A strut is a structural part of the suspension that locates the wheel. A shock absorber only damps movement.

In a strut suspension there is no upper wishbone - the strut itself takes that job, carrying cornering, braking and side loads into the body.

It normally carries the road spring around it, and on a steered axle the whole assembly rotates on a bearing in its top mount.

A conventional damper is mounted between two points and carries no locating load at all; the wishbones or links do that job.

So a failed strut can change the vehicle's geometry, while a failed damper only changes how it behaves.

This is why struts are aligned after replacement and shocks generally are not, and why a corroded strut body is a structural concern.

Because the strut defines the wheel's camber and castor, and no two components sit exactly alike.

Alignment angles are set by where the strut's lower mounting and its top mount hold the hub.

Removing and refitting the assembly changes those positions slightly even with an identical part.

Many struts have elongated or eccentric lower bolt holes specifically so camber can be adjusted, which means it must be set rather than assumed.

Skipping alignment shows up as rapid inner or outer edge tyre wear within a few thousand kilometres.

Measure with the vehicle at correct ride height and correctly laden, since both change the readings.

Genuinely dangerous - a compressed road spring stores enough energy to kill, and it releases all of it instantly if the compressor slips.

The spring must be compressed to take the load off the top mount before the centre nut can be undone.

A fixed workshop compressor with captive jaws is far safer than a pair of hook-type tools, which can slip off a corroded or tapered spring.

Corroded springs are the worst case, because rust hides cracks and the spring can fracture while under compression.

Never leave a compressed spring standing on a bench, and never use mismatched or damaged jaws.

This is trained work with the right equipment, and it is the main argument for buying the assembly complete.

Usually yes - once the labour, the spring compression risk and the mounts and boots you would otherwise reuse are counted.

A complete unit arrives as damper, spring, top mount, bearing, bump stop and boot, assembled and ready to bolt on.

It removes spring compression from the job entirely, which is the main safety and time cost.

It also renews the parts most likely to be reused out of optimism - the top mount and bearing especially, which are a common source of noise after a strut change.

Fitting time drops substantially, which for a fleet is bay time as well as labour.

The case for dismantling remains where the spring is a specific rate not offered in an assembly, or where only the damper has failed on a low-mileage vehicle.

A creak or graunch when turning at low speed, or a knock over bumps - and on a steered strut it often feels notchy through the wheel.

The top mount carries the strut load into the body through bonded rubber, and on a steered axle it contains a bearing that lets the assembly rotate.

A failed bearing binds, producing a creak or a stepped feel as the steering is turned while stationary or parking.

Perished rubber lets the strut move in its mounting, giving a knock over bumps that is easily blamed on the damper or a link.

A binding mount can also make the steering fail to self-centre, which is often misdiagnosed as an alignment fault.

Because it is inside the assembly, it is replaced with the strut - which is another argument for the complete unit.

The damper cartridge can be replaced on some designs; a bent, corroded or damaged strut body cannot be repaired and must be replaced.

Some struts are built to take a replaceable insert, so the damper is renewed inside the existing housing.

That was common on older vehicles and is now the exception rather than the rule.

Sealed struts have no serviceable internals and are replaced as a unit.

A strut body that is bent, dented or significantly corroded is scrap regardless of damper condition, because it is a structural member.

Heating, straightening or welding a strut is never acceptable - it is a safety-critical load path and any of those alters the material properties.

Yes for the damping and the ride height, and in practice for the geometry too - a new strut one side and a tired one the other makes the vehicle asymmetric.

The damper element inside is subject to the same pairing logic as any shock absorber.

If the spring is being replaced as part of a complete assembly, ride height is also involved - a new spring one side and a sagged one the other tilts the vehicle.

That height difference changes camber and the alignment on both sides, not just the one that was replaced.

Steering pull and uneven tyre wear follow quickly from either mismatch.

Front and rear are independent, so an axle pair is the unit of work rather than the whole vehicle.