Iana you are correct.

It is most obvious when encountering certain road conditions such as high bridges across river gorges like some on the Hume Highway.
These situations are generally marked with wind socks and signage.
Having said that, there are many other situations which may cause instability.
The main one encountered by caravanners is when passing a large truck in either direction.
The air pressure differential created by the moving truck and to a lesser extent by the moving van will cause the lighter weight vehicle to become unstable.

This situation is not new to any experienced caravanner and most good truck drivers are aware of the effect of their vehicle on lighter vehicles and in particular, caravans.

As you point out above, the caravan and car combination creates instability just purely by design.
The speed of the vehicle also may magnify an already dangerous situation.

As we drive down the road there are many Vanners who may not notice this, but if approaching a good truck driver that truck driver will move his vehicle to travel along the fog line so as to create a larger gap between the two vehicles and this reducing the effect caused by the low air pressure area between the two vehicles.

A word of note is dont expect this courtesy from all truck drivers as many of them are either cowboys or just really dont know or care.

A car and van combination can become the most unstable unit we may have on our roads and my view is that the drivers of these combinations need specialist training BEFORE being permitted to operate thei vehicle on a public road.

Regards

Rob

You can see why one gets a lot of dust on the back of the car.

Test aerodynamic of caravan

https://m.youtube.com/watch?v=karGqjCZOV8

 

Also with roof box on car roof

https://m.youtube.com/watch?v=sweZltfD-no

 

Every setup will be different, but it gives some indication of what is happening.

I have a crap on my roof but have done my best to tuck it all in as tight as possible. Also thinking of getting a wheel cover to have a smooth front surface.

Perhaps you should plan regular stops Whenarewethere or was that just a typo?  

You could put a aero foil on the tug, air goes right over van, less drag, no bugs on van, do not even feel a road train go by and better fuel economy  

Cheers Bob

 

 

Peter_n_Margaret wrote:

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Bobdown wrote:

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You could put a aero foil on the tug, air goes right over van, less drag, no bugs on van, do not even feel a road train go by and better fuel economy  

 

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 Or you could join the tug to the caravan and call it a motorhome....

Cheers,

Peter

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 Yes Peter, been there and done that, Jayco Conquest years ago.

There isn't a truck on the road without some sort of aerodynamics, like your in-built air foil at the front of your rig.

Cheers Bob

V1....Rotate

I think yo-all need to re read my original post, we've gone off track, funny that never happens on here.

Tony you stand on the roof of my van when we are moving you will find there is wind up there.

p.s. could be down your way next year with a house sit.
Ian

iana wrote:

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Rob, you also raise an interesting point, in as much I hear of vanners feeling the efects of trucks passing, and wondered if that was due to the air (pressure) being trapped between the two vehicles, either pressure or vacuum. With our van which is very high off the road, with plenty of clearance underneath, can allow these pressures to escape. The most we have felt with a road train, or a large semi passing is a little nudge, quite often we don't know its there until its beside us, but we have little or no effect. However I wonder how the English designed vans get on, with less escape for the air pressure.

Just furthering on my original post, it also makes one think about the advantage of having a pow profile van (pop top), reducing the turning effect, and then the opposite and that is these small full height vans that just look like a lever on wheels.

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 Hi iana 

The varying size and shape of trucks, semi trailers and their respective loads will cause varying effects on the lighter vehicle while travelling in proximity.

It is my experience that when both vehicles are travelling in the same direction there is created a low are pressure area between the two vehicles. The effect of this may be magnified with road train configurations and may also result in an extra buffeting from wind or varying air pressure due to the gap above the dolly between the two (or more) trailers.

When the two vehicles are travelling in the opposite directions the shear volume of air created by the moving truck manages to buffet the lighter caravan at the moment the two vehicles meet and then as they pass each other the movement creates a lower air pressure area between the two vehicles which will cause the lighter vehicle to be drawn toward the heavier one.

These situations may also vary with speed which is obvious but also with wind direction and speed.

All these situations need to be taken into account when towing a caravan at speed when operating with trucks and in particular, large volume semi trailers and combinations.

Apart from being a professional driver for my working life I have also owned various caravans, campers and tow vehicles.

My current van is classed as off road and is higher off the ground than any other van I have ever owned. My observation when towing this new van is that it is a lot more stable in the passing truck situation than was any other van that I had owned.

Now without more research I can not comment on wether the height is actually creating the situation of a lesser effect from passing trucks but I strongly suspect that this is so. This not only comes from my personal experience with my current van but from my observations when actually driving large truck trailer combinations.

From these observations my thoughts are that the vacuum is broken to an extent from the air flowing more freely under the van and helping to balance the varying pressures from each side of the van.

The same applies from the truck combination as well with for example a two trailer road train or B Double with irregular shaped tarped loads and on standard semi trailers will have a lesser effect than one of these newer aerodynamically designed and built combinations with large pan tec bodies with wings and wind deflectors between each unit of truck and trailers. As a driver this is a very noticeable difference between these various truck combinations. Many of these trailers are also built closer to the ground to assist with wind deflection and fuel economy.

I am probably guilty of going away from your original topic contents and enquiry but with your original observatIons of the instability varying with speed and movement is a fact and with a caravan and tow vehicle combination this situation is probably for the worse due to the simple fact that most of these combinations are a balancing act using near maximum allowances on both vehicle and caravan suspensions.

It is such a shame in this country that we as a consumer have to the most extent Boo Hooed 5th Wheeler caravans when in all honesty these combinations are just so much safer than our pig trailer caravans.

I am as guilty as anyone else as I chose to buy a conventional caravan mainly due to the cost and the general unavailability of Aussie built 5th Wheeler vans. Having bought this new van I am aware of the limitations being put on me by towing a pig trailer as opposed to a semi trailer but in the same breath I am not in a hurry and fully aware of the instability when operating a caravan in windy conditions, uneven road surfaces and in the proximity of large trucks and combinations.

Regards

Rob

-- Edited by bentaxlebabe on Sunday 22nd of November 2020 10:21:37 AM

Hi Rob,
My original post probably not clear, was that all the careful weight calculations basically go go out the window once a tug-van combination starts moving, i.e. the rotational forces created rearwards, increasing with speed. Different designs, combinations, head winds, following winds what ever. So ball weights and all change.

You bring up some interesting points paralleling with my thoughts on the effects of the passing rigs, our van is quite high off the ground. We get more buffeting effect when there is a cross wind and have rows of trees along the road, but nothing that worries us.

It would be interesting to see wind tunnel tests on some of our rigs.

PS if you travel close enough behind a truck, one can save on fuel , don't try it, just joking.

Hi iana,

An apology to be still talking trucks but searching hasnt found much info on caravan aerodynamics.

I did come across this truck which has a aero wing fitted under the trailer. It appears to create a low pressure area directly under the trailer. This would be for improved fuel economy.
My thoughts with a caravan would be a system of creating downforce might be better for stability.

Here is the link.

youtu.be/8g21AofSu_E

Regards

Rob

It would be interesting if the rear of a caravan is too symmetrical left to right which causes an oscillation. Together with an added issue maybe just enough to cause the caravan to be unstable.

Tall towers or chimneys have a spiral so the air always goes one way & up.

If you have a look at Burj Khalifa tower the sub towers step up clockwise around the building to dissipate wind energy clockwise & up.

If buildings are not designed this way, wind will destroy them.

 

Interesting, that rig i.e. the Silverline and ranger in the video clip, is similar to ours, I have to set some hearing apparatus up to listen to them, I'm a bit deaf, so have the sound turned off. Retired life is all a bit busy, and there are time I wish I had some time of my own.

I did a fair bit of "Aero dynamics" theory during my apprenticeship, we were taught clean was good. Have a look at the birds and fish, they can be blunt at the front, but they all taper cleanly to the rear. We tend to think in terms of say an arrow head being pushed through margarine, but the trouble is air, being a gas does not behave like that. The air swirls and eddy's and seeks out the low pressure areas. As for fitting aero foils, we really don't know what the effect is, unless it was tested in a wind tunnel in controlled conditions.

With regards the high pressure areas at the font of the van, we were taught that streamlining at the front (of an aircraft) was not that important, that there would be a pocket of Hi pressure air at the front, the on coming air would sense this, split into laminar flow streams and go around, but the rear was important. A bit of trivia, all goes well until we reach a speed of 550mph or so depending on altitude, the plane starts to go through the sound barrier, and **** hits the fan.

Ian, apologies for going off topic but your post reminded me of something that has puzzled me for a while. What is the origins of V1 and rotate when taking off. I would have thought that lift was a better description?????