Evidence from Snaking Tyre Marks
10ci Apology. I am new to blogging and cannot see how to prevent the excessive use of blue underlining which has appeared in my blogs. I did not introduce the above; it appears when I paste work in to the sites. Evidence from Snaking Tyre Marks One of my hypotheses is that there must be almost zero friction between tyres and the road during the middle part of each swing of a snaking caravan. (Please also note that a rigid HGV towing a large central axle trailer is the same in principle as a car towing a caravan - except that the former has brakes that can be applied when snaking leading to jack knifing takes place). The only tyre marks seen on the road after a typical snaking accident (see para 5g from my unpublished report to the ITAI), are on the extremities of each swing. I am publishing this account because from other evidence I am convinced that the person who supplied me with the information concerning the snaking tyre marks is a retired police traffic accident investigator, who still does fairly regular work for the courts as an expert witness. In addition, any RTAI carrying out a new investigation concerning a snaking trailer or caravan, will be able to compare what I have written with the photographs of the actual marks made in the accident that he/she is investigating. It follows from my hypothesis of almost zero friction that the caravan/trailer must experience some aerodynamic lift; the fact that this does not fit current theory that one needs a structure shaped as an aircraft wing to achieve lift is a problem for academia. The aerodynamic lift exists ( see my evidence above) and to make our recreational activity safer we must adapt to this situation. Standen, Phd thesis Bath University,1999, “Towed Vehicle Aerodynamics,” as a result of wind tunnel tests with a model car and caravan did show that suitable aerofoils did improve stability. He only failed to solve the problem of devising a legal/practical solution as his final aerofoils were on the side at the front of the caravan. He also is in my opinion also confirming the existence of this aerodynamic lift produced without a wing shaped structure, as a result of his wind tunnel tests.Further information:- See also my blog Para’s 5g, 9e, 9d, 10c, 10b and much further on in this blog:- Para's 10d, 11a to g (Inclusive), 12 b and c, 14a and b, 15a, 17a.
The Case against Over Run Brakes
10cii
SUMMARY 2
I wrote about this in my blog
www.caravanaccidents2.wordpress.com
Starting at paragraph 1e, and in
www.20six.co.uk/roadtrafficaccidentsparagraph 12a
The material in these blogs is an update of a paper I produced for the Institute of Traffic Accident Investigators in 2005. This was not published, but most of the material had been well circulated via the private Yahoo web site for Accident Investigators
The Selby Rail Disaster
Some years ago a Land Rover Defender towing a trailer with over run brakes and carrying a fairly large car ran on to the main railway line whilst the driver was sleeping (this was the verdict of the Court) right up to the last short distance to the point of impact with the passenger train.
See para 2c in www.caravanaccidents2.wordpress.com and para 15a in www.20six.co.uk/roadtrafficaccidents
I have owned a Land Rover Discovery for 11 years, as well as a Jaguar 21 Sailing Cruiser with road trailer for 19 years. The latter is comparable in weight with the trailer plus car in the above accident. My Discovery is mechanically almost the same as a Defender and as the latter type was not available to the police for testing the trailer which survived the accident (the train only hit the Land Rover) the police used a Discovery similar to mine to do their tests ( my information comes from an article in the Institute of Traffic Accident Investigators Journal).
I have towed the Jaguar 21 far enough to learn about towing this weighty object; I towed my previous sailing cruiser (a Bradwell 18) around 8000 miles in 12 years, between various coastal harbours and non tidal rivers in Wales and the Midlands.
I have frequently pulled my current sailing cruiser up a steep shingle beach, and of course gone down the same slope when launching. I need to engage the “locking differential mechanism” and use
the low ratio gear box to do this.
(When I had a Rover 3500 SD1 SE V8 I had to use a long rope and block and tackle to pull the boat up the same slope).
I have looked at railway embankments of different types, from the train, and from the top near roads.
I think that if I had woken up some distance down an embankment when I was driving my Discovery but not towing, I could have avoided getting on to the track. I would first of all have turned at about 45 degrees to continue at a slightly reduced speed due to the reduced gradient. When I reached the bottom I also think that I could have turned again to run parallel with the track.
The Land Rover driver who caused the Selby Rail Disaster was an experienced professional tower of trailers and would be even more familiar than I am with the dangers resulting from turning whilst towing such a heavy trailer with over run brakes. He kept the outfit going straight on as I would have done. Trying to turn would have been a worse option as at the time the train was not in view.
The police spent an enormous amount of time testing the trailer that survived the disaster intact, as the train only collided with the Land Rover. The police proved that the trailer met the requirements as laid down by parliament. If their tests had shown otherwise the man may have been given a lighter sentence or even acquitted.
It has been about two years since I wrote my report for the ITAI and I now think that the police should have run additional tests on the trailer as it may have been possible to show that although it did comply with the law, it could possibly be demonstrated that it was dangerous to turn same in such a situation. This would mean, (if demonstrated in the tests), that the trailer was a subsidiary cause of the accident and that this may have given rise to a different verdict/sentence or a “Rider” concerning over run brakes. (Roll bars are common in vehicles used for off road events, as is the use of full safety harness, so non professional testers should be well protected).
This is very much the “Science of Hindsight” many years after the event. I do not think that at the time of the crash any serious doubts had been raised concerning over run brakes. The brake actuating mechanism was first introduced about 1929 and at the speeds most trailers were pulled at in those days I am not surprised that few problems showed up. It is since the development of fast roads and the increase in the popularity of recreational towing that concern about caravan/trailer safety has increased.
If the defence solicitors had seen any evidence that this was the case I am sure they would have insisted that the matter was investigated.
I do not wish to suggest that the police or any one else reopen this case in court.
However, I do think that all road users will wish to have this matter evaluated if the hypothetical case I have outlined above fits the layout of the embankment at the crash site. If there is not a match and it is felt that the gradient at the crash site is too steep for any evasive action to have been taken, I hope that tests will be carried out on a slope more typical of most embankments, as well as on flat tarmac roads in a testing site. (I am not proposing that a railway embankment is used, unless it is a disused railway and disused railways may have great numbers of trees and not be typical of those in use).
These results would also be available as evidence for future court hearings. The Police Accident Investigators could repeat the tests if they thought it necessary.
As far as I know Bath University were not asked to investigate over run brakes.
In my Blogs (see first reference above after main headings) there is a description of a jack knifing accident, and even though I only obtained my information from the TV news and Radio 4 news, I think there is enough to show that there is a serious problem with over run brakes in the case I quote.
Tests should also be carried out to replicate the swerve made by the driver in the above accident (a) using a trailer with over run brakes and (b) using a trailer with electric brakes.
The above site published some of my observations under the heading of "Safe Towing of Caravans and Trailers."
Subsequently other respondents to the debate questioned my sanity and ability as a Physics teacher. This arose because my work in some cases has been edited so that on the basis of what I appear to have written the above claims are justified.
The most serious misrepresentation of my Physics is in the account of the Selby Rail Disaster.
Accordingly, whilst thanking a large number of the respondents to T&T .com for their constructive and helpful comments I must now disclaim any responsibility for work that appears on this site in my name.
Addendum 29-1-08
I recently observed that Google still shows up high on the list a reference to T&T.com if one enters "caravan snaking accidents" in the Google search engine.
If this is clicked it will take you to an edited account of the correspondence I mention above. This only covers a few of the topics I wrote about, but the items of mine that remain are unedited.
If you are permitted to log into the site the advanced search engine can be used. Enter "Peter W Jones" to show up all the correspondence.
Addendum 14-2-08
My items in T&T.com continue to be correctly posted.
Although I have received a considerable amount of written abuse some comments are still constructive. There have been over 330 in total.
The surfer identified as "Logiclee" has a HND in Electrical and Mechanical Engineering.
He was very sceptical concerning the value of aerofoils, but he may be changing his mind.
10d. THE SCHOOL/UNIVERSITY MINI BUS Peter W Jones AMInstP A number of years ago a tragic accident to a mini bus from Hagley RC School sent shock waves through the Midlands. Following the accident the visibility of mobile teams painting lines on the motor way was much improved. In 2002 a Land Rover caused a disaster when it got on to the main railway line. (see 2c and 15a). This was followed by extensive improvements to safety barriers near railway lines. I am trying to improve a situation which applies particularly to high aspect vehicles BEFORE another great disaster takes place. Minor accidents in this category go unreported by the media and the Police, all the time. I was Head of Science in a Comprehensive School and was named on the school mini bus insurance policy for 15 years, right up to the time I retired. However, I was only a “reserve” and was not often prevailed upon to assist some aspect of school work by driving a bus load of pupils. I was of course always aware that mini buses, like 4x4’s although to a lesser extent, were prone to overturning if you were forced to swerve in an emergency. This is due to them having a higher centre of mass than the typical car. BBC2 and Jeremy Clarkson’s programme are not well known for giving advice on road safety, but I read in a “Land Rover” magazine some years ago that he had done exactly that. He demonstrated, on TV, that a Range Rover was so prone to over turning that such an event could happen if you changed lanes on a motor way rather sharply. ( The demonstration was of course on a private road.) I now find myself, 14 years into retirement, offering advice on this comparatively minor matter, because I have recently come to the conclusion that I have some good evidence pointing towards other and more serious hazards to be aware of when driving a mini bus. Aerodynamically the trailers that are often towed by schools and colleges can easily (unintentionally) be converted so that they have caravan characteristics, unless great care is taken concerning the number and arrangement (for example) of canoes, and the weight and distribution of luggage carried. It therefore follows that virtually everything I have written in these blogs applies to the mini bus when it is a tow car towing a heavy or ungainly trailer (see 1f and 1i). I should also point out that it is most probable that school/university mini buses will be towing trailers with no brakes at all. This is quite legal as long as the trailer and its load do not weigh more than 750kg, and the trailer does not weigh more than half the weight of the mini bus. I have discovered that the reason we are still suffering from potentially dangerous over run brakes is that since they were approved for use about 1930, as far as I know, no one has seriously questioned their suitability for use on modern high speed roads. The same sort of story most probably accounts for most of our academic institutions not applying Newton’s First Law of Motion; in up to date appropriate language I can now restate this law as: If a body has no brakes it will not stop until it hits something, and the latter will be the mini bus. According to "A" level Physics by Nelkon and Parker (1958 Edition) Newton originally said "Every body continues in a state of rest or uniform motion in a straight line, unless impressed forces act on it." If a mini bus brakes when towing a trailer without brakes at about 30 mph, the force exerted by the trailer pushing on the tow car will be sufficient to bring it to a standstill. If the combination is braked gradually from 60mph, everything will be fine. If sharp braking is necessary at about 40 mph the trailer will almost certainly overtake the tow car and “jack knife.” At around 60mph sharp braking will at the least result in the trailer rising up and crashing in to the back of the tow car, and at worst it will produce a replication of the somersaulting caravan phenomena (see 3a and 6b).Compared with mini bus passengers, passengers in the rear of a 4x4 ( Range Rovers excepted) are well protected by the hefty rear door mounted spare wheels. However, canoe trailers, and other similar trailers, have structures which are likely to pierce a mini bus and injure the occupants. I often have to tow a small unbraked trailer holding about 6 "Hold alls" but as I always chain it to the tow bracket it will not go over the roof if I have to brake heavily, and a 200kg trailer is unlikely to destabilise a 2000kg Land Rover. However, this trailer is really designed for a very small car, which could easily be destabilised. I would also like to remind readers that the maximum allowed weight for an unbraked trailer is 750kgs. When I had solid fuel central heating I used to have 15 lots of 50 kg bags of anthracite delivered at the same time. One or two men trudged up my drive and round to my coal house 15 times to deliver this load. Just imagine the effect of projecting these 15 bags of anthracite at the rear of a schoolmini bus at 50mph. The courts rightly hand out stiff sentences to "yobs" throwing bricks off bridges at trains and cars travelling at high speed; the above situation may be just as potentially life threatening. I would urge all schools and Universities to disregard the implied government advice that it is safe to tow an unbraked trailer weighing up to 750kg at up to 60mph. One does not need DfT approval to fit electric brakes to these trailers (as far as I know). I hope that manufacturers who will be insensed by my criticisms will see commercial sense and realise that it would be more profitable to import and sell electric brakes than it would to devote energy to persuading "20six" to close me down. I have seen references in the Institute of Traffic Accident Investigators' Journal to the "Unbraked Trailer Syndrome." This apparently refers to some tests they ran on unbraked trailers some years ago when they found that all instances of fairly moderate braking resulted in the trailer jack knifing. However, I have been unable to obtain a copy of the Journal containing the appropriate report. I hope that some itai member reading this blog will confirm my recollection. I hope that groups of staff who use minibuses for towing trailers will meet together to examine their safety policies. I have been working at this investigation for a long time, but as the trailer in the South African Mini bus Accident was so much larger than anything I have ever seen towed by schools or colleges, it did not strike me that there was a problem that I need report to my former colleagues. However, I then remembered that in 1978 my car developed a very serious fault on the last but one day before the Easter holidays, and I needed same to take my wife four children and my sailing cruiser from Birmingham to Aberystwyth. I therefore used the school minibus, but this was not something that needed the Governors' approval as the mini bus was a long wheel base Land Rover that belonged to the Head of CDT, and he allowed this to be used for school purposes in return for "expenses." He had also ensured that the pupils made a good job of constructing a trailer to carry three racks of canoes, and a further colleague had overseen the construction of the canoes by the pupils. I am not absolutely certain of the next bit, but I think my recollection is very likely correct. The canoe trailer had no brakes, but as these two teachers were based near the school entrance they occasionally checked the roadworthiness of cycles being ridden by pupils. Anything with faulty brakes would have to be rectified or the pupil would not be allowed to ride to school on the cycle. It has taken me 30 years to realise the enormity of my error as Head of Science in not thinking of this before, but like everyone else I just assumed that if it was legal to have no trailer brakes (up to 750kg) then it must be safe. When I have managed to put some diagrams and photographs into my blogs I hope that any reasonably well educated person will be able to understand most of my work. Please add comments to the blog as you think fit and I will endeavour to clarify items if necessary. Currently, if you look at the space at the head of this series of pages you will see a square with a small flag in same.On my computer this comes out as a photograph (for identification purposes) of myself. I have some more work to do with the software.
10ciii SUMMARY 3 Testing Friction based Stabilisers for Caravans and Trailers I am strongly in favour of modifications to over run brakes to convert them to electronic/electric operation. I also wish to draw attention to the fact that if you put “electric brakes” into the Google search engine you will be introduced to the range of these available in the USA and Australia. In particular you will also be able to see that electric brakes would cost considerably less than over run brakes plus stabiliser and electric brakes have been in use for around 30 years. It is possible to apply electric brakes when a trailer is snaking and this application of the brakes will in most cases control the snaking (but see also Bath University Wind Tunnel Research showing the need for aerofoils to create down force). I hope that Caravanners and Trailer towers will test their friction based stabilisers. You do not need qualifications in Mechanical Engineering to make a reliable rough estimation. I previously wrote that you need to evaluate the amount of friction available from your stabiliser by moving it with the aid of a 2 metre length of tubular steel or timber. I also wrote that to perform this task accurately the length of timber must be equal to the distance from the tow hitch to the centre of mass of the trailer or caravan. It is possible that by looking at a caravan/trailer and its load any reasonably intelligent person can estimate the distance of the centre of mass from the tow hitch almost as accurately as I can calculate same. It will be seen therefore that for a small trailer the stabiliser test lever will be less than 2m long, but for a large twin axle van it will be very much longer. The Centre of Mass is the point where the whole of the mass acts. If the centre of mass is over the axle the caravan is balanced on its axle and zero force is needed to lift the caravan by the handle on the tow hitch. In theory the centre of mass could be found experimentally by balancing the van on a strong length of tubular steel placed under the van on axle stands. This is not advisable as the steel may damage the chassis. If the van is correctly loaded the centre of mass will be a short distance in font of the axle/axles. Calculations The first solicitor who wishes to use this method of testing a stabiliser in connection with any legal proceedings will need to engage the services of a Forensic Engineer to evaluate the method. However, if it were not for the size of the apparatus required this test could be carried out in a school or FE College as part of the Physics course for either GCSE or GCE “A” level. (The theory behind my formula may still be in some schemes of work for GCSE if Physics is taken as a separate subject.) As a previous Head of Science I would not have thought it justifiable for lab technicians to spend time on this construction, but if a member of staff with a caravan and stabiliser made the apparatus in their own time it would be a different matter. Physics teachers are always on the lookout for practical applications of the work they teach. If any school or College did carry out this procedure, as long as the teacher/lecturer in charge had a sufficiently high academic qualification in Physics or Engineering to satisfy the court, it could well be that this would be accepted as a validation of my method. In simple terms, by moving the stabiliser with the constructed lever the operator is just asking themselves the question : Does this amount of force stand any chance of having any effect at all on a caravan or trailer snaking at 50mph? According to the Haynes Caravan Manual (by John Wickersham) my stabiliser needs adjusting so that a force of 27kg is needed to just turn the arm of 0.7 m length. This gives 270N x 0.7 = 189 Nm. Similar design specifications for stabilisers which obtain their friction from a device fixed over a dry tow hitch, have never been disclosed (to the best of my knowlege). If readers look at the www site for the “Advertising Standards Authority” they will find some interesting reading. In 2003 the Swift Caravan Group took a whole page in the Caravan Club Magazine to advertise the ALKO stabilisers on their caravans, but they included a picture of a gyroscope in the advertisement. Put “SWIFT CARAVAN GROUP” into the search engine in the top right hand corner, and then click on the extract that appears, to see the full “adjudication.” Other methods of testing stabilisers have been used. The Fratilla Phd thesis of 1994 used a computer model; see para 2e, 9b and 10c of my blog www.caravanaccidents2.wordpress.com
11a Effect of air disturbance produced by hgv's and coaches. Many years ago I read in the Caravan Club Magazine a letter from a member who, early one Sunday morning, on a virtually empty motorway, had had his Range Rover and a fairly large caravan completely wrecked by an overtaking coach which had passed close to them, but had made no physical contact. He and his family escaped with no physical injuries. An e mail I received (2003) from an Australian Police Investigator (who worked for the "Major Accidents Section",) told of a very similar event, except that this one caused two fatalities. Both events produced accounts from witnesses of being "virtually sucked up and thrown out of control " by the effect of the coach/hgv which had not touched their vehicles. (11b) In a long e mail (on the effects of hgv's on caravans) I received from a University Researcher (not via the RTA Yahoo site), I was reminded of the "Ventrui Effect" ( except that he called it something else). Sailors are well aware of this because it is the reason why two sails used together (as long as they slightly over lap) have a greater driving force on the boat than the sum of the two effects when the same sails are used on their own. This is because as the wind passes in the narrow space between the sails it speeds up and air pressure is reduced. I have now come to the conclusion that this effect over rides the effect of the bow wave when two vehicles pass (at high speed) very close together ( ie almost knocking off wing mirrors). (11c) To summarise:- The Ventrui Effect can cause a serious accident when a large vehicle overtakes, or is over taken by, a caravan or similar high aspect trailer, providing that the passing vehicles are very close together. The Bow wave of a large vehicle will most probably destabilise a caravan or high aspect trailer, if the latter is moving at above its critical air speed, at the usual motor way seperation distance, and all vehicles are going in the same direction. If the vehicles are travelling in opposite directions, on a normal single class A road, I have observed no disturbance whatsoever. I have attempted to explain the Physics of this matter at the end of the next section. (11d) The following is a mainly unaltered version of the item in my itai report of 2005. . Wave Theory
An Explanation of why trailers "snake", based on well established Laws of Physics and 26 years experience of towing sailing cruisers and caravans.
Resonance
A number of phenomena studied in School Physics exhibit resonance. When something is made to vibrate it will only vibrate excessively when the frequency of vibration is the same as the natural or resonant frequency . Musicians have to be familiar with this but moving closer to caravans I would point out that if a company of troops marched over a wooden bridge were not to "change step" every few minutes there would be a probability that they would set the bridge resonating and cause it to collapse. I have several times shown a TV "Physics for Schools" programme which depicts an approx 1930's USA road supension bridge collapsing when set in resonance by a strong wind.
I would therefore suggest that one reason caravans snake excessively is that an overtaking HGV or the wind happens to have set them oscillating at their resonant frequency.
HGV Bow Wave in phase with trailer oscillations
I have many times demonstrated, using a water ripple tank, in GCSE Physics lessons, that when waves are "in phase" they will reinforce each other and produce bigger waves, and that when they are in "anti phase" two identical waves will cancel each other out and have zero effect. It is reasonable to suppose therefore that this also applies to vibrating caravans being affected by the bow waves of HGV's. If the bow wave of a HGV is in phase with the slight vibrations of a caravan a dangerous snake could be induced as the HGV overtakes. If another HGV is following too closely behind (as is often the case) and it also has a similar bow wave it could make the situation even worse for the caravaners, but if the wave happened to be in "anti phase" with the caravan vibrations it would dampen them down.The above also explains why you are less likely to get a dangerous snake on a crowded motorway than on a quieter one. Waves from various vehicles will interfere with each other and be less likely to cause trouble for trailers.
Longer Trailers are more prone to snaking
I had always thought that the length of a trailer (with a high aspect) had almost as much influence on stability as the tow car/trailer weight ratio, but after I purchased a twin axle caravan ( just over 19ft long) I became more convinced of this. I was therefore not at all surprised when I found that wave theory also explained this phenomena.
If L= length of bow wave of HGV, and T= average body length of high aspect part of trailer, the bow wave will have the maximum turning effect (giving rise to snaking) on the trailer when T=L/2; ie when the length of the trailer is half the length of the HGV bow wave. Wave theory shows that the pressure created by the wave is such that it will tend to push the front of the trailer in one direction and suck the opposite end in the other direction ( or vice versa). This will create the twisting effect which sets the trailer in oscillation. It is when T=L/2 that this occurs, but it should also be taken into account that as these effects are felt at the ends of the trailer they will have greater turning effect on a longer trailer than a short one.
Similarly, wave theory shows that this effect is also felt when :- T=3L/2, 5L/2, 7L/2 ......and so on.
No Snaking when HGV is going in opposite direction
In the cases mentioned above the overtaking HGV travelling at 60mph will pass the trailer (travelling for example at 50mph) at a relative speed of 110mph. A 20ft trailer will therefore be under the influence of the first and most energetic part of the bow wave for approx: 1.5 secs. When one is travelling on an ordinary class A road HGV's will pass close by going in the opposite direction at a relative speed of 100mph (assuming that both vehicles are moving at 50mph.) I would speculate that in this case the HGV has no unpleasant effects on the trailer because the interval of time that the latter is subject to the worst effect of the bow wave is only approx: 0.15 secs.
(11e)
Effect of sudden deflation of a Tyre
It is well known that a "blow out" on a car tyre can have a serious destabilising effect on a car, and often leads to an accident. It has therefore been assumed that the same thing applies to trailers. I do not think that this is correct. If a trailer tyre suddenly deflates there will be an increase in friction between wheel and road on that side of the trailer. This will exert a steady force which will cause the trailer to be slightly out of line with the tow car, but the trailer should continue without snaking, at a slightly skewed angle with respect to the tow car. Similarly, the fact that the trailer is slightly lower on one side should not have any effect. It is the tow car that is steering the trailer and the outfit should hold a steady course so that it can be slowed down gradually, only gently applying the brakes for the final halt. I was towing an Ace Pioneer Caravan (about 12 ft long) with a Morris Marina 1800cc at approx: 50mph, on the M5, when I had a "blow out." I managed to stop in a controlled manner as outlined above. (see photo B).
(11f)
Kinetic Energy and Gravity
You cannot ignore the Laws of Gravity. Whereas you can shut off the power of the car engine simply by taking the pressure off the accelerator pedal, you cannot shut off gravity. It is to remind drivers of this Law of Physics, amongst other things, that the Highways Agency has errected large signs on "fast roads" at the start of steep down gradients stating "TOWING VEHICLES SLOW DOWN".
The other basic law of Physics which is applicable here is the one that leads to the equation for calculating the amount of energy your outfit has because it is moving (Kinetic Energy).
KE=1/2MV(where V is squared)
M= mass and V= velocity ( for the purposes of this explanation it can be assumed that velocity = speed and mass = weight.)
Even if everyone reading this suddenly has a "flash back" to school Physics, my observations tell me that not many people remember that the effect of the weight of the trailer on the amount of energy is the same at 30mph as at 60mph, but as the speed in the above equation is raised to the power of two (ie squared), an increase of speed from 30 to 60mph (ie x2), will increase the energy by x4 (ie 2 squared).
Also, when going downhill it should be remembered, that not only is it not possible to shut off the force of gravity, but if your caravan starts to snake, as previously explained, you have no brakes if you are relying on an "over-run" brake actuating mechanism and almost no brakes if it does not snake due to overheating as previously mentioned.
(11g)
Self Generating Snakes
The energy required to set off a snake comes from the bow wave of the overtaking vehicle ( assuming the wind is neglible), but once the snake is established it can often increase in size due to another effect. As the trailer is snaking it will be travelling a greater distance than the tow car and will have to move at a higher speed to keep up with the car. This will lead to the tow car exerting a considerable force on the trailer, particularly as it reaches the outer edge of its swing; it is at this point in particular, that the trailer will be increasing its speed and this fact must give rise to the snake increasing in severity.
(12a) Over run Brakes A virtually unaltered copy of my 2005 report for the itai. A Theoretical Explanation of the Serious "Design Faults" in Over- Run Brakes see diagrams G and H Over-run trailer brakes only operate after the tow car brakes have been applied. When the trailer starts to catch up with the tow car , the telescopic part of the trailer brake mechanism will push against the brake lever and apply the trailer brakes. This only works satisfactorily when the car and trailer are "in line," which is of course nearly all of the time . However, all other cases are likely to be "emergency" stops so these small number of cases are the really vital ones. If you are on only a very slight bend when the brakes are applied there is a probability ( for instance) that your trailer will turn your car round through 180 degrees. The probability of this disaster taking place is low if your speed is low, and vice versa. Also, the probability of disaster is low if your trailer/car weight ratio is 50% ( as is the case if you are towing a small caravan with a heavy 4x4 ), but very high indeed if the previously mentioned ratio is 100%. As can be seen from the diagram (G) this can happen in two ways; the force of the trailer pushing on the tow car may be insufficient to apply the trailer brakes adequately, but in a situation when the tow car will be close to starting to slide in any case it may only need a slight extra push to send the tow car spinning further out of control. Alternatively, the momentum of the effectively "unbraked" trailer may cause it to jack knife ( ie swing round in an arc with the tow hitch as the centre of the arc.) Again, it is important to study the trailer's tyre marks The absence of tyre braking marks for the trailer in my view confirms that an "offence" has been committed because the law is that trailers between 750kg and 3500kg must have brakes, and this means that they should work at all times. If a post accident investigation shows that the brakes are in working order the absence of tyre braking marks is strong evidence that all over run brakes are likely to have the same problem and be ineffective in this situation. The left hand diagram in diagram "G" indicates the condition I found the over run brake mechanism in for my current boat trailer after the Winter of 2003/4. The boat plus trailer had been stored in a line of similar boats, on shingle, above the high water level, well up a sheltered creek, near Falmouth. Something quite big had obviously "bumped" my trailer. It would be a serious and most unusual accident on the road that caused this much damage, but the incident did remind me of the fact that every time a tow car brakes in the situation shown in the right hand diagram "G" there will always be a tendency for the mechanism to distort very slightly ( perhaps by such a small amount that the mechanism would recover its original shape), but the slight distortion would ensure that the telescopic item would "stick" and the brakes would not be applied. The size of the effect that either of the above actions will have on the tow car depends on the distance of the car's rear axle from the tow hitch (diagram G). The greater this distance the greater will be the leverage on the tow car and the larger will be the probability that the car will be swung round or even turned over. I would postulate that the chances of either of these two situations happening would be vastley reduced if the trailer brakes came on as soon as the tow car brakes were applied. In the EU/UK, as opposed to many other parts of the world, we have never learned whether this is an economically viable proposition because fitting better brakes as "original equipment" to new trailers has never been tried. (12b) Snaking The process of snaking, whether the trailer is a caravan, horse box, glider carrier, general goods carrier, or a trailer holding another car or a sailing cruiser, is most complex. The path taken by the snaking trailer, the forces exerted on it by the tow car, and the relevant forces due to other factors, contain enough Physics to warrant a further Phd thesis supported by a specially developed computer model. However, having considered these factors I have singled out one only which in my view is the most important because it is clearly the greatest of the forces which act on the trailer in a direction opposite to the direction of motion and prevent the over-run brakes being applied when a trailer is snaking. This force is the "centrifugal force" exerted on the trailer by virtue of the fact that it is snaking. The trailer can be considered to approximately describe an arc of a circle each time it swings, so it would be justifiable to use the well established formula for motion in a circle to give a good approximation of the magnitude of the centrifugal force exerted in this case. ( I would contend that if a computer model was set up to assess the centrifugal force it would be far greater than I have calculated. ) The calculations for this have been outlined on the next page. I have shown that if the trailer is only swinging a modest 15 degrees either side of the staight ahead position, and takes 2 seconds for a swing, the centrifugal force generated by a small loaded caravan weighing 1000Kg is 913 Newtons. I have also shown that this force of 913N would theoretically slow the caravan down from 50mph to 47.96 mph in 1 second. As the caravan is fixed to the tow car the net result of this is that the telescopic part of the brake actuating mechanism will be on its end stop with zero chance of applying the brakes. When snaking takes place, you therefore have NO BRAKES on your trailer. ( see also Section on Snaking Tyre Marks) Page 584 of the Caravan Club Handbook states, when advising on snaking, "..........do not accelerate............" as this would be an exceedingly hazardous procedure; but if the trailer could be "braked" independently of the car this would have the same effect and would be a safe way of controlling a snake. ( see Section on Electric Brakes.) Section 74 of the "New Expanded Edition" of the Highway Code states, " ....ease off the accelerator and reduce speed gently to regain control." In actual practise the decelaration time in these cases is very long and with a trailer behind behaving very erratically it is only the most experienced drivers who will be able to resist the natural reaction of applying the car brakes. If you apply the brakes "very gently" as the Caravan Club suggests or "slow down gently" as the Highway Code tells you, it must be remembered that you are applying the car brakes. The trailer brakes will only be applied when it catches up with the car sufficiently to compress the strong spring or hydraulic damper and move the trailer brake lever. The gentle pressure the driver applies to the car brakes is not necessarily the same pressure that is applied to the trailer brakes, particularly as the trailer is behaving most erratically when snaking, and the actual direction the trailer is facing when the brakes "take hold" determines whether the tow car is likely to be spun round by the trailer. This takes us back to my first argument above. Out line of method of calculation. Cetrifugal Force. Centrifugal Force = F = m w r , where F= force in Newtons, m= mass in Kg's w = angular velocity in radians. r = effective radius of arc of circle = 3.33metres = distance from towing hitch to centre of mass of caravan. My small Caravan has a loaded mass of 1000Kg, a shipping length of 5.7metres, a body length of 4.22 metres and the centre of mass is assumed to be just forward of the axle. Decrease in speed of trailer due to Centrifugal Force. Using v = u + at, where v = final velocity, u = initial velocity, a = acceleration produced by centrifugal force, and t = time in seconds, and assumed to be 1 second for this calculation, (the time for half a swing). The time taken for the trailer to swing when it is snaking has been estimated. On a test track this time could be measured accurately to see whether the conclusions I have reached are reasonable. I should point out that the time I have estimated is based on experience of snaking. Addendum February 2005 I have been informed by a University researcher into caravan instability that the rate of oscillation they work on for a snaking caravan is 1Hz, whereas the time for half a swing of 1 second that I mentioned above is 0.5 Hz. The centrifugal force concept therefore comes into play long before a caravan is reaching the stage when it is completely out of control.
(13a) Stabilisers An almost unaltered copy of my 2005 report for the itai
A stabiliser consists of a friction device with surfaces similar to brake linings, an adjusting nut and bolt to facilitate the maintenance of a steady pressure on the friction surfaces as they wear; an arm which is hooked on to the trailer chassis aswell an attatchment to fix the other end to the tow car. I have a Scot Stabiliser, which, according to the Haynes' Caravan Manual, should be adjusted so that a force of 27Kg (approx. 270N) is needed to move the arm against the friction of the device. The length of the arm is 0.7 metres.
The distance from the tow hitch to the centre of mass of my caravan is 3.33 metres, but this will vary slightly depending on how the caravan is loaded. Referring to the diagram (J), and applying the Principle of Moments, it can be seen that the maximum force that the stabiliser can bring into play to dampen down the caravan oscillations is 56.8 N. Using the calculations from the section on Centrifugal Force it can be shown that the velocity that the caravan is moving with as it describes (approx)an arc of a circle with the tow hitch as its centre, is 1.8 metres/second (this is an "average" velocity assuming half an oscillation takes 0.5 secs; in actual practise the velocity is zero at the beginning and end of the movement, so to give an average of 1.8m/s the max velocity in the centre will be greatly in excess of 1.8 m/s giving a much larger centrifugal force).
Using v = u + at ( v = final velocity, u = initial velocity, a = acceleration and t = time in seconds) I have calculated the acceleration when the velocity (the velocity of the caravan as it oscillates, not its forward speed) is reduced by a tenth in 0.5 seconds ( this being the time for a quarter of a swing). I have then used Force = Mass x acceleration to show that a force of 1800N is needed for this very small amount of damping, but the stabiliser cannot exert more than 56.8N as explained above.
It can be seen therefore that there is an urgent necessity to bring into use a more reliable braking system which will work when a trailer is snaking, as this is known to be an effective means of control.