John Hughes takes us through the dynamics involved in towing a caravan and how to avoid potentially dangerous situations.
At the most basic level a caravan is a trailer with a little house built on top. Vans are often long and heavy and subject to all kinds of dynamic forces when coupled to a tow vehicle travelling down the road. If these forces are not managed correctly, they can lead to extremely dangerous towing scenarios — among the most apparent being trailer sway.
In this article, we provide simplified explanations of some of these dynamics without attempting to fully cover the complexity of how they may interact with each other. We also shed light on how some of the dangers of these forces can be mitigated. You should use this information as a guide. It is imperative that you use your own judgement or seek the help of an industry expert as the dynamics will vary depending on your rig set-up.
Why does a van normally want to return to a straight line behind a tow vehicle?
When a tow vehicle is moving in an apparent straight line the path of least resistance is for the van to follow it in a straight line. However, the van is continuously exposed to external forces such as wind or bumps in the road which move it around. When a van is pushed slightly out to the side it doesn’t keep pivoting sideways. Instead, at some point, the van begins to swing back towards the centre as the car pulls on it. However, the van can keep pivoting in the opposite direction past centre, until the car tries to pull it back towards straight again. In other words, the van can experience tiny swinging motions back and forth. Usually, the forces are small and therefore the oscillations are small. Therefore, the system just settles down naturally and we may not even be aware of the tow vehicle dynamics.
A lightweight van won’t push a tow vehicle around as much as a heavy van
What is trailer sway?
Now we understand that, when we are travelling, the van is regularly oscillating at a micro level. If the energy inputs become greater, then the oscillations become greater. Some examples of greater energy inputs are strong gusts of wind, a truck driving past or a sudden change of direction of the tow car. So, these larger forces push the van further sideways before the pulling effect of the car swings the van back in the opposite direction. And because the reaction is greater the van returns further past centre creating a larger pendulum effect.
It is important to understand that a van moving around translates forces to the tow vehicle. In fact, the van can move the car on multiple axes. 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. We all know the shortest distance between two points is in a straight line. To start with we have the car going in a straight line and the van more or less going in a straight line too. 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.
Having your van and tow vehicle level is essential for optimum performance
Does the relative size of the van to the car matter?
To help explain this we will start with the outlier of towing setups. Fifth wheelers are different to everything else because the hitch point sits inside the ute tray directly over the rear axle. Virtually all other caravan towing setups involve a hitch point that sticks out behind the back of the car quite some distance from the rear axle.
We have learned that the van is capable of moving the tow vehicle around. The span between the tow vehicle’s rear axle and its tow point is a lever that multiplies the push, pull, lifting and twisting forces the van imparts on the car. Therefore, the lighter the van is the less force it imparts on the car. There is also the important side fact that lighter objects can be decelerated faster for safer braking distances.
Once we get our head around this leverage idea it doesn’t take much to see that a longer van in a swaying action will impart more force on a tow vehicle. A longer van also has more surface area which acts like a big sail in crosswids/a side wind gust. The larger the surface area, the more the van will move and the more it will move the car. The same principles apply to a taller van. It has more leverage to twist the car. A taller van also has more front-on surface area which means a head-on gust of wind will have a greater deceleration effect on the van which can then effectively pull on the car.
Why is it important that your van and your tow vehicle sit level when hitched?
The simple answer is that both the tow vehicle and the van have been designed to provide optimum handling and braking performance when sitting level. If the tow vehicle is tilting nose up, then there is insufficient load on the front axles. This will make the steering and front braking less effective, as the front tyres will have less contact with the road. If the tow vehicle is tilting rear up, there is insufficient load on the rear axles. A tow vehicle with a ‘light rear end’ has less tyre contact with the ground and is less able to resist the lateral movements of a caravan which can contribute to trailer sway pushing the car around.
The caravan angle impacts on the tow vehicle under braking. A nose-up van will tend to push the rear of the car up under braking causing a light rear end. A nose-down van will tend to push the rear of the car down under braking causing a lighter front end. Generally, a slight nose-down van would be a better scenario than nose-up as it improves rear braking performance.
In a single-axle caravan, a nose-down angle will shift more weight onto the tow ball. Conversely, a nose-up angle will shift weight off the tow ball. As the connection angle is not going to vary dramatically between a high and low hitch point, the tow ball loading will not change dramatically either way. However, an increase of 10kg or 20kg may be enough to take you over the tow ball weight limit of your tow vehicle. And reducing the towball download by 10kg or 20kg could be enough to contribute to trailer sway if other variables are not right too. More on that later.
A twin-axle van is more complex on two fronts. First of all, there are two different types of suspension systems for twin-axle vans. Load share systems (usually roller rocker leaf springs) share the load equally between the front and rear axle regardless of the angle of the van. A twin-axle van with load-share suspension will behave the same as a single-axle van described above. Non-load share systems (usually trailing arm suspension) shift more of the load to the front axle or rear axle depending on the angle of the van. If the van is nose up, the van’s front wheel will become slightly less loaded. Some of that load will be picked up by the rear axle and some of it by the front hitch point. It sounds counterintuitive, but this means a nose-up van in this scenario will have more weight on the ball. And conversely, a nose-down van will have less weight on the ball.
Secondly, if the front and rear axles are not evenly loaded, tyre contact on the road will vary from front to rear causing uneven braking forces. Also, the suspension will be unbalanced due to a shift in load distribution.
It’s not related to handling dynamics, but a nose-up van is likely to hit the ground at the rear of the van and a nose down is likely to hit the ground at the front of the van.
Keeping weight to a minimum on the rear of a van improves stability
Why do low nose weights cause trailer sway?
A low nose weight relative to the total weight of the van tells us that there is more load towards the rear of the van. Two things happen when there is more weight to the rear of the van:
- If the load towards the rear of the van is great enough it will lever the tow vehicle upward more readily in dynamic situations. As described earlier a lighter rear end is less able to resist the lateral forces of a sway making the car more likely to be pushed around by the van.
- Weight to the rear of the van will have a similar leverage effect in a lateral direction. If a force pushes the van sideways, once it gets going, the additional force will swing it further to the side before the opposing force of the car pulls the van back in the opposite direction. Because the reaction is greater the van is likely to swing further past centre creating a greater sway.
What this tells us is when packing we should aim to have as much weight as possible in the centre of the van over the axle/s after allowing for an appropriate nose weight.
A van with low nose weight may tow better with the front storage loaded
Why does tow vehicle wheelbase impact on trailer sway?
Wheelbase is the distance between the front and rear axle. A tow vehicle with a longer wheelbase can more greatly resist the dynamic inputs of the van both laterally and vertically. This means the van is less able to push the van from side to side or move the car up and down. You can imagine if a van is pushing a car sideways and lifting it simultaneously it will cause greater instability.
A ute generally has a longer wheelbase ad the hitch point is further from the rear axle
Why does the distance from the tow vehicle’s rear wheel to the towball point matter?
This is another lever, the greater this distance the more the van can push and lift the car. Refer above for the impacts of these dynamics. It is worth noting that utes tend to have a greater rear wheel-to-towball point distance compared to wagons. This disadvantage in utes is at least partially offset by them typically having longer wheelbases than wagons.
What other variables come into play?
Speed is a critical consideration. The faster you go the more forces are amplified and it’s not linear, it’s exponential. In other words, travelling at 90km/h is not a little bit safer than travelling at 100km/h, it’s a lot safer. Travelling downhill increases the chances of trailer sway and travelling uphill decreases it. This is because the tow vehicle has less capacity to pull the van back into line going downhill.
A caravan is more likely to continue to sway if the tow vehicle’s tyre pressures are too low for the conditions. A lower tyre pressure has more flex in the sidewall compared to higher pressures. A flexy tyre wall is less able to resist the lateral movement of a swaying van than a more rigid tyre wall. A major factor to keep in mind that tow vehicle rear wheels often need to run at higher pressures when hitched up to compensate for the tyre side wall deflection caused by the additional load.
Final advice
It is smart to have a healthy respect for the forces involved in towing. A knowledge of the variables that contribute to tow rig behaviour can greatly enhance your safety on the road — and the safety of other road users. We all know a tow vehicle pulls the caravan, but it is critical to understand that a caravan also imparts force on the tow vehicle causing it to move around. Variables such as speed, weight and tow rig set-up contribute to how much the van/car set-up will misbehave on the road.
In part two, we will follow up with:
- How should I respond to a swaying van?
- How does a weight distribution hitch work?
- How do van electronic stability control systems work?
- How do tow vehicle trailer stability control systems work?
If in doubt, consult a professional
An incorrect tow vehicle and trailer combination can lead to injury, death or property damage. The information provided is general in nature, not comprehensive and can only be taken as a guide. Individual discretion must be exercised and persons doing their own tow rig setups do so completely at their own risk. Publishers and creators of this content accept no responsibility for loss or damage.
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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.
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A guide to caravan towing dynamics and how to stay safe: Part 1