85% debate continuation

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Jan 31, 2018
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I agree Parksy , but I know quite a few caravanners who think or act like the 85% law is gospel and am in the process of writing a letter to the caravan clubs and magazines as they always mention and test by the 85% guidance but very little if any mention is made of the manufacturers rated towing capacity-in fact in the practical mag guidance at the back 'is my combo legal' it states piont 3' z is between 85% and 100% of x and this is quite misleading. ' i feel as there is the implication that this is law.
 

JTQ

May 7, 2005
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practical mag guidance at the back 'is my combo legal' it states piont 3' z is between 85% and 100% of x and this is quite misleading. ' i feel as there is the implication that this is law.

So, playing even safer at 80% of x, is illegal then.;)
 
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The thing about any percentage figure, whatever it is, is that it always suggests an absolute limit, regardless of whether it is backed by legislation or not. As several contributors to this thread have pointed out, there is so much more to a stable outfit than weight ratio. I won't say 'a safe outfit' because whether it is safe or not is entirely up to the driver.
Accepting the fact that other factors determine stability quite as much, if not more so than weight ratio, why doesn't one simply recommend keeping the caravan as light as possible relative to the weight of the car and advise the novice to leave a reasonable margin? Then one doesn't get involved in ever-recurring discussions about kerbweight, either.
I started my caravanning career, in retrospect and because I, probably like so many others, had never heard of any recommendation, with a weight ratio of just over 100%, but I can't say that I ever got into a scary situation because of that.
 

Parksy

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Nov 12, 2009
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Hello Parksy

It seems I did not express my self clearly enough.

I was referring to caravanners who have posted questions or points, and there have been several across the years where they have obsessed about achieving an 85% match but then have openly couldn't care about to other legal issues such as speeding, using phones, or driving without seatbelts etc.

I've been a forum moderator on this website for over 10 years, and I've seen occasional queries about the 85% match where the replies from members have clarified the situation by pointing out that the 85% figure is for guidance only with no legal standing.
I haven't noticed posts on this forum 'where they have obsessed about achieving an 85% match but then have openly couldn't care about to other legal issues such as speeding, using phones, or driving without seatbelts etc'
The majority of discussions surrounding the 85% match have as far as I can see been instigated by yourself for the reasons that you have stated.

...One of my objectives in this is to encourage people to stop and think about this weight ratio tradition. If after proper analysis of the tradition is shown to be the best way then fine, but all too often traditions become outdated and its time this one was properly reviewed and evaluated to see if its fit for purpose.
The only problem is that we on this forum have no accurate method with which the ''tradition'' as you call it can be properly evaluated to see if it's fit for purpose.
 
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If you are not over 100%( caravan weighing more than towin g vehicle), just stand a few feet away from the outfit close one eye look along your arm through your thumb and if the car and caravan are level or slightly down and all the
tyres are still nice and round off you go,
 
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Not sure I would agree but the CAMH does say that a slight nose down attitude is better. I have not seen any evidence to back this up, although in theory it would seem to have a slight advantage. With the nose down the caravan should be slightly more aerodynamic and you would not get air under the floor providing some lift and wind resistance that a nose up attitude would.
 
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Any advice regarding a level or slight nose down attitude being preferable for aerodynamic reasons is a myth. Caravans are big and boxy with an enormous frontal area creating an awful lot of drag. Therefore, they do not behave like an aerofoil section, especially at the relatively low speeds at which caravans are towed. Caravans are accompanied by so much drag and turbulence that minor changes in attitude make little or no difference. The only disadvantage of a nose up stance is the reduction in ground clearance at the back of the caravan which may, in the extreme, cause problems when negotiating speed bumps, for example.
A nose up attitude will reduce the noseweight, but this can be compensated by checking it with the caravan standing at the same attitude as when it is hooked up to the car, which is the correct procedure anyway.
 
Mar 14, 2005
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Hello Parksy.

I have occasionally started a thread about tow matching, but more often than not I joined a thread started by others, where the issue of 85% has raised its head. I am guilty of perpetuating the subject, becasue it has been held up as a precise target rather than a guidance value so many times it is a tradition, rather than a thought out process.

I too have been a member of this forum for a long time and I do recall there have been a few posts where the some people have been fretting about getting as close to 85% as possible, but perhaps in other threads have shown disregard for other legal requirements. I do refer back to it becasue it shows how skewed the some peoples view are on the subject.

A tradition may have more extensive definitions but it includes "something that is habitually done or used" and basically it means that the reason for doing it that particular way is not thought about its just done. Just consider; How many traditions (not just caravanning ones) that when investigated have proven to be wrong? Some were so abhorrent they've been made illegal! Traditions should be tested, not necessarily to prove them wrong but sometimes to confirm they are still the best way forward.

It shouldn't bee seen a subversive to challenge something that does not make logical sense. As I have indicated, I have tried to get an indication of why and how the present advice was formed.
The reluctance to comment and lack of positive information from the caravanning organisations is very unfortunate.

Even though this thread cannot possibly produce a new form of towing advice by its self, If nothing else, it's made more people actually think about what the advice is trying to do. It may even prompt members of the industry body that set the advice think its time to look again.
 
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I have seen replies to questions in this and other forums from people seeking advice regarding the suitability of their vehicle as a towcar only mentioning the 85% weight ratio recommendation, without any reference to manufacturers' or legal limits. Whilst such advice may have been given in good faith, I do believe that it is verging on the irresponsible not to provide a complete picture as there are vehicles about where the towload limit is less than 85%. There have even been posts concerning vehicles that aren't approved for towing at all, even though they may be heavy enough and yet, even there, there was only reference to the 85% figure.
That does suggest that too much emphasis is placed on the recommendation.
 
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JTQ

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I think aerodynamics needs a bit of discussion re stability, rather than being dismissed.

Aerodynamics is involved with our units, this is evidenced in the fact our caravans present “drag” which we feel when an oncoming lorry's bow wave slams the front. Also, it is more than evident in our fuel consumption when towing, often modulating as it reflects the headwind we encounter.

As the van's body frontal area is predominantly way above its axle this component of the drag will try to relieve the dynamic vertical loading on the nose. The “shadowing” and turbulence from the towing vehicle, unless that is Transit like, if anything will IMO lead to the centre of pressure being well up vertically, further decreasing the dynamic vertical nose load.

I suggest in part, this drag off setting of the dynamic vertical nose load, accounts for units with the higher static noseweights yielding more stable towing. The dynamic vertical nose load certainly needs IMO to remain positive.

This particular aerodynamic effect would IMO be little changed via setting up nose up or nose down.
I however, see potentially other air loading influences could be related to the van's longitudinal stance.

Whilst a box has little in common with the aerodynamic finesse of an aerofoil, the box's “incidence” to the air with its flat plate surfaces will still be there, both on the top and bottom. It should not be overlooked that whilst induced pressure changes might be small in magnitude they act over a very significant area.

On the top, roof surface, with the nose down the pressure point is IMO going to fall somewhat forward of half the roof's length, so add to the vertical dynamic nose load, offsetting part of the drag's affect.
Underneath, the turbulence from the front lower edge because of the incidence, will reduce the pressure on the immediate forward section of the floor, again adding to the dynamic nose load, and again offsetting the negative affects of the frontal drag.

Tip the van backwards so the floor's incidence creates a force pushing the nose further up, compliments the drag in together reducing the dynamic vertical nose load.
 
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I think aerodynamics needs a bit of discussion re stability, rather than being dismissed.

Aerodynamics is involved with our units, this is evidenced in the fact our caravans present “drag” which we feel when an oncoming lorry's bow wave slams the front. Also, it is more than evident in our fuel consumption when towing, often modulating as it reflects the headwind we encounter.

As the van's body frontal area is predominantly way above its axle this component of the drag will try to relieve the dynamic vertical loading on the nose. The “shadowing” and turbulence from the towing vehicle, unless that is Transit like, if anything will IMO lead to the centre of pressure being well up vertically, further decreasing the dynamic vertical nose load.

I suggest in part, this drag off setting of the dynamic vertical nose load, accounts for units with the higher static noseweights yielding more stable towing. The dynamic vertical nose load certainly needs IMO to remain positive.

This particular aerodynamic effect would IMO be little changed via setting up nose up or nose down.
I however, see potentially other air loading influences could be related to the van's longitudinal stance.

Whilst a box has little in common with the aerodynamic finesse of an aerofoil, the box's “incidence” to the air with its flat plate surfaces will still be there, both on the top and bottom. It should not be overlooked that whilst induced pressure changes might be small in magnitude they act over a very significant area.

On the top, roof surface, with the nose down the pressure point is IMO going to fall somewhat forward of half the roof's length, so add to the vertical dynamic nose load, offsetting part of the drag's affect.
Underneath, the turbulence from the front lower edge because of the incidence, will reduce the pressure on the immediate forward section of the floor, again adding to the dynamic nose load, and again offsetting the negative affects of the frontal drag.

Tip the van backwards so the floor's incidence creates a force pushing the nose further up, compliments the drag in together reducing the dynamic vertical nose load.

I agree with your comments regarding the need to have a substantial static noseweight in order to offset reduction due to dynamic aerodynamic drag acting on the large exposed frontal area predominantly above the roof level of the towing vehicle, but this is much higher than any effect that minor changes in attitude of the caravan will have
 
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I agree that aerodynamics do play a part in the stability issue, But its not directly related to the weight ratio which was the focus of this thread. But it is another example of a criteria that the present advice does not take into account.

I have no doubt that the aerodynamics due to the attitude of a caravan will have some effect, but the question is how much. The range for the angle of attack will be dictated by the coupled height of the trailer. EU Diretive 94/20/EC sets out the range of vertical coupled hitch heights as between "350 and 420mm", and it sets out the permitted range of trailer hitch heights for when the trailer is actually level as "(430+/-35)mm".

By looking at the extremes of these values and comparing the worst case scenario where the caravans level hitch is at its lowest limit of 395 (430-35) coupled to a car with its hitch at its highest limit 420, the nose of the caravan would be lifted to just 25mm above its level height.

The angular displacement this would create for the caravan would be determined by the horizontal distance between the hitch and the axle. The shorter the hitch to axle distance the greater the nose up attitude would be.

Even the shortest practical caravan has a hitch to axle distance of at least 2800mm
and based on those figures the greatest nose up angel that could be created will be less than 0.82 degrees!

The chances that an outfit would actually create the above scenario are very small. But even if it did I seriously doubt the effect it would have would have on stability would be more than overpowered by other more significant forces.

The frontal area of the caravan has been mentioned, and it has been suggested the effect of drag created by teh frontal area would tend to counter the applied nose load. I can vouch for that being the case, as I once collected an empty twin axle box trailer, which was fine at low speeds but as speed was increased I could detect the nose load changing to a lifting effect. I stopped and added about 15Kg of ballast to the front load area of the trailer, and all was well for the rest of the journey

However The box trailer was a true slab sided box, and the frontal area drag was definitely noticeable by the increased difficulty the tow vehicle had in getting up to normal speed. Having used the same tow vehicle for large caravans which weighed more than the empty box trailer the engine did not seem to have to work so hard to get up to speed.

It think it is reasonable to assume most of the difference would be down to shaped front that caravans have. This has two effects, firstly it allows the frontal air to be more easily directed around the sides and top of the trailer reducing drag, and secondly the angle of the front panel is often raked more than you might imagine and consequently the frontal air that is directed over the top of the caravan has less opportunity to push against the front panel opposing its motion, but the in the act of deflecting it the resultant force will largely if not totally offset the tendency tor the drag to reduce dynamic nose load.

So I'm not saying the drag as zero effect on nose load, just that it is likely to be much less than has been suggested.
 
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Personally I think drag and aero effects could have a far bigger effect on the stability of the van than we think-only conjecture from looking at how aero is used in the motorsport world. Do manufacturers wind tunnel test caravans=maybe we'll see front splitters and side skirts and wings adorn caravans in future!!!
 
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As aerodynamic effects increase with the square of the speed and caravans are towed at very modest speeds, certainly compared with the motorsport world, there is little point in making any specific provisions in this respect.
 
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I was being a bit facetious but do wonder about the stability of the thing -at 60mph there is a fair bit of drag?

Oh yes, there is a fair bit of drag, even at 60mph. I can even imagine that the drag can be so high, especially under headwind conditions, that the noseweight may dynamically turn into a negative value. However, given that there is little scope for reducing the overall width and height of the average caravan about the only thing that is likely to provide much improvement is to increase the rake of the front end, but there is obviously a limit to what can reasonably be done in that respect without encroaching on the headroom inside. Minor aerodynamic treatment, such as deflectors, etc. are not going to have an appreciable effect on something as big and boxy as a caravan being towed at no more than 60mph.
 

JTQ

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Minor aerodynamic treatment, such as deflectors, etc. are not going to have an appreciable effect on something as big and boxy as a caravan being towed at no more than 60mph.

[I accept I am going off the pro's base topic, but feel it adds some value.]

I might have shared that view up to changing from my previous Hymer to a new one in 2008, but my findings on fuel consumption gave cause to realise the detailing could well make some difference.

The earlier a 2002 Nova, featured abruptly squared off corners everywhere between front, side and roof panels. The 2008 featured rolled 100 mm radius corners, replacing the squared of the earlier, plus moving the awning rail location.
Both vans were similar in size, though the 2008 was longer, and had an appreciably % increased length "A" frame. Both had profiled fronts below the waist one folded, one moulded/
However, the 2008 was at 1900kgs a whole 150 kgs heavier than the 2002.

They were both towed by the same D3 over some thousands of miles, where religiously I monitored fuel consumption as accurately as practical. Here I used the on board computer as rarely was a tank brimming period exclusively used to tow.

The fuel consumption towing the heavier 2008 was measurably better than the 2002. Averaging about 0.75 to 1 mpg.
I accept the D3s mileage was increasing,so a minor difference could be credited to frictional changes over time, but the towing consumption figures were a step change with van change.

Aerodynamic detailing, most probably the cause.
 
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Aero detailing certainly can make some very significant differences to vehicle efficiencies. The manufacturers of large commercial van and lorry bodies have done research in the past and during the 1990's a lot of new van body details were rolled out. We saw body corners being radiused, artic trailers having hump backed roofs and other small added panels to cabs and fairings to fill the gaps between tractor units and trailers all of which all were used to help reduce air drag and raise fuel efficiencies.

I recall some caravan manufactures (I think Bailey were one) have claimed to have used wind tunnels to help with drag reduction. Moulded front panels have allowed manufacturers to incorporate drag reduction features, and if you were to compare the designs of today's caravans with products from those of only 30 years ago you will see that both the rake of the front windows and the transition from front to roof are much smoother now than before.

You may also see efficiency being aided by aero features moulded into rear panels that help to reduce turbulence as the air flow leaves the caravan roof and sides.

Unlike HGV's where the designs of tractor cabs are box like and not vastly differnt in over all size, In terms of aerodynamics, the design of tow vehicles for caravans has much greater variability.
This means that whilst HGV trailers can have aero devices incorporated with a reasonable degree of universal effect, the same cannot be assumed for caravans due to the the big differences in the way that each tow vehicle disturbs the air that will impinge on the front of a caravan.
 
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Even car makers go to serious lengths to prepare the cars for testing in the now superseded emissions/ mpg tests. Small gaps say between doors would be sealed etc And the temporary mods came off afterwards.
 
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I appreciate that improvements to aerodynamics of caravans are possible, but on the whole they are limited and one would never be able to approach anything like the drag coefficient of a modern car. Despite aerodynamic measures, they will never be sufficient to prevent reduction noseweight as speed increases, and that, after all, is what this thread is all about.

Even car makers go to serious lengths to prepare the cars for testing in the now superseded emissions/ mpg tests. Small gaps say between doors would be sealed etc And the temporary mods came off afterwards.

Emissions tests are still carried out on a rolling road, because that is the only method whereby truly reproducible standard conditions can be maintained. Only the methodology and procedures have been improved to more closely represent actual road conditions.
 
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You are right Prof, Bailey advertised their caravans are windtunnel tested most definitely in the 2016/17 era when we had our Pegasus and maybe still do. Even though it was heavier it felt notably easier ie less strain on the engine than our lighter Avondale Argente, though I think the Argente was a bit wider which could explain it.
Articulated HGv's started to have a domed ie upward curved roof and advertised on the sides it was saving fuel!
 
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My last truck that I drove was a new Merc Actros, the cab roof had a moulded wind deflector the also had side cheeks and side skirts most of our trailers had aero dynamic moulded roof with side skirts down. the trailer sides the side cheeks on the trailers are not just for wind deflection but stop other road users from going under the trailer,
 
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.......is your collective advice to forget the 85% recommendation and have your potential caravan checked for it's drag coefficiency?

My advice is to look at this excellent article by PC magazine.

ww.practicalcaravan.com/advice/50460-towing-101-part-6-how-to-deal-with-a-snaking-caravan

Puts the 85% recommendation into context with all the other precautions that should be taken.
The editorial does put things into perspective, however whilst it does reaffirm the existence of the 85% advice, it also clearly makes the point its not just the 85% that makes the difference. Its the second part about how to control the initial stages of a snake. When instability occurs what's the thing that brings it back under control? reduction of speed.

As far as I can see no one has suggested reduction of drag is the answer to a poor match.
 
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