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85% debate continuation

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Parksy

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Nov 12, 2009
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Don't take it to heart Prof, to be honest I didn't get the joke either.
I genuinely thought that Camel's caravan was unstable from what he'd written, so take no notice and scroll on past the things that are of no interest to you. 😉
 
Jan 31, 2018
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Totally agree Prof but if you read what he put-most odd. No reply to mine either. Not helpful and even could cause upset. Odd!
 
Mar 14, 2005
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Don't take it to heart Prof, to be honest I didn't get the joke either.
I genuinely thought that Camel's caravan was unstable from what he'd written, so take no notice and scroll on past the things that are of no interest to you. 😉
Thank you Parksy
Taken at face value and in the context of the thread, I still don't believe they were intended to be humourous.
 
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Sep 5, 2016
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Prof, I'm sorry about all this if its causing confusion, but I might of used the wrong term 'unstable', my point was if I leave with my outfit with a full tank of fuel in my case in weighs 94.92 kgs, and I know what my towing % is surely by the time I get to my destination with a empty tank my towing % can't be the same because the car is lighter , and I'm certainly not avoiding answering this post,
Camel
 
Mar 14, 2005
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You are correct, the car will lose weight as you drive, and in real terms the ratio between the weights of the car and trailer will change, but that is not how the industries conventional towing ratio is calculated, it does not use measured weights, it uses the trailers MTPLM divided by the cars Kerbweight. Both of these figures are paper figures, and in theory represent the worst possible scenario for your outfit.

In practice and provided the caravan is not overloaded, the outfits real world weight ratio will always be better than the industry calculation even when the fuel tank empties, because cars are rarely driven in their kerbweight condition when towing they invariably have other stuff or passengers or luggage that ensures the car will be heavier than its paper kerbweight.

This amply demonstrates one of the inherent weaknesses of the industries calculation, it does not take into account real world variables.

Even though the car loses weight, the overall balance of the car would not be affected very much, at a difference of about 60kg, it will be virtually the same as the effect of losing 1 adult passenger from the back seat. You would have to be an exceptional driver to be able to notice such a weight change when towing.

If as in the scenario you did provide, where you started the journey and all seemed well, but you found the outfit less stable at the end of your journey when the fuel load had diminished, no well matched outfit should be that sensitive, and it points to a poor match to begin with.

I'd suggest it would be more likely driver fatigue or present speed, other road conditions than the loss of fuel weight.
 
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Jan 31, 2018
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Sorry folks-got it totally wrong then. I was with you Prof in thinking there couldn't be that difference so it must be tongue in cheek. Wrong, my mistake and sorry Camel too!
 
May 7, 2012
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I think that the loss of weight from the fuel tank is never going to be noticeable in most cases simply because it is a steady drain and not an immediate loss. If there is any loss in stability it would happen gradually and you would compensate for it as it happened. If you lost 60 kg suddenly this would possibly be more noticeable, although it has never happened to me so I cannot say for sure.

I appreciate the point about the loading in the car and caravan being variable and that 85% figure is simplistic but have still to see a better suggestion. Trying to use the loading would simply be far too complicated although personally I think 90% is probably more realistic these days.
When people want advice then I cannot see you can sensibly not mention the 85% recommendation, and it is then up to them as to how they proceed from there.
 

JTQ

May 7, 2005
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I appreciate the point about the loading in the car and caravan being variable and that 85% figure is simplistic but have still to see a better suggestion.
I have, consider the vehicle's characteristics that for better or worse affect its towing suitability, primarily re stability, including but far from exclusively its kerb weight.

Most are very easy to determine, both from an eyeball check and decent sales brochure. That, quite limited skill should be well within the capabilities of honest caravan retailers.
 

JTQ

May 7, 2005
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Shouldn't be too tricky should it-trailer assist /stability control etc too.
I would much prefer the basic characteristics for towing stability are right in the first place, leaving nothing for the electronic gizmos to try and resolve.
All I ask of them is their support should an event outside normal overwhelms the towed unit's own inherent stability.
All towed units can go unstable, if circumstances push it outside of its safe operation envelope. That is where ATC etc comes into play.
 
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Mar 14, 2005
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Yes! The present advice is too simplistic, because if it is followed for some outfits, it can lead to an illegal outfit. or worse it may actually be unstable becasue it has not put into perspective the other arguably more important characteristics that should be kept in consideration.

I have no preconception that any new model advice would increase the present 85%, it may not have a fixed value, it may be derived from using technical information, and for some outfits it may be significantly less than the present figure.

Part of the difficulty with the present advice is there is no scientifically sound evidence for the methodology used to get to 85%. Giving the job to caravan dealers without a prescriptive method would mean personal opinions would be used, and that as we know will be open to considerable variation. That would be worse than the present industry advice!

There should be a method that can use a number pieces of technical data about the car and caravan that are already published to produce some form of advice that is more reflective of their capabilities and limitations.

As for equipement like ATC, These are reactive safety devices and are there should the worst happen, like safety belts. They should never be considered or used to make an inherently unsatisfactory outfit drivable.

I did discuss the issue with a couple of ex University of Bath automotive Engineering masters graduates, and whilst they recognised the issue, the scope of the project needed to investigate it properly was well beyond our personal finances.
 
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Sep 5, 2016
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Prof,
Have you ever presented your findings on this subject to the caravan bodies out there because regardless of what ever views we personally have on the 85% matching you do have substance in your view on this subject, although you have had a really deep intriguing discussion with you uni chums are they really caravanners
 
Mar 14, 2005
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Hello Camel,

As yet I have no concrete findings, becasue it needs formal research to explore the subject and to produce findings.

However as I have previously pointed out, my views on the subject are known around the industry, and any party that wanted to contact me could do so either directly or through this forum. As yet non have chosen to do so.

As I also pointed out, none of the industry organisations seem to want to take any responsibility or ownership of the present advice, so its difficult to know who to address in the industry.

Its very likely that any new advice would have to show a logical thought process and procedure, and have to show it has considered health and safety issues. This would almost certainly involve the DVSA having some input, and that could easily result in new and tighter regulations about towing vehicles and trailers and driver licencing.

If you think about it, No single industry organisation would want to be held up as the business that causes tighter regulation of the industry or towing.

I would not call the discussion I had with the Graduate engineers as deep, intriguing maybe. They were fairly brief but we quickly established that work needed could not be done on a shoe string, and it would need access to some specialist facilities. They are not chums's per say, they are acquaintances. They were not caravanners, but that is not a relevant issue, as the work would require to be run as a professional project in a scientific manner.
 
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Mar 14, 2005
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lutzschelisch.wix.com
Yes, the research carried out by Bath University in conjunction with Bailey only scratched the surface. An in-depth study would require considerably greater resources, more than what one could expect any single caravan manufacturer to provide. The car industry, due to its much greater size, would probably be more in a position to do so, but for them, the volume of vehicles actually used for towing is probably too small to justify the outlay. I would expect a comprehensive analysis of the subject to cost well in excess of a £100,000 and I can't imagine who would make such funds available.
 
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Feb 13, 2020
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Having read this topic with interest, and while the stats and figures can't and shouldn't be ignored; there are either a hell of a lot of people 'getting it right', or a hell of a lot of lucky caravanners out there. Given the roads are not littered with upturned 'vans, there must be a lot of leeway for people to remain safe when towing. As i do not believe most people get the scales out for all the junk - and quite a lot of it is peripheral stuff, that they happily throw into their car/caravan for the trip.
I have one and a half legs in JezzerB's camp, in that i have a big truck and an even lighter caravan on the back. Hence we cart all manner of stuff with us, as the weight never figures.
But, a lot of people with 'regular' combos are doing the same, with seemingly no ill effects. So are we panicking about things that are not as big as they are made out? Serious question.
 

JTQ

May 7, 2005
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Or, the suggested ratio protects newbies from all but the most awful tow cars with respect to their car's towing stability?

Which rather, misses several opportunities not least of these here in the UK running 365 days a year a more environmentally damaging bigger car simply because we want to tow with it a few times.

A wake up call is on its way with how the vehicle market is being directed to move. A realisation some of our continental friends faced years ago with taxation strictures.
 
Feb 13, 2020
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Which rather, misses several opportunities not least of these here in the UK running 365 days a year a more environmentally damaging bigger car simply because we want to tow with it a few times.
How many would have a smaller vehicle if it wasn't for the need to - safely, tow a caravan? Few of us are towing every week, or even every month, so the cars 'requirements' are only really valid 'part time'. But, you cannot have 'part time' vehicle as such. So what is the logical answer. Is there a logical answer?
For the record, mine also doubles up as a work vehicle, often full of goods. Which in effect keeps the need for a second vehicle (van) off the roads.
 
Nov 11, 2009
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How many would have a smaller vehicle if it wasn't for the need to - safely, tow a caravan? Few of us are towing every week, or even every month, so the cars 'requirements' are only really valid 'part time'. But, you cannot have 'part time' vehicle as such. So what is the logical answer. Is there a logical answer?
For the record, mine also doubles up as a work vehicle, often full of goods. Which in effect keeps the need for a second vehicle (van) off the roads.
Ever since I retired in 2000 my tow vehicle has been very much “ part time” and when not towing is used for longer journeys. There are some periods when I have to remind myself that it requires a run. So environmentally it’s not going to cause Greta T any angst. Even when the day comes and I don’t tow I would still keep the larger car as it’s easier to get in and out of than the smaller Kia Rio. The Subaru also being petrol is less polluting re NOX and PMs although admittedly it’s CO2 isn’t up there with the best.:whistle:
 
Feb 13, 2020
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Not for a minute am i suggesting anyone should start feeling guilty or attempt to justify their actions over what is essentially a hobby which offers a lot of well-being. In fact, i see anything anyone does as regards 'cutting back' on what and how they drive, as three parts pointless. Until the really big polluters of the planet decide to get their collective backsides in gear, to my mind the UK's 'contribution' makes little or no difference in the great scheme of things. OK, it doesn't harm to get the really old and knackered engines off the road, but trying to get everything 'electrified' so we all don't die of fumes, in the coming years is pie in the sky, and just pandering to the likes of 'Miss T'. who is only spouting what others are feeding her.
 

JTQ

May 7, 2005
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How many would have a smaller vehicle if it wasn't for the need to - safely, tow a caravan? Few of us are towing every week, or even every month, so the cars 'requirements' are only really valid 'part time'. But, you cannot have 'part time' vehicle as such. So what is the logical answer. Is there a logical answer?
Aside from those of us that have two vehicles where the towing one has little use outside that role, I suspect most single vehicle users who could have a lighter vehicle, would do so?

The logical answer to me to using a lighter vehicle is to better understand the vehicle's towing attributes and weaknesses and use that knowledge as a modifier for the 85% mass ratio. Thus, as we can see elsewhere vans towed with vehicle 90, 95%, without encountering issues. Plus, knowing we now have the advantage of ATC, though I emphasise not a stabilising tool, but as a last resort device to cull instability should it occur. Something unknown when the 85% mass ratio was first tabled as a guide.

It would be great if academics busied themselves developing the mathematical models to explore towed unit stability or experimenters physically modelled towed units to gain a deep insight.
However, I contend some aspects are self evident in pushing the natural frequency more safely out of our way, as a snake is all about oscillations and how they manifest or decay.
A mass ratio we all know can help, but other self evident features are the overhang and wheelbase, the less of one the more of the other must improve the match. Then, even a casual glance tells you the vehicle is squat or lanky, so near 100% indicates if the CoG is lower helping stability. A glance at the tyres fitted tell if the vehicle's lateral stiffness is designed to be comfortably high or otherwise.

I don't see it challenging to see the attributes a vehicle has and if you are damned to keep down to an 85% mass ration or could with little risk ease up on that.
 
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Mar 14, 2005
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lutzschelisch.wix.com
The logical answer to me to using a lighter vehicle is to better understand the vehicle's towing attributes and weaknesses and use that knowledge as a modifier for the 85% mass ratio. Thus, as we can see elsewhere vans towed with vehicle 90, 95%, without encountering issues.
I'd challenge anyone to tell the difference between the way an outfit handles at 90% compared with 85%. Even 95% would only really be noticeable under adverse conditions. It's only when you get to 100% and beyond that things start getting appreciably more demanding for the driver, always assuming that other variables remain unchanged.
 
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JTQ

May 7, 2005
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I agree entirely Lutz, but the guidance here is so firmly entrenched at 85%, where it need not be thought quite as critical as some like to paint it.
As I have tried to labour, consider the vehicle in question, not only its weight.
 
Mar 14, 2005
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lutzschelisch.wix.com
I agree entirely Lutz, but the guidance here is so firmly entrenched at 85%, where it need not be thought quite as critical as some like to paint it.
As I have tried to labour, consider the vehicle in question, not only its weight.
Absolutely, but it can be difficult to predict with any certainty how one's existing car will handle with a caravan on the back if one has never done it before. It is even more difficult if you're in the market for a new car, because more likely than not you won't have the opportunity to do a test drive with it together with the caravan prior to purchase. You can only go by common sense and a little appreciation of technical matters that will tell you, for instance, that high lateral stiffness of the rear suspension and a short rear overhang are conducive to a stable outfit. But how are you to judge lateral stiffness, for example? The best that you can do is to give the back of the car a sideways shove as it stands in the showroom in the hope that the salesperson doesn't object. Low aspect ratio tyres are bit more obvious, as is the rear overhang, but an element of uncertainty will always remain. Usually it's a matter of pot luck in the hope that the outfit handles OK. At best, you'll be satisfied with your purchase and enjoy the new-found freedom of caravanning. At worst, you may find that a change of car or caravan is the only solution, but even then how can you be sure that another combination will be appreciably better or whether or not you are just expecting too much?
 
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Mar 14, 2005
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Yes, the research carried out by Bath University in conjunction with Bailey only scratched the surface. An in-depth study would require considerably greater resources, more than what one could expect any single caravan manufacturer to provide. The car industry, due to its much greater size, would probably be more in a position to do so, but for them, the volume of vehicles actually used for towing is probably too small to justify the outlay. I would expect a comprehensive analysis of the subject to cost well in excess of a £100,000 and I can't imagine who would make such funds available.
We thought it would cost considerably more than that if it were to be thoroughly carried out.
 

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