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Trailer Brake Basics

Make sure you can stop before you start

Greg R. Whale
Oct 20, 2002
Like trailers themselves, trailer brakes come in a variety of sizes and system designs. Conversely, there is at least one constant: Without proper brakes and control of them, your trailer becomes an oversized yo-yo.
Modern consumer tow vehicles - pickups or sport/utilities - are frequently called upon to tow a trailer. Horses, boats, mobile houses, utility and stakebeds, race cars, and any manner of two-stroke buzz bombs need a trailer to get to the boat ramp, campground, or track. Since many of these combinations end up weighing more than the tow vehicle, and most states and vehicle manufacturers require them on trailers over a minimal weight, trailers have brakes.
Laws regarding brakes on towed vehicles vary by state and type of trailer. Some states require brakes on any trailer or anything over a set weight (loaded), while others use the type of trailer and/or number of axles as guidelines. In many cases, any trailer over two axles must have brakes on at least two of those axles.
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Tow-vehicle manufacturers have their own guidelines regarding trailer weights and brakes, with all maximum tow ratings given for braked trailers. Generally speaking, the non-braked trailer weight allowed by a manufacturer is a small fraction of the tow rating with trailer brakes, and some go as far as listing tow maximums separately for boats and other trailers because boats have much less frontal area. Further variations can be found with four-wheel drives that occasionally list towing weights for both high- and low-range operation. In Truck Trend road-test specifications, the maximum tow rating listed is for a braked trailer and normal high-range gearing. As we prefer to err on the conservative side, it's also our opinion that - manufacturer guidelines and laws notwithstanding - any multi-axle trailer or single axle that you can't move around on your own should have brakes.
The medium that supplies brake-action power on trailer brakes is air, hydraulic or electric. Commercial vehicles rely on air and hydraulic systems where the tow vehicle and trailer use the same circuit. Virtually all recreational trailers and boats use either hydraulic surge brakes or electric brakes. Regardless of system, the end result is a pressurized application of one material (the pad or shoes) against another (the drum or disc) to convert mechanical energy into heat energy. The heat energy then dissipates into the atmosphere, unless you've added wheels or fenders that limit airflow to the brakes or you've overheated them by excessive application, at which point the heat is transferred to wheel bearings, seals, and the fluid itself.
Hydraulic surge brakes are most popular on boat and watercraft trailers. Sealed hydraulic systems deal better with immersion than electricity, and the normal trailer weight is determined more by the boat on it than by how much you load in it. This brake system is self-contained on the trailer, consisting of a master cylinder on the coupler and brake lines and wheel units much like on your truck.
When a tow vehicle slows down, the trailer wants to push against it, and as the trailer pushes against the coupler, a lever inside it exerts pressure on the master cylinder just as pushing the brake pedal in your truck does. As the cylinder is pushed upon, hydraulic fluid is pressurized and goes through the lines, pushing the calipers or shoes against the discs or drums, thereby slowing the trailer. The more the tow vehicle slows, the more pressure is exerted on the trailer brakes, and many brake systems are sensitive enough to engage when you downshift or engage the exhaust brake on a diesel engine.
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Surge brakes historically have used drum brakes, but disc brakes are becoming popular because of benefits like heat and water-shedding ability and no adjustment required for wear. Surge brakes are usually set up right from delivery with the boat and require little to no adjustment for braking force. However, they may require adjustment to the shoes for wear, need bleeding - especially when parked for long periods of time, and the coupler must be kept lubricated so it slides easily under load. When surge brakes become troublesome, the culprit is often low fluid level or a bent or pinched brake line.
Since surge brakes engage when the trailer is pushing on the tow vehicle, they also tend to engage when backing up. Often a minimal, smooth throttle input will get past that, although many new trailers come with a fifth wire that will release the brake pressure for backing under load. For long descents, surge brakes should be treated like truck brakes, using regular short applications of moderate braking with intervals off for cooling them.
Except for the reverse-release signal wire, which can be triggered by back-up lights, there's no wiring for hydraulic surge brakes. However, you'll still have to wire the trailer lights.
The controller is in standby mode until the brake-lamp switch is activated by pressing lightly on the brake pedal (most vehicles illuminate the brake lights before the brakes actually start working). Roughly a tenth of a second after that lamp circuit is energized, the controller sends a variable amount of voltage to the trailer-brake solenoids, which convert it to mechanical energy so the brakes can convert the motion of the trailer (kinetic energy) into heat. It's important to know the brake-lamp circuit is the trigger for electric trailer brakes for two reasons: One, if the light switch or its wiring is bad, there will be no automatic trailer braking; and two, as long as your foot is on the brake pedal the trailer brakes are activated (you're wasting voltage sitting at a traffic light on flat ground).
While routine maintenance of electric trailer brakes is less involved than hydraulic surge brakes, the electric units require more investment in wiring and time. And the simplest part of that, for any trailer, is a good ground connection; the tow ball is not an acceptable substitute.
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Trailer brakes add only one wire to those required for the lights and trailer-battery charge. The controller usually needs four wires: 12VDC +/-, the signal wire from the brake-lamp switch, and an output to the trailer. Many new trucks come with a dedicated four-pin connector under the dash specifically for brake controllers, while others include it as part of a towing package. Truck/trailer connectors themselves have yet to be standardized, though most RVs use a seven-pin plug and most vehicle manufacturers include one as part of a towing package. Given the increasing complexity of modern truck electrics and the fact that the majority come with separate turn and brake-lamp circuits (while trailers are still primarily a combination turn/brake), we'd opt to pay the nominal sum and let the factory build in the tow wiring from day one.
Regardless of design, trailer brakes should be checked prior to every trip. This requires a short drive instead of a visual inspection. Trailer brakes are subject to changes in temperature, humidity, and wear just like any friction device. Trailer brakes should never be considered an excuse to further overload a tow vehicle or a trailer.
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All trailers should also consider a break-away device (again, requirements vary by state). On hydraulic surge systems, the break-away is usually a chain attached the tow vehicle. When the chain is pulled taught, as when a hitch failed, the chain pulls on the brake cylinder, applying heavy braking force and stopping the trailer. On electric brake systems, the break-away is powered by a smaller, secondary battery on the trailer itself that will engage the trailer brakes if it disconnects from the trailer.
Sanely driven and properly maintained, trailers tend to stay behind the tow vehicle, but both Murphy and Newton suggest that an object in motion tends to keep going. If anything in your tow hitch fails and the trailer becomes disconnected from the truck, a break-away will keep it in the mirrors and not the side windows.

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