Trailer Brakes Explained

Trailer brakes are required above a weight threshold in almost every state, and the threshold is lower than most buyers expect, commonly between 1,500 and 3,000 pounds. The two systems you will meet are electric brakes, which need a controller in the tow vehicle, and surge brakes, which are hydraulic and self-actuating. Electric is far more common on the trailers sold in this catalog.

Electric brakes

An electromagnet in each brake drum is energised by a controller in the tow vehicle. When you press the brake pedal the controller sends proportional current to the trailer brakes. This is the standard on utility, dump, equipment and enclosed trailers, and it is why a brake controller is a normal part of a tow setup.

Surge brakes

A hydraulic actuator in the coupler senses the trailer pushing forward against the tow vehicle under deceleration and applies the brakes mechanically. No controller and no wiring. Common on boat trailers because it survives immersion, and it is why you will occasionally meet one on a used unit.

Breakaway systems

A battery on the trailer and a cable to the tow vehicle. If the trailer separates, the pin pulls and the brakes apply on full. The cable attaches to the vehicle, not to the hitch, because if the hitch is what came off then a cable attached to it does nothing. This is one of the most common setup errors.

Adjusting electric brakes

Most electric drum brakes are self-adjusting in reverse, but they still drift. Jack the wheel clear, turn the star adjuster until the drum will not rotate, then back it off until the wheel turns with light drag. Do both sides equally, because uneven adjustment pulls the trailer under braking.

Controller gain

Set gain on a loaded trailer, not an empty one. At around 25 mph, apply the manual lever alone. The trailer should slow firmly without locking. Too low and the tow vehicle does all the work. Too high and the trailer brakes grab, which is its own hazard on a wet surface.