A tow vehicle’s brakes are sized for the vehicle and its payload, not for an extra several thousand pounds pushing from behind. Without trailer brakes, a heavy trailer can nearly double stopping distance and overheat the vehicle’s brakes on long descents. Thetowing stopping distance calculator shows the difference for your rig. Rules about when trailer brakes are required vary by state, province and country, and many vehicle manufacturers set their own threshold in the owner’s manual, so check both.
Types of trailer brakes
Electric drum brakes
The most common type on travel trailers, cargo and utility trailers. Inside each drum, an electromagnet is energized by the tow vehicle’s brake controller. The magnet grabs the spinning drum face and is dragged around, which pushes a lever that expands the brake shoes against the drum. More current means a stronger grab and stronger braking. They need a brake controller in the tow vehicle and periodic adjustment as the shoes wear.
Electric-over-hydraulic brakes
An electric signal from the controller drives a hydraulic pump on the trailer, which operates disc or drum brakes like a car’s. They are common on heavier trailers and on boat trailers that want corrosion-resistant discs. They also need a compatible controller.
Surge brakes
Self-contained hydraulic brakes activated by the trailer itself. When the tow vehicle slows, the trailer pushes forward against a sliding coupler, which presses a master cylinder and applies the brakes. No controller is needed, which is why surge brakes are popular on boat and rental trailers. Because they react to the trailer pushing, they brake a little later than electric brakes, and backing up hills may need a lockout. Some vehicle manufacturers limit the trailer weight with surge brakes, so check the manual.
Brake controllers
A brake controller sits in the tow vehicle, often under the dash, and decides how much power to send to electric brakes. Many newer trucks include one integrated in the dashboard.
- Proportional (inertia-based) controllers sense how hard the vehicle is decelerating and send matching power. Gentle stops get gentle trailer braking; panic stops get full braking. They feel smooth and are the usual recommendation for heavier trailers.
- Time-delay controllers ramp up to a preset power over a set time after you press the brake pedal, regardless of how hard you press. They are simpler and cheaper, but braking feels less natural and can be harsh or late.
Every controller has a gain setting (the maximum power sent to the brakes) and a manual lever or buttonthat applies the trailer brakes alone. The manual control is also the key tool for stopping trailer sway; seetrailer sway: causes and prevention.
How to set the gain
- Load the trailer as you will tow it. A loaded trailer needs more gain than an empty one.
- Find a dry, level, empty paved area.
- Drive at about 20–25 mph (30–40 km/h) and squeeze the controller’s manual lever all the way.
- If the trailer wheels lock (you hear tire skid or see it in the mirror), reduce the gain. If the trailer barely slows the rig, increase it.
- The goal is the strongest braking just short of lockup, with the rig stopping smoothly and the trailer not pushing the tow vehicle.
- Then test normal stops with the brake pedal. The rig should stop as one unit, without a jerk from the trailer braking too early or a push from it braking too late.
Proportional controllers may also need a level or “boost” setting; follow the controller’s instructions. Re-check the gain whenever the load changes significantly and when road conditions are wet or slippery.
Breakaway systems
Trailers with electric brakes usually carry a breakaway switch and a small battery. A cable runs from the switch to the tow vehicle; if the trailer separates, the cable pulls a pin and the battery applies the brakes fully. Attach the cable to the tow vehicle’s hitch, not to the safety chains or the ball mount, so it pulls only in a real separation. Keep the battery charged and test the switch before trips by pulling the pin and trying to move the trailer slowly; then reinsert it.
Maintenance
- Adjust drum brakes as the manufacturer recommends; worn, unadjusted shoes reduce braking a lot. Some have self-adjusters.
- Inspect magnets, shoes and drums when servicing the wheel bearings, which are in the same hub.
- Check wiring and the connector: corroded pins and weak ground wires are the most common cause of weak electric brakes.
- Boat trailers: rinse brakes after salt water and inspect more often; corrosion is the enemy.
- New brakes need bedding in: a series of moderate stops lets shoes seat against the drums.
Signs something is wrong
- The controller shows no connection, or an overload or short message.
- The trailer pushes the tow vehicle in normal stops.
- One trailer wheel locks while others don’t, or the trailer pulls to one side when braking.
- A hub is much hotter than the others after a drive (brake dragging or bearing problem).
Weak trailer brakes are easy to miss in everyday driving and only show up in an emergency. Testing them with the manual lever at the start of every trip takes seconds; it is part of our pre-trip towing checklist.
This guide is general information. Your vehicle, hitch, trailer and tire manufacturers’ ratings and instructions always come first, and towing rules vary by state, province and country. See our disclaimer.