I pulled onto the freeway towing our 25-foot sailboat. I looked in my mirror, and I was shocked to see heavy smoke coming from all four trailer wheels. I quickly pulled to the side of the road, and slowly drove to the next off-ramp. The brake components were very hot, and the wheel hubs were starting to get hot. I inspected all visible brake components, but could find nothing amiss.
The next day I tried towing again to see if the problem would repeat, and it did, with all four wheels smoking. Since the brakes were getting hot before the hubs, the source of the heat must have been the brakes, not the bearings. I inspected all the brake components again, but still did not find anything out of the ordinary. I was stumped.
This was going to require more sleuthing. My trailer is equipped with a surge brake system, with disc brakes at each of the wheels. Determining the cause of the problem was going to require a good understanding of the theory of surge brake operation and the design of the actual hardware installed on my trailer.
After doing some research, I found there is a wide variation in the design of boat trailers and brake systems. Boat manufacturers generally do not make trailers for the boat. Rather, boat manufacturers utilize the many independent trailer manufacturers located throughout the country. Also, there is little standardization of trailer design. Manufacturers usually produce trailers by bolting together components they select from the many component manufacturers, including axles, brakes, actuators, wheels, fenders, lights, bunk boards, and rollers.
Further complicating the picture, not all states have the same legal requirements for trailers. Therefore, retailers may sell a single model of boat with different models or brands of trailers depending on location. As a result, a single model of boat can be paired with a variety of trailer designs and components. Moreover, the amount of information available about a trailer and its components varies greatly by manufacturer.
Surge brake systems are the most common type of brakes used on the 7 million boat trailers in the US. In contrast to travel trailers that ordinarily use electric brakes, surge brakes activate mechanically, operating independently of the tow vehicle brakes. This eliminates problems that water causes with electric brake components when submerging the trailer during launching.
Surge brakes use hydraulic pressure to activate the brakes. The system consists of an actuator/coupler assembly, where the front of the coupler attaches to the ball on the tow vehicle. The back of the coupler has a sliding connection to the actuator, and the actuator is permanently attached to the trailer frame. The actuator contains a master cylinder, which connects to the individual trailer brakes via hydraulic brake lines. The individual brakes can be either drum or disc, similar to a car.