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Will your brake controller work with advanced safety aids when you’re towing a trailer?

Modern cars have advanced safety aids like AEB, and you want to make sure they still work when you’re towing a trailer or caravan.

TOWING TRAILERS has always been deceptively easy. Ever heard someone say “didn’t even know it was there”? That’s true until you actually run into some sort of trouble, like an emergency stop or an avoidance manoeuvre. It is then you might notice there’s a trailer on the back, when stopping takes a lot longer, the rear of the car tries overtake the front, you just push straight on forwards off the road, or a crosswind sets up sway giving you a few moments of terror before disaster.

There’s many different ways to avoid this sort of trouble, which we’ll cover in another article. However, the basics are – minimise distance between rear wheels and towball, use a 10% towball mass, reduce weight, centralise weight (not the same as just focusing on towball mass), ensure your trailer tyres are not fossils, tow level, and tow no more than 80% of the max rated towing load because the maximum quoted is a figure from marketing, not engineering.

The actual skill in towing is the setting up of the vehicle, far more than amazing car control reactions once you run into problems. Amazing tow drivers use their amazing setup skills to avoid situations where their amazing driving skills aren’t needed.  And a big part of that trailer setup is braking.

A trailer brake is a pretty simple concept; a system that applies the brakes to the trailer. You need to do this because the towcar’s brakes aren’t designed to slow the mass of the towcar plus trailer, and if the trailer starts to sway then the absolute best way to deal with the situation is to apply the trailer brakes and leave the towcar brakes alone – “accelerating out” may be the Aussie bloke’s solution to any given road emergency, but sadly all it does is get you to the scene of the accident that much quicker.

When it comes to trailer towing you need brakes on your trailer when your vehicle manufacturer says you do. A common misunderstanding is that limit is 750kg; that’s actually the maximum unbraked tow mass and the common limit for medium-large 4X4s, not a general rule. Many roadcars and softroaders require trailer brakes at less than 750kg, and while we’re on the subject, beware of being sold trailers with a 740kg tare mass with no brakes…as of course once loaded they’ll go above 750kg.

Above 2000kg the law (and sense, it’s rare but the two agree in this case for a change) says you need independently controlled brakes with a breakaway system that applies the brakes automatically if the trailer disconnects from the towcar.

There are ovverun brakes which are activated by the trailer compressing an activation system as the towcar slows down quicker than the trailer, but those are of little use offroad and not as effective onroad as proper manually applied brakes. And as they aren’t independently controlled. they can’t be used for trailers over 2000kg.

What you need for safe towing is an electric brake controller that applies the trailer brakes when you brake the towcar, and one that allows you to vary the gain – how hard the trailer brakes are applied when you touch the brake pedal. There’s also a manual switch so you can apply the trailer brakes without needing to touch the towcar brakes, useful offroad and essential for dealing with trailer sway.

Now back to the subject of electronic safety aids and braking. Modern vehicles have many electronic safety aids, and these have to be considered when towing. Here’s the main ones that involve braking:

  • Autonomous Emergency Braking (AEB) –  automatically detects an impending collision, and then applies the brakes automatically to help mitigate the resulting crash. Please, please, please do not rely on this system. In my testing, it will only stop in time with a unladen vehicle on a flat, level surface with good trip, at less than around 40km/h, in good visibility. More on that here.
  • Adaptive Cruise Control (ACC) – monitors the vehicle ahead and maintains a constant gap to it regardless of speed. More on ACC here.
  • Emergency brake signals – during an emergency stop the hazard lights and in some cases the brake lights flash at high speed. Mercedes call them “adaptive brake lights”.
  • Trailer Sway Control (TSC) – sensors on the car detect trailer sway, and applies brakes to the towcar’s rear wheels individually to cancel it out. While TSC is a great safety feature, if it activates consistently then consider it an early warning and you need to rethink your trailer setup, load distribution or driving style because sooner or later it won’t be able to save you.
  • Electronic Stability control (ESC) – applies brakes to individual wheels to prevent understeer and oversteer. Full explanation here.
  • ABS – prevents wheel lockup under braking, along with companion tech EBD and EBA. Explanation here.

ESC and TSC will still activate when trailers are connected, and they do not attempt to brake to any significant degree so there’s no worries about trailer braking. The braking in ESC is more to correct a skid than slow the car down. There’s also no problem with TSC, or stability control systems built into the trailer itself. The ABS system operates when braking, it’s not a direct cause of braking in itself.

The concern is with AEB, ACC and the brake signals. The problem is that now we have the potential for the vehicle to apply its brakes without the driver doing anything. The risk is that if the brake controller activates the trailer brakes by detecting when the brake pedal is pressed…then the trailer brakes aren’t going to be applied in the case of the AEB and ACC.  And if you go off the brake light, then if those flash under emergency braking there’s a risk that the trailer brakes may be pulsed.

Fortunately, the manufacturers have thought about this and their towing systems are compatible with braking systems.

We checked with Land Rover, Mercedes and Subaru, all of whom have vehicles with AEB, ACC and are commonly used for towing. All three of them said the systems work when towing…but only with factory approved wiring, and that typically takes the brake feed from the brakelights.

As for flashing brakelights; not all cars have that feature, some just flash the hazards, and Mercedes told us “that feature is disabled when a towbar is fitted” – and that’s when a bar is fitted, not when a trailer is connected.

Subaru said: “with our Eyesight system (which controls AEB – autonomous emergency braking – and adaptive cruise control) if the brakes are activated the brake lights are switched on. Therefore if the aftermarket electric brake controller can sense brake light operation (both conventional globes or LEDs) the electric trailer brakes can be activated. Current Subaru vehicles in Australia don’t use ‘flashing’ brake lights under any form of braking.”

Now here is the point to check. If brake controllers are wired up outside of the factory design wiring systems and activation points, then all bets are off. Basically, you can’t wire up a modern vehicle in the same way you would a GQ Patrol. Many modern cars have built-in trailer wiring, for example Land Rover’s Discovery – and we heard of one case where that wiring had been completely bypassed, and with it the vehicle’s safety aids. The odd thing is that running the extra wires would have been hours more work! When Ford’s Ranger PX came out there was quite some effort required to figure out how to wire that in for a trailer too.

We asked Redarc, makers of electric brake controllers about this issue and they said:

“Not all AEB or ACC indicate via the stop lamps but these are not an issue for electric trailer brake controllers (ETBC). It is the systems that do indicate via the stop lamps that are significant to ETBCs. The ETBCs utilise the illumination of the stop lamps to detect braking. Under emergency braking it is critical that the trailer braking is maximised however if the ETBC is not AEB compatible, the flashing of the stop lamps can switch off or at least significantly reduce the trailer braking.

Therefore customers with vehicles that do flash the stop lamps under AEB need to ensure the ETBC fitted is AEB compatible.”

The bottom line is that if your car has advanced safety aids, then make absolutely sure they work with your brake controller. That requires a three-way check; the car manufacturer, the trailer brake manufacturer, and the installer. Gain assurances from all three, and then do some testing yourself, never a bad idea as you don’t want the first time you apply the brakes hard to be in a real emergency. Trailer towing is easy, until you run into a problem.

Further reading

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Robert Pepper

Robert Pepper