Safe towing of Caravans and Trailers

[peter_5297]

From Peter W Jones AMInstP

Retired Head of Science in a Birmingham Comprehensive School.

We purchased our first caravan in 1965. It was a Donnington Castle. It measured about 28ft x 8ft and had brakes and road wheels, although we never towed it ourselves. We had this for 20 years.

Currently we have had a 1986 Twin Axle Lunar Delta 590 for about 15 years; a 1994 Swift Challenger 400 SE for about 10 years; a 1982 Jaguar 21 Sailing cruiser (with road trailer)

for about 21 years, and a new Daxara 750kg trailer with brakes

purchased in May 2007.

I have been looking into the Technical aspects of towing caravans for some time. A summary of what I have concluded is contained in the first two pages of

www.caravanaccidents.wordpress.com

and I would like to hear what readers of Pactical Caravan think about this.

The rest of the above web site expands on the introduction.

For those with technical experience I have also produced two more web sites details of which are given at the end of the first one. I intend to add photographs and diagrams in due course. (I am a novice at Blogging).

I was most impressed by details I read in Practical Caravan of

of a successful trial of a caravan pneumatic brake carried out

over a period of twelve months by a Practical Caravan Journalist.

 

[ProfJohnL]

Hello Peter.

I have speed read your postings and supporting documents. I can agree immediately with the conclusion that the dynamics of towing need to be more fully examined, with the hope that improvements to both vehicles, trailers and guidance will follow.

You make some interesting points. but I need to spend more time reading and cross referencing to content before I draw my own conclusions and comment.

 

[Lutz]

While I would also welcome further research into the dynamics of caravans, I have serious reservations about some of the statements made in the blog.

1. The Bath University findings regarding stabilisers only investigated the performance of passive (frictional and/or viscous) stabilisers, and did not even consider active systems such as the AlKo ATC or LEAS.

2. The polar moments of inertia of towcar and caravan have at least as much affect on dynamic handling performance as the height of the centre of gravity of the caravan.

3. Any electrical or hydraulic braking systems for caravans would require devices in the towcar to provide proportional braking control. Otherwise, the brakes could only be made to be either on or off, or they would just be electric or hydraulic variants of the simple mechanical overrun brake. Only when sophisticated 'drive-by-wire' systems become a production reality sometime in the future would proper proportional control be economical as it could be integrated in the electronic towcar braking system at little extra cost. Until such time, we will have to resign ourselves to the overrun brake, either mechanical, electrical or hydraulic, as the only cost-effective measure.

4. I see very little connection between the aerodynamic characteristics of aircraft and those of caravans. The sheer frontal area of the caravan will always cause an enormous pressure at speed and this has the effect of lifting the front end, thereby reducing the necessary noseweight, not any aerofoil properties which are negligible compared with their significant drag. Caravans would have to be designed with much more sloping front ends to offset the drag effect but this would make them significantly longer and/or result in lost useable space inside.

 

[peter_5297]

While I would also welcome further research into the dynamics of caravans, I have serious reservations about some of the statements made in the blog.

1. The Bath University findings regarding stabilisers only investigated the performance of passive (frictional and/or viscous) stabilisers, and did not even consider active systems such as the AlKo ATC or LEAS.

2. The polar moments of inertia of towcar and caravan have at least as much affect on dynamic handling performance as the height of the centre of gravity of the caravan.

3. Any electrical or hydraulic braking systems for caravans would require devices in the towcar to provide proportional braking control. Otherwise, the brakes could only be made to be either on or off, or they would just be electric or hydraulic variants of the simple mechanical overrun brake. Only when sophisticated 'drive-by-wire' systems become a production reality sometime in the future would proper proportional control be economical as it could be integrated in the electronic towcar braking system at little extra cost. Until such time, we will have to resign ourselves to the overrun brake, either mechanical, electrical or hydraulic, as the only cost-effective measure.

4. I see very little connection between the aerodynamic characteristics of aircraft and those of caravans. The sheer frontal area of the caravan will always cause an enormous pressure at speed and this has the effect of lifting the front end, thereby reducing the necessary noseweight, not any aerofoil properties which are negligible compared with their significant drag. Caravans would have to be designed with much more sloping front ends to offset the drag effect but this would make them significantly longer and/or result in lost useable space inside.

From Peter W Jones

Thanks for the detailed conideration of some of my ideas.

1. Friction based stabilisers are the only type I have ever seen actually in use. This is based on many years of observation.

I would be interested to hear about statistics for the use of all types of stabilisers.

ALKO claim 80% of the UK trailer/caravan chassis market. They fit their frition based stabilisers to many new caravan chassis.

I therefore still feel that this type of stabiliser is the one to investigate in particular.

In considering the effectiveness of the friction based stabiliser the distance of the centre of mass of the trailer/caravan from the tow hitch is the only relevant factor.

Height of the centre of mass is important when considering the chances of a particular trailer/caravan overturning and problems with "pitching."

3. Electric brakes are well established in Australia and sold there by ALKO (according to their web site). Why are they not available in the EU?

 

[peter_5297]

While I would also welcome further research into the dynamics of caravans, I have serious reservations about some of the statements made in the blog.

1. The Bath University findings regarding stabilisers only investigated the performance of passive (frictional and/or viscous) stabilisers, and did not even consider active systems such as the AlKo ATC or LEAS.

2. The polar moments of inertia of towcar and caravan have at least as much affect on dynamic handling performance as the height of the centre of gravity of the caravan.

3. Any electrical or hydraulic braking systems for caravans would require devices in the towcar to provide proportional braking control. Otherwise, the brakes could only be made to be either on or off, or they would just be electric or hydraulic variants of the simple mechanical overrun brake. Only when sophisticated 'drive-by-wire' systems become a production reality sometime in the future would proper proportional control be economical as it could be integrated in the electronic towcar braking system at little extra cost. Until such time, we will have to resign ourselves to the overrun brake, either mechanical, electrical or hydraulic, as the only cost-effective measure.

4. I see very little connection between the aerodynamic characteristics of aircraft and those of caravans. The sheer frontal area of the caravan will always cause an enormous pressure at speed and this has the effect of lifting the front end, thereby reducing the necessary noseweight, not any aerofoil properties which are negligible compared with their significant drag. Caravans would have to be designed with much more sloping front ends to offset the drag effect but this would make them significantly longer and/or result in lost useable space inside.

From Peter W Jones

Comments continued

4. Caravan towers are not informed by the manufacturers of the critical air speed at which a caravan/trailer is likely to have zero road friction due to aerodynamic lift.

I have only made a comparison with aircraft to estimate this critical speed for caravans/trailers. In the absence of other methods this is the best one available. The stalling speed of an aircraft is likely to be close to the above mentioned critical speed for caravans/trailers of the same weight.

I base this estimation on the critical speed I have estimated (by observation on the road) for my own 1000kg caravan (50 mph). The latter is the same approximately as the stalling speed of aircraft of 1000kg weight.

It would be far preferable if we were given the correct figures.

They are easily established with equipment available to university researchers.

 

[ProfJohnL]

Hello Peter,

I have sympathy with the broad thrust of your arguments and more authoritative research would be welcome,

Perhaps the photos and diagrams mentioned but not supplied would help us to fully appreciate and understand all your points, but I list some of the points that are not clear or I cannot accept at face value..

It would be interesting to know what question was actually posed to the CC about braking systems. It might make more sense of the answer you quote.

Cost is almost certainly the biggest factor that is limiting the introduction of advanced braking systems. Jones Venning will follow they do not lead.

It is not clear why you single out schools and University minibuses towing trailers and the phenomenon of trailer lift off under heavy braking this must apply to all towing outfits. And why is this not more frequently reported - How frequently does this occur?

Whilst it is true that there are far fewer towed miles, what is the evidence that suggests that incidents per mile are higher than the average for un-towed miles?

I for one have known and understood that 'stabilisers' do not magically prevent instability; they only defer the onset under deteriorating circumstances.

The aerodynamic effects in play whilst lorries overtake are many fold, and each car and a caravan combination will have broadly similar overall characteristics, but the devil is in the detail differences. Bath University's results may have used a particularly sensitive combination, and what is important to the caravanner is how their outfit performs not an average one.

Like Lutz, I can see no advantage in comparing the aerodynamics of a car and caravan combination to an aircraft. One is designed to fly high in the sky the other is not. The 'critical air speed' is effectively hypothetical, as it will change quite radically depending on the aerodynamic slip-stream of the towing vehicle. It will also change depending on how the caravan and car are loaded. You must also consider the ground and coander effect of aerofoils close to large continuous surfaces. Not a realistic proposition for normal mortals.

 

[Lutz]

At all realistic speeds, any lift created by air flow will never be large enough to reduce the caravan's axle load to zero. However, it is conceivable that the noseweight may actually reverse, i.e. pull the back end of the towcar up, if the aerodynamic drag is large enough.

 

[peter_5297]

At all realistic speeds, any lift created by air flow will never be large enough to reduce the caravan's axle load to zero. However, it is conceivable that the noseweight may actually reverse, i.e. pull the back end of the towcar up, if the aerodynamic drag is large enough.

From Peter W Jones

If the caravan moves sideways, as when on an icy road, the road friction must be aproaching zero and as the tyre characteristics

have not changed this must be due to lift which, I call aerodynamic lift as it can only be due to air. It must therefore be possible to get lift without a wing shape.

 

[peter_5297]

From Peter W Jones

Thanks for the detailed conideration of some of my ideas.

1. Friction based stabilisers are the only type I have ever seen actually in use. This is based on many years of observation.

I would be interested to hear about statistics for the use of all types of stabilisers.

ALKO claim 80% of the UK trailer/caravan chassis market. They fit their frition based stabilisers to many new caravan chassis.

I therefore still feel that this type of stabiliser is the one to investigate in particular.

In considering the effectiveness of the friction based stabiliser the distance of the centre of mass of the trailer/caravan from the tow hitch is the only relevant factor.

Height of the centre of mass is important when considering the chances of a particular trailer/caravan overturning and problems with "pitching."

3. Electric brakes are well established in Australia and sold there by ALKO (according to their web site). Why are they not available in the EU?

From Peter W Jones

Point 4

I did not say that the aerodynamics of caravans were the same as

air craft.

On a the basis of an estimate or guess I have said that if my caravan of 1000kg has (on the basis of my road assessment over along period of time) a maximum safe speed of 50 mph it is worth making comparisons between heavier aircraft and heavier trailers.

 

[peter_5297]

Hello Peter,

I have sympathy with the broad thrust of your arguments and more authoritative research would be welcome,

Perhaps the photos and diagrams mentioned but not supplied would help us to fully appreciate and understand all your points, but I list some of the points that are not clear or I cannot accept at face value..

It would be interesting to know what question was actually posed to the CC about braking systems. It might make more sense of the answer you quote.

Cost is almost certainly the biggest factor that is limiting the introduction of advanced braking systems. Jones Venning will follow they do not lead.

It is not clear why you single out schools and University minibuses towing trailers and the phenomenon of trailer lift off under heavy braking this must apply to all towing outfits. And why is this not more frequently reported - How frequently does this occur?

Whilst it is true that there are far fewer towed miles, what is the evidence that suggests that incidents per mile are higher than the average for un-towed miles?

I for one have known and understood that 'stabilisers' do not magically prevent instability; they only defer the onset under deteriorating circumstances.

The aerodynamic effects in play whilst lorries overtake are many fold, and each car and a caravan combination will have broadly similar overall characteristics, but the devil is in the detail differences. Bath University's results may have used a particularly sensitive combination, and what is important to the caravanner is how their outfit performs not an average one.

Like Lutz, I can see no advantage in comparing the aerodynamics of a car and caravan combination to an aircraft. One is designed to fly high in the sky the other is not. The 'critical air speed' is effectively hypothetical, as it will change quite radically depending on the aerodynamic slip-stream of the towing vehicle. It will also change depending on how the caravan and car are loaded. You must also consider the ground and coander effect of aerofoils close to large continuous surfaces. Not a realistic proposition for normal mortals.

From peter W Jones

I have only recently started blogging and it will take some time getting to grips with feeding my diagrams in. Tony Blair and David Cameron have huge amounts of assistance for doing this sort of thing.

The stabilisers I have referred to were on the website (private members only)of the Caravan Club.

I do not think that electric brakes would cost as much as the over run system. The high quality telescopic device for the latter must be quite expensive and I cannot see that a solenoid

system inside the brake hub will cost more than the complicated reversing system plus the mechanical device for pushing on the brake shoes.

There are USA websites advertising both types of brake sold as kits to make up trailers. My further estimate could be checked.

I will do it myslf in due course.

My comments about unbraked trailers, as you say, apply to all of them. I picked on Schools in particular as I am sure that no school would do this if they were aware of the situation. There are enormous problems in schools currently assessing risk factors for out of school visits.

As far as I know there is no precise evidence on towed miles.

I have said that the CC's estimate of miles covered makes a case. The evidence from the Highways agency notices makes the case very strong for there being a serious problem with caravan instability. On the M5 near Birmingham there are two notices on seperate stretches. They show a drawing of an upturned caravan and a line saying "11 accidents of this type on this stretch of road last year.

I have replied elsewhere to the point covering the complexity of assessing air speed. All the factors for car caravan combinations would need putting in a date base so that you could feed in your variables and get a suggested maximum safe speed.

Some further control is needed even now over reckless driving. Insurance companies must make their financial penalties (extra insurance costs) for any proved to be driving without due care and attention much clearer.

This should be extended to badly loaded trailers and poorly matched combinations (For instance).

I think aerofoils will need to be quite high above the caravan roof. They were very high indeed on the Formula I cars when they started.

This of course is why the trucking companies are keeping quiet.

HGV's found to be particularly vulnerable to snaking could hardly have much of an aerofoil. They only just get under the bridges as it is and as far as I know the trailers do not have electronic brakes.

 

[peter_5297]

Hello Peter,

I have sympathy with the broad thrust of your arguments and more authoritative research would be welcome,

Perhaps the photos and diagrams mentioned but not supplied would help us to fully appreciate and understand all your points, but I list some of the points that are not clear or I cannot accept at face value..

It would be interesting to know what question was actually posed to the CC about braking systems. It might make more sense of the answer you quote.

Cost is almost certainly the biggest factor that is limiting the introduction of advanced braking systems. Jones Venning will follow they do not lead.

It is not clear why you single out schools and University minibuses towing trailers and the phenomenon of trailer lift off under heavy braking this must apply to all towing outfits. And why is this not more frequently reported - How frequently does this occur?

Whilst it is true that there are far fewer towed miles, what is the evidence that suggests that incidents per mile are higher than the average for un-towed miles?

I for one have known and understood that 'stabilisers' do not magically prevent instability; they only defer the onset under deteriorating circumstances.

The aerodynamic effects in play whilst lorries overtake are many fold, and each car and a caravan combination will have broadly similar overall characteristics, but the devil is in the detail differences. Bath University's results may have used a particularly sensitive combination, and what is important to the caravanner is how their outfit performs not an average one.

Like Lutz, I can see no advantage in comparing the aerodynamics of a car and caravan combination to an aircraft. One is designed to fly high in the sky the other is not. The 'critical air speed' is effectively hypothetical, as it will change quite radically depending on the aerodynamic slip-stream of the towing vehicle. It will also change depending on how the caravan and car are loaded. You must also consider the ground and coander effect of aerofoils close to large continuous surfaces. Not a realistic proposition for normal mortals.

From Peter W Jones

I am pasting in below the item on this subject I have just put on T&T.com

From Peter W Jones

Sorry. I should have said snaking accidents CAN be the cause of a tyre failure.

When I read these posts I really thought that some of my material had been selectively edited (as is allowed by the site rules) and that

I had been cut of from access to the site. I immediately added my replies to the beginning of my blog

www.caravanaccidents.wordpress.com

If, when I next visit T&T I am still allowed access, I shall add to my blog a suitable apology for what I have said about T&T and Prac Caravan Mag.

We are tomorrow returning to our caravan, but I shall endeavour to answer the rest of the points now or subsequently raised by accessing via wireless connection or a computer in a public library.

I will try to make a contribution at least once per week.

I left the ITAI because I felt that they could not help me further and I could not help them.

I joined instead the Society of Expert Witnesses. I was admitted as an Associate Member on the basis of my 1973 AMInstP and my 1952 Teaching Certificate (which included a Distinction in Advanced Science) after a course at what is now Chester University. Account was also taken of the work I have done on Trailer Instability.

I have set myself up as a "Consultant for Towed Vehicle Accidents" and hope to have a few cases to set up a legal precedent so that someone much younger can easily carry on when I give up. I am doing this in the interests of Caravan/trailer safety. Any money I earn in excess of expenses I would donate to a charity of my choice. As I am "non profit making" I hope my advertisement is in order on this site.

My induction consisted of reading the bulky file of legal advice supplied by SEW. I was also provided with a Mentor, who was a well qualified Engineer

who had had an extensive career as a an Expert Witness for RTA's. He gave me much advice and thought I was well qualified for the narrow area I had chosen. I circulated suitable RTA solicitors all over the midlands but the only one to phone me asked me to assess the speed of a car before the accident. He had clearly not even read my Job Title.

At the end of last year I therefore told SEW that I would not renew my Associate Membership ( as the fee is not an insignificant amount).

I was surprised to have a reply to the effect that if I wished to do any expert witness work in the future I would immediately be readmitted.

Now before anyone gets any more exaggerated ideas about flying caravans my situation is as follows:-

If someone runs out of luck or makes a mistake and gets killed or seriously injured in a caravan or trailer snaking accident, I am available to be consulted by their Solicitors or Traffic Accident Investigator.

Any disagreements between experts are usually settled out of court, but if they could not be I would have to appear in court. I would also have to appear if asked to by a judge.

I would be sent the file of evidence. At this stage SEW would again assist

by arranging for a mentor to check what I proposed to say or write and advise accordingly.

My next case would be treated in the same way and if I showed myself to be competent I would then proceed without assistance unless I requested same.

A typical case that I would accept is outlined on

www.caravanaccidents2.wordpress.com Para 3a

In this case it was absolutely clear that the couple were given no warnings

concerning the stabiliser or the over run brakes when they purchased the brand new caravan and stabiliser.

I know that when the stabiliser they purchased is sold "off the shelf" in a shop there is a "small print" disclaimer from the manufacturer. The latter is that vague that my opinion is that the manufacturer could not be held liable

if my test showed the stabiliser to be absolutely useless. It is therefore the duty of the retailer to explain the limitations of such stabilisers.

Turning to the over run brakes I have never heard of any claims being made

at the point of sale. I feel there is strong evidence to show that there is a need to warn of the limitations of over run brakes at the point of sale and I would wish to examine with great care the evidence submitted concerning the written and spoken words in this transaction.

All the above is hypothetical. What I would do would have to be approved by SEW.

I submitted my stabiliser test to Birmingham Trading Standards about 18 months ago. They took the advice of an Engineer at the appropriate government department and the latter refused to accept my test and did not give a reason. I have a copy of the letter about this sent to Birmingham Trading Standards.

After the Summer recess I shall ask my MP to look into this. If Trading Standards accepted my test it would save me a great deal of further effort.

 

[peter_5297]

At all realistic speeds, any lift created by air flow will never be large enough to reduce the caravan's axle load to zero. However, it is conceivable that the noseweight may actually reverse, i.e. pull the back end of the towcar up, if the aerodynamic drag is large enough.

From Peter W Jones

This may not be the right place for the following, but I will put it in anyway.

I sent yesterday the following, to T&T.com

From Peter W Jones

Sorry. I should have said snaking accidents CAN be the cause of a tyre failure.

When I read these posts I really thought that some of my material had been selectively edited (as is allowed by the site rules) and that

I had been cut off from access to the site. I immediately added my replies to the beginning of my blog

www.caravanaccidents.wordpress.com

If, when I next visit T&T I am still allowed access, I shall add to my blog a suitable apology for what I have said about T&T and Prac Caravan Mag.

We are tomorrow returning to our caravan, but I shall endeavour to answer the rest of the points now or subsequently raised by accessing via wireless connection or a computer in a public library.

I will try to make a contribution at least once per week.

I left the ITAI because I felt that they could not help me further and I could not help them.

I joined instead the Society of Expert Witnesses. I was admitted as an Associate Member on the basis of my 1973 AMInstP and my 1952 Teaching Certificate (which included a Distinction in Advanced Science) after a course at what is now Chester University. Account was also taken of the work I have done on Trailer Instability.

I have set myself up as a "Consultant for Towed Vehicle Accidents" and hope to have a few cases to set up a legal precedent so that someone much younger can easily carry on when I give up. I am doing this in the interests of Caravan/trailer safety. Any money I earn in excess of expenses I would donate to a charity of my choice. As I am "non profit making" I hope my advertisement is in order on this site.

My induction consisted of reading the bulky file of legal advice supplied by SEW. I was also provided with a Mentor, who was a well qualified Engineer

who had had an extensive career as a an Expert Witness for RTA's. He gave me much advice and thought I was well qualified for the narrow area I had chosen. I circulated suitable RTA solicitors all over the midlands but the only one to phone me asked me to assess the speed of a car before the accident. He had clearly not even read my Job Title.

At the end of last year I therefore told SEW that I would not renew my Associate Membership ( as the fee is not an insignificant amount).

I was surprised to have a reply to the effect that if I wished to do any expert witness work in the future I would immediately be readmitted.

Now before anyone gets any more exaggerated ideas about flying caravans my situation is as follows:-

If someone runs out of luck or makes a mistake and gets killed or seriously injured in a caravan or trailer snaking accident, I am available to be consulted by their Solicitors or Traffic Accident Investigator.

Any disagreements between experts are usually settled out of court, but if they could not be I would have to appear in court. I would also have to appear if asked to by a judge.

I would be sent the file of evidence. At this stage SEW would again assist

by arranging for a mentor to check what I proposed to say or write and advise accordingly.

My next case would be treated in the same way and if I showed myself to be competent I would then proceed without assistance unless I requested same.

A typical case that I would accept is outlined on

www.caravanaccidents2.wordpress.com Para 3a

In this case it was absolutely clear that the couple were given no warnings

concerning the stabiliser or the over run brakes when they purchased the brand new caravan and stabiliser.

I know that when the stabiliser they purchased is sold "off the shelf" in a shop there is a "small print" disclaimer from the manufacturer. The latter is that vague that my opinion is that the manufacturer could not be held liable

if my test showed the stabiliser to be absolutely useless. It is therefore the duty of the retailer to explain the limitations of such stabilisers.

Turning to the over run brakes I have never heard of any claims being made

at the point of sale. I feel there is strong evidence to show that there is a need to warn of the limitations of over run brakes at the point of sale and I would wish to examine with great care the evidence submitted concerning the written and spoken words in this transaction.

All the above is hypothetical. What I would do would have to be approved by SEW.

I submitted my stabiliser test to Birmingham Trading Standards about 18 months ago. They took the advice of an Engineer at the appropriate government department and the latter refused to accept my test and did not give a reason. I have a copy of the letter about this sent to Birmingham Trading Standards.

After the Summer recess I shall ask my MP to look into this. If Trading Standards accepted my test it would save me a great deal of further effort.

End of post sent to www.touringandtenting.com

I had a similar problem with this site at about the same time.

As I said above I will apologise and alter my blog comments if I am still connected to this site when I next access in about 10 days. The problem, of course, may have been caused by the fact that I am new to blogging and writing on the net

 

[peter_5297]

At all realistic speeds, any lift created by air flow will never be large enough to reduce the caravan's axle load to zero. However, it is conceivable that the noseweight may actually reverse, i.e. pull the back end of the towcar up, if the aerodynamic drag is large enough.

From Peter W Jones

With ref to comments from Lutz I think people who oppose my conclusions need to explain why snaking caravans leave no tyre marks on the road when they are virtually moving almost sideways.

See my blog

www.20six.co.uk/roadtrafficaccidents