Understanding the dynamics of towing: what is a Dutch Roll?

ByRV DailyJuly 22, 2020
5 MINUTE READ
Understanding the dynamics of towing: what is a Dutch Roll?

We’ve looked at the basics of towing dynamics in several parts – one, two and three) and now we’re adding in a couple of effects. Namely, what happens on inclines and what’s a Dutch Roll?

Dutch Roll is a tendency for the caravan to not only pitch forward and backwards but roll from side to side as well. This is usually most common on motorways where the inside lane has been worn down to two grooves by heavy goods vehicles. What happens is one side of the caravan will settle into a groove and the other side will ride on the edge of the adjacent groove. This will set up a rolling motion in the caravan and will have the effect of trying to move the hitch from side to side. As the vehicle and caravan travel forward, the undulations will also set up a rocking backwards and forwards motion in the caravan, which has the effect of first lightening the load (nose weight) on the hitch, then increasing it. The combined effect of this is the caravan hitch will move in a circular motion first sideways then down, then sideways in the opposite direction and finally upwards again, repeating continuously. In some vehicles, this can be felt more than in others, and sometimes you can have a passenger that normally is okay, but when you are towing, might complain of feeling a little travel sick.

Usually, the effects of this can be minimised by changing your speed slightly. You might also notice a similar effect as a large vehicle overtakes you, it first draws you towards it as the flow of air around you and the vehicle causes a pressure drop between the vehicles, then a push away as the airflow changes and the new flow creates a pressure wave between the two vehicles.

Read next: How to be overtaken safely with a caravan

Wind loading and nose weight

One effect that is sometimes not considered is the effect of actually driving on the nose weight of the caravan. Any tow vehicle, no matter how aerodynamic is when towing, actually towing the equivalent of a house brick aerodynamically.

The effect of wind on a caravan

There are some things that can be done to reduce the aerodynamic drag by the caravan designers when towing. The closer the caravan is to the rear of the car (sometimes called short-coupled) reduces the turbulence created between the car and caravan as the car moves through the air. This is why sometimes you see semi-trailer units fitted with fibreglass infill pieces behind the cab to reduce the gap between the rear of the cab and the front of the trailer. For most caravans, there is little we can do, but we need to consider the effects on the caravan.

We are in effect, towing a flat wall into the wind, the effect this has on the caravan is to try to pitch it backwards on its axle, which as we know from working out the nose weight, moves the effective centre of gravity rearwards and as a consequence, will reduce the nose weight on the hitch.

Drag increases with speed, in fact, drag is proportional to the square of speed ( R ∝ v2 ) so, in simple terms, if you are doing 30 km/h and accelerate to 60km/h, the drag at 60km/h will be four times the drag that it was at 30km/h.

Now the formula for calculating power required to overcome aerodynamic drag is: Power To Overcome Drag Formula

What this means is if you double your speed the drag increases by a factor of four, but as you are now doing the work twice as fast (you have doubled your speed) the power required increases by a factor of eight. Effectively you will need eight times as much power from your engine to overcome the drag at 60km/h than you did at 30km/h to maintain a constant speed.

What does this mean for nose weight? Well in basic terms, at 60km/h there will be more force trying to tip the caravan backwards. It’s not quite that cut and dried though. As the force of the wind on the front of the caravan tries to tip it backwards (red arrows below), it is stopped by the hitch, it can’t move up because it’s attached to the towing vehicle, therefore, the force is translated into two elements: one trying to move the hitch upwards reducing the nose weight. The second is converted into a force pushing down on the axle (yellow arrow below) which will also increase friction between the tyres and the road.

It’s not quite simple enough to calculate these forces and effects. The effects are known by caravan designers and they try to mitigate some of these by designing the caravan to be as aerodynamic as possible and reduce the turbulent air at the rear of the caravan as much as possible. Again, they have to consider what will be towing the caravan, everything from sedans and wagons up to large 4X4s. They all will have a different effect on the airflow over and around the caravan.

The effects of inclines

The effect of an incline also has an effect on towing. If you park your caravan across an incline, and un-hitch the car, if you were then to take the handbrake off, the caravan will turn, so its hitch (assuming that the nose weight is correct) to point downhill… shortly before rolling off into the sunset! Let’s have a look why.

The effect of an incline on a caravan

The centre of gravity on an incline is angled directly downwards when looking from the front, and if viewed from above slightly off to one side in the direction of the downwards slope. If you were to take the hand brake off, as the centre of gravity is in front of the axle (it has to be because of the nose weight) there will always be a tendency for the hitch to move down the slope.

Rvd Tech Talk Diagram 23

Now, if you are towing across the incline, the caravan will always want to push the rear of the car in the direction of the downhill slope. Farmers driving tractors are very aware of this and when they are towing farm machinery and traversing a slope, especially a wet grassy one, they always take extra care. It is highly unlikely that you will encounter an extreme slope, but to know about the effects beforehand is always a safety bonus.

Remember earlier we looked at Dutch Roll… well, this is one of the factors that can contribute to it. If you have one wheel in a rut caused by big trucks and the caravan’s other wheel is not quite in the other rut, all things being equal, the caravan hitch will push ever so slightly in one direction, this can most often be felt as the caravan ‘hunts’ from one rut to another, i.e. one wheel running in the rut, then moving over slightly and the other wheel then running in the rut. This has the effect of the hitch pushing the rear of the car from side to side as it transfers from one rut to another.

So now you know what a Dutch Roll is all about, as well as more on understanding the dynamics of towing, hopefully, your own perception of what’s happening around you, the safer your adventures will be. More to come!

This series was written by and reproduced with kind permission of Simon Barlow at Caravan Chronicles.

Main image Anthony Kilner.