I recently had to delve into the brakes on a trailer and was quite surprised at how they work. With all of the advances in technology, it’s a wonder that this method still exists as the majority of trailer brakes are still a drum style arrangement in this world of disk brakes. There are however conversion kits to electric over hydraulic disk brakes.
Drum trailer brakes use a magnet to adhere to the rotating drum (See How electric brakes work). The magnet floats loosely on mounting studs. There is just enough movement for the magnet to push against the back brake shoe forcing it against the drum. This is a departure from traditional drum brakes whereas a hydraulic piston pushes both the front and back brake shoes against the drum. Still, the back brake shoe does most of the work in hydraulic brakes. For electric trailer brakes, the magnet pushes against the front brake shoe when going in reverse. In order for this arrangement to work, the magnet is mounted to the bottom of the brake backing plate.
“How Electric Brakes Work” courtesy of the Hitchweb Team3
When the brake isn’t powerful enough to stop the vehicle, the magnets slide along the drum creating friction. This seems like a hokey way of doing things as wear can result on the magnet. The magnet is powered by a brake module that provides adjustable gain to optimize the braking based on load. I find that my brakes often lock the wheel when the trailer is unloaded regardless of the module setting. Also, you can hear the brakes vibrating when applied. These brakes are either on or off, creating a common tugging feeling when the brake is applied.
With the advancement in technology, one would think that servo motor style brakes would be available. The advantage would also be the ability to push both brake shoes against the hub. A quick search turned up nothing. Similarly, regenerative braking would make sense especially to capture the energy in RV batteries. Although an article turned up about regenerative braking, I didn’t find much in the way of products.
Disk brakes increase stopping power and are easier to repair. However, trailers often sit unused, which to me means the disks may get rusty faster than the closed drum style. Still, there are conversion kits for disk brakes. In this style of brake, an electric actuator pushes a hydraulic fluid, creating pressure for the caliper to squeeze the brake pads against the disk.
Electric Over Hydraulic Actuator courtesy of etrailer.com5
Electric brakes require an electrical connection to the trailer. New vehicles often come prewired with this feature to a seven-pole style connector. On the old Scout, I had to run the line and add a connector in addition to my four-wire setup.
In this world of sophisticated electronics, it’s amazing that old style electromechanical devices are still prominent. It matters less when I feel that tug knowing the trailer is dragging me to a stop rather than pushing me into an intersection.
References
- Trailer Disc Brake Conversion – Electric Over Hydraulic Disc Brakes – WOW!, Road Less Traveled site
- How Electric Brakes Work, Written by Hitchweb Team, Monday, January 7, 2013
- How Electric Trailer Brakes Work, by ghostses, YouTube video, published on Oct 14, 2015
- Are You Ready For: The Hybrid Camper Trailer?, By Edward Niedermeyer on August 21, 2011
- Parts Needed to Add Electric Over Hydraulic Disc Brakes to a Trailer, etrailer.com website
The electrical and mechanical aspects of most trailer brakes lend themselves to anti-lock methods. Over 15 years ago, my car hauling trailer was augmented with a slotted wheel and magnetic pickup on each wheel for sensing wheel speed. Some simple solid state relays provided a way to release the appropriate brake when a while diverged from the active trailer model. It worked well and even an 8bit microcontroller could handle the math. Trailer brakes are relatively simple and work well in most cases when maintained. Passive controllers could use some work for those of us that want the quickest response feasible.