One thing that seems to unnerve people new to towing is being overtaken. The feeling that the tow vehicle and caravan are moving around seemingly uncontrolled when a large vehicle overtakes. Knowing what is actually happening when this occurs will help.
If you can remember back to your science lessons I’m sure at some time someone mentioned a Dutch chap called Daniel Bernoulli who lived in Switzerland and had a bit of a thing for maths and fluids, and I don’t mean beer… although he might have done, I just don’t know. To cut a long story short he came up with a principle, which basically says – well this is my simple version – air (or fluid) moving faster on one side of an object than the other will create a force due to differing pressure. Accelerate the gas or fluid and the pressure drops relative to the other side of the object.
The best demonstration of this is an aircraft wing. The shape of the wing causes air to move faster over the curved top surface than the flat underside of the wing, therefore the pressure above the wing is lower. In essence the aircraft wing is sucked up from above and pushed up from below. (OK if you are Sheldon Cooper you will have a better explanation)
What’s this got to do with being overtaken?
If you look at the drawing below, it shows (blue arrows) the airflow around the tow vehicle and caravan as an approaching vehicle starts to overtake. Air is being forced round the front of the two vehicles by virtue of them moving forward. The air between the two vehicles is being squeezed and therefore accelerates. as the air between the two vehicles is moving faster than the air on the opposite side, there is a pressure difference, the pressure between the two vehicles is lower (ventrui effect).
As the pressure is lower the rear of the caravan is drawn towards the overtaking vehicle. For the moment we will only consider single axle caravans. The ‘pivot point’ is effectively the axle – you know how easy it is when the jockey wheel is down to turn your caravan round by pushing on the rear side corner – well the same thing is happening here. The rear corner of the caravan is being drawn (red arrow) into the low pressure airflow. This has the effect of turning the caravan and applying a side load (red arrow) on the tow ball which tries to turn the vehicle.
So what you feel as an overtaking vehicle draws level with the rear end of the caravan is the ‘feeling’ of being pulled or sucked in towards the other lane. If you haven’t been keeping an eye on your mirrors for overtaking vehicles this is when people new to towing start to get a little nervous.
Lets follow the overtake
As the overtaking vehicle draws level with the axle of the caravan, the forces are reduced due to the length of the ‘lever’ being reduced (you would find it hard to turn your caravan round by pushing on the side just behind the wheel for instance) and the effect it has on the tow vehicle is also reduced.
Now as the overtaking vehicle still moves forward, the low pressure effect is still there but this time it is acting in front of the caravans axle and has the opposite effect. It draws the front of the caravan in, so applying a force tending to turn the vehicle away although is has less effect. This is the second part of making a novice tower nervous. By now the driver has usually applied some corrective steering input to move away from the vehicle after finding themselves being ‘drawn towards’ the overtaking vehicle, the force now being applied amplifies this and they find that the move over in the lane a little quicker than anticipated.
Why does it have less effect?
If you look at the drawing you can see that the airflow is still faster along the length of the gap between the two vehicles, so are still some forces acting on the rear of the caravan. However, by now some of the air between the two vehicles is spilling out of the gap over the roof of the caravan and overtaking vehicle therefore reducing the pressure difference slightly.
By the time the front of the overtaking vehicle get level with the gap between the towing vehicle and caravan, most of the effects are over. Towing vehicles tend to be more aerodynamic than caravans and have a lower roof line allowing the faster moving air to spill out more easily reducing the overall effects.
Twin Axle Caravans
Now now doubt you have heard “Twin axle caravans are more stable” well when it comes down to being overtaken it’s probably correct. A single axle caravan resting on its wheels and jockey wheel is fairly easy to turn just by pushing on one of the corners, it will easily turn on its own axle. Now trying to do that with a twin axle caravan would be almost impossible unless you are a person that turns bright green when getting slightly annoyed. It’s the same when being overtaken, the forces in play are generally not large enough to have much effect on a twin axle caravan, and that’s why the statement “Twin axle caravans are more stable” is often heard.
5th Wheel trailers
With 5th wheel trailers, the forces are still there, but as the hitch (pivot) point between the tow vehicle and trailer is on or in front of the centre line of the rear axle of the towing vehicle it is generally unaffected.
Well caravans that have a longer overhang behind the rear axle will tend to magnify the effects and tow vehicles that have a longer rear axle to tow ball distance again will feel the effects more (to understand why read: Understanding the dynamics of towing – Pt 2)
Different vehicles overtaking will have different effects. An articulated lorry is likely to be passing you around 5 to 10 MPH faster, but a large commercial van – (Ford Transit, Mercedes Sprinter etc) or coach could be passing you with a 20 MPH or more speed difference and will have just as much effect or more due to the greater speed even though it’s lighter in weight.
What can be done to reduce the effects of being overtaken?
Keeping an eye on your rear view mirrors is important. Knowing exactly when a vehicle is going to overtake will allow you to prepare for the sideways movement. Giving yourself a greater sideways gap between you and the overtaking vehicle by moving over in your lane will also reduce the effects. Easing off the accelerator slightly so that the overtaking vehicle passes slightly quicker to will also help.
For people new to towing, the big thing is not to over react and over steer, knowing what’s happening when you are overtaken is the first step in safer towing.
This not only affects you when being overtaken, but also when overtaking. If you overtake a large vehicle, the effects are just the same. In Europe where towing speeds in some countries are higher, then you must be extra careful. I suspect, although don’t have any hard evidence, that this event was caused by the driver of the tow vehicle being caught out when they over took the HGV and possibly reacting by over steering.
Video (c) by the original poster/youtube