Because a bonded rubber bush twists rather than rotating - it has to be clamped in the position it will normally sit in.

The rubber is bonded to both sleeves, so all relative movement is taken up by the rubber deforming.

Tighten with the suspension hanging and the rubber is locked in a twisted state permanently.

Once the vehicle is on its wheels, the bush fights that twist through every movement, and it tears out within months.

The method is to support the hub at normal ride height before final tightening, or lower the vehicle onto its wheels first where access allows.

Polyurethane bushes rotate on the sleeve, so they are far less sensitive - which is one of their genuine practical advantages.

Knocking over bumps, vague and wandering steering, an alignment that will not hold, and clunks as drive or braking loads reverse.

A worn bush lets the component it mounts move under load, so the wheel's position changes with acceleration, braking and cornering.

That produces steering that feels delayed and a vehicle that needs constant small corrections.

The knock is the sleeve moving in the perished rubber, and it is loudest over sharp edges and speed bumps.

An alignment that goes out of specification soon after being set is a classic bush symptom, because the geometry is no longer being held.

Visually: cracked, perished or missing rubber, the inner sleeve sitting off-centre, or fluid weeping from a hydraulic bush.

Rubber for isolation and comfort, which is what most vehicles want. Polyurethane where precise geometry control matters more than noise.

Rubber absorbs vibration and road noise well and is what the vehicle was engineered around.

Polyurethane deflects less under load, so the alignment stays where it was set and the steering feels sharper.

It transmits considerably more noise and harshness into the cabin, which on a vehicle driven all day is a real comfort cost.

It also needs the correct grease at fitting, and sometimes periodically, or it squeaks - the most common complaint after fitting.

For working and passenger-carrying vehicles rubber is usually the right answer; polyurethane earns its place on specific problem mountings rather than throughout.

A fluid-filled bush that provides tuned damping as well as compliance - and it cannot be substituted with a plain rubber bush without changing the vehicle's behaviour.

Internal chambers of fluid connected by passages give a rate that varies with the speed of the movement, damping specific frequencies.

They are typically fitted where fore-and-aft compliance under braking has to be controlled without making the mounting stiff.

When one fails it usually leaks visibly, and the vehicle develops a shudder under braking or a boom at certain speeds.

Fitting a solid rubber bush in its place gives a stiffer mounting and loses the damping the designer specified.

They are more expensive and are often only available as part of a complete arm, which is worth establishing before quoting.

Generally yes, by assembly and by axle - they are the same age, and replacing one stiffens a single point in a system designed to work together.

Bushes on the same arm or subframe have shared the same loads, heat and age.

A new bush next to a perished one takes a disproportionate share of the load and wears faster.

Mismatched compliance side to side makes the vehicle behave asymmetrically, in the same way mismatched dampers do.

Since the labour to reach them is most of the cost, doing the set while access exists is markedly cheaper than returning.

The exception is a single bush damaged by a specific event rather than by age.

With a press or a purpose-made drawbolt tool, using the correct lubricant - not with heat or a hammer.

Most bushes are an interference fit in their housing and must be pressed out and in squarely.

A hydraulic press with correct support is the workshop method; drawbolt tools do the same job in situ where the arm cannot be removed.

Burning rubber bushes out is common practice and is genuinely hazardous - the fumes are toxic and it can distort the housing.

Use the lubricant the bush manufacturer specifies. Ordinary grease attacks rubber, and the wrong lubricant on polyurethane causes the squeak it was meant to prevent.

Check the housing for distortion and corrosion before pressing a new bush in; a damaged housing will not hold it.

Yes - they hold the geometry, so worn bushes let it move under load and prevent an alignment from holding.

Alignment is measured statically, but the vehicle is driven dynamically, and it is the bushes that decide how much the geometry changes under load.

A vehicle with worn bushes can measure perfectly on the alignment rig and be out of specification the moment it brakes or corners.

That is why repeat alignment complaints so often trace back to bushes rather than to the alignment.

Bushes should be inspected before an alignment is attempted, not after it fails to hold.

After replacing them, align the vehicle - the new bushes will sit the components slightly differently from the worn ones.