Peter W Jones MInstP

The article below is mainly a copy of the relevant 2005 unpublished report that I sent to the itai. However, I have recently added a few “updates.”

Edit Jan 2014




It was my reading of the Bath University Phd thesis by Standen that has been the greatest help in understanding the behaviour of “Towed Vehicles.


However, it was the dedicated members of the internet group that have provided the support and stimulated the motivation which has made me quite determined, over a considerable period of time, to complete this enquiry. ( even over a long period whilst the NHS successfully treated me for cancer). The two most notable examples of this assistance came from a New York Police Traffic Accident Investigator and a UK Private Accident Consultant(Early 2003)



.On reading my circulated account of the first accident described above (plus my “proof” based on Newtonian Physics of the non functioning of over run Brakes during snaking) the New York man wrote to say that in the USA over run brakes were no longer allowed on commercial trailers due to the centrifugal force generated during snaking preventing the application of the brakes. There are of course other design faults with over run brakes




The UK consultant (a retired UK Police investigator) sent me a description of the tyre marks made on the road by a typical snaking caravan. He indicated that no one had been able to explain this phenomena. I eventually managed to explain that burst tyres could be the result of snaking accidents and not their cause. Details of this account are in paragraph 17a




I read in a January 2007 issue of the Caravan Club Magazine that officials of the Club have now accepted that burst tyres may not cause snaking and the snaking may actually be the cause of the burst tyres




The UK consultant also sent me an excellent photograph of an accident where a caravan had lifted that far into the air when the tow car braked suddenly, that the following car had run under the elevated caravan before it dropped on to the following car. However, it seemed this accident was the result of faulty brakes. I cannot of course publish this photograph as it would identify the consultant and the motorist. This caravan was very similar to my current small touring van. I have included a photograph of this van in my blog. My daughter‘s caravan was smaller than the one involved in this accident, and also had a roof section which dropped down to reduce wind resistance on the road.




AIR SPEED INDICATORS To tow safely I think that all tow vehicles should be fitted with an air speed indicator..


The speed of the wind varies enormously; the forecasted speeds are only “averages;” even on the open sea this can be observed, and on land the matter is complicated by valleys, bridges, and when you come to the end of a tree screen and suddenly feel the full force of the wind..


Air pilots have to learn very early that if their plane leaves the ground at 60mph in still air, with a head wind of 30mph it will leave the ground at 30 mph ( although the air speed indicator would still show 60mph at take off). All aircraft have to carry a handbook available to pilots, showing the lowest air speed at which an aircraft can fly (called the stalling speed), and the take off speed (when an air craft reaches this air speed the pilot must never abort the take off).


.Our road vehicles would come a step nearer aircraft safety levels if each vehicle handbook stated clearly the “air speed” at which the aerodynamic lift was sufficient to reduce the tyre friction to almost zero and leave the vehicle open to being pushed sideways. With the aid of road observations over a long period of time I think that this critical speed for my small caravan (weight 1000kg, 4m body length, single axle) is 50mph air speed. I came to the conclusion in about August 2003..


. At a private Caravan Club Meeting held at the Thistle Hotel, East Midlands Airport, at the end of February 2004, the Vice Chairman of the CC (responsible for technical matters) told members present the speed at which a caravan under test had been virtually “flying.” Also present on the platform was the MD of a very large Group of Caravan Manufacturers. I was that surprised by this admission, that when allowed to speak I repeated what had just been said to emphasise the significance of the revelation. I expected the remark to be retracted, but it was not. No further elucidation was given on the matter of critical speed for caravans; it has also never been explained in the Technical Information Section of the Club Handbook.


Also in the air pilots handbooks is the maximum allowed side wind component ( in mph, or knots) for landing. Clearly the obvious thing to do is land your aircraft straight into the wind. Light aircraft may be able to do this as they can land on firm grass, but larger aircraft must land on solid runways which do not always run in the right direction. This even applies to Jumbo Jets.


.If the side wind component is above the prescribed figure the pilot has to divert to another air port where they have a runway available to meet the requirements. A side wind will blow an aircraft sideways, but as it touches the ground it will go in the direction of the run way. This could place such an extra strain on the undercarriage that the latter could collapse. I will explain the matter of wind components in more detail in the Science addendum.


When there is a head wind of 30 mph blowing at 45 degrees to the direction of travel there will be a direct head wind component of approx 20mph and a side wind component of the same speed. If you are towing the type of caravan mentioned above the critical speed will be reached at 30mph (20+30=50mph air speed).


I think the side wind component of 20mph would then set the caravan snaking as I have explained above, but we really need this critical side wind component to be accurately quoted in the handbooks, as it is for air craft


Quite apart from the matter of aerodynamic lift, information should be given concerning the side wind speed required to overturn each high aspect vehicle.

.. (7f).

. Every time a caravan or high aspect hgv takes to the road in strong winds the driver does not know whether the forecasted wind is likely to overturn the vehicle. Even the weather forecasters remind us about certain winds being likely to produce structural damage to buildings, but there must be a policy of pretending that high aspect vehicles cannot possibly be at risk, as I rarely hear the roads mentioned at such times.

NB 3-2-08 I have recently noticed an increase in the number of warnings given on weather forecasts when it is possible that the strength of the wind could over turn high aspect vehicles.

.(7g). (7h)

.I have seen two TV adverts when car manufacturers have claimed, in general terms, that the shape of their new model is such that it stands a much better chance of sticking on the ground than models made by their rivals. The public should not accept such vague statements. We need technical details commensurate with the computer age in which we live.

.(8a) (8b) (8c).

. HGV’s need similar wind deflectors to caravans, particularly the trailers of huge articulated lorries. Some people will need reminding that an articulated lorry is two vehicles. A hefty tractor towing a very long trailer.

Edit Feb 2011; Note that about 2 years ago I suggested that creating turbulence over the top of HGV trailers could possibly eliminate aerodynamic lift, and thus the need for aerofoils to produce down force. However, this solution would not work for F1 racing cars as they have no large top surface.

. (8d)

.Flat bed trailers are a greater problem, as anything loaded on changes the critical airspeed, particularly if it has a high wind resistance.


.According to a Boeing 737 has a take off speed of 150mph when it weighs 45.36 tonnes. I would estimate the stalling speed at about 120mph.

This would mean that an HGV with trailer weighing the same would most probably become unstable due to aerodynamic lift at 60 mph when there was a head wind of 60mph. I make this very rough prediction as no information has been released by the Road Haulage Industry or academia and I feel certain that these critical values could easily be calculated or found in road tests.


However, all HGV’s of this size are two vehicles:- tractor and trailer, and first you would need to consider the trailer, as it will have a different critical air speed to that of the towing vehicle.


. The Standen Phd thesis of 1999 had involved very extensive wind tunnel research. Standen used scale models of tow car, caravan and an HGV. He managed to measure the actual force generated by the bow wave of the hgv.

Results from Bath University (Standen)

The Standen thesis, page 65, states, “Therefore during the overtaking action the rear end of the caravan is drawn in towards the HGV as it is approached, then when the front of the HGV is just forward of the rear of the caravan the two vehicles are pushed apart.” This variation in pressure is characteristic of a “wave motion” and readers will recognise in any case that the action of the HGV is similar to my previous account. The fact that the researcher did not comment on this fact is simply due to him looking for another item at the time and concentrating on the accuracy of his measurements, which was the principle reason for his research. He made these observations using scale models of tow car/caravan and the overtaking HGV in the Bath University wind tunnel. I would conclude that this result proves that HGV’s produce waves, as I understand the above quotation from Standen to apply to a number of his experimental wind tunnel results, and probably all of them. In “Conclusions” on page 67 Standen also states, ” The action of an HGV lorry overtaking a car-caravan combination was shown to have a strong aerodynamic influence on the caravan body. The magnitude of the aerodynamic forces and moments acting on the caravan are likely to be strong enough to affect the lateral stability of the system.” My ideas have been developed on the basis of observations “on the road” so it is gratifying to know that wind tunnel measurements confirm that HGV’s can destabilise caravans.


. In support of my view of this matter I suggest a repeat of what I unintentionally first observed on a “B” road near Meridan. In a “Torrential rain storm” I pulled off the road into a layby and got into the caravan without winding down the corner steadies and found that even when I was standing over the axle the van shook quite violently every time a car or light van passed at about 50mph. I further later observed (when sitting in the stationary Land rover) that similar traffic had no effect on the Land Rover, although it was hitched to the caravan.


. Standen also did many tests to try and find a practical wind deflector that would cause the caravan to be subjected to a “Down Force” by the wind and so counteract the aerodynamic lift. He did not succeed in finding a practical solution, but he did make a significant discovery. He found that using wind deflectors to create down force at the side of the van at the front, the stability of the van was much improved. It therefore only needs someone to develop a practical down force creator for caravans, and the latter will begin to close the large gap in technology between themselves and Formula 1 racing cars.

.(9e) ..

The formula 1 cars have one deflector at the front near the ground and one above the level of the drivers’ head at the rear. The Million Euro Bugatti Veyran sports car (made by VW) has a computer controlled variable pitch deflector at the rear. The reason why six years after Standen’s results were published nothing has been done is that Caravan Club Officials have not kept the membership informed of developments.


. This matter also applies to hgv’s with trailers, paticularly those with approximately central axles. Fast Trains are suitably shaped at the front to produce some down force, but by how much is not well known.. (Some older Virgin trains actually have a backward sloping front end which vastly increases aerodynamic lift. I hope their speed is suitably regulated particularly when going under Victorian railway bridges which due to “wind gusting effects” in these confined spaces will greatly increase the air speed of the train.

There was an accident to a French high speed train a few years ago. It jumped off the track as it passed under a bridge and the engineers could find nothing wrong with the track.
Railways have an excellent safety record when compared with the roads, but I think they could improve their performance still further if they used a version of the anemometers that skippers of sailing cruisers have used for decades.


.There is a need to monitor rail wind speeds at critical points, particularly the many Victorian bridges still in use. It also amazes me that the only evidence I see on Motor Ways of wind monitoring is wind socks, which I first saw in use on air fields pre ww2. All vulnerable points need monitoring with automatic devices feeding their results to the police motor way control centres. When the police close roads because of “strong winds” it must be pure guesswork on their part. They need better support to eliminate unnecessary closures. Motorists need to have more information so that they can make decisions based on a first rate scientific knowledge.

. (10a)

.Standen also found in his wind tunnel experiments evidence that when scaled to motorway distances lead him to believe that one could move away from an hgv on seeing it approaching to reduce its effect. I read in the Caravan Cub magazine that he had never towed a caravan. If he had I think he would have reviewed this result. This is the only one of his results that has been reported by the caravan club. It is still in the latest edition of the CC handbook ( page 622 under “Snaking”. (2005)

. (10b)

.Standen also explained the effect of the wind. He argued that high aspect trailers act like a wind vane when subjected to a wind ( eg) at 45degrees to the direction of travel. He wrote that the trailer would swing into line with the wind, but the swing would continue until, due to the wind’s reduced effect the trailer would then swing back, so setting up a snake.

.(10c) .


.. Bath University Department of Mechanical Engineering, Phd Theses.

(1) Fratilla. 1994 “Lateral Stability of Passenger Car/Caravan Combinations.” Page 159 Results of Tests on Stabilisers.

(2)Standen. 1999. “Towed Vehicle Aerodynamics” Page 65 Results that I think show that HGV’s produce waves (as opposed to turbulence).Page 67 Conclusions for wind tunnel tests on the effect of an HGV bow wave on a Caravan. Complete chapter on investigations into creating “down force” on a caravan with wind deflectors. The above can be obtained on microfiche from the British Library by a local UK public library.

(2a) Vibration Isolation Characteristics of Caravan Trailer Suspension Systems and Factors Leading to Moisture Ingress; a M Phil Dissertation by JMA McDonald, University of Bath, 1999. (All the above were sponsored by the Caravan Club)

(3) “The Air Pilots’ Manual” by Trevor Than. Page 292. Take off Performance Tables for a Gulf Stream AA-5A Cheeta. The book is in the Aeronautical section of the Birmingham Central Reference Library. Also volumes one and four of a later edition by the same author.

(4) The Highway Code. “New Expanded Edition.” (Driving Standards Agency) See Science addendum.

(5) Most of the school level Physics and Maths is derived from Sir Isaac Newton, Laws of Mechanics, 1686.

(6) Test Report from Caravan Club Magazine on an electronic/ electric caravan brake system.
Edit Feb 2011
Note the more recent Caravan and Camping Club road test of this device in

(7) Negative reference; Bath University have not investigated over run brakes.

(8) The Haynes Caravan Manual by John Wickersham. (adjusting caravan stabilisers.)

(9) The Haynes Manual for the Land Rover Discovery (Tdi).

(10) “Impact” the Journal of the ITAI (Institute of Traffic Accident Investigators) (2002). (see (Report by the investigator who attended the scene of the Selby Road/Rail Disaster)

(11) A University academic wrote an article for itai “Impact” which was published in the Winter 2000 edition. I have been unable to confirm this but I understand that some of his conclusions for caravans were as follows:- “Drivers should be aware that under steady conditions at speeds above 50 mph, a small change due to a sudden wind gust or a steering manoeuvre may lead to uncontrolled behaviour,” and also “exceeding the legal speed limits with a caravan is potentially much more serious than similar behaviour with a car. This is particularly true if the towed load and its distribution exceed the values discussed in this article.” (Similar values are also given in the Caravan Club Handbook).

(12) The Handbook of the Caravan Club. (Technical Information Section) See (2005)Page 637 Stabilisers Page 622 How to avoid the effect of the bow wave of an HGV.

(13) Camping and Caravanning Club Magazine Page 11 January 2005. A two page article about the leading Stabilisers on the market (up to 450 pounds stirling each). The article praised the Bath University Research but did not quote Fratilla’s 1994 result on stabilisers.

(14) Stabilisers; 11 page document available to Caravan Club members only by internet download. (see 9b above and also:-. and

for paragraphs 10ci to 18b see

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