A rumble that tracks road speed and changes when the vehicle is steered gently left and right, because cornering transfers load between sides.

The noise rises and falls with road speed and is unaffected by engine speed or gear selection, which separates it from driveline noise.

Swerving gently at a steady speed loads the outer wheel and unloads the inner. A bearing that gets louder when loaded is the failing one.

So a noise that worsens turning left is usually the right-hand bearing, and vice versa.

With the wheel raised, spinning it by hand may reveal roughness, and rocking it top to bottom may reveal play - though a badly worn bearing can have neither.

Tyre cupping from worn dampers produces a very similar rumble and is the most common misdiagnosis, so check the tread before condemning the bearing.

It is decided by the design, not by preference - adjustable taper rollers, press-fit sealed bearings and bolt-on hub units are three different arrangements.

Adjustable taper roller bearings are packed with grease and set to a specified preload or endfloat, and remain standard on trailers and heavy axles.

Press-fit sealed bearings sit in the hub carrier and need a press, usually a new circlip, and often removal of the carrier from the vehicle.

Complete hub assemblies bolt on with four or five fasteners and include the bearing, flange and often the ABS encoder.

Hub assemblies cost more as parts and much less in labour, and they remove the risk of damaging a bearing during pressing.

For a fleet, the labour saving and the reduced chance of a comeback usually make the complete unit the better economics where it is offered.

Because the sensor ring is often a magnetic encoder built into the bearing seal, and it is easily damaged or fitted the wrong way round.

Many modern bearings carry a magnetised encoder ring as part of the seal rather than a separate toothed ring.

It has a defined orientation, and fitting the unit reversed puts the encoder away from the sensor.

The magnetism is damaged by contact with tools, by dropping the unit, and by placing it near anything magnetic - including a magnetic parts tray.

Debris on the encoder or the sensor tip gives intermittent faults that look electrical.

Handle the unit by its edges, keep the seal face protected until fitting, check the orientation mark, and clean the sensor bore before assembly.

Critical. It sets the bearing's preload, the figures are often very high with an angle stage, and the nut is usually single-use.

On many sealed units the hub nut clamps the bearing's inner race and directly determines whether it runs correctly.

Under-torque lets the race move, which destroys the bearing quickly and can loosen the hub.

Over-torque crushes the bearing, giving a unit that runs hot and fails within months.

Torques are frequently far higher than other suspension fasteners and need a suitable torque wrench and often an angle gauge.

Staked and single-use nuts must be replaced, and the staking done properly where the design calls for it.

To a specified preload or endfloat, after being packed with the correct grease - and the setting is what determines their life.

The usual method is to tighten the nut while rotating the hub to seat the rollers, then back off and set the final figure.

Some designs specify a small endfloat measured with a dial gauge; others specify a rolling torque.

Too tight and the bearing runs hot and fails rapidly; too loose and it hammers and wears the races.

The correct grease and the correct quantity matter - overpacking causes churning and heat, underpacking starves the rollers.

This is the arrangement on most trailers, so it belongs on the trailer inspection schedule, along with checking hub temperature after a run.

Yes - unlike dampers and springs, bearings do not need to match, so replacing the failed side alone is entirely legitimate.

A wheel bearing's condition does not affect the vehicle's balance or geometry the way a damper or spring does.

There is therefore no handling reason to replace both sides.

In practice the opposite side has done the same work, so checking it while the vehicle is raised is worthwhile.

Where the second bearing is also noisy or rough, doing both saves a second visit and a second set of hub nuts.

Record which side was replaced and when. A pattern of bearings failing on one side points at load, kerbing or a sealing problem rather than at bearing quality.

Contamination past a damaged seal, incorrect fitting loads, wrong torque, and impact damage from kerbs and potholes.

Water and grit past a failed seal are the commonest cause, and they are why pressure washing directly at the hub is a bad habit.

Pressing a bearing in by its outer race and out by its inner race - or the reverse - passes the fitting load through the rolling elements and brinells the races.

Incorrect hub nut torque either crushes the bearing or lets it move.

Kerb strikes and potholes impose shock loads that dent the raceways, and the resulting noise appears weeks later with no apparent cause.

Excessive wheel offset from non-standard wheels increases the load on the bearing permanently, which is a slow but reliable way to shorten its life.