It holds the hub at a fixed distance from the body while letting it move vertically through a controlled arc - which is what sets the alignment angles.

The arm pivots on the body or subframe at its inner end and carries the hub at its outer end.

Its length and mounting positions determine camber change through the suspension's travel, and therefore how the tyre meets the road as the vehicle rolls.

It also reacts braking and acceleration loads, which is why the rear bush on many lower arms is larger and hydraulically damped.

Double-wishbone suspensions use upper and lower arms; strut suspensions use a lower arm with the strut taking the upper location.

Because it defines geometry, anything that changes the arm - wear, damage or replacement - changes the alignment.

Wandering, a vague or delayed steering response, a knock over bumps, and pulling under braking.

A worn bush lets the arm shift in its mounting under load, so the wheel's position changes as forces change.

Under braking that shift alters castor and toe, which is what makes the vehicle pull to one side.

Over bumps the movement produces a knock that is easily blamed on the damper or an anti-roll bar link.

Visually, look for cracked, perished or displaced rubber, and for the inner sleeve sitting off-centre in the bush.

Levering the arm gently while watching the bush shows movement that is invisible with the vehicle on its wheels.

On some arms yes, on many modern ones no - the joints are pressed or riveted in and the arm is supplied complete.

Older and heavier vehicles commonly use bolt-in ball joints and pressed bushes that a workshop can renew with a press.

Many modern light vehicle arms have the ball joint staked or riveted in place, and the manufacturer supplies only the complete arm.

Aftermarket joints for such arms exist and usually require drilling out rivets and bolting the replacement in, which is legitimate where the arm is sound and the kit is designed for it.

Pressing bushes needs the right tooling; heating or hammering them in damages both the bush and the arm.

Establish which case applies before quoting - the difference between a bush and a complete arm is substantial.

Only where the sharper response is genuinely wanted. They transmit more noise and vibration and need periodic lubrication.

Rubber bushes are bonded to their sleeves and deform to allow movement, which isolates noise and vibration well.

Polyurethane bushes rotate on the sleeve rather than twisting, so they resist deflection under load and keep the geometry more stable.

That gives a more precise feel and more transmitted harshness, which on a working vehicle is usually the wrong trade.

They also need the correct grease at fitting and sometimes periodically, or they squeak - a common complaint after fitting.

Because they rotate rather than twist, they are less sensitive to being torqued at ride height, which is their one maintenance advantage.

Because a bonded rubber bush twists rather than rotating - clamped in the wrong position it is permanently pre-loaded and tears itself apart.

The rubber is bonded to both the inner and outer sleeve, so relative movement is accommodated by the rubber deforming.

Tightening with the suspension hanging locks the rubber in a twisted state.

Once the vehicle is lowered, the bush works against that twist through every suspension movement for the rest of its life.

The result is a bush that fails within months, which then gets blamed on part quality.

Support the hub at normal ride height before final tightening, or lower the vehicle onto its wheels first where access allows.

No. It is a safety-critical component and straightening alters the material properties without any way to verify the result.

Arms bend from kerb strikes and potholes, and the bend is frequently slight enough to be invisible.

The symptom is an alignment that cannot be brought into specification, or one that needs an implausible amount of adjustment.

Heating and bending a steel arm alters the heat treatment locally; cold straightening work-hardens material that has already been overloaded.

Aluminium arms are worse - they are cast or forged and crack rather than bend, and any visible deformation means the part is scrap.

Where alignment will not come into specification, measure the arm against a new one or against the opposite side before chasing adjustment further.

A wheel alignment always, plus checking the joints and links it works alongside, and using new fasteners where they are single-use.

Alignment is not optional - the arm defines the geometry and no two parts sit identically.

Many arms have an eccentric camber bolt at the inner mounting, so the setting has to be established rather than assumed.

Anti-roll bar links, tie rod ends and the opposite side's arm are all worth checking while the corner is apart.

Suspension fasteners are commonly torque-to-yield or self-locking and are frequently single-use; the manufacturer's procedure names which.

Corroded fasteners on an older vehicle should be replaced whether single-use or not, because they will not torque accurately.