85% debate continuation

[Parksy]

Dave Motton has continued in his role as towcar editor of Practical Caravan Magazine but what may be so widely known is that he was once the magazine editor before Nigel Donnelly took over.

 

[camel]

Personally with the contacts this forum has with practical caravan mag I think the Prof plus with the support of the forum could approach the magazine with the thoughts of the forum on the 85% issue plus the caravan matching concerns to see if they could approach who ever is responsible. for this figure to see could someone say eureka it is now. 95%, just a thought,

 

[ProfJohnL]

Hello again Camel

David Mottons views were published back in 2015, and even his high profile in the media didn't start a revolution.

As I have stated previously, I have used my industry contacts to try and find who has responsibility for the present advice. No one, I repeat no one admits to knowing for sure who either as an individual or as an organisation controls the advice.

 

[Beehpee]

If someone were to bring about a change in recommended weights ratio, no matter who it was who brought about the change it would still only be exactly that, a recommendation and not law, it would then follow that people would then quote the new figure as advice/rule/law or anything else they chose to call it and the argument would then continue for the next decade, knowing who brought about the change would not mean everyone would adhere to it or agree with it, it would probably make little difference if by some miracle it was to become a law, people disregard all sorts of laws every minute of every day, laws are only as good as how well they are enforced, maybe Boris might allocate enough extra money so each police force could have a dedicated team to check caravan weights and ratio to the tow car, he might but that’s probably as likely as this subject ever being agreed on, personally I’m quite happy with the well known RULE or is it LAW of having to have no more than 85% weight ratio.

BP

 

[Lutz]

Why get so worked up about the 85%? I think that it's accepted that it has no scientific backing and therefore a purely arbitrary figure that can be ignored if one so wishes. On the other hand, one doesn't need to be very technically qualified to appreciate that as light as possible caravan and as heavy as possible towcar is obviously going to be beneficial and just leave it at that. What about all those people who have never heard of an 85% recommendation in the first place because they only made themselves aware of the legal requirements, and I am sure that there are quite a lot?

 

[JezzerB]

Don't know why it winds me up-but it does; there do seem to be people who get very hung up about it and won't tow at 86% or more ever , at all-might be just who I have spoken to in the past. What irks me, and I know it's advice-is the word 'rule' in the advice- ie back of practical mag-'what is the 85% rule?'

 

[Parksy]

The term 85% Rule is a colloquialism, the recommendation is called a Rule because its a rule of thumb, a rough and ready guideline.
I have continued to reinforce the 85% figure on this thread because it's existing advice.
I'm not bothered about the name of the person who first advised the 85% figure, it's given to this day by every reputable source that a novice would be likely to follow.
If the advice, rule, recommendation or whatever is still considered by both main clubs to be good enough for their members then it's good enough for me to reinforce on this forum.
In the real world I'd say that very few caravanners adhere strictly to the 85% ratio.
From what I see every time that we are on sites, there are not all that many caravanners who stay within their legal weight limits with the amounts of gear that they carry, never mind following the 85% advice 🙄

 

[JTQ]

In the real world I'd say that very few caravanners adhere strictly to the 85% ratio.
From what I see every time that we are on sites, there are not all that many caravanners who stay within their legal weight limits with the amounts of gear that they carry, never mind following the 85% advice 🙄

No argument on that point, and I would add have no idea of their vehicle's actual kerb weight, a brochure minimum spec value at best, and no idea at all of their van's actual loaded weight.

 

[otherclive]

The term 85% Rule is a colloquialism, the recommendation is called a Rule because its a rule of thumb, a rough and ready guideline.
I have continued to reinforce the 85% figure on this thread because it's existing advice.
I'm not bothered about the name of the person who first advised the 85% figure, it's given to this day by every reputable source that a novice would be likely to follow.
If the advice, rule, recommendation or whatever is still considered by both main clubs to be good enough for their members then it's good enough for me to reinforce on this forum.
In the real world I'd say that very few caravanners adhere strictly to the 85% ratio.
From what I see every time that we are on sites, there are not all that many caravanners who stay within their legal weight limits with the amounts of gear that they carry, never mind following the 85% advice 🙄

Such a relief to have someone who says it as it is. Thank you Parksy.

 

[JezzerB]

I fully understand that and would discuss it with a newbie just like you-however the 'rule' word to some people seems to be that's the law-misunderstanding I know but we Brits try to obey rules! i reckon rules are meant to be broken!

 

[Damian]

My one and only post on the subject having skimmed through most, if not all, and reached the conclusion that there is a modicum of OCD at play by the Prof over the mechanics of the 85% thing.

As for rules, the following is sensible I think.

Quote " Rules are for the guidance of wise men and the obedience of fools. Brickhill 1954, p. 44. Note: (also quoted as "...for the obedience of fools and the guidance of wise men.")

 

[JezzerB]

Very true!

 

[Gafferbill]

OCD or not!

.......without doubt the best post in this thread is the first one by PJL especially with his recent edit of February 22nd.

 

[Raywood]

I do not think anyone controls the advice. It was produced I understand after the CC sponsored some research at Bath University and seems to have then been adopted generally by the caravan comunity. Newcomers do need some advice on the subject and unless someone with authority comes up with a better recommendation then It has to stand.

 

[Lutz]

The 85% weight ratio recommendation originated well before the first research paper published by Bath University concerning the dynamics of towed vehicles back in May 2003. Besides, even that paper didn't make any mention of a weight ratio limit, but concentrated more on investigations into the self-damping properties of outfits under various conditions.

 

[ProfJohnL]

Several posts references have suggested the University of Bath have studied caravan stability and suggesting they have done specific work on behalf of the CC to establish or verify the present industry advice. As far as I can establish this is not strictly true. I have not found any reference to the CC commissioning any research at UoB.

2003
A study of caravan unsteady aerodynamics, J Darling, P M Standen.,

and in 2009
An experimental investigation of car—trailer high-speed stability, J Darling, D Tilley, B Gao

The Abstract posted on the UoB's research portal includes this important statement;
"It is interesting to see that the trailer mass alone does not dramatically affect the high-speed stability, as this runs contrary to current guidelines relating to limits on the relative mass of the car and trailer."

Dr. Jos Darling was the senior academic in both these studies, but the studies were graduate final projects.

I have found other references to J. Darling which predate these two projects, and it seems these were not conducted at UoB. He appears in the following list.

I have discovered there are several studies that look have at various aspects of outfit stability. These are from various respected academic or engineering portals.

References

1980 - ‘Handling characteristics simulations of car—trailer systems.’ Anderson, R. J., Kurtz, E. F.

1983 - ‘Some factors influencing the stability of car/caravan combinations.’
Bevan, B. G., Smith, N. P., Ashley, C. In Proceedings of the IMechE Conference on Road vehicle handling, IMechE Conference Publication C123, Nuneaton, Warwickshire, UK, 24–26 May 1983,

1993 - 'Method of test for lateral stability of passenger car/trailer combinations' (London British Standards Institution 1993)

1996 - ‘Simulation of coupled car and caravan handling behaviour.’
Fratila, D., Darling, J. Veh. System Dynamics

2002 - ‘Car—caravan snaking. Part 1: The influence of pintle pin friction.’
Sharp, R. S., Fernandez, M. A. A. Proc. Instn Mech. Engrs, Part C: J. Mechanical Engineering Science

2003 - ‘A study of caravan unsteady aerodynamics.’
Darling, J., Standen, P. M. Proc. Instn Mech. Engrs, Part D: J. Automobile Engineering

2003 - ‘Parametric study on vehicle—trailer dynamics for stability control.’ Deng, W., Kang, X.

2004 - ‘Trailer stabilization through active braking of the towing vehicle.’
Williams, J. M., Mohn, F.-W.

Most of us have seen the video of the model car and trailer with adjustable loads. There is an almost identical demonstration from America. I don't know which was first.

 

[Lutz]

Most of us have seen the video of the model car and trailer with adjustable loads. There is an almost identical demonstration from America. I don't know which was first.

One should note, however, that the adjustable loads on the model trailer were extremely heavy relative to the unladen weight of the model. It would be roughly the equivalent of moving the entire payload of the caravan fore and aft from one far end to the other, so the video only serves to illustrate an extreme situation which would hardly be encountered to that extent in reality.

 

[JTQ]

The Abstract posted on the UoB's research portal includes this important statement;
"It is interesting to see that the trailer mass alone does not dramatically affect the high-speed stability, as this runs contrary to current guidelines relating to limits on the relative mass of the car and trailer."

That is really going to upset some.
It also proves, if it is sound research, that the original guidance our clubs etc promote, never was soundly based.

Science being what science is, that original finding would be as repeatable today as when originally determined, if it ever was scientifically determined before being so well promoted.

 

[JezzerB]

Which is exactly my findings-towing very happily and stably(is that a word) at 113% of kerbweight. Dacia Duster 1.6 petrol, with a Pegasus Verona.
And also why I took no notice of the 'rear is light' brigade when first towing our Bucanneer with the Nav-it most definitely isn't tail twitchy-worryingly i don't know that 200kg of 8ft wide caravan is on the back=thank heavens for a rear view mirror that shows you can't see any cars behind!

 

[ProfJohnL]

One should note, however, that the adjustable loads on the model trailer were extremely heavy relative to the unladen weight of the model. It would be roughly the equivalent of moving the entire payload of the caravan fore and aft from one far end to the other, so the video only serves to illustrate an extreme situation which would hardly be encountered to that extent in reality.

I made that exact same point in a previous thread, but if it serves to make caravanners think more carefully about loading, its no bad thing.

 

[Raywood]

I think the point about weight at the back is more that if you do get the back end moving it can then start to provide a pendulum effect and be harder to control. It is one of those things you have to watch in trying to get the balance correct and make sure weight at the back is not excessive. Weight in the centre is bound to be be safer than at the rear, but like the towing ratio it is only part of the equation.

 

[Dustydog]

The models used at the University of Bath are important but as the Prof pointed out there are so many variables that not one idea stands alone as the final solution.
Come on guys , I go back towing 40 years and there has been nothing at all that says I have got it wrong. In simple non technical terms in 1978 I was advised, a nose weight I could just about physically pick up myself, the caravan should be lighter than the car, no ratio given, and the longer the caravan axle from the tow bar the better . En route magazine at that time suggested 85% ratio but it was never substantiated.
Today the only real variable is the available speed of the modern tug. 40 years ago 50 mph was thrashing the beast.

 

[JezzerB]

Car design has come on a fair bit in every way too though, stability control on both car and caravan,suspension and abs etc etc.

 

[Raywood]

I do agree that design and stability have moved on since the 85% figure was introduced. Personally I would accept 90% as a starting point, as caution is the best approach for newcomers. I do still say that the clubs and magazines say 85% even though it is probably outdated and leave them to decide where to go, after all any figure is an opinion.

 

[Raywood]

The towing limit is more a statement of the drive line strength rather than anything else. If the towing limit looks very high for caravans it would still work if you were towing a broken down vehicle across town. The problem with the towing limit is that it applies in lots of circumstances where it may not be a problem if the towing vehicle is lighter than whatever is towed.