In part one, John Hughes delved into the dynamic forces that come into play when towing a caravan and, in particular, the variables that contribute to trailer sway. In this second part of his feature, he explores driver responses and the tech available to counter caravan sway.
Before we dive in, let’s run through a quick summary of what is going on when a caravan sways. A van can often experience tiny swinging back and forth as it reacts to external forces such as high winds or bumps in the road. Usually, the forces are small, and the swaying is small. Therefore, the system just settles down naturally, and we may not even be aware of the dynamics.
If the energy inputs become greater, then the swaying becomes greater. Some examples are strong gusts of wind, large vehicles or trucks driving past or a sudden change of direction of the towing vehicle. These larger forces push the van further sideways before the pulling effect of the vehicle swings the van back in the opposite direction. And because the reaction is greater, the van swings further back past centre creating a larger pendulum effect.
It is important to understand that a van moving around translates forces to the tow car. The van can push the car, pull on it, lift it up, push it down and even twist it. If the forces are great enough something more dramatic can take over.
Testing caravan sway with an AL-KO ESC system
We all know the shortest distance between two points is in a straight line. To start, we have the car going in a straight line and the van more or less following in a straight line. Then as the van begins to sway more it is travelling in a curved line out to one side and then another curved line out to the other side. As the van and the car are attached, the van continues to keep up with the car and the only way it can do that is to travel faster than the car. Now the van is able to push the car around more than normal. Ultimately, if this oscillation continues the van will try to overtake the tow vehicle. If this happens jack-knifing or even rig rollover can occur.
Obviously, it is best to avoid major trailer sway rather than rely on measures to react to it. Correct weight distribution is one of the most important things you can do to prevent sway. Again, this is covered in part one which we would encourage you to read. However, the reality is that sway does happen, and it is reassuring to know there are a number of lines of defence at your disposal. The concept that a swaying van is travelling faster than the tow car is a really critical one because many of the solutions to combatting sway are based on slowing the van down at a greater rate than the car.
Driver responses and how to avoid caravan sway
Having spent a fair amount of time towing vans in a variety of conditions, I have experienced uncomfortable sway on a couple of occasions. Having survived to tell the story my main advice is to try to remain calm and don’t make any major driver inputs. Sway can happen very quickly and with significant forces involved it is not possible to yank on the steering wheel to muscle your way out of it. In theory, increasing the speed of the tow vehicle to match the van could straighten the van out but in practice, the last thing you want when things are potentially going pear-shaped is to add more speed into the equation. Rapidly hitting the car brakes is not a good idea either as it may increase the pendulum effect or cause a loss of traction.
The recommended approach (and it has worked for me twice) is to gently back off the acceleration and just try to keep pointing the car straight. Ideally, you should also simultaneously slow the caravan down independently of the tow vehicle. Caravans are fitted with their own brakes because the car brakes alone are inadequate to stop a towing rig. A device known as an electronic brake controller is fitted to the car. This activates the caravan/trailer brakes whenever you put your foot on the car brakes. Brake controllers also have a switch that enables you to activate the caravan brakes without applying the car brakes. Slowing the van down at a greater rate than the tow vehicle is the best defence against trailer sway. I must confess during my two major sway experiences, unfamiliarity with the press vehicles meant I failed to have the muscle memory to hit the manual brake override. It is also unnerving to remove a hand from the steering wheel in these conditions. There are a couple of things you can do to increase the chance of hitting the override. The first is to regularly practice applying the manual override when it is safe to do so until it becomes second nature to react automatically if the time comes. The second is to familiarise responsible passengers with the concept and locate the electric brake controller such that a passenger can easily reach it.
Ensuring you are familiar with the location of your brake controller override is vital (REDARC Tow-Pro Elite V3)
Caravan mounted electronic control stability systems
We just learned that brake controllers give you the ability to manually apply the caravan brakes to slow the van down faster than the car. Caravan-mounted electronic sway control systems (ESC) essentially do the same thing except automatically. They do it way better than you or I will ever do it manually for a multiple of reasons:
- The automatic system is guaranteed to activate the van brakes when required whereas humans like me regularly fail to apply manual override brakes at all.
- The reaction time of the automatic system is dramatically faster than any human, enabling the sway to be nipped in the bud before it becomes a big problem.
- Automatic systems can detect and respond to small sways that a human may not perceive as a potential issue again preventing a big problem.
- Newer generation automatic systems are much more capable because they can apply the caravan brakes on only one side of the van to do a better job of arresting a sway issue.
- Some automatic systems are even smarter because they can turn the brakes on and off rapidly to react to situations.
How do these automatic systems work?
No doubt the software is extremely sophisticated, but the concept is quite simple. The van is fitted with a sway sensor. This is essentially a device that detects rapid changes in movement. In this case, it detects side-to-side movement otherwise known as trailer yaw. The yaw may have been created by the van being pushed around or it may be from significant evasive manoeuvres instigated by the driver. If the yaw exceeds programmed parameters, it sends a signal and activates the electric brakes.
Sensors inside the drum brakes measure wheel speed as the notches rotate past the sensor
Earlier generation systems (such as the original AL-KO ESC) apply all of the caravan’s brakes equally and with a predetermined braking force until the sensor detects the yawing has stopped. While these systems are effective and scrub off speed quickly, applying the brakes on both sides of the van is not the best way to pull the van back into a straight line.
Newer generation iterations (such as those by Tuson, BMPRO and Camec) use a system known as asymmetrical braking. If the van is swinging to the left the system will apply the brakes on the left only to bring the van back into a straight line and vice versa. This will straighten the van faster than a symmetric braking system but may take longer to slow the van down because you are only using half the braking. The sensors also respond to varying degrees of lateral force where small forces trigger moderate braking, and larger forces trigger harder braking.
Currently, most sway control devices are not compatible with vans fitted with disc brakes. One exception is a system available through AL-KO, which delivers symmetrical braking. The market take-up is currently small as it has only been tested for compatibility with a limited number of disc brake systems.
The most sophisticated trailer sway system currently available also incorporates Anti-lock Braking System (ABS) in addition to asymmetric braking. The system has been developed by Bosch and is marketed through brands such as AL-KO and Cruisemaster. While new to caravans, the technology has been available in cars for decades. It prevents a vehicle from skidding when the brakes are applied. This increases safety dramatically because a skidding vehicle has longer stopping distances and will also tend to veer off sideways. ABS works by comparing the speed at which the wheels are turning and the speed at which the vehicle is moving. If the system detects that a wheel is rotating significantly slower than the vehicle it interprets this as an impending skid. The system will momentarily back off the brakes to stop the skid and then reapply the brakes. An additional benefit of avoiding skids is preventing flat spots from developing on your tyres.
AL-KO was the first to launch a combined ESC and ABS system in the Tow Assist
AL-KO Bosch ABS sway control test
Alternatively, if the system detects one wheel rotating significantly faster than other wheels it will apply more braking to that wheel until wheel speeds are matched. This process of backing off and reapplying the brakes is repeated multiple times per second until the wheel speed matches the vehicle speed. ABS enables the most effective braking during sway events but is also a big advantage in general emergency braking.
Tow vehicle integrated trailer sway control systems
Many tow vehicles now have trailer sway control systems integrated into them. The principles are the same in that yaw sensors detect excessive lateral movement in the car and automatically apply the brakes. Earlier generation systems apply the brakes in the car only. This action is not arresting trailer sway at the source but applying the brakes in the car asymmetrically helps keep the car straight and resist the swaying movement. Engine torque is also reduced to slow things down. If your van has trailer sway control, in principle, there is no conflict between the car and van systems as the van sensors work to straighten out the van and the car sensors work to straighten out the car.
Newer generation systems (such as the new Ford Ranger) include factory-fitted integrated brake controllers in the car with more sophisticated safety braking. When a sway is detected, the car will activate the caravan brakes as well as the car brakes. It’s a huge step forward if your caravan does not have its own sway control system. However, because the van does not have its own yaw sensor to determine which brakes will mitigate sway the best, it will only apply symmetrical braking to the van. This is not as effective as asymmetric braking at straightening the van. It will also not be able to deliver ABS-style braking.
Ford integrates a brake controller into the impressive F-150 and Ranger ranges
Some aftermarket brake controllers, such as Caravan World favourite the REDARC Tow-Pro Elite V3, also enable the antisway braking action initiated by the car to send a signal to the caravan to brake even if you don’t put your foot on the brake or hit the manual override button. This is because the tow vehicle antisway braking also activates the tow vehicle brake lights. The brake controller picks up the signal through the car brake light loom and activates the caravan brakes. Again, this effectively provides the caravan with symmetrical braking sway control.
The REDARC Tow-Pro brake controller is a Caravan World favourite
Final advice for dealing with caravan sway
The best way to manage caravan sway is to avoid it in the first place through correct set-up of your tow rig and driving to the road and weather conditions. However, if sway does occur it is reassuring to know there are layers of protection available through both the van and tow car to bring the rig under control. There are a number of systems available on the market with varying functionality that can offer peace of mind and extra safety. Hopefully, the information in this article will help you understand the concepts to find the best system/s for you.
The best way to manage caravan sway is to have a good setup and to drive to the conditions
THE NEXT STEP
If you want to learn the latest caravan news, find the most innovative new caravans and camping gear or get inspired to plan your next adventure on the open road to some of Australia's best getaway destinations subscribe to our weekly newsletter. We promise to send you only the best content.
To learn more about caravan weights and what all the terminology (aggregate trailer mass, gross combination mass, gross trailer mass, tare weight, kerb weight etc.) means, then check out this guide.
Related articles:
A guide to caravan towing dynamics and how to stay safe: Part 1
Towing essentials | The best gear to improve your safety
A guide to caravan sway and how to stay safe: part 2