85% debate continuation

[Parksy]

This is a wide ranging discussion with many interesting points being raised.
It's quite clear that the much vaunted 85% advice is no guarantee of a safe and stable towing experience, but for a novice about to choose their first caravan the figure is not a bad starting point as long as other factors which can affect stability are also taken into consideration.
Correct loading is of paramount importance, I once met someone who had bought their first caravan, a Lunar if I remember correctly.
They hadn't towed before and after the dealers hand over they hitched the Lunar to their Discovery and began a relatively short journey home with part of the journey along the northbound M5 between Jn 6 and Jn 2.
They didn't make it!
I spoke to the gentleman concerned a few weeks after the incident in which the caravan was written off and the Discovery was damaged and I asked about loading, had he put his awning right at the rear of the caravan for example?
No, the awning was actually in the car, the caravan was quite empty and the only additional load was the leisure battery which was secured inside the battery box.
The gent hadn't been speeding but he told me that when he noticed that the caravan had begun to move from side to side he'd increased his speed because some idiot tv motoring pundit (possibly Clarkson) had said that this was the best way to get out of a snaking situation.
Of course, we all know that this was a bad mistake, he should have taken his feet off everything to let the speed bleed off and he just might have kept his caravan shiny side up if he was lucky.
I don't know for sure but I think that a low noseweight and ruts in the carriageway caused by 44 tonne trucks had started the instability and he made matters much worse by accelerating.
My towcar / caravan ratio is about 90% with the awning over the 2 axles toward the front axle and I get the noseweight as close as possible to the limit of 100kg.
I normally get a safe and comfortable tow with this set up, although with no on board stability or traction aids I use seat of the pants experience to detect any sign of road ruts or crosswinds early on to adjust my driving accordingly.
The 85% advice when taken by itself is worthless i.m.o .
Towing safety involves many factors, not least correct tyre pressures and a well maintained outfit, with correct loading and defensive driving techniques all having an important part to play.

 

[Lutz]

You're absolutely right, Parksy. Unfortunately, although it would actually be necessary to do so, little attempt is made to differentiate when making a weight ratio recommendation. Perhaps this is because the recommendation, as such, is aimed primarily as an aid to the novice and the novice may be more than confused if one were more specific to suit each and every variable. For that reason, the sources that make the recommendation try to keep things simple. On the other hand, using a little bit of common sense should tell anyone that if the manufacturer's towing limit as, say, over 120% of the kerbweight of the car, then exceeding 85% by a not unreasonable amount should not be an absolute 'no-no'. One just has to bear in mind that the manufacturer's towing limit is not usually specific to the type of trailer and big boxy caravans are much more critical than other types of trailer. Appropriate allowances must therefore be made accordingly.

 

[ProfJohnL]

Hello Dusty,

In fairness to all the previous posters on this thread the Dr Jos Darling at the University of Bath has carried out extensive research into “ trailer” caravan stability.

Dr. Jos Darling is a Senior Lecturer in the Department of Mechanical Engineering · Centre for Power Transmission and Motion Control at the University of Bath. If you Google his name there are several references and pieces about him.

The department has been involved with several projects involvoing Bailey caravans. However as far as I have been able to establish, there has only been one study into caravan stability, and that was a graduate paper which has been widely referenced in caravanning circles.

If you read the paper, you will see it was related to one car and one caravan, and as such its detailed findings cannot be applied to all outfits. But it's general precepts are valuable and confirm that load distribution and speed are significant variables that affect towing stability. The project was never designed to investigate the validity or otherwise of the industries weight ratio advice.

 

[Dustydog]

The project was never designed to investigate the validity or otherwise of the industries weight ratio advice.
[/QUOTE]

Thanks Prof

For those who haven’t read the thesis the above is correct. It has always been about how a caravan tows. Weight ratios is one of many aspects but not a prime mover. In fact this thread has probably generated more technical debate.

 

[Raywood]

Twin axles are generally regarded as more stable, presumably because the second axle produces more resistance to sideways movements which helps a lot. The second axle also makes them heavier so they should be more resistant to side winds although the usually larger total area of the side wall could counter this to some extent.
I do wonder having seen a twin axle almost jacknife in front of me and recover, if a single axle model of the same size might have had a different outcome. I will never know and have no wish to find out though.

 

[Lutz]

Twin axles are generally regarded as more stable, presumably because the second axle produces more resistance to sideways movements which helps a lot. The second axle also makes them heavier so they should be more resistant to side winds although the usually larger total area of the side wall could counter this to some extent.
I do wonder having seen a twin axle almost jacknife in front of me and recover, if a single axle model of the same size might have had a different outcome. I will never know and have no wish to find out though.

The second axle of a twin only increases the unladen weight, not necessarily the laden weight. Nowadays 2000kg single axle caravans aren't unusual and that's more than many twins.

 

[Gafferbill]

..........I would be willing to bet (based on observation) that most caravan out fit owners (including those that deem themselves to be experienced) have no idea what their towing ratio is likely to be with any accuracy.
They simply don't know what their caravan weighs and most appear to be overloaded when you see what is carried.
I would argue that the weight ratio of the caravan to the tow car is the most important aspect in achieving safe towing closely followed by keeping speed down and appropriate to the driving conditions.
I would agree that correct loading and adequate maintenance are also important.
For me caravanning is a leisure activity and I want my tow car to have a kerbweight at least 15% heavier than my caravan actually weighs.
I also want as much weight as I need to carry over the rear axle of my tow car and the caravan noseweight as close to the maximum allowed as possible.
I am more than happy to heed the advice given by the various clubs and towing organisations.

 

[otherclive]

..........I would be willing to bet (based on observation) that most caravan out fit owners (including those that deem themselves to be experienced) have no idea what their towing ratio is likely to be with any accuracy.
They simply don't know what their caravan weighs and most appear to be overloaded when you see what is carried.
I would argue that the weight ratio of the caravan to the tow car is the most important aspect in achieving safe towing closely followed by keeping speed down and appropriate to the driving conditions.
I would agree that correct loading and adequate maintenance are also important.
For me caravanning is a leisure activity and I want my tow car to have a kerbweight at least 15% heavier than my caravan actually weighs.
I also want as much weight as I need to carry over the rear axle of my tow car and the caravan noseweight as close to the maximum allowed as possible.
I am more than happy to heed the advice given by the various clubs and towing organisations.

Why not have the heavy weights in the car as near the middle as is practicable. That improves the car's balance, unless your name is Porsche or does your car have a large overhang?

 

[Gafferbill]

Clive.......... most cars have a higher front axle loading than the rear in their natural state. This is not ideal when towing as it is the rear axle of the towing vehicle that gets pushed out of alignment when outfit instability occurs.
Adding any weight (rear seat passengers, fuel in the tank or load) to the rear axle will all help to keep the rear end firmly planted and aligned.
You often see reports of large heavy 4x4's towing large caravans that have come to grief.
The question is asked....."How did that happen ?"..........almost without doubt no weight being carried over the back axle of the 4x4 allowing the weight of the unstable caravan to rear steer the 4x4 which no driver would be able to control.

 

[Lutz]

Clive.......... most cars have a higher front axle loading than the rear in their natural state. This is not ideal when towing as it is the rear axle of the towing vehicle that gets pushed out of alignment when outfit instability occurs.
Adding any weight (rear seat passengers, fuel in the tank or load) to the rear axle will all help to keep the rear end firmly planted and aligned.
You often see reports of large heavy 4x4's towing large caravans that have come to grief.
The question is asked....."How did that happen ?"..........almost without doubt no weight being carried over the back axle of the 4x4 allowing the weight of the unstable caravan to rear steer the 4x4 which no driver would be able to control.

I would suggest that if large 4x4's come to grief it is more because of their higher centre of gravity, inherently laterally more compliant suspension (albeit a benefit in off-road use) and higher aspect ratio tyres. Also, I suspect that to some drivers their very size and weight leads to a false sense of security.

 

[JTQ]

Also, I suspect that to some drivers their very size and weight leads to a false sense of security.

Miss guided by the "guidance" .

 

[JezzerB]

I'm with Lutz and Clive here-if you load up the rear of the car you effectively put more weight over the rear axle-and increase the chance with the caravan load on the towball of car too of bounce at the rear and lifting the front wheels; if you have a rear wheel drive or 4x4 that isn't too important but if you are front wheel drive you might have traction issues-I remember my Grandfather-in the 70's cursing the new fangled front wheel drive Renault 12 with soft suspension-fully loaded ie 5 adults-2 front 3 back and a boot full of food, drink and luggage for a cottage holiday, as it struggled to get a grip and pull away on hills!

 

[Lutz]

I’ll never forget taking the caravan towed by a front wheel drive car to Iceland at a time when the main road around the island wasn’t fully surfaced. When I couldn’t make it up the top of a hill because the front wheels of the car lost grip on the loose stones I had to unhitch, turn car and caravan around on the narrow road, rehitch and return to the bottom of the hill and take a run at it. On one occasion I had to accelerate to 70mph in order to make it to the top.

I understand that the ring road around Iceland is now fully surfaced all the way

 

[JTQ]

When I couldn’t make it up the top of a hill because the front wheels of the car lost grip on the loose stones

I have been known to "sit" on our Saab's bonnet, to get traction. Those were the days! Proper Saab days, "the engineers car" as they were known.

 

[ProfJohnL]

Gafferbill
I agree that very few if any caravan drivers know exactly what their outfit weighs and what it's real world ratio actually is. To do so would require the outfit to be weighed before each journey, something that would be a total culture shock and impractical.

You later suggest that you always like to have your car weighing 15% more than your caravan. What intrigues me is how you arrived at 15%, why not 10% or 20%? Something must have drawn you to choose that figure?

 

[JTQ]

You later suggest that you always like to have your car weighing 15% more than your caravan. What intrigues me is how you arrived at 15%, why not 10% or 20%? Something must have drawn you to choose that figure?

(100 minus 15)/100 =85% 😉

 

[camel]

My car and caravan could be a perfect match when I set off on every journey but the further I go down the road the more unstable it gets, what does a tank of fuel weigh by the time I arrive down in Dorset I have an unstable outfit, how Do I get round this should I have on board water ballast tanks,

 

[Lutz]

My car and caravan could be a perfect match when I set off on every journey but the further I go down the road the more unstable it gets, what does a tank of fuel weigh by the time I arrive down in Dorset I have an unstable outfit, how Do I get round this should I have on board water ballast tanks,

If the weight of a tank of fuel makes the difference between a stable and a potentially unstable one then there is something inherently wrong with the the outfit in the first place. It shouldn't be that sensitive.

 

[camel]

60 lts of diesel 49.93 kgs I don't all that sensitive,

 

[Lutz]

60 lts of diesel 49.93 kgs I don't all that sensitive,

Due to all the other things that are loaded into the car when towing the caravan, it will undoubtedly weigh more even with an empty fuel tank than the kerbweight upon which the weight ratio is based.

That aside, 50kg shouldn't make a noticeable difference.

 

[Parksy]

My car and caravan could be a perfect match when I set off on every journey but the further I go down the road the more unstable it gets, what does a tank of fuel weigh by the time I arrive down in Dorset I have an unstable outfit, how Do I get round this should I have on board water ballast tanks,

In what way does your outfit become unstable Camel?

 

[camel]

The vehicle is lighter thought you would of known that,

 

[ProfJohnL]

My car and caravan could be a perfect match when I set off on every journey but the further I go down the road the more unstable it gets, what does a tank of fuel weigh by the time I arrive down in Dorset I have an unstable outfit, how Do I get round this should I have on board water ballast tanks,

Is this a genuine question from your experience? or is it a wind up?

Giving you the benefit of the doubt, and if the difference in handling does deteriorate to the extent it becomes unstable by the end of your journey simply because of the use of fuel, then it raises serious question of what makes you think the outfit was adequate matched at the start of the journey.

 

[Lutz]

The vehicle is lighter thought you would of known that,

But as I pointed out earlier, it’ll still be heavier than the kerbweight upon which the weight ratio is based.
50kg less fuel is more likely than not less than 5% of the actual weight of the car. One would have to be a very sharp individual to detect such a small difference in the way the outfit handles.

 

[camel]

Is this a genuine question from your experience? or is it a wind up?

Giving you the benefit of the doubt, and if the difference in handling does deteriorate to the extent it becomes unstable by the end of your journey simply because of the use of fuel, then it raises serious question of what makes you think the outfit was adequate matched at the start of the journey.

Prof, Do you not know where I'm coming from?, think about it,