Margin of Air: Trailer Brake Controller

As I was looking at trailers that were large enough for this project I realized that I was going to need a brake controller for my car.

Unfortunately Subaru was not accomidating enough to provide integrated brake controllers in my Outback. The new Forester seems to have access for integrated brake controllers. But on the recommendation of my dad I looked into the Max Brake controller. See http://www.maxbrake.com/ for the product website.

The Max Brake controller mounted under my steering wheel.

Most other brake controllers are based on accelerometers, the more expensive ones are 3 axis mems gyors which learn their orientation, and the cheaper ones have historically been simple pendulums. As the vehicle slows down, the brakes are applied on the trailer. This concept has some significant downsides. It has a bit of phase lag as the trailer brakes won't respond until the car has started slowing down. And as I am going to be towing the trailer with my car and not a truck, the mass ratio between the car and trailer is even worse, making the emergency case worse.
The other similar solution is surge brakes. These work by having a spring built into the tongue of the trailer and when the hitch pushes forward against the car the brakes are applied. This happens when braking, or when backing up. So there's a way to disable it for backing up hill. And the surge brakes also have the same issue of a delayed response requiring the car to start slowing down before reacting. And only reacting in relationship to how much the car is slowing down.

The acceleration based systems fail if the braking is improperly tuned. In the case they are too powerful they will lead to oscillations as the trailer overbrakes at first, then slows down, stops braking, speeds up, overbrakes, stops braking, speeds up, .... The same effect can happen with the accelerometer based brakes, though often not as pronounced. And likewise if the gain is improperly set too low. The trailer will just never brake as hard as you want.

In contract, the design of the Max Brake is to tap into the hydraulic brake line of the car, thus directly sensing the drivers braking command. After calibrating the sensor to know what the pressure in the brake line is for full and zero braking, there is one knob to determine the ratio of trailer braking to car braking, such that for any amount of brake pressure you will have a repeatable trailer brake pressure.

The easiest way to see the value of this system is to understand how it overcomes the oscillating trailer issue. If you brake gently at a constant force on the brake pedal, the trailer will brake gently and constantly too. Tuning the knob on the controller will adjust the ratio of vehicle braking to trailer braking, but they will both hold constant if the brake is constant. Importantly this will work for both regular braking and emergency braking. When the acceleration based systems are tuned well for normal driving it is hard to make sure that they respond proportionally to your desired braking in an emergency. And it brakes down in the case that the vehicle starts losing traction, say if the trailer is to conservative in it's braking.

PS I did have an idiot cut me off when I was coming home from the dealership on I-5 and was very glad that the trailer brakes were super responsive.

Here are some pictures of my setup. I bought the Max Brake controller and then had Kurt and Dorn's of Palo Alto install it because I didn't want to work on my brake lines without training/practice. They were able install it in a few hours.