What do you think of the 85% rule?

[Raywood]

Before towing anything heavier than your car you should check your insurance as at least one firm excludes claims where a trailer heavier than the car is being towed. I can only assume this has been added because of one or more serious accidents they have had to pay out on.
When people quote the towing limit they should be aware this has nothing to do with safety but is based on the vehicles ability to start on a 12% hill five times.
I have seen a vehicle snake in front of me to the extent we expected it to be wrecked although by some miracle the driver did manage to control it. Having seen this I would never want to be in that position and getting the weights and loading right has to be a priority for us.

 

[NigelHutson]

So how many cars are designed to tow a large 1-2 ton box?

Cars are designed as people carriers with a slight nod to a few quirky people who want to tow caravans.
By sticking a caravan on the back of a car the driver and passengers are exposing themselves to a whole host of extra risks and it is all about minimising those risks.
We all learn by experience and from guidance from experienced practitioners.

In the case of the 85% towing ratio .......this is guidance given by a whole host of experienced practitioners in caravanning.
You will always get people who question this guidance for various reasons and many will tell you they have ignored it for years without consequence.
These people are right in that the 85% guidance is not the law or the only factor to be taken into consideration.
I prefer to think of this guidance in a different way and think it should be.......... the towing vehicle's kerb weight should weigh 15% more than the caravan MTPLM.

I for one think this is a relevant and useful cut off figure to aim at.
The best advice is that the caravan should be as light as possible in relation to the towing vehicle.
With that in mind ........I tow at 62% ratio.

Hi Bill,

Whilst not getting into the debate about percentages, you'd be surprised at how many vehicles we see being tested for towing each year at MIRA whilst doing the Tow Car Awards event. Unsurprisingly those manufacturers generally do well at the event, where we do try to push the cars beyond what you would normally have to do in every day towing (otherwise, I suspect most would be good these days).

A couple of years ago I had the pleasure of driving a high up representative from a car manufacturer, and when I asked them how they develop their cars for towing, his reply went along the lines of, "Well, we don't test them specifically, but we know they'll be good." I had all on to stop myself saying that it showed, because most of them that we've tested have been ****!

Nigel.

 

[colin-yorkshire]

without going over old ground, or duplicating what the Prof has written, I still do not understand why so many appear to believe that some untested undefined and in most cases unattchievable figure is so important.
there is no data or testing that proves a 50% tow ratio is any more stable than 100% never mind sticking with a figure somewhere in the middle, it is more important to instruct new towers in the art of loading, weight distribution, and driving technique.
I read some of the replies and think OMG where does that come from!! OK so some advocates state they consider it is a good starting point but why !! it is almost impossible to define, without the use of a weighbridge, there are very few specific vehicle weights published to work from as trim level, models, ect vary so much, then theres the van using the MTPLM, is a maximum but how much does it actually weigh, you cannot assume it is the MIRO plus user payload as most vans do not stipulate what the MIRO contains, and of course there the noseweight to deduct from van weight to find the tow load, once you have gone to the trouble of finding out what the true weights are.
given all the parameters involved, a quick guess could be anything from 75 to 90% so why be specific at 85%. it makes no sense to me at all.
a unit that is towed at 50% can be just as unstable as anything at a higher ratio. hell even a few PSI in the tyres makes a difference, there are of course the diehards who defend it stating they have always used it and never had a problem so therefore it must be a good idea, but if you have never towed at a higher ratio how do you know.
and of course there is the van jumping around on the road and swaying wildly [as seen by all] there are so many things that can cause this, but just because the van may be a biggish one may have nothing to do with it's ratio to the tow car just driver error, the question should how it got there in the first place.
on a personal note my last outfit was approximately 73% did it tow better that the last one at 95% NO. in some respects it was worse but had nothing to do with the weight ratio.

 

[JonnyG]

this makes a good read... https://www.caravanclub.co.uk/media/1022856/choice-of-towcar-mo.pdf
As i am amazed at some of the responses I've read... criticizing the 85% rule or harping on how unscientific it is!!!!
to me it all makes good sense and a good starting point for towing although i try to stay well below that rule of thumb figure as that also makes even more sense to me.until someone lets me borrow their outfit and test its limits on a test track or Motorsport circuit then that's the mainstream of info that i and most other have as a guideline to how to tow safely with concern to vehicles weight ratio's....sometimes there is no scientific one fits all answer as there are too many permutations and this is one of them. soo rule of thumb guidelines are the best you can get. .
take it or leave it....

 

[colin-yorkshire]

This is a bit long winded but shows the dynamics involved when passing or being passed by a lorry,
http://caravanchronicles.com/guides/understanding-the-dynamics-of-towing-being-overtaken/

Hutch

hi Hutch sorry just seen this, in reply, most of us who have towed a while are aware of this syndrome the worse one is the car transporter where the air flow is not constant as it moves around the load, [aerodynamic cars] one can often get sudden changes in air flow and vortexes, that tend to throw the van around, more than other HGV's.
one controversial method to avoid this adopted by many towers is to drive just over the middle of the lane your in slightly to the right, this makes the overtaking vehicle give you more room by also driving slightly to the right, then as the vehicle begins to overtake move slightly to the left this increases the gap between the two units and cuts down the effect explained in the link. yes I said it was controversial but it does work.

 

[JonnyG]

This is a bit long winded but shows the dynamics involved when passing or being passed by a lorry,
http://caravanchronicles.com/guides/understanding-the-dynamics-of-towing-being-overtaken/

Hutch

hi Hutch sorry just seen this, in reply, most of us who have towed a while are aware of this syndrome the worse one is the car transporter where the air flow is not constant as it moves around the load, [aerodynamic cars] one can often get sudden changes in air flow and vortexes, that tend to throw the van around, more than other HGV's.
one controversial method to avoid this adopted by many towers is to drive just over the middle of the lane your in slightly to the right, this makes the overtaking vehicle give you more room by also driving slightly to the right, then as the vehicle begins to overtake move slightly to the left this increases the gap between the two units and cuts down the effect explained in the link. yes I said it was controversial but it does work.

hi Colin. i think these days its best if you are being overtaken by a large vehicle to just move over to the left of your lane...given some of the animosity i read on some HGV forums about this practice of trying to force an HGV vehicle to the right of its lane is now well known. i certainly wouldn't try these day to do it. i move to the left of my lane, no way would i try to move to the right of my lane first knowing the hgv driver might not move right, worse read several threads with posters saying they would be inclined to move to the left of there lanes!
which when you clear you head and think about it is no worse than purposely trying to force an HGV lorry to move to its right..

 

[Dustydog]

Having read all the posts if I was a Newbie would I want to be a caravanner. ?
Reviewing all the posts what comes out is twin axles are more stable.
There is no specific weight ratio that makes for a good tow.
It's all down to the driver.

Going forward has anyone a few simple suggestions that will help the Newbie become a safe tugger? ?

 

[JonnyG]

i reckon the basic stuff is still here for all to see,but it might take quite some time for a newbie to read through all the material and sort out what was what..when i took to caravaning some 14 years now i joined both the caravan clubs.and read up on their websites which still are full of good info,as is this site and the magazine it backs up..
problem is you will find posters whose knowledge exceeds the basics by a long way and that in itself can be a problem as they question the basics or rule of thumb or recommendation from a position of gathered knowledge,very useful at times but not necessarily of help to a newbie . would i still take up caraving today ? yes but i would be a little more confused than i was 15 years ago..

 

[Hutch]

A Dusty Dog sugest, notes to newcomers,

Take your time , enjoy your trip, 5 mph less means you get there 5 miutes later , giving you less stress to argue over putting up the awning,
Normally getting on site 20 minutes from un hitch to beer, or if being the 5 o'clock un hitch where everything goes wrong 1 to 2 hours. Keep smiling.
Hutch.

 

[colin-yorkshire]

This is a bit long winded but shows the dynamics involved when passing or being passed by a lorry,
http://caravanchronicles.com/guides/understanding-the-dynamics-of-towing-being-overtaken/

Hutch

hi Hutch sorry just seen this, in reply, most of us who have towed a while are aware of this syndrome the worse one is the car transporter where the air flow is not constant as it moves around the load, [aerodynamic cars] one can often get sudden changes in air flow and vortexes, that tend to throw the van around, more than other HGV's.
one controversial method to avoid this adopted by many towers is to drive just over the middle of the lane your in slightly to the right, this makes the overtaking vehicle give you more room by also driving slightly to the right, then as the vehicle begins to overtake move slightly to the left this increases the gap between the two units and cuts down the effect explained in the link. yes I said it was controversial but it does work.

hi Colin. i think these days its best if you are being overtaken by a large vehicle to just move over to the left of your lane...given some of the animosity i read on some HGV forums about this practice of trying to force an HGV vehicle to the right of its lane is now well known. i certainly wouldn't try these day to do it. i move to the left of my lane, no way would i try to move to the right of my lane first knowing the hgv driver might not move right, worse read several threads with posters saying they would be inclined to move to the left of there lanes!
which when you clear you head and think about it is no worse than purposely trying to force an HGV lorry to move to its right..

hi JonnyG. I wasn't advocating forcing HGV's to move to the right, merely stating that if one drove slightly to the right, the overtaking vehicle would give more of a space in the act of overtaking. [ever noticed the difference in the gap HGV's give to other HGV's in relation to other road users] if not take note,
one simply then eases left increasing the gap it has no effect at all on the overtaking vehicle as they proceed as normal in their own lane as if overtaking another HGV. :lol:
not that it happens often [well not to me anyway] as the HGV's are doing 55mph and I'm doing 60mph so do not get passed often, except if joining the carrageway or boxed in by a line of HGV's overtaking and then it doesn't seem to matter as the forces exerted on the van are different and have less effect. one thing I would not do is move right over to the left of the inside lane as this is where all the debris is and the van is wider than the car.

as for the link yes I have read it quite a few times as it turns out, as the one published by the C&CC which is the same but different in the way one is supposed to calculate the 85% , it mentions using the car unladen weight calculated by the actual tow load of the van not it's MTPLM but does not mention the effect noseweight has on tow load, the C&CC on the other hand recommend the MTPLM should be no more than 85% of the loaded car.
as one can easily see the two recommendations are similar but in practice vary wildly as is the NCC that suggest you use the u/w of the car and the MTPLM as yet a third way of obtaining 85%.
while all 3 suggest the same thing, in practice the results would vary greatly so the recommendation is not universal, while you may agree in principal with it, which one is supposed to be right, that is why I never advocate it too many variables, to consider I also noted that in the link it goes to great lengths to explain why it is a good idea and why they recommend it but in 18pages of text never mention once the need to do correct loading and pre towing checks.

what does worry me a bit is when a newbie in their first post state they have just taken delivery of a 6mtr caravan and while having never towed anything before want information as to what car they should get to tow it with.
the answer I would love to give [but never have] to ask where they live and where they are intending to go with it so I can keep well away from the area.

 

[ProfJohnL]

This is a bit long winded but shows the dynamics involved when passing or being passed by a lorry,
http://caravanchronicles.com/guides/understanding-the-dynamics-of-towing-being-overtaken/

Hutch

This is indeed an interesting blog, and much of what it contains is quite good, but just because its nicely presented doesn't mean its free from errors, and indeed the author has not fully understood the way to measure nose , load correctly, he does not arrange to set the hitch height to the same as the coupled height,

He also implies that towing with the caravan in an attitude of nose up will compromise braking ability, This is not the case because the imposed thrust of the caravan is generated by the caravan's centre of gravity which will always be above the hitch height and forward of the caravan axle thus it will will create a turning moment that increases the nose load during braking. This attitude of the caravan has no bearing on the braking effect of the tow vehicle.

He also implies that because a TA is difficult to turn by hand it must be more stable when coupled. That is not necessarily so because the mechanism that that resists turning a caravan on its axis is actually the tow hitch of the car. consequently any force applied to the side of the caravan will not spin it on its axis but try to turn it around the tow hitch. The caravan tyres would have to be scrubbed sideways to move the caravan. The force necessary to achieve this is fundamentally the weight of the caravan, acting through the tyres. Extra tyres simply means the load is spread over a greater surface area but with a lower pressure, and consequently the actual side force to over come this will be virtually the same for a SA and a TA. which means there is no significant difference the a caravan resistance to sideways movement by the number of axles.

 

[Dustydog]

The following thread discusses in a lot of detail which tends to support both the technical physics and the overall general experience of a twin axle.
Note our old friend Lutz speaks his mind :cheer:
http://www.caravantalk.co.uk/community/topic/92003-single-or-twin-axle-which-is-most-stable-for-towing/

The coefficient of friction makes sense yet in another world perhaps it doesn't.
A six coupled steam engine gave better adhesion on the tracks than a four coupled unit of the same weight. So some may argue the TA is more stable than a SA?
Forgetting the physics for the moment the general concensus of opinion is the TA is a more stable tow. Now why should so many seasoned caravanners say that when allegedly the physics says no :silly: :blink:

 

[Martin24]

Okay so I never have professed to be technically minded and have been shot down more than once for my flippant answers, especially regarding nose weight.
When I first went from single axle to twin axle I noticed a huge improvement in how stable the outfit felt and that's what I'm going to go by.
I used to be very hung up on the 85% rule which I had grown up with. I still agree with it for novice towers as well to some extent. What I have always been hung up on and I feel is the most important thing for stability is loading the outfit correctly.
Now I know I don't do much of the towing these days but we are both in agreement that our current outfit is the most stable in all respects we have ever had overall.
Just done the figures and hey ho we're at 90% so this leads me to question why?
Twin axle tick, narrower van so less wind resistance tick, proper loading every time tick, a vehicle designed with work in mind over comfort and convenience tick and keeping to the speed limit tick. All in my mind more important than whether we're running an outfit at 85% or 90% There you never thought I'd say that did you?!

 

[ProfJohnL]

Hello Dusty,

The information for new towers is there, it just needs to be quoted and used in a proper context, and not given exaggerated relevance and perceived authority.

The thread you point to is another example of a caravanning herd mentality. i.e. an individual taking action (e.g. buying a twin axle caravan) based on assumptions that are hearsay rather than properly considered mechanical and scientific processes.

Now I’m not saying don’t buy a TA if that’s what suits you, but if your purchase is being driven by thoughts of better stability, better control if a blow out occurs, then think again. There is no physical reason to believe a TA will be any better than an SA.

However there has been a groundswell of comments that suggest that TA’s do have improved attributes, but I suspect two things have happened, first is when caravanners move to a TA, it is usually a bigger caravan, which naturally has better stability because of its length and mass, so comparisons made are not apples to apples.

And secondly, people have an expectation based on the groundswell that a TA will be more stable, and so it is easy to convince yourself yours is more stable. Humans are notorious for not being able accurately recall and clinically compare items that have similar performance, so they can easily be swayed by preconceptions that A might be better than B, or vice-versa, when in fact there may be no actual difference or the opposite may be true. You must also consider that the external factors that may excite instability are unlikely to be exactly the same on any two trips, so it’s really difficult to make truly balanced comparisons.

Other misconceptions expressed in the thread such as “four must be better than two”, “more rubber on the road must equate to better grip” etc fail to consider the facts and underlying scientific principles. For example the difference of rolling friction vs compared to lateral grip.

I have no connection with Lutz except we both seem to share a healthy scepticism for the numerous traditions and common preconceptions made about towing caravans. I miss his valuable insights and contributions here on PC Forum.

 

[colin-yorkshire]

I miss his valuable insights and contributions here on PC Forum.

As do we all John,

never had a T/A van so have no experience of them, although I once had a double axle boat trailer, cannot say I noticed any difference in towing, although it was not like for like in respect of a caravan, as they have the axles amidships where boat trailers tend to be further back [as that is where the most weight is]

 

[seth1]

What ever happened to Lutz?Never hear of him anymore.

 

[Gafferbill]

Nothing .......he just migrated to the Caravan Talk forum.

 

[seth1]

Right,thanks.

 

[Gafferbill]

..........as is usual with this topic some people have missed the whole point of the 85% guideline.

They immediately confuse the issue by bringing in other aspects of caravan towing safety.
Surely the 85% guidance is only relevant when a towcar is being matched to a caravan or visa versa.
I does not preclude or diminish all the other aspects of safe towing that come into play once a matching choice has been made.
All the main interested parties such as the Caravan Club, Camping and Caravanning Club, National Caravan Council, AA and RAC, repeat this guidance as a good ratio to aim at as do many Caravan orientated magazines.
These bodies are well known and contactable and have proven reputations.

If I was a new tower I think I would listen to such bodies rather than people who post on forums and go out of their way to remain anonymous.
I include myself in this category even though I have met face to face with quite a few members of this forum.

 

[Dustydog]

..........as is usual with this topic some people have missed the whole point of the 85% guideline.

They immediately confuse the issue by bringing in other aspects of caravan towing safety.
Surely the 85% guidance is only relevant when a towcar is being matched to a caravan or visa versa.
I does not preclude or diminish all the other aspects of safe towing that come into play once a matching choice has been made.
All the main interested parties such as the Caravan Club, Camping and Caravanning Club, National Caravan Council, AA and RAC, repeat this guidance as a good ratio to aim at as do many Caravan orientated magazines.
These bodies are well known and contactable and have proven reputations.

If I was a new tower I think I would listen to such bodies rather than people who post on forums and go out of their way to remain anonymous.
I include myself in this category even though I have met face to face with quite a few members of this forum.

Oh dear Bill
So you too are a Herd Mentalist 😉
At the risk of upsetting the Prof I have to agree with Martin the TA still takes some beating on the general stability front.

 

[seth1]

Im just glad im not a newbie.Reading certain posts can be quite scary.But then if people choose to believe certain posts more fool them.

 

[colin-yorkshire]

..........as is usual with this topic some people have missed the whole point of the 85% guideline.
All the main interested parties such as the Caravan Club, Camping and Caravanning Club, National Caravan Council, AA and RAC, repeat this guidance as a good ratio to aim at as do many Caravan orientated magazines.
These bodies are well known and contactable and have proven reputations.

If I was a new tower I think I would listen to such bodies rather than people who post on forums and go out of their way to remain anonymous.

Bill, I think the problem is just because something becomes acceptable does not make it right, time and again I have repeated the inconsistancies of the recommendation, the bodies you quote all make a point of saying 85% and then go on to stipulate how to calculate it in as many different ways as there is the people that recommend it,
the results in the calculations using the different methods give a wide variation of the weights that by themselves are meaningless, for example using the calculations provided by the C&CC to achieve 85%, and then do the same calculations as provided by the CC [ie, as done on my last unit] the weight difference was 600kg, or 40%.
so according to one I would have been over the 85% by 300kg or 20% and by the other under 300kg and towing at 65% they both cannot be right, so one has to assume both are wrong.
the recommendation is not universal, and therefore just a best guess.
the other point surely has to be what is safe as the recommendation is on safety grounds, are they really saying
a new caravan tower, on their first few trips are any safer towing a 6 mtr 1600 kg caravan just because they have a hummer to tow it with, and it is any safer than any other vehicle just because it is a 85% or lower match.
I think not Bill. no where in any of the publications do they address other aspects of towing a caravan safely this is not confusing the issue, but stating the obvious a poorly loaded 50% matched unit will tow less well than 90% unit correctly loaded to some of us this is more important than any mythical figure dreamed up decades ago, anonymously declared or not,

 

[Gafferbill]

Colin .......I believe your last tow car was a Vauxhall Meriva.
I believe that this model had a maximum legal towing ratio of 85% as laid down by the manufacturer.
Why do you think that level was set?
I would think that Vauxhaul decided that that was the maximum weight of caravan they wanted that model to tow.
I say ......good on them!

It is a pity more manufactures don't quote a more realistic maximum towing weight limit for a particular model.

 

[ProfJohnL]

.................
It is a pity more manufactures don't quote a more realistic maximum towing weight limit for a particular model.

Hello Gaffer,

Can you define what a more realistic towing weight actually is?
How would you derive your chosen value? Would it apply to all trailers, or just caravans or indeed all sizes of caravans?
What would be the measure of success or failure?

The maximum towed weight limit for any vehicle is not contrived by a committee, or someone holding a finger in the air, but as the result of a set of repeatable tests. (notwithstanding the VW emissions debacle).

There would be no reason for a car manufacturer to artificially reduce a towed weight limit, as that would reduce its appeal to the widest possible market.

Perhaps the caravan Industry would like to take a leaf out of the car industries book and actually tell us how they came up with the 85% guidance figure - It seems the origin of the guidance is as anonymous as you and I.

 

[colin-yorkshire]

hi Bill, yes that is correct it was a Meriva still is as I kept it, the trailer limit is in fact 82%. due to the TUV approval for the rear floor plan and tow bar attachment, [thats why it has a 50kg nose limit] and problems with the cooling system.on 100BHP common rail turbo diesel engines.
the later models that had the changed rear end and double radiators and fans had the tow limit increased by 125kg yet the model was 150kg lighter, making the type approval nearer 90% with a 60kg nose limit. I do not think Vauxhall/Opel would have had a recommendation from the British Caravan Club in mind when they set the trailer limit somehow. the calculations I did was based [as one would] on the vehicle U/L weight, @ 85% and running weight/ tow load [as the other one would] @85% completely ignoring the plated weights on the car.

but you do raise a good point though, the figures on the weight plate ARE more important than working out a figure based on the vehicle/caravan weight ratio, as these are legal limits not recommendations. some cars have a well reduced figure and some cannot be used to tow with at all.