By the method the design requires - a load-bearing joint must be unloaded first, or the vehicle's weight holds the play closed and hides it.

On a strut suspension the lower joint is generally non-load-bearing, and the wheel is rocked with the suspension at normal ride height.

On a double-wishbone suspension the joint carries the weight, and the correct method supports the control arm to unload it before checking.

Rocking the wheel top to bottom shows vertical play; rocking it side to side at the front and rear edges shows steering play, which is usually a tie rod rather than a ball joint.

A pry bar under the tyre with the joint unloaded is the standard way to reveal axial movement.

Manufacturers publish permissible movement figures. Where they do, measure rather than judge, because 'a bit of play' is not a specification.

Because it can separate completely, dropping the hub and folding the wheel under the vehicle - a sudden loss of control rather than a degradation.

Most suspension wear degrades handling progressively and gives warning.

A ball joint that pulls out of its socket removes the location of the hub entirely, and it happens without a useful warning period once the joint is badly worn.

On a load-bearing joint the corner of the vehicle drops onto the wheel or the road.

This is why roadworthiness regimes inspect them specifically, and why detectable play is a defect rather than an advisory in most of them.

It is also why a knocking noise from a corner is worth investigating immediately rather than at the next service.

A knock over bumps and at low speed over rough ground, sometimes a creak on turning, and often uneven tyre wear before either.

The knock comes from the ball moving in a worn socket as loads reverse, so it is loudest over undulations and speed bumps.

A dry joint whose grease has escaped through a split boot creaks or groans as the suspension moves, particularly at low speed.

Play in a load-bearing joint changes camber under load, which scrubs the tyre and shows as edge wear.

Steering that wanders or feels vague can be a ball joint, though a tie rod end is the more common cause of that particular symptom.

None of these is conclusive on its own, which is why the physical check with the joint correctly loaded is the actual test.

Only if the split is fresh and no contamination has entered - and in practice that is rarely provable, so the joint goes.

The boot retains the grease and excludes water and grit; both functions are lost the moment it tears.

Grit inside a ball joint is abrasive paste, and it wears the bearing surfaces irreversibly.

Many modern joints are sealed for life with no separate boot available, which settles the question.

Where a boot is available and the split is genuinely recent, cleaning, repacking and re-booting is legitimate on heavy vehicles where joints are expensive.

Given the failure consequence, the balance of risk favours replacement on anything safety-critical and reasonably priced.

If they are actually greased on schedule, yes - and if they are not, they can fail sooner than a sealed joint.

A greasable joint has a nipple that lets fresh grease be pumped in, purging contamination out past the boot lip.

That is genuinely effective and is why heavy commercial vehicles still use them extensively.

It depends entirely on the greasing happening at the specified interval, which on a mixed fleet is a real operational question rather than a given.

Over-greasing bursts the boot, which converts a maintainable joint into a contaminated one - so it needs doing correctly, not just often.

Sealed joints remove the maintenance dependency and are the norm on light vehicles, with a life set at manufacture.

The joint is held by a friction fit in a tapered bore, not by the nut - a damaged or dirty taper will not hold however hard the nut is torqued.

The stud's taper wedges into a matching taper in the knuckle or arm, and that wedge carries the load.

The nut's job is to pull the taper home and keep it there, not to clamp the joint like a bolt.

Hammering the stud or levering the joint apart burrs and deforms the taper, and a deformed taper allows the new joint to work loose.

Use a proper splitter or press, clean both tapers thoroughly before assembly, and fit them dry.

Torque to specification and fit a new split pin or nut where the design uses one - castellated nuts and their pins are single-use.

Always. A load-bearing ball joint locates the hub, so replacing one changes camber and often castor.

Alignment is set by where the locating components hold the wheel, and the ball joint is one of them.

Even an identical replacement sits slightly differently, and a worn joint had drifted from specification before it was removed.

Skipping the alignment produces rapid, uneven tyre wear that costs more than the alignment did.

Check the opposite side at the same time - joints wear in pairs because they have done the same work.

Measure with the vehicle at correct ride height and in the specified load state, since both change the readings.