It uses spring bars as levers to transfer load forward, restoring weight to the front axle that the tongue weight had lifted off.

Tongue weight applied behind the rear axle levers the front axle upward.

Spring bars running from the hitch head to brackets on the trailer A-frame are tensioned, applying an opposing moment.

That moment rotates load forward onto the front axle and rearward onto the trailer's own axles.

The combination then sits level, and the front axle regains the grip it needs for steering and braking.

It redistributes existing weight; it does not raise the vehicle's towing capacity, its tongue weight limit or any plated weight.

No. It redistributes load - every rating still applies unchanged.

The vehicle's maximum towing weight, its tongue weight limit and its plated axle weights are all set by the manufacturer.

A weight distribution hitch improves how the combination behaves within those limits; it does not extend them.

Some manufacturers do specify that a weight distribution system must be used above a certain trailer weight, which is a requirement rather than a capacity increase.

The hitch and bars have their own ratings, which must suit the tongue weight in use.

Bars rated far above the actual tongue weight are as wrong as under-rated ones, because they cannot be tensioned appropriately.

Insufficient tongue weight is the usual cause, and it is almost always a loading problem rather than an equipment problem.

A trailer loaded too far to the rear has its centre of gravity behind the ideal point, which makes it unstable in yaw.

A disturbance - a crosswind, an overtaking lorry, a camber change - starts an oscillation that the trailer's own momentum amplifies.

Speed makes it dramatically worse, and sway that is manageable at one speed can be uncontrollable slightly faster.

Other contributors are low tyre pressures, worn dampers on the towing vehicle, and excessive rear overhang on the trailer.

The fix is to move load forward until the tongue weight is within the recommended proportion, and to reduce speed.

No, and treating it as one is dangerous - it is a second line of defence against a disturbance, not a cure for a badly loaded trailer.

Friction and cam-based devices resist yaw movement and damp an oscillation once it starts.

They cannot change where the trailer's centre of gravity is.

A trailer that sways because it is rear-loaded will still sway with sway control fitted; the device only delays the onset.

That delay can be worse than no device, because the driver discovers the problem at a higher speed with less margin.

Load the trailer correctly, verify the tongue weight, and treat the equipment as protection against the disturbances loading cannot prevent.

By measurement - record the vehicle's front and rear arch heights unhitched, then tension the bars until the front measurement is close to its original figure.

Park on level ground and measure from the ground to a fixed point on each wheel arch, front and rear, with the vehicle unhitched and loaded as it will travel.

Hitch the trailer without tensioning the bars and measure again; the front will have risen and the rear dropped.

Tension the bars progressively and re-measure until the front arch height returns to close to the unhitched figure.

The hitch head angle and the number of chain links or the bracket position are the adjustments, and manufacturers publish target figures.

Re-check after the first journey and whenever the load changes significantly, because the setting depends on the actual tongue weight.

Usually, but not always - some vehicle manufacturers restrict certain types, and the handbook governs.

Many modern vehicles have trailer stability programmes that detect sway and apply individual brakes to damp it.

Those systems generally work alongside weight distribution equipment without conflict.

Some manufacturers specifically advise against friction sway control devices with their electronic systems, on the basis that the friction interferes with the sway signature the system is looking for.

A few restrict weight distribution systems on certain models, particularly where the rear suspension is self-levelling.

Check the vehicle handbook for each platform in a fleet rather than assuming, and record the answer so it is not rediscovered every time.

Lubricate the friction points as the manufacturer directs, inspect the bars and brackets for wear and cracks, and check the trunnions and chains.

Spring bar sockets and trunnions carry high loads and need the specified grease - though friction sway control surfaces must be kept clean and dry, which is the opposite requirement.

Inspect the bars for bending or cracking, particularly near the ends where stress concentrates.

Check A-frame brackets for movement and their bolts for tightness, since they carry the full bar tension.

Chains, hooks and lift brackets wear and should be replaced rather than run to failure.

Remove or slacken the bars before reversing sharply on some designs, and before manoeuvring on very uneven ground, or the bars can be overloaded.