Trailer Brakes and Brake Controllers in Plain English

Stopping a heavy trailer takes more than your truck's brakes. That's why many trailers have their own brakes, and why you need a brake controller to run them. If the terms sound confusing, here's the plain-English version.

Why trailers have brakes

Your truck's brakes are designed to stop your truck, plus some extra margin. A loaded trailer can weigh as much as your truck or more. Without trailer brakes, all that weight pushes on your vehicle when you stop, making stopping distances longer and wearing out your brakes fast. It can also make the trailer push the back of your vehicle sideways on a slippery road.

Trailer brakes let the trailer do its share of the stopping.

Types of trailer brakes

• Electric brakes: the most common on utility, landscape, and equipment trailers. A magnet inside each brake drum is powered through the trailer's wiring plug, and that applies the brake shoes.
• Surge brakes: common on boat trailers. When the tow vehicle slows, the trailer pushes forward on the coupler, which squeezes a hydraulic cylinder and applies the brakes. No controller needed.
• Electric-over-hydraulic: an electric signal runs a pump that applies hydraulic brakes. Often seen on heavier trailers.

This article focuses on electric brakes, since they're what most people run into.

What a brake controller does

A brake controller is a small unit mounted in your vehicle, usually under the dash. When you press the brake pedal, it sends power to the trailer brakes through the 7-way plug. Many newer trucks have a controller built into the dash.

There are two main kinds:

• Proportional (inertia-based): senses how hard your vehicle is slowing down and applies the trailer brakes to match. Brake gently, and the trailer brakes gently. Stop hard, and it brakes hard. This feels smooth and natural.
• Time-delayed: applies a set amount of braking that builds up over a short time after you press the pedal. It's simpler and usually cheaper, but doesn't respond as naturally.

Both kinds have a manual lever or button that lets you apply only the trailer brakes. That's important for stopping trailer sway.

Setting the gain

The gain is how much power the controller sends to the trailer brakes. Too low, and the trailer pushes your vehicle. Too high, and the trailer wheels can lock up.

A common way to set it (always check your controller's manual):

1. Hitch up the loaded trailer and find a flat, empty, dry area.
2. Drive at about 20 to 25 mph.
3. Squeeze the manual lever all the way.
4. If the trailer wheels lock up, turn the gain down. If the stop feels weak, turn it up.
5. Aim for the most braking you can get just before the wheels lock.

Set the gain again whenever the load changes a lot. An empty trailer needs less gain than a fully loaded one.

The breakaway kit

Trailers with electric brakes usually have a breakaway kit: a small battery on the trailer and a switch with a cable.

The cable attaches to your tow vehicle, not to the safety chains. If the trailer ever separates from your vehicle, the cable pulls a pin out of the switch, and the battery applies the trailer brakes on its own. That keeps a runaway trailer from rolling down the road.

Before each trip:

• Make sure the breakaway cable is attached to the tow vehicle
• Make sure it has enough slack for turns, but not so much that it drags
• Check that the breakaway battery is charged, if your trailer has a test button or gauge

Signs your trailer brakes need attention

• The controller shows no trailer connected (check the plug first, see our trailer lights guide)
• One side grabs or pulls
• Weak braking even with the gain turned up
• Grinding or squealing from the wheels
• A burning smell or very hot drums after normal driving

What Michigan law says

Michigan's vehicle code has brake rules for trailers, including when trailers must have brakes. They depend on the trailer's weight and when it was sold. Our Michigan towing laws article covers what we verified, with links to the official text.

Our flatbed needs a brake controller

Our 24 ft flatbed is rated at 10,000 lb GVWR, and it needs a properly rated tow vehicle and a working brake controller. If you're not sure whether your truck has one, check the dash or under it, or send us a message before pickup. Our Which Trailer Do I Need? guide explains the requirements for each trailer.

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