In this ultimate guide to towing series, Robert Pepper discusses the different types of trailer brakes and how you should operate them.
Your car has brakes because otherwise, it’d be more than a little dangerous! But the car’s brakes are designed to stop the car alone, not deal with the extra weight of the trailer. That is why your car has a braked tow rating and an unbraked tow rating for trailers with and without brakes. Typically, for a medium to large 4WD, that’ll be 750kg unbraked and maybe 3,000kg braked. It is a myth that every vehicle can tow 750kg unbraked – they can’t. Subaru Foresters, for example, may be around 400kg unbraked. Some very light trailers have no brakes at all.
Because your trailer is heavy, it will overwhelm the tow car’s brakes, so the trailer needs to do its own braking. So, just add brakes to the trailer, and the job is done. Well, yes, but then they need to be activated, and that’s where the complexity starts to come in.
The two basic types of trailer brakes
Overrun brakes
There are two basic types of trailer brakes – overrun and electric. The overrun brakes are simple. Imagine a trailer and tow car both travelling at 60km/h. The tow car brakes. This causes the trailer to ‘run into’ the tow car, and that deceleration difference activates the brake. The overrun brakes have no manual control and there is no way for the driver to change the braking force. The overrun brakes should get applied harder the more the tow car brakes. They are legal on trailers up to 2,000kg GTM, after which independent brakes are usually required, and these are typically electric.
However, my advice is that independent brakes are a good safety idea on trailers lighter than the maximum limit, and you should be looking at independent brakes any time the trailer weight gets up beyond 50% of the vehicle’s weight. I think the law should be relative to vehicle tare, not an absolute value. The Venn diagrams of what’s legal and what’s safe do not necessarily overlap.
Electric brakes
Electric trailer brakes are more complex and do require driving technique knowledge. The trailer brakes need to act in concert with the tow car, and that is why you have an electric brake controller that detects when the tow car brakes are applied and applies the brakes in turn to the trailer. But how much braking to apply? Further complexity. Every braking controller is different, but they all operate fundamentally the same way. We’ll use the popular REDARC TowPro Elite 3 as our example.
It has two modes, user-controlled mode and proportional mode:
User-controlled mode
You select a braking force from 0 to 10, with 10 being the highest. When the tow car brakes, the trailer brakes are applied according to the controller setting, regardless of how hard you brake the tow car.
Proportional mode
The trailer brakes are applied in proportion to how hard the tow car is braked. Light braking on the tow car equals light braking on the trailer.
When to use the proportional mode
For on-road use, you’d typically use proportional mode, as it simply makes sense to have the trailer braked at the same level as the tow car. But there is always a but, and the caveat here is that the proportioning is correct.
Say the tow car is braked such that it decelerates at X, you want the trailer decelerating at X as well, not half X, or twice X, and for various reasons, that’s not necessarily a given. The way to check this is to do some brisk braking without the trailer attached, then use manual mode and do the same braking, starting at 5, then trying with 4 and 6 and so on. Give it a few minutes between stops, as otherwise, you’ll overheat your brakes. You’re looking for a braking feel where you minimise the trailer feel. That’s your benchmark. Then, switch into proportional mode and see if it feels the same as the optimised manual mode or close to it.
When to use user-defined mode
So when do you use user-defined? When you want a different braking force on the trailer than on the tow car. For example, when driving off-road up a steep hill, you might need to back down, and you might well want more braking force on the tow car than the trailer. Conversely, when driving down a steep hill, it’d be the reverse, with more braking on the trailer than the tow car. You may also dial up the trailer braking on a long downhill.
When to use manual override
Then you have the manual override, and the primary use here is to combat trailer sway or snaking. This is when the trailer wags, snakes or sways relative to the tow car, so the trailer is actually travelling faster than the tow car. So how do you fix it? Slow the trailer relative to the tow car, which means applying the trailer brakes only. You do that by pressing the controller button, or it may be a slider depending on the model. That will apply the trailer brakes, and if you don’t touch the foot brakes, the trailer will slow down relative to the tow car and the sway is fixed.
However, you need to ensure the controller sends enough braking force to the trailer. Let’s say the controller is in manual mode and set to 1, which is very low. You press it, and very little braking force is applied to the trailer, maybe not enough. Conversely, with some trailers, the controller set to ten might just lock the trailer wheels which isn’t ideal either. So it’s not just a case of “activate the trailer brakes manually”, it’s understanding what’s going on. We’ll cover that more in the forthcoming sway article.
Car braking when the driver hasn’t used the brakes
We now come to the car braking when the driver hasn’t used the brakes. How so? Modern cars have numerous safety systems which activate the brakes without the driver touching the brake pedal. One example is AEB, or Autonomous Emergency Braking, where the car detects a problem ahead and applies the brakes even if the driver does not. From a driver’s perspective, there is no additional technique required, but installers of brake controllers need to be sure to take the brake signal from the brake lights, not the brake pedal, or whatever else the carmaker recommends.
Don’t forget to maintain them
Finally, maintenance. Trailer brakes need maintenance and adjusting, even more than your towcar brakes, just another reason to have your caravan booked in for regular servicing. It’s when you make that emergency stop, or find yourself with the trailer snaking that you’ll be happy you understand how your trailer brakes work and how to operate them in all conditions.
