Howdy –
Had a fellow ask me how to adjust and use trailer brake controls and as we talked it came to me that he didn’t really know the difference between a timer control or a proportional unit or any of the other methods of trailer brake control so I figured I’d drop a few words about the subject. Now it sounds simple but as Sir Issac Newton proved, once you get a mass moving it wants to continue to travel until resistance finally slows it to a stop. The word to remember is “Inertia”. Our trailers are that mass behind us and once put into motion it they want to keep moving. So we live with the “Curse of the brake controller” when we tow. Of course none of us tail-draggers are pulling an overweight trailer so our tow vehicle should stop it just fine – right ? Sure, just like nobody plays computer games !! The federal government’s rules for towing say that the tow vehicle of any trailer is only responsible for the first 2,000-pounds of trailer weight. All trailers over 2,000-pounds require that a controlled braking system be installed.
Brake controllers have gone from a simple timing device to pendulums to the modern accelerometer driven devices most of us use today. The overriding theory has always been that you want your trailer brakes to engage at precisely, or slightly ahead, of the tow-vehicle’s brakes. The trick is how to accomplish the proper timing. If the tow vehicle begins stopping before the trailer you have an instant case of “Tail wagging the Dog”. If the trailer brakes lock-up or begin grabbing then you have tire wear and unsafe braking, not to mention the “bobble-head” effect to the driver and passengers. Every manufacturer has had to fall back on the one common event that occurs every time a tow vehicle’s brakes are applied and that’s stepping on the tow vehicles brake peddle.
Your car or truck’s brake light comes on the moment the brake pedal is pushed. The light signal is slightly before any brake pressure is actually applied thereby giving the trailer brake controller a bit of a head start in the stopping process. Brake control manufacturers take advantage of this by tapping into brake light wire and sending a small amount of power to the trailer brakes the moment it’s detected. That’s how your trailer stops “ahead” of the tow vehicle. An alternative is a cable link from the brake pedal to the controllers, again to detect movement of the pedal.
All brake controllers are a compromise in function. The smarter the controller the better, but the driver may still have to “tune” the controller to fit the conditions. To have smooth stopping at slow speed we want the tow vehicle to do most of the stopping and have just enough brakes on the trailer to keep is straight and assist in slowing down. In an emergency we want the trailer brakes to be much more aggressive. On most brake controllers, the driver adjusts this initial power output with the GAIN control. That way, the power can be reduced in city driving to keep the trailer brakes from “grabbing”. The safety issue here is that the driver can fail to turn the gain back up for high-speed driving, which can result in an accident in a panic stop situation or the driver sets the gain to high which makes the trailer try to stop the tow vehicle. Now having said all this, the biggest fly in the ointment is often the driver over “adjusting” the controller instead of letting it do it’s job.
Since all current controllers are operated by the brake light signal or a strain-gage cable running from the brake peddle to the controller to make them function the biggest differences is in how the power is metered to the brakes. Time-controlled systems use Gain and Timer settings. These controllers don’t care about the outside world, only how much gain, or power, to apply and over how much time to apply that gain. Strain-gage and cable type controllers are essentially doing the same thing, except they detect the slight “tug” on the cable instead of brakes light power to detect brake pedal movement.
Now I know that some of you, er, more senior folks are going to brag about the “master cylinder tap” along with your uniquely tuned resistance coil. In this setup the controller was directly connected to the tow vehicle’s master brake cylinder and pressure from the cylinder was what determined the voltage from the controller. A resistance coil was used to “smooth” the power output and keep the controller from dumping all the power directly into the trailer brakes. The advent of ABS or anti-locking brake systems did away with this type of system because the ABS unit can mis-read braking conditions if a controller is directly attached to it.
Smarter systems use a pendulum to measure braking Inertia from the tow vehicle’s slowing down. During normal braking in town there isn’t much inertia so the controller only puts a small amount of voltage to the trailer brakes. However radical braking, such as a panic stop, forces more rapid movement of the pendulum and more power is applied to the trailer’s brakes. There is a set-up procedure that includes leveling the unit’s pendulum and setting the gain control, but this is easily done, and unless ya monkey with it it doesn’t need to be done again.
We live in a computer age and the top of the line systems use accelerometers that “sense” movement of the towing vehicle. Their computer-driver brains automatically adjust to real world conditions and compensate for them. You can even set the amount of power and time advance that the controller provides to the brakes. Once the accelerometer or other input device takes over, the power sent to the brakes depends on two factors: the reading from the input device and the gain set by the driver. In the case of an accelerometer based controller, the deceleration of the tow-vehicle is measured and, depending how rapidly the tow-vehicle is slowing, power is sent to the trailer brakes. Again, this output is limited by the gain setting. This is true of any brake controller, whether it taps into the brake line, brake lights, cables to the brake pedal, or uses an accelerometer to control power to the trailer brakes. In accordance with federal law all brake controllers have a over-ride system that suspends the normal controller function and applies power immediately to the trailer brakes, but with most controllers this is limited by the amount of total power the gain control is set for.
One of the best solutions to all the problems of trailer braking is the electric/hydraulic disc brakes for the trailer. In this system the trailer has automotive disk brakes installed. These brakes are hydraulically operated, just like on a car but instead of a vacuum booster an electric motor operates a hydraulic pump mounted in the trailer which provides the pressure to the disk brakes. Hydraulic brakes, however, have their own problems, the biggest being the slight delay which can be as much as a half second between pressing the brake pedal and the activation of the hydraulic actuator. Ironically, the best brake controllers for this type of system have always been the low cost timer or pendulum based controllers, which don’t allow for all conditions and are still limited by the gain setting.
The goal of any of these controllers is to make the trailer brakes work in conjunction with the tow-vehicle brakes under all the conditions we find on the open road. Easy? No! Always remember, the biggest factor in trailer brake control is still the nut holding the steering wheel !
Latter – –
The Old Ranger
