Electric trailer brakes: when the law needs them and how they work

The brake rules, step by step

These limits are in gross trailer mass (GTM), the weight on the trailer's axles. See tare, ATM and GTM.

Safety chains

Safety chain rules use ATM instead:

• Up to 2,500 kg ATM: at least one safety chain to AS 4177.4.
• Over 2,500 kg to 3,500 kg ATM: two safety chains to AS 4177.4.
• Over 3,500 kg ATM: two chains of a heavier grade and size.

The tow vehicle's limits still apply

The trailer's brakes do not raise the vehicle's ratings. An unbraked trailer must also be within the vehicle's unbraked towing capacity, often 750 kg and sometimes less. Check all four limits in the calculator.

How electric trailer brakes work

Electric brakes are drum brakes with an electromagnet inside each drum. When the tow vehicle brakes, the brake controller sends a current down the brake wire, pin 5 on a 7-pin plug. The magnet is pulled against the spinning drum face, and that drag moves a lever that pushes the brake shoes out against the drum. More current means a stronger pull and harder braking. Because the controller decides the current, electric brakes can be adjusted to suit the trailer's weight and road conditions, and many controllers let the driver apply the trailer brakes alone with a manual lever, which is the tool the NSW Government's sway advice relies on.

Electric, override or electric-over-hydraulic

Over 2,000 kg, brakes must work on all wheels and be controlled from the tow vehicle, which is why override brakes drop out at that point.

The brake controller

Electric brakes need a controller in the tow vehicle. Proportional controllers sense how hard the vehicle is stopping and apply the trailer brakes to match; time-based controllers ramp the brakes up over a set time once the brake pedal is pressed. Proportional controllers generally feel smoother. Either way, the controller's gain must be set for the loaded trailer, so the van does its share of stopping without locking its wheels. Our electric brake controller guide covers the choice and installation.

Setting the gain, simply

On a quiet, flat, dry road, with the van loaded as you travel:

1. Set the controller to a middle setting.
2. At about 30 to 40 km/h, apply the controller's manual lever alone.
3. If the van's wheels lock or the rig jerks hard, reduce the gain; if you barely feel the van slowing the car, increase it.
4. Aim for firm braking from the van without lock-up, then test a normal stop with the pedal.

Recheck the setting when the load changes a lot, and in the wet. The controller's manual has the exact method for that model.

The breakaway system

Above 2,000 kg GTM, a breakaway system must apply the trailer brakes automatically if the van separates from the car, and keep them on for at least 15 minutes. It usually uses a small battery on the van and a switch with a pin attached to the car by a lanyard. Check before each trip that the breakaway battery is charged, many systems have a test button or indicator, and that the lanyard is attached to the car, not to the safety chains, so it pulls the pin if the coupling fails.

Looking after electric brakes

• Check the magnets and shoes at the intervals the axle maker sets; magnets wear and need replacing.
• Adjust the shoes if the brakes feel weaker over time; many electric drum brakes need periodic manual adjustment.
• Service the wheel bearings with the brakes, because the drum comes off anyway.
• Look after the wiring: a corroded pin 5 or a poor earth on pin 3 weakens braking long before the brakes themselves wear.
• After water crossings or beach driving, have the drums checked for water, sand and corrosion.

Do I need trailer brakes on a box trailer?

Most single-axle box trailers stay under 750 kg GTM when loaded with garden waste or a small load, so no brakes are required. A loaded tandem box trailer, a car trailer or a small boat trailer can easily pass 750 kg, at which point brakes are required. Weigh it loaded, as it will be used, rather than relying on the empty weight. Our tare weight guide explains the weights involved.

Safety chains in practice

Brakes stop the van; safety chains keep it attached if the coupling fails. Cross the chains under the drawbar so they form a cradle that catches the coupling if it drops, attach them to the towbar's chain points with rated shackles, and leave enough slack to turn but not so much that they drag on the road. Check the rating stamped on the chain and shackles matches what the trailer's ATM requires under the rules above. Transport for NSW also lists safety chains among the equipment a towing vehicle must have.

Brakes and a heavier van: check the tow vehicle too

A van that needs electric brakes is usually heavy enough that the tow vehicle's own limits come into play. The NSW Government's guide of 5 to 10 per cent of the loaded van on the tow ball means a 2,500 kg van can put up to 250 kg on the car. Check the car's braked towing capacity, its tow ball limit and its GVM with that ball weight included, using our tow ball weight guide.

Brakes and steep descents

Long descents are where trailer brakes earn their keep. Transport for NSW's advice for towing is to use a lower gear when travelling downhill to control speed and reduce the risk of brake failure. Let the engine hold the rig back and use the brakes in firm, short applications rather than riding them all the way down; that keeps the drums of both car and van from overheating. If the van starts to push or sway on the way down, a gentle application of the trailer brakes alone with the controller's manual lever helps pull the combination straight.

Before every trip: a two-minute brake check

1. Hitch up and connect the plug, the breakaway lanyard and the safety chains.
2. Check the controller shows the trailer is connected.
3. Roll forward slowly and apply the controller's manual lever; you should feel the van pull back.
4. Check the breakaway battery indicator or test button.
5. After the first few kilometres, feel the van's wheel hubs at a stop: one much hotter than the others can mean a dragging brake or a bearing problem.

Buying a used van: what to look for

On a secondhand caravan or camper, ask when the brakes and bearings were last serviced and ask for the invoice. Look behind the wheels for rust and for signs of grease on the backing plates, check the breakaway battery and switch work, and ask to test the brakes on a short tow with your own controller. Worn magnets and shoes are not expensive to replace, but a seller who cannot say when they were last checked is telling you something about how the van was maintained.

Common faults and what they usually mean

Most faults are electrical rather than mechanical, and most of those are at the plug or the earth. Our 7 pin trailer plug wiring guide shows which pin does what.

Hubs and water

Brakes and bearings share the hub, and both dislike water. After a creek crossing, a boat ramp or a beach run, water and sand can get into the drum. Drive gently for a short distance applying the trailer brakes lightly to dry them, and have the hubs checked at the next service if the van has been in salt water. Many owners of off-road campers fit sealed hubs or carry spare bearings for remote trips.

How much braking does the van do?

With the gain set well, the van's brakes should do roughly their share of stopping the van's own weight, so the car is not pushed from behind. You feel it as a smooth, settled stop with no shove at the end. If the car seems to be dragging a heavy van to a halt, or the van tugs the car sideways when braking, the gain or the brakes need attention. Lighter loads need less gain; a fully loaded van on a wet road needs a careful check that the wheels do not lock. The aim is balance, not maximum braking from the van.

Brakes are part of the weight picture

Electric brakes let a heavy van stop safely, but they do not change any weight limit. A braked van still has to stay within its ATM and GTM, the towbar and coupling ratings, and the tow vehicle's towing capacity and GCM. Brakes, weights and the hitch all work together; get one wrong and the others cannot make up for it.

Upgrading from override to electric brakes

Owners of camper trailers and boat trailers sometimes move from override brakes to electric or electric-over-hydraulic brakes, often because the trailer has grown heavier with accessories and is approaching or passing 2,000 kg GTM. Above that figure, override brakes are no longer allowed and a breakaway system is required. The change involves new brake assemblies, a controller in the tow vehicle and wiring to pin 5. Have it done by a trailer specialist, and check whether the change needs to be noted or certified for registration.

Common questions