February 28, 2010





(2)  removed


        (including new introduction dated 29/04/12)






and in the second part of  www.hgvaccidentstatistics.wordpress.com 

and their teacher should consider asking the Governors’ permission to send some of the comments to the West Midlands Police Authority and also request that in future accident statistics should be published regularly on the web.

Some clues to a few of  the problems will be found on my twitter site which can be accessed by putting “Peter Jones MInstP on twitter” in to Google.

See also the rest of my blog  www.caravanaccidents3.wordpress.com    and



December 20, 2008
See below for explanation

See below for explanation

Peter W Jones MInstP

See also     



Peter W Jones  MInstP
GCSE. Physics. Single Subject

It has been suggested that caravans/trailers (including HGV trailers) would be much safer if tow vehicle drivers were aware of the velocity of the wind blowing at them from the side which tends to make the trailer snake or jack knife.
As the faster the trailer goes the more likely it is to snake, it would help if the tow vehicle driver knew the trailer air velocity.

A normal anemometer ( used on sailing cruisers for instance) gives a read out of wind speed and direction ( ie velocity).
 The diagram on Page 185 of OCR Gateway “Science for OCR GCSE Biology, Chemistry and Physics B” shows how the writer  calculates the resultant of two velocities. A similar thing can be done in reverse. Starting off with V (the wind velocity acting on the trailer), the wind velocity acting on the side of the trailer would then be Vh and Vv  would be the trailer air velocity.
The wind velocity acting on the trailer has been “resolved” into two components at right angles to each other to better understand the effect of the wind on the trailer.
If the diagram shown in the text book were drawn to scale the two resolved velocities could be found from the  diagram. See also the above diagram.

As shown in the text book these values can be calculated.
If the ordinary anemometers had their software modified they would in effect do this calculation and the revised anemometers would then give a read out of air velocity and side wind velocity.
The company in the UK that manufactures a suitable anemometer has already confirmed that the above proposal could be acted on.

Trains also need this type of anemometer to improve safety on our railway net work, particularly if they are “high speed trains.” At the time of writing anemometers modified as above would be prohibitively expensive for recreational trailer towers, but for railway trains and HGV trailer towing vehicles the cost would be more in line with that of the rolling stock. In addition, when Governments catch up with their knowledge of Physics/aerodynamics they will legislate to make these anemometers compulsory.

If the Met Office improved the availability of their excellent wind forecasts on the internet, by making them available via telephone, there would be far fewer accidents and recreational towers would also benefit in this case. 

The above needs a brief mention to GCSE Physics classes. It is the type of item which shows the relevance of Physics to current problems and is likely to generate interest in studying Physics for “A” level or following up an interest in engineering problems.

Advice for HGV Articulated Lorry Drivers.

See above diagram

This  topic could also be revisited in either “A” level Maths or Physics classes. From the above diagram it can be seen how the HGV tractor plus trailer is subject to an air speed of 80mph and 56mph of this is contributed by the forward motion of the HGV. HGV’s are currently restricted to 56mph road speed, but year 12/13 students will understand that, as with air craft, the critical speed is the air speed.

Currently accidents arising from this type of problem are not investigated with anything like the rigour applied to aircraft accidents. In my opinion this is one of the reasons why air travel is the safest.

The Select Committee on Transport of the House of Commons have recently produced a very long report containing numerous suggestions for improving road safety, one of which is that the investigation of road accidents needs to be more  transparent.

 This means ( in my opinion) that  results of  road accident investigations which reveal important new information should be published on the internet as they are for air accidents.  

This is much more likely to happen if the Select Committee on Transport are supplied with extra evidence to support their requests to the DfT. It is the job of the Select Committee to criticise the Government. Year 12 and 13 students in particular (if supported by appropriately qualified physics/mechanical engineering  teachers), could provide some of this extra evidence.

In the past such activities would have required enormous effort to write every thing in good handwriting and illustrate same with immaculate  diagrams. Today it should be possible to use work done with the aid of a computer as part of normal course work for exams. It would just need to be printed so that teachers and students could sign the documents and send them off by post. 

As local councils in the UK control the Police Authorities via nominated councillors who are members of these authorities, the details mentioned above should be sent to these councillors.

Physics extra curricular activities.

Most insurance companies advise that drivers should never admit liability for an accident; lorry drivers can not therefore be expected to divulge details of a jack kifing /snaking hgv trailer accidents.

There are virtually no anemometers at the side of roads or railway tracks in the UK and currently no advice is issued by manufacturers concerning critical air speeds and side wind speeds for road trailers and trains.  However, there are always many other witnesses of accidents who will be only too willing to assist in any enquiries which may lead to a better understanding of the science which could produce  a reduction in accidents.

When an accident is reported via the media students can ascertain from


the likely wind gusting velocities for the region and add this information to the eye witness accounts. I am sure that local media will assist with such activities carried out by schools/colleges.

The above  procedures  would need the approval of  Governors, Head Teachers, and Parents.

See www.caravanaccidents3.wordpress.com 

Scroll down to item on “Air Speed Indicators” for further information

see further items  on gcse/gce “A” level in  www.schoolminibusaccidents.wordpress.com


December 19, 2008


Edit  29/04/12

The Caravan Industry/Clubs have been telling caravan/small trailer towers about extensive research carried out on caravans since about 1990. The Industry/clubs have a virtual monopoly on anything published on caravans/trailers in the UK as about 1.5 million club members are supplied with a “free” monthly magazine . In addition the few nominally independent caravan magazines depend on caravan industry advertising for their survival and naturally their editorial policy is to stick to the industry policy which, summarised, is:-  “The extensive caravan research carried out at Bath University fully supports all caravan industry policies on safety.”

The truth of the matter is that the clubs/industry had not even divulged that it was Bath University who were doing the research until I raised this matter at a large caravan club meeting in 2003. The clubs have never, to the best of my knowlegde, revealed the fact that Bath researched caravan/trailer stabilisers and they have not revealed that Bath were not asked to investigate caravan/trailer brakes.

Teachers have naturally relied on this published material when  they are faced with towing a trailer with the school mini bus; furthermore, like myself, many teachers tow caravans/trailers on a recreational basis. Hence the reason why I think it would be a good idea if Physics teachers referred to caravans/trailers when teaching, for example,  Wave theory, resolution of velocities, conservation of momentum, energy of a moving body, Newton’s Laws, friction, levers and Taking moments about a point.

Old type of leaf stabiliser


See text below for explanation of above dagram



Peter W Jones MInstP
GCE “A” level/GCSE Maths/Physics
A Caravan/Trailer Stabiliser Test
Although the position of the centre of mass of a caravan/trailer can be ascertained approximately for towing purposes quite easily, for a Scientific test to have “academic” respectability the CoM position needs to be found by calculation. This procedure would fit very well into the course work for “A” level Maths (Theoretical Mechanics).
Calculation of the distance of the centre of mass of a caravan/ trailer from the tow hitch.
F= maximum allowed weight on tow hitch ( see tow cars’ handbook for drivers)
d= distance of tow hitch from axle ( or the mid point between the axles)
x= distance of centre of mass from axle (or mid point between axles)
W= weight of caravan/trailer
Using the Principle of Moments, for equilibrium :-
Fd = Wx
x = Fd/W
ie x = F multiplied by d divided by W
It is then necessary to construct a lever of length = d- x
For a small 4m body length caravan this came out to be 3.33 m
The construction of the lever is quite a lengthy procedure, particularly as the old type of “leaf” stabilisers need one type of lever and the more modern ALKO stabilisers need a different type made from a length of tubular steel.
Furthermore a tow car is needed to fix the stabiliser in place and it would be a good idea to have at least one caravan available so that the lever could be seen in place at the side of the caravan.
It can be seen therefore that this exercise needs a joint effort between adjacent schools/colleges with the proceedings being recorded on DVD for later perusal in the lecture room. The costs of this experiment could easily be recouped by the sale of copies of the DVD as long as purchasers agreed that they would only copy the DVD for their own internal school/college use.
To test the ALKO stabiliser (fixed permanently to the caravan draw bar,) simply remove same from the caravan draw bar.
It is usually only held in place by two large bolts and nuts.
In the case of the small caravan for which the calculation has been carried out, the tubular steel would have to be 3.33m long.

Whether using wood or steel, the lever weight is considerable and the end away from the stabiliser needs supporting on wheels (furniture castors are ideal if your car park has a good surface).

Fix the stabiliser to the car in the normal way. You should find that you can easily move the stabiliser from side to side by gripping the long lever between finger and thumb 3.33 meters from the tow ball. Even this test over estimates the effectiveness of the stabiliser as there is also the friction between the ground and the castor wheels.

Do you think the stabiliser is a good means of reducing snaking?

Before carrying out the test check that the force of friction that the stabiliser can exert is the same as stated in the Haynes Caravan manual.

Alternatively Caravan Club Members can gain access to special “Technical Advice” at http://www.caravanclub.co.uk

Several pages of advice on stabilisers will be found which includes the specified force needed to turn the stabiliser arm for all types available except the ALKO, which can not in any case be adjusted.

The force meters usually available in science laboratories may be difficult to connect to the stabiliser arm. Caravanners use Bath Room Scales. Students who know their own weight can check the accuracy of the scales used. If a compression type of force meter is available this can also be used to check the bath room scales.

The photograph above shows one of the old type of leaf stabilisers.

The friction plate adjusting nut is clearly visible to the right of the tow ball.

Similar stabilisers will still be found for sale in caravan and motor spares shops.

Which type of stabiliser has the greatest amount of frictional force

(or torque) to act against the snaking trailer/caravan?

How can a tow car driver reduce the possibility of an HGV bow wave setting off a caravan/trailer snake?

Why do think that snakes produced when overtaking an HGV are much more severe than those produced when the caravan/trailer is over taken?

It may happen that a slight snake could be set up by one over taking HGV and a following HGV’s bow wave may actually “kill” the first snake. Explain the theory of this event.

How can a wind induced snake be avoided ?

All answers to the above can be found by reading the worksheets on trailer snaking for GCSE Physics.

Bath University on Stabilisers



Scroll down to paragraph 36

Do you think the Government (The DfT) should warn the public concerning the lack of reliability of these stabilisers?

Explain your reasons for reaching a decision.

Describe how you came to see the lever test demonstrated.

For further advice on GCSE Physics see



Note that the above work may be covered at GCSE Physics level if triple science is studied.

It may then also appear in an “A” level Maths course with more sophisticated calculations.

All the Physics items covered could be in either GCSE or “A” level depending on whether triple or double Science is studied for GCSE.


Suggested activities for students of GCE “A” level Physics

(1) Trailers up to 750 kg with no brakes

Advanced Level Physics by Nelkon and Parker (1958 )

Page 12 Newton’s First Law of Motion. (1686)

“Every body continues in its state of rest or uniform motion in a straight line, unless impressed forces act on it.”

Discuss the advisability of repealing the current UK law allowing trailers up to 750 kg to travel at up to 60 mph without brakes. Is it advisable to incorporate Newton’s First Law into UK law?

To be less obtuse; does it make sense to put belts round driver and passengers and tow a trailer with no brakes?


www.20six.co.uk/roadtrafficaccidents   or, when this is “off line”   see
 Paragraphs 11d to 12c
Pick one or two of the following items and use your knowledge of Physics/Maths to explain whether you support or oppose the writers’ conclusions.
(a) Resonance and Snaking Caravans.
(b) Longer trailers are more prone to snaking.
(C) There is no snaking effect on a caravan/trailer when it passes an HGV going in the opposite direction on a single carriageway road.
(d) Sudden deflation of a caravan/trailer tyre.
(e) Kinetic Energy and Gravity.
(f) Over run Brakes
Note Paragraphs 10cii and 12a of “20six” “The Case Against Over Run Brakes.”
     See also www.caravanaccidents2.wordpress.com
See the photograph of a typical luggage trailer and notes on use of same.
(g) Snaking and “centrifugal force.”
(h) See para 10ci of “20six”
“Evidence from snaking tyre marks”
Do you think that the fact that there are only tyre marks made on the road during snaking at the outside of each swing is evidence pointing towards “aerodynamic lift ” being a factor in caravan snaking?
(i) See para 12c of “20six” above for an alternative approach to investigating over run brakes and stabilisers. I used these approximations  before I became aware of the Bath University research or I devised my simple stabiliser test. It would be helpful if some one else examined this method and expressed an opinion as to whether it has any validity.
(j)  See para 17b of “20six” or www.itai2005report.wordpress.com  on overloading of caravans (and trailers).
 (k) The Vetrui Effect.

        This can, in certain circumstances, over ride the bow wave  effect of an overtaking coach/HGV.

See www.20six.co.uk/roadtrafficaccidents   or   www.itai2005report.wordpress.com   Paragraph 11b

(5) Elementary Theory of Flight.

Although I have never seen the above topic included in studies for either GCSE, GCE “O” or “A” level Physics, it often appears in the more advanced textbooks used by students. In addition, this topic could be treated as a Maths problem only and would then possibly provide additional follow up work to items covered in either GCSE or “A” level Maths.

I have a copy of “Higher Physics” by E Nightingale  MSc (Vict),  FInstP, ARCO. (1959) but I am certain similar work will be found in current books and in reliable sites “on line” which I am sure current studens will very rapidly find and put to good use.

Due to the number of erroneous formula and accounts quoted by caravan industry executives, employees and ordinary caravanners (all writing under pseudonyms) on the internet, I must set the record straight by quoting from Nightingale.

Page 46.

“Application of Mechanical Principles to Flight. ( by E Nightingale)

In order to understand, in a simple way, the forces involved in flight, let us consider AB ( see diagram below) to be the section of a plane surface travelling through the air towards the left. The pulling force T, called the thrust, has to overcome the resistance or drag D caused by the air. The force of the air against AB also has a vertical component which is responsible for part at least of the lift, L, which, if sufficiently strong to overpower the weight, W, of AB, causes the plane surface to rise.

In level flight at constant speed it is evident that (a) L=W, (b) T=D.

If the resultant pulling force T-D > 0, acceleration will result.

If L-W > 0 the plane will rise.”


As we are interested in staying in touch with the ground to stay safe on the road L – W must be < 0.


 Elementary Theory of Flight (cont)

Nightingale then goes on to describe how he used apparatus supplied by Philip Harris and Co Ltd to deduce experimentally that the drag and lift were proportional to the square of the wind speed and also point out that this result had been confirmed by extensive investigations.
Furthermore Nightingale also showed that when the angle of the wing to the horizontal was about 15 degrees the streamline flow over the upper wing surface was changed to turbulence and the lift due to reduction in air pressure became non existent.
Whilst it is self evident that formula 1 racing cars have made good use of aerofoils to create down force and keep themselves in better contact with the road, it seems to me that it may be preferable that all large road vehicles, particularly trailers, could be made safer if turbulence could be induced over their upper surfaces to reduce the aerodynamic lift. This may be preferable to creating down force using aerofoils.
Reducing pressure underneath vehicles by the use of plastic skirts to increase air speed has been carried out successfully in some cases, but it is a non starter for trailers with central axles as they can not be relied on to stay horizontal.
The issue of speed on our roads has become  very controversial  as many claim that they can drive safely at high speeds.
I pointed out some time ago that the amount of energy that a moving vehicle has by virtue of its speed is proportional to the square of the speed. This energy has to be dissipated as heat energy via the brakes (or converted into electricity and stored in the battery, assuming this is possible) if one is to stop and if we now add that the aerodynamic lift is also proportional to the square of the air speed and remember that this lift will reduce the effectiveness of the brakes, we must urgently address these problems to make our roads safer.
see also