The Lowdown On Trailer BrakesBy Tim Murphy
Published: Spring 2014
Maintaining a trailer is simple, but if you don't take care of the little things, you could be in for a big letdown.
Boat trailers live in two notoriously hostile environments: on the highway and in the water. That means regular maintenance — especially of bearings, brakes, and wiring — is crucial to keep them safe and legal. Tires are a critical component, too, and should be stamped with an "ST" classification, for "special trailer." Dedicated trailer tires are more durable and resist abrasion and impact better than passenger "P" tires or light-truck "LT" tires; they also bounce less. Make sure the tire is neither overinflated nor under-inflated, and that the weight on the trailer doesn't exceed its capacity. Divide the trailer's total gross weight (measure it at a public weigh station) by the number of tires. For more on tire wear, see "How To Know When Your Tires Are Shot".
Maintaining Trailer Brakes
Most states require brakes to be fitted on trailers with a gross weight over 3,000 pounds. Many experts recommend them on trailers rated for 1,500 pounds and above. Trailer brakes can be mechanical, electrical, or hydraulic. Hydraulic "surge brakes" are the most popular technology. The entire system is contained in the trailer itself; any number of different vehicles can tow it. Like other hydraulic systems, this one is based on the fact that liquids cannot be compressed. A master cylinder is located inside the trailer's coupler device. When the towing vehicle brakes, inertia from the decelerating trailer drives a piston inside the master cylinder, increasing the pressure of the hydraulic fluid all through the brake lines.
At the other end of the brake lines, this pressurized fluid drives the braking device — either calipers in the case of disc brakes or a brake cylinder in the case of drum brakes. Disc brakes consist of a rotor oriented in the axis of the wheel. During braking, a piston drives calipers that squeeze against the rotor, creating friction to stop the wheel. By contrast, drum brakes consist of a wide cylinder section. During braking, a curved shoe inside the drum presses against the drum, slowing the wheel.
Disc brakes tend to have greater stopping power and greater fade resistance than drum brakes, and function better in wet conditions. They're also self-adjusting and easier to inspect and maintain. That said, their rotors are typically made of steel, which is prone to corrosion. Rinse metal parts thoroughly with fresh water after every launching.
Brake pads are a routine-wear item. Follow manufacturer's instructions for indications of when it's time for replacement. Brake lines are another regular-service item. Fluid levels should match the manufacturer's recommendation. If they call for, say, new "DOT 3 brake fluid," don't substitute anything else; seals and other components depend on the right blend. If brake lines need to be either replaced or bled, carefully follow manufacturer's instructions.
Tim Murphy is the coauthor of Fundamentals of Marine Service Technology (ABYC, 2012), from which this article is adapted.
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