongated shackle or a sheared centre bolt all let the axle shift relative to the chassis.

Symptoms are tyre scrub on one side, a vehicle that tracks slightly sideways, and handling that has changed without any obvious cause.

Measure from a fixed chassis reference to the axle on both sides and compare - the difference is the misalignment.

The centre bolt is what holds the leaves in register and locates the pack on the axle seat; a sheared one is a common and easily missed failure.

On multi-axle vehicles and trailers, axle-to-axle squareness matters as much as axle-to-chassis, and misalignment there destroys tyres quickly.

They clamp the spring pack to the axle. They relax after fitting and must be re-torqued after the first laden run and then at service.

U-bolt clamping force is what stops the axle moving on the spring; the centre bolt only locates it.

New U-bolts stretch and settle, so the initial torque falls within the first period of use.

Loose U-bolts let the axle work on the spring seat, which starts as a knock and ends with a broken centre bolt or a shifted axle.

They are usually single-use, because tightening stretches them into the plastic region and re-using one gives an unreliable clamp load.

Torque them in the specified sequence and to the specified figure, then re-check - it is a small job that prevents an expensive failure.

Where a vehicle must ride acceptably empty and carry properly when full, yes - that is precisely the problem they solve.

A helper or overload leaf sits clear of the main pack and only comes into contact once the suspension has deflected under load.

Below that point the vehicle rides on the main pack; above it, the rate rises sharply.

That avoids the usual compromise of a spring stiff enough to carry the load and unbearable when empty.

It does not increase the vehicle's legal payload, which is set by its plated weights - a distinction that matters and is frequently misunderstood.

Air helper bags are the adjustable alternative, letting the support be varied with the load rather than being fixed by geometry.

It is traditional practice and still done, but a re-arched spring rarely holds its shape for long and the metallurgy is a gamble.

Re-arching cold-works or heats the leaves to restore the original curvature.

Heating risks altering the temper, and a spring that has lost its temper sags again quickly or breaks.

Cold re-arching stresses metal that has already fatigued for years.

Where new springs are available and affordable, they are the better answer, and for a fleet the predictability is worth more than the saving.

Re-arching keeps its place where springs are simply not obtainable, which for some older and specialist vehicles is a real situation.

Inspection of the leaves, eye bushes, shackles, centre bolt and U-bolts, plus lubrication at any grease points fitted.

Look for cracked or broken leaves, particularly the main leaf and its eyes, which is where failure is most consequential.

Eye bushes and shackle pins wear and produce knocking and axle movement; many have grease points that belong on the lubrication schedule.

Interleaf squeak on a multi-leaf pack indicates dry, corroded leaves, and some designs use liners or anti-friction pads that can be renewed.

Check for leaves that have shifted out of register, which means the centre bolt has failed or the U-bolts are loose.

Record ride height and axle squareness at service - both change slowly and both are invisible without measurement.